Showcasing research from France-Japan Collaboration
by Professors C. Jouvet (U. Aix-Marseille), M. Miyazaki(Ochanomizu U) and M. Fujii (TokyoTech).

As featured in:

Revealing the role of excited state proton transfer (ESPT)in excited state hydrogen transfer (ESHT): systematic
study in phenol–(NH3)n clusters
A general model of excited state hydrogen transfer (ESHT)
which unifies the excited state proton transfer (ESPT) andthe ESHT mechanism is presented. It reveals the hiddenroll of ESPT in controlling the reaction rate of the ESHT.
For this purpose, experimental and theoretical works
on the excited state dynamics of phenol-(NH3)n clusters
and related molecular systems over the last 30 years are
overviewed. The systematic change from ESHT to decoupledelectron/proton transfer according to the number of solvent
molecules in the clusters is rationalized by the general modelof ESHT including the role of ESPT.

See Christophe Jouvet,
Masaaki Fujii et al.,Chem. Sci., 2021, 12, 3836.

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Revealing the role of excited state proton transfer(ESPT) in excited state hydrogen transfer (ESHT):
systematic study in phenol–(NH3)n clusters
Christophe Jouvet,*ab Mitsuhiko Miyazaki

cd

and Masaaki Fujii*bd

Excited State Hydrogen Transfer (ESHT), proposed at the end of the 20th century by the corresponding
authors, has been observed in many neutral or protonated molecules and become a new paradigm tounderstand excited state dynamics/photochemistry of aromatic molecules. For example, a signiﬁcantnumber of photoinduced proton-transfer reactions from X–H bonds have been re-deﬁned as ESHT,
including those of phenol, indole, tryptophan, aromatic amino acid cations and so on. Photo-protection
mechanisms of biomolecules, such as isolated nucleic acids of DNA, are also discussed in terms of ESHT.
Therefore, a systematic and up-to-date description of ESHT mechanism is important for researchers inchemistry, biology and related ﬁelds. In this review, we will present a general model of ESHT which
uniﬁes the excited state proton transfer (ESPT) and the ESHT mechanisms and reveals the hidden role of
ESPT in controlling the reaction rate of ESHT. For this purpose, we give an overview of experimental andtheoretical work on the excited state dynamics of phenol–(NH3)n clusters and related molecular systems.
Received 17th December 2020
Accepted 25th January 2021

The dynamics has a signiﬁcant dependence on the number of solvent molecules in the molecularcluster. Three-color picosecond time-resolved IR/near IR spectroscopy has revealed that ESHT becomes
an electron transfer followed by a proton transfer in highly solvated clusters. The systematic change

DOI: 10.1039/d0sc06877bfrom ESHT to decoupled electron/proton transfer according to the number of solvent molecules is

rsc.li/chemical-science

rationalized by a general model of ESHT including the role of ESPT.

a

CNRS, Aix Marseille Université, Physique des Interactions Ioniques et Moleculaires(PIIM), UMR 7345, 13397 Marseille Cedex, France. E-mail: christophe.jouvet@
univ-amu.frbWorld Research Hub Initiatives, Institute of Innovative Research, Tokyo Institute ofTechnology, 4259-R1-15, Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan

During my scientic career at
CNRS, I worked on the photochemistry of molecules andclusters. Starting at the University Paris-Iud I developed
a method to catch the intermediate state of a bimolecular
photochemical
reactionbyfreezing the collision pair ina van der Waals complex. Using
molecular cluster I evidenced therole of the hydrogen loss reaction in the excited state ofaromatic molecules which lead to hydrogen transfer. Thishydrogen dynamics is now recognized as responsible for a lot of
non-radiative processes in aromatic subsisted molecules, including
protonated ones that I study now at the Aix-Marseille University.

3836 | Chem. Sci., 2021, 12, 3836–3856

cNatural Science Division, Faculty of Core Research, Ochanomizu University, 2-1-1
Ohtsuka, Bunkyo-ku, Tokyo 112-8610, JapandLaboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo
Institute of Technology, 4259-R1-15, Nagatsuta-cho, Midori-ku, Yokohama 2268503, Japan. E-mail: mfujii@res.titech.ac.jp

Mitsuhiko Miyazaki received his
PhD in chemistry in 2004 fromTohoku University. Aer postdoctoral research at Kyoto
University and Institute for
Molecular Science, he held
a position of Assistant Professor
at Tokyo Institute of Technology
in 2005. He was appointed
Associate Professor at Ochanomizu University in 2019. He was
granted by the Humboldt Foundation as an experienced scientist, and stayed in Technische Universität Berlin 2017–2019. He
was awarded by The Spectroscopical Society of Japan (2013),
Japan Society for Molecular Science (2015). He has been studying
intermolecular interactions and reaction dynamics using molecular clusters according to advanced laser spectroscopic techniques.

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1. Introduction
Excited state proton transfer (ESPT) and excited state hydrogentransfer (ESHT) play important roles in photochemistry andbiology. These photochemical reactions have been explored for
30 years via the spectroscopy of molecular clusters. The initialidea was that aromatic alcohols are more acidic in the excitedstate than in the ground state and therefore, in a molecularcluster, it should be possible to induce a proton transfer reaction by optical excitation, i.e. ESPT. However, following developments in experimental techniques and progress incomputational chemistry, controversial results were reported,
particularly in phenol (PhOH) hydrogen-bonded (H-bonded)clusters, and nally ESHT took over ESPT in the descriptionof many photochemical processes. This research topic – started
in around 1985 – has since provided many surprises, with theresults having had a signicant impact on our understanding of
the photochemistry and photophysics of substituted aromatic
molecules, including biomolecules.
Due to the importance of this topic, as will be shown below,
a substantial number of papers on ESHT and ESPT have been
published. Despite all that has been published on this subject
over a period of around forty years, it seems that only the mostrecent studies have given rise to an accurate understanding of
ESHT. It should also be noted that the understanding of theexperimental results is now facilitated by ab initio calculationsof excited states, a technique that has made considerable
progress in recent years. Theoretical calculations well reproduce
not only the energy of the electronic transitions but also theintensities of the active vibrational modes in spectroscopy.
From the theoretical calculations and advanced experimental
results, we can understand the relation between ESHT and
ESPT, which has not been previously discussed in a systematicmanner. In this review, the role of the ESPT underlying the
ESHT process will be discussed using the benchmark molecularsystem PhOH–(NH3)n (n ¼ 1–5). The review will begin by giving

Masaaki Fujii obtained Dr. Sci.
from Tohoku University (1987).
He was appointed as Professor ofTokyo Institute of Technology in2003 via Assistant Professor of
Tohoku U., Visiting Scientist ofCornell U., Associate Professor of
Waseda U., Professor of Institutefor Molecular Science andDirector of Laser Research Center
for Molecular Science. He has
been studying spectroscopy anddynamics of isolated moleculesand clusters by developing novel multicolor laser spectroscopy
including picosecond time-resolved UV-UV-IR spectroscopy. He wasawarded by Chemical Society of Japan (2014), The SpectroscopicalSociety of Japan (2015), Japan Society for Molecular Science (2018)
and Humboldt Foundation (2019).

© 2021 The Author(s). Published by the Royal Society of Chemistry

Chemical Sciencean overview of progress in this research eld from the initialidea of ESPT to the establishment of ESHT.

2. Research history from ESPT to
ESHT
2.1

Proton transfer in the condensed phase

Since the pioneering work of Förster,1 the protolytic processes(displacement of the acid–base equilibrium or proton transfer
in hydrogen-bonded complexes) induced by optical electronic
excitation have been studied in the condensed phase.2 Emission
from neutral photoexcited 1-naphthol (1-NpOH*) is shiedfrom its gas phase value at 350 nm in the near ultraviolet (UV) to
the visible (blue) in an aqueous solution. The blue emissionaer UV excitation was initially interpreted as the formation of
the naphtholate anion3 and accompanying proton release fromthe excited state, i.e. ESPT. This implies a dramatic decrease inpKa (9.1 (S0) to 0.5 (S1)) upon photoexcitation to the rst excitedstate of 1-NpOH. The decrease of pKa was also found in aromatichydroxy, amino, and carboxy derivatives, and thus ESPT was
established in the condensed phase.
2.2

Gas phase PT

Since the 1980s, molecular beam laser spectroscopy has been
applied to aromatic molecular clusters and ESPT has beenstudied in both the frequency and time domains. A combination of several factors explains the success of this technique.
Collisional relaxation can be neglected in a molecular beam.
The number of molecules in the cluster is clearly specied whencoupling with mass-selected multiphoton ionization detection.
The relative orientations and geometries of molecules in thecluster are well dened because of the low temperature.
Furthermore, an excited state reaction implies that the opticalexcitation is triggering the reaction, thus the reaction dynamics
can be followed in real time, from a well-dened t ¼ 0 of thereaction. Due to these advantages, the ESPT mechanism can berevealed at the molecular level by using molecular beam laser
spectroscopy.
2.3 ESPT in naphthol/phenol–ammonia clusters and thecontradictions
Leutwyler and coworker4 provided the rst example of ESPT in
the gas phase by the study of the uorescence spectra of AH–B
clusters (AH ¼ 1-NpOH, B ¼ (NH3)n, (piperidine)n). A red-shiedemission assigned to the proton-transfer process AH*–B /A*–(HB)+ was observed only from the excited state of suﬃciently large clusters (n $ 4 or 2 for ammonia or piperidine,
respectively). The observation of a threshold number of solvent
molecules for an intracluster ESPT attracted the researchgroups specializing in time-resolved spectroscopy combinedwith the mass-selected multiphoton ionization technique. The
dynamics in S1 were measured by picosecond (ps) pump–probe
spectroscopy of NpOH–(NH3)n. However, not only n ¼ 4 but also
n ¼ 3 were reported to be the threshold size of ESPT in NpOH–(NH3)n.4–7 High-resolution time-of-ight mass spectroscopy
revealed the inuence of NH3 evaporation aer ionization in
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these experiments, and eventually it was concluded that thethreshold size was n ¼ 5.8 This threshold size was also supported by recent theoretical calculations.9
It was expected that solvated clusters of PhOH and NpOH
show similar behavior as far as ESPT is concerned. For PhOH
solvated clusters, ESPT was postulated from the lowering of theionization potential which was measured by the two-color
resonance-enhanced multiphoton ionization (REMPI) technique.10,11 Since a strong H-bond lowers the ionization potentialof the proton donor (acid), one can expect a large shi of theionization thresholds when the ion pair is formed in the clusters. As expected, a strong lowering of the IP was found in
PhOH–(NH3)n at n ¼ 4 and was interpreted as the ionizationfrom the proton-transferred structure (PT structure).
There was, however, a notable diﬀerence between PhOH–(NH3)n and NpOH–(NH3)n clusters: free NH4+(NH3)n1 clusters
formed aer photoexcitation were detected only for PhOH–(NH3)n clusters and not for NpOH–(NH3)n clusters.12 Moreover,a ps rise time of the NH4+(NH3)n1 signal was similar to the
decay signal of the parent PhOH–(NH3)n. The formation of
NH4+(NH3)n1 clusters aer the ionization of the PhOH–(NH3)ncluster was observed, and the energy dependence of the ps
dynamics was probed. The observation of NH4+(NH3)n1 implies
the presence of a high barrier in the ionic state at a size where
the barrier is very small in the excited state. However, it is wellestablished that the acidity of ionic species is larger than that ofneutrals13 thus the barrier should be lower than that in S1. In
addition, Schmitt14 et al. found that the action spectra of PhOH–(NH3)n clusters obtained by monitoring NH4+(NH3)n1 show
well-resolved vibronic structures in S1. This posed a problem, as
no ESPT is expected for small cluster sizes (n # 3), whereas theappearance of protonated fragments was supposed to bea signature of the proton transfer mechanism either in the
excited or in the ionic state. These contradictions suggest thepresence of dynamics other than ESPT. A full discussion ofthese contradictions can be found in two reviews.12,15

2.4

Discovery of ESHT

These contradictions were resolved upon the discovery of ESHTat the end of the 20th century. The rst key experiment was theobservation of the cNH4(NH3)n hypervalent cluster produced byexcitation of the S1 state of PhOH–(NH3)n.16,17 In 1999, Pino et al.
observed the formation of a long-lived species from the S1
excited state by a delayed ionization of PhOH–(NH3)n clusters.16
Serendipitously, they measured the pump–probe photoionization mass spectra not only at a short delay (Dt ¼ 0) but also atvery long delay (200 nanosecond (ns)), which is obviously longer
than the lifetime of the parent cluster in S1 (see Fig. 1). Whenthey measured the mass spectra without delay Dt ¼ 0, bothparent ions PhOH–(NH3)n and fragment ions NH4+(NH3)n1
were detected (see Fig. 1). The typical lifetime of hydrogenbonded PhOH clusters is 10 ns therefore all the signals
should be very weak with a delay of 200 ns. Indeed, the mass
peaks of parent ions PhOH+–(NH3)n become signicantly
weaker. However, even with such a long delay, the mass peaks ofthe fragment ions remain. This indicates that long-lived neutral

3838 | Chem. Sci., 2021, 12, 3836–3856

Review

Fig. 1 Demonstration of the formation of cNH4(NH3)n1 clusters froma two color ionization of PhOH–(NH3)n via S1.16,17 – The lower trace:
a second ns laser at 355 nm, with no delay, ionizes the PhOH*–(NH3)nexcited by the ﬁrst laser (281.75 nm) within the S1 lifetime. Simultaneously, NH4+(NH3)n cluster ions are observed. – Upper trace: The
second (ionization) laser is delayed by 200 ns from the ﬁrst laser. It
cannot excite the PhOH*–(NH3)n, of which the lifetime is a ns, thus
PhOH+–(NH3)n mostly disappears. In contrast, NH4+(NH3)n ionsremain at almost the same intensity. This suggests the production of
long-lived species, i.e. cNH4(NH3)n1. Reproduced from ref. 17 withpermission from the PCCP owner Societies, copyright 2000.

species are generated from the photoexcited PhOH–(NH3)n, andphotoionization of these species produces NH4+(NH3)n1. Fromthe various possibilities of long-lived species (including a triplet
state of the clusters), they concluded that these neutral species
are hydrogenated ammonia clusters generated by neutralcleaving of the OH bond, followed by a successive H atomtransfer to the NH3 moiety: PhOH–(NH3)n / PhOc + cNH4(NH3)n1. This result was soon conrmed18 by studying theelectronic spectra of the reaction products using UV-near
infrared (NIR)/IR-UV0 three-color ion dip spectroscopy. The
NIR transition is the signature of a surplus electron in cNH4(NH3)n1, which occupies the 3s orbital and gives rise to Rydbergtransitions, such as 3p–3s in the NIR region. Fig. 2 displays the
NIR spectra obtained by UV-NIR-UV ion dip spectroscopy of
PhOH–(NH3)n (n ¼ 2–5). Regardless of the number of ammonia
molecules in the cluster, broad NIR transitions are observed.
The spectra are very similar to the NIR spectra of cNH4(NH3)n1generated by photodissociation of pure NH3 clusters,19,20 which
are traced in the same gure in dotted lines. This strongly
supports the formation of cNH4(NH3)n1 and neutral cleaving of
the OH bond aer photodissociation, i.e. ESHT. The infrared(IR) spectra measured by UV-IR-UV spectroscopy also conrmed
the formation of cNH4(NH3)n1.21
2.5

The S.D.D.J. model

Theoretical approaches also support the existence of ESHT. Ab
initio calculations were performed on PhOH–(H2O),22,23 witha diﬀuse function on the H atom in order to properly describe© 2021 The Author(s). Published by the Royal Society of ChemistryView Article Online

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Review

Electronic spectra of the reaction products from photo-excited
PhOH–(NH3)n are shown in color.18 Electronic spectra of the reaction
products from photo-excited pure (NH3)n clusters assigned to theexcitation of the cNH4(NH3)n radical cluster are shown in black.19,20
Reproduced from ref. 18 with the permission of AIP Publishing,copyright 2002 and from ref. 20 with permission of Springer Nature,
copyright 1999.
Fig. 2the Rydberg character of the 1ps* state (see Fig. 3 for the orbital
shapes). The calculations revealed a new predissociation
pathway from the S1 (pp*) state to a low-lying 1ps* statethrough a conical intersection, which leads to a concerted
electron- and proton-transfer reaction from the chromophore to

Fig. 3 Orbitals involved in the PhOH–NH3 system. In the ground state,
two electrons are localized in the p orbital. In the ﬁrst excited state, thepp* state, one electron is promoted into the p* orbital. In the second
excited state, the ps* state, one electron is localized in the very diﬀuse
s* orbital on the NH3 moiety.

© 2021 The Author(s). Published by the Royal Society of Chemistry

Chemical Science
the solvent. Theoretical calculations also show that the excitedstate reaction is endothermic in PhOH–(H2O) complex but
exothermic in the PhOH–NH3 complex. Thus, ESHT in PhOH–(NH3)n clusters is also validated by the theoretical results.
Combining experimental and theoretical results, a generalmodel – the SDDJ model23 – was proposed in 2002. In thismodel, a key role is played by excited singlet states of ps*
character, which have repulsive potential-energy functions withrespect to the stretching of OH or NH bonds (see Fig. 4). The
1
ps* potential-energy functions intersect not only the boundpotential-energy functions of the 1pp* excited states, but alsothat of the electronic ground state. The S1 surface typicallyexhibits a local minimum of 1pp* character in the vicinity of theequilibrium geometry of the ground state. The second excitedstate is a dissociative 1ps* which induces a barrier on the S1
surface by its conical intersection with the 1pp* potential
surface. The excited state lifetime is governed by the rate oftunneling through the barrier from the pp* to the ps*.23–25 The
barrier is also responsible for pronounced isotope eﬀects on theuorescence lifetime and quantum yield.26 The ps* statecrosses the ground state at a greater distance, leading to thesecond conical intersection. Classically, at this point, the Hatom can pursue the ps* potential curve and will retain quitea high kinetic energy or be trapped on the ground state surface
leading to a hot molecule which undergoes statistical evaporation of the H atom. From the theoretical point of view, thepotential energy surfaces calculated by diﬀerent methods arequite similar and vary only in details.27–31
2.6 H-loss in the free PhOH molecule and its relation to
ESHT
ESHT was established through the experimental and theoreticalstudies performed on solvated clusters. In monomers, the ESHTof clusters turns into an H loss channel from the excited state. H
atom dissociation in photoexcited PhOH has been wellFig. 4 Potential energy proﬁles of the lowest 1pp* states (squares), thelowest 1ps* state (triangles) and the electronic ground state (circles) as
a function of the OH stretch (PhOH) or NH stretch (indole, pyrrole)
reaction coordinate. Geometries have been optimized in the excitedelectronic states at the CASSCF level; the PE proﬁles have been obtained with the CASPT2 method. Reproduced from ref. 23 withpermission from the PCCP owner Societies, copyright 2002.

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Chemical Sciencecharacterized by the measurement of the kinetic energy of the Hatom. A bimodal distribution is observed. For the rst component, the fast H atoms, the kinetic energy reects the vibrational
progression in the remaining PhOc along the out of planevibration. Here, out of plane vibrations are necessary in order tocouple the pp* and the ps* states because of the diﬀerence inCs symmetry.29,32–34 Such a well-dened and structured kinetic
energy distribution is characteristic of bond rupture on
a dissociative potential. The H-loss channel is also evidenced by
the formation of the PhOc radical, as detected by VUV photoionization.35,36 The second component of the distribution, theslow H atoms, is assigned to a statistical fragmentation which
occurs aer internal conversion following the crossing of the
ps* and the ground state. By measuring both the ps transientsof parent and fragment and the total translational energy
distribution of products as a function of the reaction time, it
was evidenced that both the slow and fast components of the H
fragment have the same tunneling origin and that the groundstate is populated at the second ps*–S0 crossing and not
through a direct pp*–S0 internal conversion process.24,37
The H-loss dynamics have also been studied in the time
domain. The ps time-evolution of the S1 excited state of PhOH
shows that the lifetime decreases from 2 ns at the band origin
000 down to 600 ps at the band located 3500 cm1 above theorigin.38 These surveys show that the excitation of vibrational
levels of A00 symmetry shortens the lifetime, mainly through S1–S2 (ps*) vibronic coupling. At high energy, the direct excitationof the ps* leads to very fast dynamics of about 150 fs at 207 nm(excess energy of 12 000 cm1).39,40 The dynamics of the PhOH
monomer has become a good example to test diﬀerent theoretical methods for excited state dynamics, such as thequantum wave packet propagation method41 or the multistate
semiclassical trajectory method. Xu31 et al. clearly conrm thathydrogen tunneling from the pp* to the ps* potential surface
is the main reaction mechanism in the PhOH H-loss photodissociation at the 000 band origin, and agree with experimentson the bimodal nature of the kinetic energy spectral distribution. They also show that the low kinetic energy release is due toboth statistical dissociations to the ground state and direct
dissociation to the rst excited state of the phenoxyl radical.
This interpretation is consistent with the translational energy
distributions and branching ratios of the phenoxyl radicals
produced in the diﬀerent electronic states.38
Besides pp* to ps* internal conversion, another mechanism to explain H loss was proposed for the PhOH monomer.33
In this model, the rst step was S1–S0 internal conversion andthe H loss was assumed to take place on the ground state
surface. This model could not account for experimental observations such as the reaction in the ammonia cluster or thedecrease of the reaction time upon the intermolecular OH/N
vibration excitation and increase upon deuteration.15 Thismodel was ultimately dismissed following the demonstration of
the correlation of the excited state lifetime of PhOH and its
derivatives with the pp*/ps* gap.24,34,37 It should be stressedthat the cNH4(NH3)n clusters can be seen as a very goodmessenger for characterizing the H loss on a ps* potential.
Indeed, the H transfer requires a fairly direct process (the3840 | Chem. Sci., 2021, 12, 3836–3856

Review
dynamics occur on a repulsive surface) since a process going
through hot ground state molecules (H loss mediated byinternal conversion to the ground state33) would lead toa statistical process (Intramolecular Vibrational Redistribution:
IVR) and the immediate evaporation of the whole cluster orsome of the NH3 molecules. A strong driving force is imperativein order to attach the H atom and repel the cNH4(NH3)n clusterfrom the chromophore, and this cannot be achieved ina statistical process. One can also suppose that the kinetic
energy of the cNH4(NH3)n clusters follows an impulsive type
distribution, but the conrmatory experiment has yet to be
performed. As we will show below, the variation of the dynamics
with cluster size can be very well understand within this pp*/
ps* internal conversion framework.
2.7

Forgotten ESPT behind ESHT

Through extensive experimental and theoretical research, ESHT (orH loss in a monomer) has been characterized in terms of tunnelingor internal conversion from the optically bright S1 pp* state to thedissociative ps* potential surface. In solvated clusters, the dissociative ps* potential will bring the H atom to the solvent moiety,the process terminating in H atom transfer. For monomers, theps* state crosses the ground state potential surface, and internalconversion to the ground state is induced. This is the quenchingprocess of photoexcited aromatic molecules, and is discussed as thebuilt-in mechanism of aromatic biomolecules which have toremain photostable upon UV excitation. The rst publications
concerning ESHT described this reaction as the “Forgotten
channel”16,17 but now it is a popularly invoked photochemical
process, particularly in gas phase studies. On the other hand, ESPT
has become a somewhat “forgotten” process now, and the relation
between ESPT and ESHT has yet not been discussed, although bothprocesses should be considered in the description of aromatic
molecular systems. We should discuss their relations in aromatic
molecular systems: which of ESHT or ESPT occurs, why so, where is
the key on the potential surface, and how do ESHT and ESPT
inuence each other. PhOH–(NH3)n clusters, at the origin of the
ESHT issue, are one of the best systems with which to unravel these
aspects, because the pp* and ps* states are systematicallycontrolled by the number of NH3 molecules in the cluster, and thereaction products cNH4(NH3)n1 are well characterized by bothexperimental spectroscopy and theory. In particular, their low
ionization potentials42 and characteristic near infrared electronic
absorption20 are important for experiments. Here, we will present
the photodynamics for individual clusters of increasing size anddiscuss pertinent aspects of the ESHT/ESPT issue accordingly.

3.

Methods

3.1

Experimental

Although the experimental setups used to study PhOH–(NH3)nclusters have been slightly diﬀerent depending upon theexperimental groups, the typical conditions are always thesame. Clusters are produced by expansion of a gas mixture of
NH3-seeded He/Ne which has been owed over a room
temperature reservoir containing PhOH. The backing pressure© 2021 The Author(s). Published by the Royal Society of ChemistryView Article Online

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Review
is typically 2 bars and the nozzle diameter is around 500 mm.
The cluster size is varied by changing the backing pressure andthe partial pressure of NH3 in the mixture, with the highest
pressures giving rise to the largest clusters. The clusters arethen laser excited and ionized in a second chamber, between
the extraction plates of a TOF mass spectrometer.
Picosecond time-resolved IR/NIR (ps-TRIR or ps TRNIR)
spectroscopy of PhOH–(NH3)n was performed by utilizing timeresolved ultraviolet (UV)-IR/NIR-UV0 ion dip spectroscopy. The
principle of the spectroscopy and details of the spectrometer aredescribed elsewhere.43 The clusters obtained in the supersonic
expansion have been studied by diﬀerent excitation schemes
which are illustrated in Fig. 5. Briey, PhOH–(NH3)n was excited
to the pp* state by a ps UV pulse, nexc, and the ESHT reaction was
triggered. The reaction product, cNH4(NH3)n1, was ionized bya ns UV laser pulse, nion, which was red at (typically) 30 ns aer
nexc. The population of the reaction product was monitored atmasses of NH4+(NH3)n1 using a time-of-ight mass spectrometer. A ps tunable NIR laser, nNIR, whose frequency was xed tothat of the 3p–3s Rydberg transition of cNH4(NH3)n1 in the
NIR region, was red for Dt ps aer nexc. The population ofcNH4(NH3)n1 was depleted by excitation to the 3p Rydberg state.
If nNIR is suﬃciently strong, population of cNH4(NH3)n1 isgenerated until the nNIR irradiation (at Dt) is completely depleted.
Then, the ion signal of NH4+(NH3)n1 corresponds to the amountof cNH4(NH3)n1 produced aer Dt ps until 30 ns aer from thenexc irradiation. Therefore, the ESHT dynamics can be monitored
by measuring the intensity of cNH4+(NH3)n1 as a function of Dt.

Chemical Science
This spectroscopy has several variations. If nNIR is changed toa pulse from a tunable ps IR laser, nIR, the time-resolved
vibrational (IR) spectra can be measured. In the time-domain,
we can measure the femtosecond dynamics by replacing nexc
and nNIR by femtosecond UV and NIR laser pulses. The static IR
and NIR spectra can be measured by using ns tunable lasers for
all three laser pulses. It should be noted that the femtosecond
UV/NIR lasers do not have high enough energy resolution to
distinguish individual vibrational transitions. Thus, femtosecond spectroscopy is used only in the measurement of timeresolved electronic spectra in the NIR region.

3.2

TheoreticalThe rst calculations on the PhOH–NH3 system showing
evidence of the role of the ps* and of the H transfer process
were performed by Sobolewski et al.44 but these phenomena
were inferred long ago by Evleth and collaborators on the basisof semi empirical molecular-orbital calculations.45 Using multiconguration methods (CASSCF/CASPT2) the calculations performed on PhOH–solvent, where solvent is NH3 or H2O, showedthat the reaction of excited PhOH-solvent was exothermic forNH3 clusters and endothermic for H2O clusters. The role of the
ps* state could only be evidenced by the addition of diﬀuse
functions in the basis set, necessary to fully describe the Rydberg character of this state. It was also shown that there is nobarrier to the proton transfer in the ionic state (PhOH+–NH3),
adding one more piece of information dismissing the ESPTFig. 5 Diﬀerent experimental schemes for studying the reaction dynamics in PhOH–(NH3)n clusters: (a) simple pump–probe experiment: thisallows the study of the decay of the parent ion, (b) ionization after a long delay between the pump and the probe. The excited state parent ionshave decayed and only the reaction product can be ionized, (c) three color experiment. A NIR laser between the ﬁrst and second UV lasers excites
the cNH4(NH3)n1 product immediately after the H transfer and leads to the depopulation of the reaction product. Reproduced from ref. 54 withpermission from the PCCP Owner Societies, copyright 2020.

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Chemical Sciencemechanism discussed in an earlier paper.13 It should be notedthat earlier calculations were focused on the ESPT process anddid not mention the ps* state.46,47 For clusters, the ground stategeometries and the ground state vibrations have been mostly
calculated at the MP2 level, and these calculations were used toassign the vibrational bands in the IR hole burning experiments
and determine the cluster structure.21,43 For the excited state,
two main methods were used: either (CASSCF/CASPT2) or
Coupled Cluster (CC2). Both methods give good agreement withexperiments. With CC2, the absorption of the excited state canbe calculated and the nature of the excited state (pp* or ps*)compared with the experimental results.24 For large clusters,
DFT/M06-2X/cc-pVTZ has also been used, with these calculations predicting a stable ground state proton transfer structure(zwitterionic structure) for PhOH–(NH3)n where n > 8.48
It is interesting to note that most of the theoretical work is
published alongside experiments. The experimental and theoretical work has made progress together, nicely illustrating theinterest of studying clusters, for which both approaches are
directly comparable.

4. Experimental ﬁndings
The ESHT mechanism has been observed to vary as a functionof cluster size. As we will describe below, the study of eachcluster size of PhOH–(NH3)n reveals some new information on
the reaction dynamics, from the n ¼ 1 complex in which the roleof the intermolecular vibration is evidenced, to the n ¼ 5 cluster
in which the role of the proton transfer state in the ESHT isdemonstrated.
4.1 PhOH–(NH3)1: lifetime variation with intermolecularvibration
For the PhOH–(NH3)1 complex (hereaer “the n ¼ 1 complex”),
the time-evolution of the H transfer reaction can only bemeasured with the pump–probe scheme monitoring the decayof the parent ion. Indeed, since the lifetime of the cNH4 productis very short (13 ps), i.e. a lot shorter than the tunneling timefrom the pp* to the ps* potential surface, the concentration of
the product is negligible. One should mention that this method(Fig. 5a) measures the lifetime of the parent ion aer the initial
excitation pulse, which corresponds to the time for thetunneling, and subsequent dissociation, to occur. The latter(dissociation) corresponds to the time necessary for the cNH4
moiety to be far enough from the PhOc. Examples of such aneﬀect are demonstrated in the case of the NaI49 diatomic
molecule or in NaI–(NH3)n clusters.50
The excited state lifetime of the n ¼ 1 complex has beenmeasured to be 1.2 ns (ref. 51) which is shorter than the lifetimeof the free PhOH molecule (2 ns (ref. 52) or 2.2 ns (ref. 24)). The
excitation of the intermolecular vibration at +182 cm1 above
the origin leads to a shortening of the lifetime down to 391 
100 ps. This vibration is the intermolecular OH/N stretchingmode which is directly connected to the reaction path. Indeed,
one can expect that when the OH/N distance decreases, thetunneling barrier decreases, as observed. This observation is3842 | Chem. Sci., 2021, 12, 3836–3856

Reviewvery convincing evidence that the reaction proceeds via
a tunneling mechanism. A similar observation was also madefor the PhOH–(NH3)2 complex.51
Another strong argument for a tunneling process in thereaction was the observation of the change in the reaction speed
upon deuteration. For the n ¼ 1 deuterated complex, the lifetime is 7 ns (compared to 1.2 ns for the hydrogenated species),and for the n ¼ 2 cluster, it is 7 ns instead of 400 ps.15 Thus, thesimplest explanation for this observation is the presence ofa tunneling barrier.

4.2 PhOH–(NH3)2: biexponential lifetime and the role of thetriplet state
The S1 lifetime of PhOH–(NH3)2 was measured by two groups
using a pump–probe ionization method.51,53 Both groups
monitored the parent ion, PhOH+–(NH3)2, and measured the
decay of S1 by the loss of the parent ion (pump–probe method,
see Fig. 5a). The reported lifetimes are signicantly diﬀerent: 80
ps (ref. 53) and 400 ps.51 The main reason for the diﬀerence isthe simultaneous excitation of larger clusters. Even aer careful
preparation of the experimental conditions, it is very diﬃcult to
avoid the coexistence of clusters of various sizes. The transitionsof the PhOH–(NH3)n (n ¼ 2–5) clusters appear in almost thesame frequency range, thus, multiple species are invariably
excited at the same time. Furthermore, larger clusters (n > 2)
easily dissociate aer ionization, and contaminate the PhOH+–(NH3)2 mass channel. The latter group51 subtracted the contributions from the larger clusters by measuring the timeevolution of the background signal, and obtained the longer
lifetime. This lifetime is the convolution of the tunneling andthe dissociation times. The pure H transfer lifetime (i.e.
tunneling in this case) was measured by time-resolved UV-NIRUV ion dip spectroscopy (Fig. 5c) which indicates the H transfer
directly by NIR absorption. This method is insensitive tointerference from larger clusters, thus the diﬀerence in the
lifetimes corresponds to the dissociation lifetime.
All the time-evolutions of cNH4NH3 formation detected by
NIR absorption are biexponential.54 Here, the biexponential
time evolution from the S1 origin means that an electronic
relaxation would be feasible to explain this behavior, while IVR
makes no contribution (Fig. 6a). The single exponential t to theearly part of the time-evolution from the S1 origin gives a lifetime of 270 ps, thus the 130 ps diﬀerence from the lifetime
measured by the conventional method51 is the contribution of
cNH4NH3 dissociation.
Two reaction paths and a contribution from the unknown
state X in one of the paths are assumed to explain the biexponential time-evolution (see below).

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Review

Fig. 6 Biexponential appearance of the cNH4NH3 product on the pps
time scale from the (a) S1 origin (log scale on vertical axis) and (b) S1
overtone of the butterﬂy vibration (40 cm1 excess vibrational energy,
linear scale on vertical axis) taken from ref. 54. A single exponential(267 ps) and biexponential ﬁt (s1 ¼ 268 ps, s2 ¼ 440 ps) are shown by
the solid straight line (a) and curve (b), respectively. Reproduced fromref. 54 with permission from the PCCP Owner Societies, copyright2020.

The rst reaction path with reaction rate k1 is the simple
ESHT reaction via the conical intersection from S1 pp* to ps*.
The second path including the state X also has an ESHT reaction for the decay of state X, but the formation of state X occurs
via electronic relaxation. By solving the rate equations54 andtting with a biexponential growth, the observed timeevolutions are tted for the time-evolutions from several
vibronic levels. Here, we assumed that k2 is common for the
dynamics from all vibronic levels, and applied a global t.54 The
example of the tted results for the time-evolution from theovertone of the buttery vibration in S1 is shown in Fig. 6b alongwith the observed time-evolution on a linear vertical scale. The
tted values are s1 ¼ 1/k1 ¼ 268 ps and s3 ¼ 1/k3 ¼ 440 ps, andthe common parameter of s2 ¼ 1/k2 converges at 125 ps. s1 ands3 vary according to the vibronic levels, but all of them are on
the order of hundreds of ps.

© 2021 The Author(s). Published by the Royal Society of Chemistry

Chemical Science
What is state X? As mentioned above, IVR is not responsible
for the biexponential behavior, thus a vibrational bath mode is
not responsible. The high energy vibrational states of S0 are also
not a suitable candidate for X, because such states relax quickly
by the fast IVR process, thus it is not possible to access the ps*state which promotes the ESHT reaction. A triplet state (3pp*)of the PhOH–(NH3)2 cluster is also not reasonable, because k2
must be the rate of the intersystem crossing, which would bemuch slower than ps.
The last candidate is a triplet ps* state (3ps*). The s* orbitalis diﬀuse and located far away from the cluster (see Fig. 3).
Thus, the 3ps* is expected to be energetically degenerate withthe 1ps*. As a consequence, there is almost no energy gap in theintersystem crossing and thus the relaxation can be accelerated
to the sub-ns order.55 The potential surface of 3ps* crosses thatof 3pp* at a longer OH distance, and a bound state can beproduced because of the avoided crossing. Then, those molecules converted to 3ps* can be trapped in this bound state
before dissociating to PhOc + cNH4NH3 along the repulsive 3ps*
potential surface (see Fig. 7).
This model was conrmed by the theoretical calculations of
the potential curves of 1,3pp* and 1,3ps* along the OH distanceshown in Fig. 7. The potential curves were obtained bystretching the OH distance around the excited state equilibrium
distance. The black curve represents the 1pp* state and a local
minimum at ROH ¼ 1.7 Å corresponds to the proton transfer
state. This PT state is not energetically accessible from the pp*
obtained by ground state excitation. The blue curve showsa triplet 3pp* state that does not exhibit a local PT state
minimum. The red and pink curves correspond to the singlet
and triplet 1,3ps* states, respectively. The potential curves ofthe singlet and triplet ps* states are almost degenerate in

Fig. 7 Potential curves of the PhOH–(NH3)2 cluster along the OH
bond length ROH in diﬀerent electronic states and spin multiplicities
obtained at the RI-CC2/aug-cc-pVDZ level. Cs symmetry has been
imposed for the calculations, thus avoided crossing should beconsidered between 1,3pp* and 1,3ps* states. Reproduced from ref. 54
with permission from the PCCP Owner Societies, copyright 2020.

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Reviewenergy. The dissociative potential curves of 1,3ps* cross those ofboth 1pp* and 3pp*, thus conrming the mechanism detailed
above: the biexponential time-evolutions are due to the triplet
ps* formation and successive trapping in the triplet manifold.
4.3 PhOH–(NH3)3: isomerization within the reaction
products
The PhOH–(NH3)2 cluster discussed above has no isomers, andthe reaction product of ESHT is a single species, cNH4(NH3),
rendering the study of this species relatively simple. For clusters
containing more than two ammonia molecules, the ESHT
reaction becomes complicated in two ways. The rst is thecoexistence of several conformers of PhOH–(NH3)n. Excitationof diﬀerent conformers leads to a diﬀerence in the initialgeometry of the cluster undergoing the ESHT reaction. This
complication can be minimized if the electronic transition ofeach conformer is well separated in energy and can be selectively excited. The second point is the conformational variety of
the reaction product cNH4(NH3)n1 (n $ 3). Isomerization
between these conformers is also possible, according to theexcess energy available aer ESHT. This second point isessential and cannot be avoided experimentally.
For PhOH–(NH3)3, hole burning spectroscopy indicates that
this cluster exists in only a single geometry in the ground state.56
Its cyclic hydrogen-bonded structure, shown in the lower le ofFig. 8, was determined by interpretation of the IR spectrum withthe help of theoretical calculations.21
Fig. 9a shows the NIR transition probing the electronic
structure obtained by scanning the nNIR frequency in the UV-IR-

Fig. 8 ESHT reaction scheme for the PhOH–(NH3)3 cluster. The initialcluster (lower left) reacts at a rate k1 to give the PhOc/cNH4(NH3)2
product (lower right). The cNH4(NH3)2 dissociates from PhOc (upper
right) and then isomerizes to NH3cNH4NH3, the most stable isomer(upper left) at a rate k2. Reproduced from ref. 43 with the permission of
AIP Publishing, copyright 2007.

3844 | Chem. Sci., 2021, 12, 3836–3856

Fig. 9 Time-evolution of the reactive products of photoexcited
PhOH–(NH3)3. (a) Electronic spectrum of the cNH4(NH3)2 cluster. The
structure at 12000 cm1 is cNH4–NH3–NH3 while that at lower energy
is NH3–cNH4–NH3. The time-evolution curves (b and c) show thatcNH4–NH3–NH3 is obtained before the generation of NH3–cNH4–
NH3. Reproduced from ref. 43 with the permission of AIP Publishing,copyright 2007.

UV pump–probe spectroscopy of PhOH–(NH3)3. The NIR
absorptions at 7800 cm1 and 9000 cm1 are assigned to 3p–3stransitions of NH3–cNH4–NH3 while those at 11 000 cm1 and
12 000 cm1 originate from that of cNH4–NH3–NH3. The NIR
absorption of cNH4–NH3–NH3 (Fig. 9c) rises faster than that ofNH3–cNH4–NH3 (Fig. 9b). Thus, the time-evolution shows thefast generation of cNH4–NH3–NH3 rather than of NH3–cNH4–
NH3. Another important point is that both species exist evenaer a long delay time. This suggests an equilibrium between
the two species.
The fast formation of cNH4–NH3–NH3 demonstrates thememory eﬀect of the initial geometry in the parent PhOH–(NH3)3 cluster (see Fig. 8). Aer photoexcitation to S1, O–H bondradical dissociation in the cyclic PhOH–(NH3)3 induces H atomtransfer to the terminal NH3. This reaction generates the initial
product cNH4–NH3–NH3 exclusively. The cNH4–NH3–NH3
species is less stable and thus isomerizes to the more stable
NH3–cNH4–NH3 (the barrier height is 3.1 kcal mol1 (ref. 43 and63)). The back reaction to the initial product under an isolated
condition, leads to an equilibrium between the two species atlong delay times.

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Review
The three-state model is conrmed by the application of rateequations,43 using the rate constants k1 (ESHT), k2 (isomerization), and k2 (back-reaction) shown in Fig. 8. The details of theanalysis and tting procedures are reported in the reference,thus we will show only the tted results here. Fig. 10 shows thetime-evolution of (a) the meta-stable product cNH4–NH3–NH3and (b) the nal product NH3–cNH4–NH3. The experimental
time evolutions were obtained by the averaging of the time
evolutions of both the NIR and IR spectra. The tted curves
obtained by the global t of the rate equation analysis areshown by solid curves as well as the residual errors between theobservations and the t functions. The ts reasonably reproduce the time-evolution of the meta-stable and the nal products with 24 ps, 6 ps, and 9 ps for the inverse of rate constants 1/
k1, 1/k2 and 1/k2, respectively. Thus, we conclude that theinitial formation of the metastable product is due to thememory eﬀect, followed by isomerization to the most stablespecies, and nal equilibrium is attained by the back reaction.

4.4

PhOH–(NH3)4: an even more complex system

While time-resolved spectroscopy has been applied to ESHT
reactions in PhOH–(NH3)2,3,5 clusters, no time-resolved spectroscopy has yet been reported for the PhOH–(NH3)4 system. To
complete the systematic study of PhOH–(NH3)n, the ps TRIR
and TRNIR spectroscopy of PhOH–(NH3)4 has been performed
for this review. Before presenting the time resolved data, wewould like to summarize the previous nanosecond spectroscopyof PhOH–(NH3)4 associated with theoretical calculations.21,56
The UV action spectra of PhOH–(NH3)4 obtained by themonitoring by delayed ionization detection of the product with(a) ps and (b) ns lasers are shown in Fig. 11 (see Fig. 5b for the
spectroscopic method). Four color hole burning spectroscopy

Fig. 10 The time evolutions of the products (a) cNH4–NH3–NH3monitored at 12 000 cm1, (b) the ﬁnal product NH3–cNH4–NH3
monitored at 6000 cm1, for ESHT in the PhOH–(NH3)3 cluster. The
ﬁtted results are shown as solid curves along with the residual.
Reproduced from ref. 43 with permission of AIP Publishing, copyright2007.

© 2021 The Author(s). Published by the Royal Society of Chemistry

Chemical ScienceFig. 11 Action spectrum of the PhOH–(NH3)4 cluster obtained by (a)a ps laser which has a larger band width and (b) a ns laser of higher
spectral resolution. The ﬁrst photon, which is scanned in wavelength,
excites the cluster to S1 and the second photon ionizes the cNH4(NH3)3
reaction product. From the IR-UV experiment, three isomers (A, B andC) have been identiﬁed.

shows that PhOH–(NH3)4 has three isomers, whose bands areindicated in the gure.56 Due to the lower spectral resolution of
the ps laser (15 cm1), a low frequency progression of isomer C
cannot be resolved in Fig. 11a. Also, sharp and weak bands ofisomers A and B cannot be seen in Fig. 11a. As a result, theESHT time-evolution can be measured only for isomer C.
Theoretical calculations at the MP2/6-31++G(d,p) level showthat a cyclic hydrogen bonded structure is the most stable forthis cluster. In the early stages of the ESHT studies in PhOH–(NH3)n, the coexistence of isomers was not known. For thisreason, the IR spectrum obtained by labeling the band at
35 348 cm1 (isomer A) was assigned to the cyclic hydrogenbonded structure. However, this assignment should be reconsidered because the most stable species should appear
more strongly than other isomers. In this case, isomer A issignicantly weaker than isomer C. Also, no IR spectra ofisomers B and C have been measured. Thus, we proceed in theanalysis with a tentative assignment of the isomer C as the cyclichydrogen-bonded structure.
The cyclic hydrogen-bonded structure shown in Fig. 12 issimilar to that of PhOH–(NH3)3. Fig. 12 also shows possible
conformations of the reaction products cNH4(NH3)3 with theirrelative energies. If ESHT takes place in the parent cluster of thecyclic structure, the H atom will transfer to the terminal NH3 inthe ammonia moiety. In this case, only the meta-stable productcNH4–NH3–NH3–NH3 (4c in Fig. 12) is produced. The NIR spectrum of reaction products from PhOH–(NH3)4 measured by ns
laser systems is also shown in the gure. Theoretically predicted
3p–3s Rydberg transitions for the products 4a to 4d are also
indicated in the spectrum. The spectrum clearly shows the coexistence of all four reaction products 4a to 4d. Therefore, it isreasonable to consider the isomerization of the meta-stableproduct 4c to the other products. However, it is too complicatedto trace all the possible isomerization paths and equilibria aer
ESHT because of the multiple isomers of cNH4(NH3)3. Thus, we
neglected the contributions of species 4b and 4c, and applied thethree-state model to the photoexcited PhOH–(NH3)4, the metastable product 4c and the most stable product for this cluster, 4d.

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Review

Fig. 12 Reaction scheme for the PhOH–(NH3)4 cluster. – The initial cluster (upper left) reacts at the rate k1 to give the PhOc–cNH4(NH3)3 species.
The initial ESHT product starting from cNH4(NH3)3 (4c) evolves to structures 4d and 4b and then isomerizes to the most stable C3v isomer 4a(upper right). – Lower trace: electronic spectrum of the cNH4(NH3)3 species, which includes the contributions of the four isomers. The colored
sticks are the calculated excited states of the diﬀerent isomers. Reproduced from ref. 18 and 63 with the permission of AIP Publishing, copyright2002 and 2003, respectively.

Time-evolutions of the meta-stable product 4c and the moststable product 4d were monitored by following the NIR
absorption at 11 000 cm1 and 7000 cm1, respectively. The
observed time-evolutions and the best t of the three-statemodel are shown in Fig. 13. The absorption of the meta-stableproduct 4c rises quickly aer the photoexcitation to S1 anddecays gradually aer 30 ps. On the other hand, the absorptionof the most stable product 4d increases slowly. This is essentially the same mechanism as that observed in PhOH–(NH3)3:
selective generation of the metastable 4c product due to thememory eﬀect, and successive isomerization towards the moststable product, 4d. Good reproduction of the observed timeevolutions by the rate equations conrms the equivalent
ESHT mechanism to PhOH–(NH3)3. The inverse of the rate
constants 1/k1, 1/k2 and 1/k2 are determined to be 15 ps, 25 ps
and 38 ps, respectively.
As compared to PhOH–(NH3)3, 1/k1, the time constant for thetunneling is two times faster (15 ps instead of 30 ps) whereas theisomerization constants (1/k2 and 1/k2) are longer (25/38 ps
instead of 6/9 ps for PhOH–(NH3)3). The shorter tunneling time
implies that the tunneling barrier for PhOH–(NH3)4 is smaller
than that in PhOH–(NH3)3, in agreement with the interpretation
given in Section 4.7 “General model for ESHT and ESPT”.
Concerning the isomerization, the barriers are nearly the samefor cNH4(NH3)2 and cNH4(NH3)3 so the slower reaction rate canbe accounted for by the higher density of states in cNH4(NH3)3
compared to NH4(NH3)2.
4.5

PhOH–(NH3)5: the return of proton transfer

S1–S0 electronic spectra of PhOH–(NH3)n (n # 4) exhibit wellresolved structures. This is consistent with the picosecond
order of the ESHT reaction rate. In contrast, PhOH–(NH3)5 gives
3846 | Chem. Sci., 2021, 12, 3836–3856

rise to a broad absorption, which indicates a signicant changeof the reaction mechanism for this cluster. Here, we would liketo summarize the structural information obtained from nanosecond “static” spectroscopy. The structure of the PhOH–(NH3)5
cluster in S0 has been assigned to a bicyclic hydrogen-bonded

Fig. 13 Picosecond time-evolutions of reaction products of PhOH–(NH3)4 measured by the transitions in the IR (NH vibrations at
3180 cm1 (red curve)) and NIR (electronic excitation). Blue curve –excitation at 11 000 cm1 – probes the cNH4(NH3)3 in the chain
structure (4c). Green curve – excitation at 7000 cm1 – probes themost stable C3v structure (4a): this structure is obtained at longer delay
times.

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Review
structure (see Fig. 14).21 Photoexcitation of the cluster triggers
O–H cleaving by ESHT, with the H atom transferred to thenearest ammonia in the ammonia moiety. From this geometry,the expected nascent product is the structure where the hammonia molecule is hydrogen bonded to one of the NH3 in
cNH4(NH3)3 in C3v symmetry, which is shown in Fig. 14 as C3v+1.
However, the NIR spectrum obtained by ns UV-NIR-UV pump–probe spectroscopy (Fig. 2 and lower right panel in Fig. 14)
shows a strong and broad band at 6000 cm1 which is
assigned to cNH4(NH3)3 in the most stable tetrahedral structure
Td, not C3v+1. Therefore, several intermediate species are expected to play a role in the ESHT reaction of PhOH–(NH3)5.
The time-evolutions of the NIR signals at 6000 cm1,7000 cm1 and 8000 cm1 are shown in Fig. 14.57,58 The signal at
6000 cm1, which corresponds to the most stable Td product,
grows gradually and becomes constant at around 100 ps. The
signal at 8000 cm1 rises rapidly and decays slowly. The ultrafast rise of the signal is consistent with the broad spectralfeature of the UV absorption transition of PhOH–(NH3)5. The
time-evolution at 7000 cm1 shows a sharp rise and the signal isconstant aer the rise. This is explained by an overlapping of
the decreasing signal at 8000 cm1 and the increasing signal of
the Td species at 6000 cm1. Quantum chemical calculations

Chemical Sciencesuggest two possible transient species in this region. One is thenascent ESHT product of C3v+1 symmetry, of which the 3px,y,z–
3s transitions are shown by blue bars in Fig. 14a. Another is the
4p Rydberg–s* transition of the CT complex originating fromthe occupation of an electron in the ps* orbital, of which thetransition energy is 0.85 eV (6900 cm1). These two species are
considered potential candidates for the 8000 cm1 absorption.
In both cases, this signal is a signature of electron transfer fromthe aromatic p* to the s* orbitals.
The NIR transitions are a good indicator of electron transfer.
On the other hand, the real N–H bond formation dynamics canbe observed independently by IR spectroscopy in the 3 mmregion. Fig. 15 shows the ps-TRIR spectra of PhOH–(NH3)5. The
theoretical IR spectra of both the C3v+1 and Td products are also
shown in Fig. 15b. Here, the Td product will exhibit intense
vibrational transitions at 2965 cm1 (H-bonded NH stretchingof cNH4) and 3260 cm1 (free NH stretching of NH3), while theC3v+1 species is calculated to give rise to IR absorption at
3217 cm1 (free NH stretching of NH3). The ps-TRIR spectra
show a strong transition at 3200 cm1 and a broad absorption
at 3000 cm1. Time-evolutions at 2983 cm1 (A in Fig. 15a),3165 cm1 (B) and 3248 cm1 (C) are shown in Fig. 15c. Bothsignals at A and C gradually grow in intensity with increasing

Fig. 14 H transfer dynamics of PhOH–(NH3)5: (a) absorption spectrum of the Td cNH4(NH3)4 species, the most stable structure, (b) time
evolutions of the electronic structures: at 8000 cm1 excitation of the charge transfer state within PhOH–(NH3)5, at 6000 cm1 the ﬁnal Td
structure, and at 7000 cm1 the overlap of both contributions. The right panel shows a scheme of the reaction events and time evolution of theelectronic structures. (c) shows the population evolution deduced from the ﬁt. The C3v+1 structure is barely populated. Reproduced from ref. 58
with the permission of Wiley-VCH Verlag GmbH & Co, copyright 2018.

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Fig. 15 H transfer dynamics of PhOH–(NH3)5 monitored through the free NH vibrations. Left: (a) experimentally-derived time-evolution of the IR
spectrum as a function of the delay between the pump and the probe, and (b) the calculated transitions for the two postulated geometries. Right:
time-evolution of the vibrational absorption at diﬀerent wavelengths shown in the left-hand spectra. In contrast to the electronic absorption,
there are no very short dynamics. Reproduced from ref. 58 with the permission of Wiley-VCH Verlag GmbH & Co, copyright 2018.

delay time, and no sharp rise was found. The signal at B should
be sensitive to the presence of the nascent C3v+1 product.
However, the time-evolution does not show an ultrafast rise,and can be tted by single exponential functions with lifetimesof 20 ps. This suggests that the ps-TRIR spectra reect mainly
the vibrational transitions of the Td product.
The right panel in Fig. 14 summarizes the ESHT reaction
mechanism and spectroscopic signatures for the possible
species involved. The time-evolutions are analyzed by rateequations with the rate constants kS1–CT, kCT–C3v+1, andkisomerization. The global t reproduced the observed timeevolutions, as shown by black solid curves in Fig. 14 and 15,
with 1/kS1–CT  3 ps, 1/kCTC3v+1 ¼ 20.2 ps, and 1/kisomerization(¼1/kC3v+1–Td) ¼ 0.4 ps. The population changes of CT, C3v+1 andTd species extracted from the t are also illustrated in Fig. 14c.
The t shows that the electron transfer from PhOH to theammonia moiety is very fast (3 ps). However, the protontransfer is slower (20 ps) and is decoupled from the electrontransfer. It is clearly a diﬀerent mechanism compared to thatobserved in the smaller clusters (n # 4). It should be noted thatthe time resolved experiment in the femtosecond regime later
determined the time constant of the initial electron transfer as
370 fs.59
The electron-proton decoupling for n ¼ 5 is rationalized by
the strong distortion of the potential surfaces of the S1 and S2
states. Fig. 16 shows the calculated potential energy curves obtained by stretching the OH distance, ROH.58 The other coordinates are xed at the geometry in S0, of which the symmetry is
C1. Due to the C1 symmetry, p/s symmetry does not exist, but wedistinguish the major character of the excited states from theshape of their orbitals. If the orbital is mainly localized on thearomatic ring, like the valence orbital, we call it p or p* and it is3848 | Chem. Sci., 2021, 12, 3836–3856

denoted by blue squares. In the case where the orbital is diﬀuse
like the Rydberg orbital, it is denoted as s* and is indicated by
red circles. Where ROH is smaller than 1.2 Å, S1 and S2 have pp*
and ps* characters, respectively. This character is reversed
beyond the conical intersection (CI) at 1.2 Å and the S1
becomes ps* in character. From the Franck–Condon (FC)position (1.0 Å), the reaction takes place without any barrier in
S1. This means that the proton moves fast along the reaction
coordinate. When the proton crosses over the CI, the characterof S1 changes to ps*. This means that the p* electron istransferred to the Rydberg-like s* orbital, and the CT complex isgenerated. The CT complex gives rise to the NIR transitions at
8000 cm1 which corresponds to the fast rise of 370 fs.59 At
that time, the proton – which goes over the CI with signicant
speed – proceeds into the S2 state rather than following the S1
potential curve. Here, S2 has pp* character beyond the CI, thus
this corresponds to the classical ESPT reaction. The proton canbe trapped in S2 for a while, and due to the Coulomb attraction
this cluster cannot dissociate. The S2 state relaxes by internalconversion to S1 (of ps* character), which immediately causes
dissociation and the successive formation of the nal Td
product. In this scenario, the observed 20 ps lifetime for theformation of the Td product corresponds to the rate of the IC,
i.e. the lifetime of S2. Further detailed discussion allowinga systematic understanding of the ESHT mechanism will begiven in the next section.

4.6

Larger clusters: proton transfer in the ground state

ESHT has not been detected in PhOH–(NH3)n when n ¼ 6 orlarger, because NH4+(NH3)n1 (n $ 6) are not observed in the
mass-spectrum aer two-photon ionization of PhOH–(NH3)n via© 2021 The Author(s). Published by the Royal Society of ChemistryView Article Online

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ReviewChemical ScienceFig. 17 Vertical excitation energies of pp* and ps* states in PhOH–(NH3)n clusters as a function of the number of ammonia molecules. Forn # 5 the calculated ground state structure is PhOH–(NH3)n and for n >5 the zwitterionic PhO–(NH4+)(NH3)n1 structure. For these structures the lowest energy excited state is the ps* state, which has
a Rydberg character, a very small oscillator strength and a very long
lifetime (around 50 ns).60 Reproduced from ref. 60 with permission
from the PCCP Owner Societies, copyright 2009.

Fig. 16 Excited state potential curve for the PhOH–(NH3)5 cluster
along the OH coordinate. The cluster is excited in the Franck–Condon(FC) region (D) on the pp* surface (black curve). It evolves within 3 ps
toward the Conical Intersection (CI). Then, diabatically, it can continue
to the pp* proton transfer state (E) or stay on the adiabatic surface andrelax to the bottom of the ps* state (F). The tunneling barrier present
for the smaller cluster sizes has disappeared, since the pp* protontransfer energy (E) is lower in energy than the pp* in the FC region (D).
Reproduced from ref. 58 with the permission of Wiley-VCH Verlag
GmbH & Co, copyright 2018.

the S1 state (see Fig. 1, for n ¼ 6, 7, .). The absence of NH4+(NH3)n1 suggests two possibilities: ESPT or proton transfer in
the neutral ground state (GSPT). However, theoretical calculations show that pp* states which promote ESPT are higher inenergy than the ps* state for clusters of n $ 6, as shown in
Fig. 17. Thus, ESPT cannot be responsible for the absence of thereaction products NH4+(NH3)n1.
GSPT is reasonable to explain the observed results. At
a certain size, the proton aﬃnity of the ammonia moiety in thecluster will be large enough to induce proton transfer from
PhOH in the ground state, leading to the formation of a zwitterion. When the zwitterionic cluster PhO–NH4+(NH3)n1 is
excited, there is no driving force to dissociate the cluster, since
the most stable NH4+ structure is already formed and thissystem relaxes in the excited state by evaporation of NH3 fragments.12,15 This is consistent with the absence of the fragments
NH4+(NH3)n1.
The obvious question, just like in the case of ESPT, is: what is
the threshold size for GSPT? The results of the mass-spectra© 2021 The Author(s). Published by the Royal Society of Chemistry

naturally suggest the threshold size n ¼ 6. This is supportedby the measurement of the ionization potential (IP) of PhOH–(NH3)n (ref. 61) through a one photon experiment usingsynchrotron light to avoid the excited state reaction in a two
photon process through S1. The measured IP drops for n ¼ 6
and larger clusters. This can be explained by the formation of
the PT structure in the ground state. It is clear from calculations13,44,60 and experiments that the most stable form in theionic state is the PT structure (PhO–NH4+) by around 0.8 eVcompared to the PhOH+–NH3 form. Thus, a PT structure in S0
has a good FC factor to access the adiabatic ionic minimum (i.e.
the PT structure), whereas the non-PT structure of ground state
PhOH–NH3 cannot access the adiabatic IP (PT structure) in theion and this leads to a higher IP.
However, the size-selected IR spectra of PhOH–(NH3)n (n ¼
0–11) measured by IR-UV dip spectroscopy raised a doubt about
the threshold size of n ¼ 6.62 The IR spectra showed the signature of PhOH vibrations (such as C–O stretching) even forclusters of n ¼ 6 and the expected marker band (C]O stretching) of PhO was not found. The disappearance of the C–O
stretching band was not clear, even in larger sized clusters,
whereas the C–O–H bending vibration, another vibrationalsignature of PhOH, disappeared at n $ 9. From these results, it
was suggested that n ¼ 6 may be the beginning of GSPT, but thatboth PT and non-PT structures co-exist for n ¼ 6–8, and all theco-existing species become PT structures at n ¼ 9.
A recent theoretical paper,48 at the M06-2X/cc-pVTZ level oftheory, has revealed that clusters of n $ 9 exhibit zwitterionic
structures. In this paper, a rather impressive number of cluster
structures were investigated and the energy of zwitterionic
structures becomes comparable to the non-PT structures only
for clusters of n $ 9. There is obviously a discrepancy compared
to previous studies and a few possibilities could be responsible

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Review

for this. One is that the evaporation processes following either
ionization or excited state reaction are very important and havebeen overly neglected in the experimental interpretations. In
this hypothesis, the ions observed at n ¼ 6 come from larger
ones: this may be tested experimentally but has not been doneyet. Another possibility might be that the DFT method is not
appropriate for describing such a system and the energy ofsome structures can be tested with other methods (MP2,
CCSD(T), .), although this requires huge computational powerat the present time. These possibilities have not been investigated, leaving work to be done by the next generation.
4.7

General model for ESHT and ESPTWe have determined the following three points on the ESHT
process mainly from experiments: (1) the reaction speed
increases as the cluster size increases, (2) the reaction mechanism changes at n ¼ 5 and (3) the H transfer proceeds througha tunneling mechanism for small clusters (n < 5).

Table 1 Size dependence of the ESHT time constants of PhOH–(NH3)n clusters

Size (n)s1s2Reference

1
2
3
4
5

1.23 ns (HT)267 ps (HT)23 ps (HT)15 ps (HT)370 fs (ET)

—

51
54
43
This paper
59

7 ps (isomerization)
25 ps (isomerization)
20 ps (PT)Let us discuss the cluster size dependence of the ESHT lifetime, which is summarized in Table 1. The ESHT lifetimes for n¼ 2, 3, 4, and 5 (electron transfer ET and proton transfer PT) aremeasured by the rise of the reaction products cNH4(NH3)n1,while that of n ¼ 1 is extracted from the pump–probe experiment in which the size selection of the cluster is not perfectbecause of the evaporation of NH3 aer ionization. Thus, thelifetime of n ¼ 1 may not be “clean” in comparison to othervalues, however, the order of the lifetime can be trusted. Thus,
the ESHT lifetime exhibits a tendency to drop signicantly uponincreasing the number of solvent molecules in the cluster:
nanosecond at n ¼ 1, sub-nanosecond at n ¼ 2, tens of ps at n ¼
3 and 4, and nally femtosecond for n ¼ 5.
At rst, this change was attributed to the eﬃcient stabilization of ps* by the higher solvation. It sounds reasonablebecause the diﬀuse s* electron can easily be stabilized bysolvents. Nevertheless, such a sharp stabilization was not foundin the theoretical calculations;60,63 the absolute energy is lowered by the solvation but the energy gaps between pp* and ps*
are not changed much at the vertical excitation.
This is shown in Fig. 18. Here, the clusters are maintained inCs symmetry and the initial structures are optimized in the
ground state.60 The clusters are then optimized in the pp*(A0 )state and the potentials are obtained by stretching the OH
distance. For the PhOH–(NH3)5, there is no barrier, so that the
FC pp* region optimization requires the constraint of the OH
distance at 1 Å. It should be noted that the potential curves for n¼ 5 (1–5 in the gure) are essentially the same as those in Fig. 16
but are calculated with simpler assumptions, such as Cs

Fig. 18 Potential curve along the OH coordinate in clusters with Cs symmetry as a function of the cluster size. In red, the ps* potential, and inblack, the pp* state. Along this pp* state there are two minima for small sizes: the FC region corresponding to the excited PhOH*–(NH3)nspecies, and at longer OH distance the PhO*–(NH4+)(NH3)n1 proton transferred (PT) minimum. As the cluster size increases the barrier
diminishes and is no longer present for n ¼ 5. The ps* potential does not change signiﬁcantly with the cluster size and in particular the energy
gap between the pp* in the FC region and the ps* stays nearly constant.

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Review
symmetry. As one can see, the ps* potential curves at long O–H
distances have a minimum which corresponds to the attachment of the H to N (i.e. formation of cNH4) but along the O–N
coordinate, this ps* state is dissociative and this is responsible
for the separation of the cNH4(NH3)n1 cluster away from PhOc;
the one dimension calculations does not take into account thisdissociation of cNH4(NH3)n1 from the clusters. Although thesecurves cannot be compared directly with the experiments, anessential picture emerges from these calculations:(a) At the equilibrium of the pp* geometry (around the FC
region), the pp*/ps* gap does not change as the cluster size
increases, being 0.5 eV for the free molecule, 0.4 for n ¼ 2, 3, 4
and rising back to 0.5 eV for n ¼ 5.
(b) The PT structures (pp* at long OH distance) gradually
decrease in energy, being of comparable energy to the pp* statefor n ¼ 3 and being lower in energy for n $ 4.
(c) The minimum of the PT structure (pp*) is obtained at
1.75 A for n ¼ 1 (1–1 in Fig. 18) and is shied to 1.5 Å for n ¼ 5,
indicating a shrinking of the O–N distance as the cluster size
increases in the PT structure.
(d) Since the reaction is controlled by tunneling,21 both (b)and (c) favor a lowering of the barrier and thus the HT reaction
becomes faster.
In Fig. 18, it can be seen that the shape of the ps* remains
approximately the same as the cluster size changes, although
the energies are shied by the cluster size. This is opposite to
the pattern observed for the pp* state. The pp* potential curves
each have two minima, except for n ¼ 5: one at around the FC
region and another for the PT (PhO–NH+4(NH3)n1) structuresat greater OH distance. In contrast to the ps* case, along theO–N coordinate this is a bound state because ionic character
prevents fragmentation. For n ¼ 1–3, the minima in the FC
region are the global minima but the relative energies of the PT
minima gradually decrease and become comparable at n ¼ 3.
The PT minimum becomes the global minimum at n ¼ 4 andfurther stabilization of the PT structure makes the pp* potential barrier-less at n ¼ 5.
The H transfer tunneling barrier along the OH coordinate
is generated by the potential crossing (conical intersection)
between the pp* and the ps* potential curves. Therefore, theheight of the barrier strongly depends on the steepness of the
pp* state as the OH is stretched. Another barrier in thissystem is that to the PT structure in the pp* potential curves.
For small sizes (n ¼ 1), the PT structure is higher in energy
than the FC region and the barrier to the PT structure on the
pp* is also quite high. Thus, the PT reaction cannot takeplace aer optical excitation to S1. The HT reaction is possible
but the barrier is quite high because of the steep pp*
potential. This is a reasonable explanation for the slowest HT
reaction in the series. As the cluster size increases, the PT
structure and the barrier on the pp* potential decrease,which also suppresses the barrier height of the H transfer.
This tendency terminates in the absence of any barrier for n $5. Gradual suppression of the barrier can be used to rationalize the size dependence of the ESHT rate as shown in Table
1. This means that the ESHT dynamics vary according to the© 2021 The Author(s). Published by the Royal Society of Chemistry

Chemical Sciencepp* potential curve, which in turn is controlled by thestability of the PT structure.
This systematic understanding of the ESHT reaction rate
provides a comprehensive understanding of the relation
between ESHT and ESPT. It is interesting to note that the initialidea of ESPT which started this research in the 1980s was dismissed at the beginning of the 21st century. However, thisreview has revealed “The return of ESPT” in a quite diﬀerent
form as being a key mechanism in ESHT.

4.8

Other examples

4.8.1 Naphthol. Despite the extensive study of ESPT (see 1.
Introduction), no ESHT has been reported in solvated naphthol clusters. Although naphthol and PhOH are both aromatic
alcohols, their excited state dynamics are quite diﬀerent. The
main diﬀerence in electronic structure between naphthol and
PhOH is that the pp* is lower in energy (3.8 eV) in naphthol
than in PhOH (4.5 eV). From our calculations, it appears thatthe pp*/ps* gap at the pp* geometry is signicantly higher in
1-naphthol (0.95 eV at the aug-cc-pVDZ/CC2 level, compared
to 0.52 eV in PhOH21). At the ground state geometry, the ps*
in naphthol is around 4.9 eV compared to 5.4 eV in PhOH.
This can be understood with very simple arguments. The ps*state potential is a dissociative curve along the OH coordinate.
There is no reason for the shape of the curve to change when
the aromatic ring is changed, thus the energy of the ps* stateat the OH equilibrium geometry (1.0 Å) is only dependent on
the OH dissociation energy. In this crude approximation, theenergy of the ps* is a constant. In contrast, the energy of the
pp* lowers as the size of the aromatic (particle in a bigger box)
– and thus the pp*/ps* energy gap – increases, which gives
rise to a higher barrier in naphthol than that in PhOH. This
gives a simple but comprehensive explanation as to why ESPT,
rather than ESHT, takes place in naphthol. This simple
scheme has also been used to explain the variation of theexcited state lifetime of protonated aromatic amino acids, inwhich the ps* state is related to the dissociation of the NH
bond.644.8.2 Substituted PhOH. The H transfer mechanism has
been studied in several PhOH derivatives.65 Most of the molecules have been studied in their complexes with a single solvent
molecule such as ammonia and amines. The rst goal was todemonstrate that ESHT occurs in various aromatics, and themechanisms were discussed in terms of tunneling. It was
shown that the excited state lifetimes of uorophenols, methylphenols, and their complexes with ammonia were remarkably
well correlated with the calculated pp*/ps* energy gaps, thelifetime being shorter when the gap is smaller, i.e. the barrierheight smaller. In addition to the pp*/ps* gap, the coupling of
the pp* and the ps* is an important parameter to control thereaction rate. In Cs symmetry, the pp* and the ps* states
belong to diﬀerent symmetry representations A0 and A00 ,respectively, thus can be coupled only through out of planevibrations27,28 and in particular the out of plane torsion.28 In
most of the complexes studied, the Cs symmetry of the system isconserved and thus the relationship between the lifetime andChem. Sci., 2021, 12, 3836–3856 | 3851

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the barrier height is relatively regular. In contrast, the excitedstate lifetime of the cis-ortho-uorophenol–NH3 complex24,66 isvery short due to the breaking of the Cs symmetry in the
structure of this complex.
4.8.3 Pyrrole–NH3 complex. In pyrrole (PyH), the lowest
energy excited state is the ps* state which leads to a direct
dissociation67 within 80 fs.68,69 In the PyH–ammonia cluster,70
the experimental results reveal that the excited state hydrogentransfer reaction cannot be considered as a simple N–H bondrupture along a repulsive potential energy curve, as was at rst
suggested by comparison with the free molecule. The results of
the lifetime measurements as well as the competition between
the H-atom transfer reaction and the evaporation imply that thereaction proceeds via a fairly long lived (10–30 ps), relatively
deep intermediate state. The excited state potential energy
surface calculated at the CASSCF level agrees with these
conclusions: two minima corresponding to very diﬀerent
structures are apparent and the reaction path aer the initial
excitation can be described in four steps. The rst step isa contraction of the N–N distance accompanied by a vibrationalmotion of the pyrrolic N–H bond. Then, at the appropriate N–N
distance, an electron transfer from PyH to the solvent occurs,
immediately followed by proton transfer from PyH to ammonia(concerted electron/proton), which produces the radical pair Pyc– cNH4 with a very diﬀuse Rydberg electron cloud around theammonium. Then the cluster breaks into the two separate
radicals.
4.8.4 Indole–NH3 complex. In indole, as for PhOH, the Hloss in the free molecule71 and the hydrogen transfer to the NH3cluster have been demonstrated.72–77 In the free molecule, the Hloss through the ps* state71 has been observed but is also
contaminated by H atoms generated by one or more (unintended but unavoidable) multiphoton processes. UV excitationof indole–(NH3)n clusters leads to a H atom transfer reaction
and the formation of cNH4(NH3)n radicals. With an excess ofa few tenths of an electron volt, the reaction proceeds very
quickly in the 100 ps regime. As for PhOH, the H transfer
process explains why the excited-state lifetime is short (subnanosecond) in indole-ammonia clusters as compared to allother indole clusters (with water, methanol, etc.) where thelifetime is much longer.
The lifetime of the rst excited state in the 3-methylindole–
NH3 complex is strongly dependent on the intermolecularvibrations. As experimentally observed,73 the excitation of the
NH/N stretching coordinate is expected to favor the H transfer
reaction. High-level ab initio optimization of the equilibriumgeometry in the S1 state and calculated vibrational frequencies
compared with the experimental observations seem to indicatethat the in-plane bending vibration also favors the H transferfrom the aromatic chromophore (3-methylindole) to the solvent(ammonia), the reaction being 530 ps on the 0–0, 145 ps for 1
quantum in the N–H/NH3 stretch and 205 ps for 1 quantum in
the N–H/NH3 bend.
4.8.5 Hydrogen transfer and protonated molecules. Unexpectedly, the competition between the PT and the H transfer
mechanisms has been found in the excited state dynamics ofprotonated aromatic amino acids. In the ground state, if the3852 | Chem. Sci., 2021, 12, 3836–3856

Review
proton is localized on the amino group, electronic excitation
triggers the proton transfer onto the aromatic ring and this leadto Ca–Cb bond rupture. In an intermediate energy range, thehydrogen transfer between the protonated amino group and thecarboxylic group leads to CO2 and H2O loss, while H loss was
experimentally evidenced upon excitation at high energy.78

5.

Concluding remarks

PhOH–(NH3)n is a very rich system in which many subtleties of
the reaction dynamics of the hydrogen mediated reaction exist.
The excited state dynamics is systematically changed by the sizeof the ammonia moiety, of which the basicity increases with thenumber of molecules. Although this idea, initiated in Leutwyler's group4,79 was used for the study of ESPT in clusters, it also
explains systematic variation in the ESHT mechanism. This isbecause the change in the basicity distorts the potential surfaceof the optically active pp* state due to stabilization of the PT
local minimum. The potential distortion gives rise to a signicant change in the tunneling barrier to H atom release, which is
produced by the crossing to the ps* potential surface.
The lowering in energy of the PT structure is strongly size
dependent. For n ¼ 1, the PT structure at O–H ¼ 1.8 Å is higher
in energy by 0.15 eV than the FC structure (PhOH*–(NH3)n), andits energy decreases as the cluster size increases. At n ¼ 4, thisstructure seems to be more stable than that of the FC, althougha barrier to PT transfer still exists, and at n ¼ 5 the energy of the
PT structure is so low (0.22 eV) that the PT reaction becomesbarrierless. In contrast to the pp* state, the ps* state is relatively insensitive to the cluster size, primarily because thepotential shape of this repulsive state is essentially independentof the molecular system. This insensitive character is given by
the energy of its asymptote at long distance. It corresponds tothe bond dissociation energy of the OH (NH) which is more orless the same for many molecules (OH bond energy 4.8 eV, or
3.2 for NH). Thus, the systematic change of the pp* potential
surface directly gives the clear size dependence of the ESHT
mechanism, as well as the ESPT (rather than ESHT) reaction in
naphthol.
Quite unexpectedly, the role of the hypervalent dissociative
Rydberg state – evidenced experimentally and theoretically for
the PhOH–(NH3)n system – has been found to be key toprocesses in the excited states of many protonated ions such as
the aromatic amino acids. In these cases, the excess proton on
the amino group (NH3+) can easily accept an electron aer
optical excitation to a hypervalent dissociative state which leads
to H loss or H transfer depending on the nearby atom. This rst
step controls both the ensuing long term dynamics, and theselectivity of the fragmentation processes.78 This work isa benchmark, demonstrating the role of the excited state reaction in leading to the formation of radical pairs through the Hloss, which was too oen neglected before. The H loss fromaromatic molecule is now quite a standard paradigm. Extensionof the ESHT reaction to describe other processes is now occurring, notably in the description of excited state H abstraction byaromatic molecules, which is the reverse process of ESHT and isdemonstrated in photoexcited pyridine–H2O system.80–83 These© 2021 The Author(s). Published by the Royal Society of ChemistryView Article Online

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Review
reverse process studies are still in their infancy but might growrapidly to resolve the world energy crisis. We hope that the upto-date mechanism of the ESHT reaction and re-denition of
the role of ESPT will thus provide a rm base for the understanding and design of this new direction of excited state
dynamics.

Author contributions
C. J., M. M. and M. F. conceived and designed the project. M. M.
measured and analysed the spectra. C. J. performed and analysed the quantum chemical calculations. C. J. and M. F. cowrote the paper.

Conﬂicts of interest
There are no conicts to declare.

Acknowledgements
The authors are deeply indebted to their collaborators who were
the work force of all these studies. This group is an international team which includes Dr C. Dedonder (U. Aix-Marseille),
Dr G. Gregoire (U. Paris-Saclay), Dr S. Martrenchard (U. ParisSud), and Dr D. Solgadi (U. Paris-Sud) from France, Prof. G. A.
Pino (Cordova U.) from Argentina, Prof. Dr A. L. Sobolewski(Inst. Physics Polish Acad. Sci.) from Poland, Prof. Dr W.
Domcke (T. U. München) from Germany, and Prof. Shun-ichi
Ishiuchi (Tokyo Tech.), Prof. Makoto Sakai (Okayama Sci. U),
Prof. Kenro Hashimoto (Open U. Japan), and Mr Ryuhei Ohara(Tokyo Tech.) from Japan, as well as their graduate students.
The authors would like to thank Dr Jennifer Noble (U. AixMarseille) for careful reading of the manuscript and her
useful comments. This work was supported in part by KAKENHI(JP205104008) on innovative area (2503), KAKENHI(JP19H05527, JP18H01938, JP19K23624) of the JSPS, World
Research Hub Initiatives at Tokyo Institute of Technology, theCooperative Research Program of the “Network Joint ResearchCenter for Materials and Devices” from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan,
and the RIKEN Pioneering Project, “Fundamental Principles
Underlying the Hierarchy of Matter: A Comprehensive Experimental Study”. We also acknowledge the use of the computing
facility cluster Meso-LUM of the LUMAT federation (LUMAT FR
2764).

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