pubs.acs.org/JACS

ArticleUltrafast Ring-Opening Reaction of 1,3-Cyclohexadiene:
Identiﬁcation of Nonadiabatic Pathway via Doubly Excited State
Shutaro Karashima, Alexander Humeniuk, Ryuta Uenishi, Takuya Horio, Manabu Kanno, Tetsuro Ohta,
Junichi Nishitani, Roland Mitric,́ and Toshinori Suzuki*
Cite This: https://doi.org/10.1021/jacs.1c01896

ACCESS

Metrics & More

Read Online

Article Recommendations

sı Supporting Information*

ABSTRACT: The photoinduced ring-opening reaction of 1,3cyclohexadiene (CHD) to produce 1,3,5-hexatriene (HT) plays anessential role in the photobiological synthesis of vitamin D3 in theskin. This reaction follows the Woodward−Hoﬀmann rule, and C5−
C6 bond rupture via an electronically excited state occurs withconrotatory motion of the end CH2 groups. However, it is noted thatthe photoexcited S1(π,π*) state of CHD is not electronicallycorrelated with the ground state of HT, and the reaction must
proceed via nonadiabatic transitions. In the present study, we haveclearly observed the nonadiabatic reaction pathway via the doubly
excited state of CHD using ultrafast extreme UV photoelectron
spectroscopy. The results indicate that the reaction occurs in only 68
fs and creates product vibrational coherence. Extensive computational simulations support the interpretation of experimental results
and provide further insights into the electronic dynamics in this paradigmatic electrocyclic ring-opening reaction.

■

INTRODUCTION
The photoinduced ring-opening reaction of 1,3-cyclohexadiene(CHD) to produce 1,3,5-hexatriene (HT) is a well-known
example of electrocyclic chemical reactions,1 and it plays acentral role in the photobiological synthesis of vitamin D3 inthe skin by facilitating photoisomerization of 7-dehydrocholesterol to previtamin D3. The ring-opening reaction of CHD
follows the Woodward−Hoﬀmann rule, and C5−C6 bondrupture via the electronically excited state occurs withconrotatory motion of the end CH2 groups.2 The reaction
starts from the S1(π,π*) state of CHD reached by aHOMO(π)−LUMO(π*) transition in the ultraviolet (UV)region, and the photoexcited state ultimately leads to theground electronic state (S0) of HT3 with the quantum yield ofapproximately 0.5.4−8 However, it should be noted that theS1(π,π*) state of CHD is not electronically correlated with the
S0 state of HT; therefore, the reaction must be facilitated by
nonadiabatic transitions among diﬀerent electronic states(Figure 1).
The well-known feature of this paradigmatic ring-opening
reaction is that the energy ordering of the HOMO(12a) andLUMO(11b) is reversed between the reactant and product.
(Here, the notations a and b of the molecular orbital symmetry
are the irreducible representations in the C2 point group. We
employ these notations in order to explain the orbital
correlation between CHD and HT.) Consequently, the S0state of HT with double occupancy of the 11b orbital iscorrelated with the doubly excited S2 state of CHD. In the© XXXX American Chemical Society

1960s, van der Lugt and Oosterhoﬀ performed a theoreticalanalysis of a related ring-closing reaction from butadiene tocyclobutene, and they pointed out that the reaction ismediated by a conical intersection (CoIn) of the potential
energy surface with a doubly excited state;9 the samemechanism has been expected for the ring-opening reactionof CHD. It is noted that the ground state of CHD with the
double occupancy of the 12a orbital is electronically correlatedwith the doubly excited state of HT so that the two potential
energy surfaces indicated in black and blue in Figure 1 undergo
strong avoided crossing under the C2 symmetry, making the
S1−S0 energy gap to be prohibitively large for eﬃcient
nonadiabatic transition (it is seen in our ab initio calculations
shown in Figure 3). Therefore, the reaction has been predicted
to involve the symmetry-breaking nuclear motion to access the
S1/S0 seam of crossing under non-C2 symmetry.10
Experimental attempts to observe the ring-opening reactionof CHD in real time started decades ago, when Fuß andcolleagues3,11,12 performed ultrafast photoionization mass
spectrometry using strong ﬁeld ionization. Their experimental
results inspired a number of subsequent experimental andReceived: February 17, 2021

A

https://doi.org/10.1021/jacs.1c01896
J. Am. Chem. Soc. XXXX, XXX, XXX−XXXJournal of the American Chemical Society

pubs.acs.org/JACS

Articleﬁnal states of the reaction products. High temporal resolutionof measurements is crucial for clear identiﬁcation of thereaction path; therefore, a ﬁlamentation four-wave mixing(FFWM) deep UV (DUV) light source24 and a high-orderharmonic generation (HHG) EUV light source were combinedto achieve their cross-correlation time of 48 ± 2 fs in thisstudy. In Supporting Information section S6, we also present
the results obtained using time-resolved photoelectron imaging(TRPEI) with UV pump and vacuum UV (6.3 or 7.4 eV)probe pulses with a shorter cross-correlation time of 31−32 fs,
which fully support the results obtained with EUV probe
pulses presented below.
In the discussion presented below, we employ the following
nomenclature for the electronic states. We consider only thevalence singlet states and indicate them with the symbol Sn (n= 0, 1, 2, ...), in which n starts from 0 in ascending order ofelectronic energy. (The Rydberg states are conventionallyneglected in studies of the ring-opening reaction of CHD fromthe S1 state because these states play a negligible role.)
Furthermore, we use single and double asterisks to indicate
predominantly singly and doubly excited electronic characters,
respectively. Therefore, when the ﬁrst excited singlet state witha singly excited electronic character undergoes avoided
crossing with the doubly excited electronic state, the notation
changes from S1* to S1**. We will later discuss the doubly
excited electronic characters of the S1 and S2 states more fully
based on quantum chemical calculations. Since the C2
symmetry of CHD is lifted during the reaction, we use theirreducible representation in the C2 point group only when it isindispensable for explaining the character of molecular orbitals
and the propensity of electronic transitions. Figure 1 employs
both the nomenclature Sn (n = 0, 1, 2, ...) with asterisks and themolecular orbital symmetries under the C2 symmetry in order
to explain the correlation of electronic states between CHD
and HT.

Figure 1. Experimental setup and schematic diagram of ring-opening
reaction of CHD. Deep UV pump (3ω, 267 nm, 4.6 eV) and extreme
UV probe (14ω, 57 nm, 21.7 eV) pulses were employed (TM,toroidal mirror; CoIn, conical intersection; CHD, 1,3-cyclohexadiene;HT, 1,3,5-hexatriene). The orbital symmetries are indicated assuming
the C2 symmetry to illustrate the correlation of the orbitals between
CHD and HT. C2 symmetry holds only between the Franck−Condon
region and S2/S1 CoIn. The symmetry-breaking motions play animportant role to access the S1/S0 seam of crossing under non-C2
symmetry.

■

theoretical studies on this reaction.3−8,10−21 Time-resolved
photoelectron spectroscopy (TRPES),5,18,19 ultrafast near-edge
X-ray absorption ﬁne structure spectroscopy20 and ultrafast
electron diﬀraction experiments21 have been performed to
study the reaction. Nevertheless, the reaction pathway via the
doubly excited state has yet to be unambiguously identiﬁed.
One of the diﬃculties with these previous experiments wastheir relatively low temporal resolution. As described in thepresent paper, the nonadiabatic transition occurs within 30 fs
after photoexcitation, and it is therefore crucial to have asuﬃciently high time-resolution to clearly capture the nonadiabatic transition. In this regard, the cross-correlation time
(typically 100−150 fs) of the pump and probe pulses in pastultrafast spectroscopy experiments on this system was
insuﬃcient except for the experiment by Fuß and colleagues
using 13 fs pulses.11 Furthermore, mass spectrometry and Xray/electron diﬀraction experiments are generally insensitive tothe electronic character of a molecule so that they are not bestsuited for observation of nonadiabatic transitions.
The objective of the present study is to perform real-timeobservation of the electronic dynamics of the ring-opening
reaction of CHD and to establish a mechanistic picture of thisparadigmatic ring-opening reaction. To this end, TRPES was
employed with extreme UV (EUV) probe pulses (21.7 eV).22
The EUV pulses ionize all the chemical species involved in thereaction;23 therefore, EUV photoelectron spectroscopy enables
complete observation of the dynamics from the Franck−
Condon (FC) region in the S1(π,π*) state of CHD until the

RESULTS AND DISCUSSION
Figure 2A shows a two-dimensional (2D) map of EUV
photoelectron spectra measured as a function of the pump−probe delay time. The pump pulses excited CHD to the S1*state, and the EUV probe pulses induced photoemission fromall transient species in all electronic states. The spectra areplotted against the electron binding energy (eBE), given by thediﬀerence between the EUV photon energy and the measured
electron kinetic energy (eKE). The three negative-intensity
bands indicated in violet in Figure 2A are the ground-statebleach (depopulation) induced by the UV pump pulses, which
can be conﬁrmed by comparing with an EUV photoelectron
spectrum of CHD at room temperature shown in the same
ﬁgure. The positive signals at short pump−probe delay times(<100 fs) are primarily from the excited electronic state of
CHD, while those at longer delay times are from the reaction
products.
Figure 2B shows an enlarged view of the rectangular region
outlined by the white dashed lines in Figure 2A. The
photoelectron signal for S1* starts from three eBE values of
3.8, 6.2, and 7.1 eV at time zero. The lowest eBE band isassociated with the D0 ← S1* ionization transition; the initial
eBE value of 3.8 eV at the time origin agrees well with thevertical ionization energy of CHD of 8.4 eV measured using
our apparatus. The photoabsorption spectrum of matrixisolated CHD+ cations exhibits two bands at around 2.62 and3.20 eV,25 which can be assigned to the ﬁrst and second
B

https://doi.org/10.1021/jacs.1c01896
J. Am. Chem. Soc. XXXX, XXX, XXX−XXXJournal of the American Chemical Society

pubs.acs.org/JACS

Articleopening reaction path, while the energy of the stable cationicground state D0 increases along the path (Figure 4B). The S1*
signal intensity integrated over an energy range of 3.0−5.5 eV,
as shown with red open circles in Figure 2C, exhibits a single
exponential decay. The time proﬁles shown in Figure 2C were
determined with experimental uncertainties of 1%. As
described in Supporting Information section S2, the highﬁdelity of our measurement enabled estimation of the time
constants with a standard deviation of about 10%, whichcorresponds to several femtoseconds. The proﬁle of S1* wasreasonably well expressed with a latency time and exponential
decay time constant of 10 ± 3 and 21 ± 2 fs, respectively, fromwhich the 1/e lifetime of S1* was determined to be 31 fs. As
presented in Supporting Information section S6-2, we also
performed TRPEI experiments using vacuum UV pulses, which
provided slightly diﬀerent estimates of the latency time anddecay time constant of 17 ± 4 and 17 ± 6 fs, respectively;
however, these values also provide an S1* lifetime of 34 fs.
Thus, we conclude that the S1* population decreases to 1/e inapproximately 31−34 fs.
The photoemission band intensity in the next eBE region of
5.5−6.8 eV exhibits diﬀerent temporal behavior. As shown in
Figure 2C, the integrated intensity in this energy range appears
time-delayed with respect to the signal in the 3.0−5.5 eV range
and decays more slowly. If only the D1 ← S1* transition occurs
in the 5.5−6.8 eV region, the time proﬁles observed for the tworegions of 3.0−5.5 and 5.5−6.8 eV are expected to be thesame. However, the observed diﬀerence between the time
proﬁles indicates that another photoemission band corresponding to D0 ← S1** transition exists in the 5.5−6.8 eV
region. We will conﬁrm this assignment later using quantum
chemical calculations. The individual contributions of the D1← S1* and D0 ← S1** transitions in the 5.5−6.8 eV region are
separated as follows. The S1* population proﬁle was alreadydetermined from the D0 ← S1* signal in the 3.0−5.5 eVregion; therefore, it can be employed to estimate the D1 ← S1*
time proﬁle in the 5.5−6.8 eV region, except for its absoluteintensity. The formation curve for the S1** population must be
the same as the decay curve for S1* because the overall
population in the S1 manifold is maintained in this time range.
Thus, what are unknown here are the relative ionization crosssections for the D1 ← S1* and D0 ← S1** transitions and the
decay time constant for S1**. By assuming latency and risetimes of 10 and 21 fs for S1**, respectively, the decay time forS1** was estimated to be 37 ± 2 fs. (As described in
Supporting Information section S6-2, TRPEI estimates the
lifetime of S1** to be 27 ± 10 fs.) Figure 2D shows spectralfeatures of the separated D1 ← S1* and D0 ← S1** bands (thedetails of the spectral ﬁtting method are described in
Supporting Information section S5). The intensity of the D1← S1* band is stronger than expected from the electron
conﬁguration shown in Table 1, for the reason explained later.
An important point raised by van der Lugt and Oosterhoﬀ
regarding the electrocyclic reaction is the involvement of the
doubly excited state.9 However, the character of eachelectronic state possibly varies with nuclear displacements sothat one may ask whether the S2** and S1** states really havea doubly excited electronic character. It would be of interest toinvestigate this point using conﬁguration interaction (CI)
calculations in order to establish this concept more ﬁrmly.
Figure 3A shows one-dimensional potential energy curves
calculated for CHD using the extended multistate complete
active space second-order perturbation theory (XMS-

Figure 2. Experimental photoelectron spectra. (A) 2D map of EUV
photoelectron spectra with the photoelectron spectrum of the groundstate CHD shown on the right. The negative signals correspond to theground-state bleach. (B) Enlarged 2D map of the area indicated withdashed lines in (A). (C) Time proﬁles of the photoelectron signal
intensities integrated over 3.0−5.5 and 5.5−6.8 eV regions (opencircles) and their least-squares ﬁtting (solid lines). Dashed lines
indicate the deconvoluted components of S1* and S1**. The residuesof the least-squares ﬁtting are shown with dots above the proﬁles.
These time proﬁles were measured with the experimental
uncertainties of 1%. (D) 2D maps of the contributions from S1*and S1**. See Supporting Information section S5 for details.

excited cationic states (D1 and D2). The observed eBE valuesof 6.2 and 7.1 eV agree reasonably well with those expected forD1 ← S1* and D2 ← S1*, respectively. As the delay time isincreased, the D0 ← S1* band shifts rapidly to higher eBE
because the electronic energy of S1* decreases along the ringC

https://doi.org/10.1021/jacs.1c01896
J. Am. Chem. Soc. XXXX, XXX, XXX−XXXJournal of the American Chemical Society

pubs.acs.org/JACS

ArticleTable 1. Electronic Energies of CHD Calculated Using XMS-CASPT2 under C2 Symmetry and Corresponding Experimental
Values (eV)at 11A equilibrium geometryat 21A equilibrium geometrystateleading conﬁguration(s)

calcd

exptl

leading conﬁguration(s)

calcd

32B
22B

(9b)2(11a)2(10b)2(12a)0(11b)1(9b)1(11a)2(10b)2(12a)2(11b)0

11.78
11.60

11.8a
11.8b

13.72
13.09

22A

(9b)2(11a)1(10b)2(12a)2(11b)0

11.11

11.3b

12B
12A
21A

(9b)2(11a)2(10b)1(12a)2(11b)0(9b)2(11a)2(10b)2(12a)1(11b)0(9b)2(11a)2(10b)1(12a)2(11b)1(9b)2(11a)2(10b)2(12a)0(11b)2
3s Rydberg stated(9b)2(11a)2(10b)2(12a)1(11b)1(9b)2(11a)2(10b)2(12a)2(11b)0

10.70
8.37
6.06c

10.7b
8.25b

(10a)2(10b)2(11a)1(11b)1(12a)1(10a)2(10b)1(11a)2(11b)2(12a)0(10a)2(10b)1(11a)2(11b)0(12a)2(10a)2(10b)2(11a)1(11b)2(12a)0(10a)2(10b)2(11a)1(11b)0(12a)2(10a)2(10b)2(11a)2(11b)1(12a)0(10a)2(10b)2(11a)2(11b)0(12a)1(10a)2(10b)2(11a)2(11b)2(12a)0(10a)2(10b)2(11a)2(11b)0(12a)2

5.87d
4.88
0

5.39e
4.94f
0

(10a)2(10b)2(11a)2(11b)1(12a)1(10a)2(10b)2(11a)2(11b)2(12a)0(10a)2(10b)2(11a)2(11b)0(12a)2

4.21
2.83

11B
11A

13.16
9.98
10.00
3.80

a

This work. The details of computational method are described in the Computational Simulation section. The absorption spectrum of the CHD
cation in a freon matrix25 suggests 11.5 eV. The 32B and 22B are almost degenerate, and their energetic orders are diﬃcult to determine. bHe I
PES.26 cClassiﬁed as 41A at the FC geometry. dThe leading conﬁguration is ...(9b)2(11a)2(10b)2(12a)1(11b)0(13a)1. The state energy wascalculated in the multistate-multireference scheme of XMS-CASPT2 with an active space of four electrons in eight orbitals, namely, two π and two
π* orbitals in addition to 3s, 3px, 3py, and 3pz Rydberg orbitals. 13a is of 3s Rydberg character. eResonance enhanced multiphoton ionization.27
f
Optical and electron energy loss spectroscopies.28

lists the leading electron conﬁgurations of these states forequilibrium geometries of 11A and 21A. It can be seen that theelectronic character of S2(21A) in the ground-state equilibriumgeometry contains a fair amount of [(10b)1(12a)2(11b)1]
conﬁguration in addition to the doubly excited conﬁguration of[(10b)2(12a)0(11b)2]. Is the fraction of the doubly excitedcharacter of 21A smaller than our expectation? In order toclarify this point further, we evaluated the fraction of the
doubly excited characters of [(10b)2(12a)0(11b)2] and[(10b)2(12a)2(11b)0] in the electronic wave functions of 11A
and 21A using the CI coeﬃcients obtained using XMSCASPT2 calculations. Figure 3B shows the squared CI
coeﬃcients of the doubly excited conﬁgurations in theelectronic wave functions of the 11A and 21A states as afunction of the C5−C6 distance. The results indicate that the(11b)2 doubly excited character in the 21A state increases as
the C5−C6 bond is elongated to 2.0 Å, where the 21A potential
energy curve crosses the 11B curve. Thus, the calculations
conﬁrmed that the second valence-excited singlet state has adoubly excited electronic character near the S2/S1 surface
crossing region. Since most of the nuclear trajectories from theFranck−Condon region to the S2/S1 minimum energy conicalintersection (MECI) pass the region close to the C2 symmetry,
the above argument assuming C2 symmetry is valid for an
ensemble of nuclear trajectories in this region of the potential
energy surface, even without the symmetry restriction assumed
here.
Previously, this reaction has been studied using UV twophoton ionization from the excited state. Two-photon TRPES
studies revealed the sequential appearance of resonanceenhanced two-photon ionization via Rydberg states fromhigh to low principal quantum numbers (n);5,19 however, theorigin of these signals was not well understood. In order torevisit this problem, we performed two-photon UV-TRPEI
with a higher time-resolution (58 fs) than in previous studies
and obtained the results shown in Figure 4A. It is seen that the
6s Rydberg state has a maximum at a delay time of 13 fs, whilethe 3p state has a maximum at 35 fs. One of the key criteria for

Figure 3. (A) Potential energy curves of 11A, 11B, 21A, 12A, 12B, 22A,
22B, and 32B for CHD calculated assuming C2 symmetry. (B) Squared
CI coeﬃcients of the doubly excited conﬁgurations in the electronic
wave functions of the 11A and 21A states.

CASPT2) method as a function of the C5−C6 distance byassuming that C2 symmetry is maintained. Under C2 symmetry,
the S1(11B) and S2(21A) potential energy curves are crossed
because they belong to diﬀerent irreducible representations.
On the other hand, the 21A and S0(11A) states form a strong
avoided crossing at around a C5−C6 distance of 2.3 Å. Table 1
Dhttps://doi.org/10.1021/jacs.1c01896
J. Am. Chem. Soc. XXXX, XXX, XXX−XXXJournal of the American Chemical Society

pubs.acs.org/JACS

Articlethe assignment of these Rydberg signals is their appearance
times; as the experimental results shown in Figure 2 revealed,
the photoexcited CHD is in the S1* state within this time
range. Therefore, this clearly represents ionization from the S1*state. It is noted that the energy diﬀerence between the
Rydberg state and S1* is expressed by
E Ryd − E(S1*) = eBE(S1*) − R /(n − δ)2where δ is a quantum defect and R is the Rydberg constant.
Thus, the excitation energies from S1* to the Rydberg series
increases exactly with eBE(S1*). We have already seen the
rapid increase in eBE(S1*) in Figure 2. Consequently, theRydberg state in resonance with the UV probe photon energy
progressively shifts to lower Rydberg states with decreasing
values of n − δ (see Figure 4B). Presumably, part of the twophoton ionization via 3p Rydberg states after 30 fs is from
S1**.
In order to conﬁrm our interpretation of the EUV
photoelectron spectra and to gain further insight into the
dynamics, we employed two theoretical analyses. In the ﬁrst
approach, we selected the most probable ring-opening reaction
path estimated by geodesical interpolation29 among themolecular geometries of FC, S2/S1 MECI, S1/S0 MECI
(non-C2-symmetry), and the equilibrium geometries of the
HT isomers, and we calculated the eBE values and photoionization cross-sections along the path using the XMSCASPT2 method, as shown in Figure 5A. The highest cationicstate considered here is D4. Superimposed with diﬀerent colors
in this ﬁgure are the ionization cross-sections estimated fromthe norm of Dyson orbitals. The value on the horizontal axis is
the total arc length for nuclei to move along this reaction pathso that the value extends up to 25 Å. A semilog plot is usedhere to present rather congested trajectories for diﬀerent

Figure 4. TRPES with two-photon ionization. (A) 2D map of thephotoelectron spectra measured with the resonance-enhanced twophoton ionization via Rydberg states using 394 nm probe pulses. (B)Schematic diagram of photoionization from S1* with EUV and UVprobe pulses.

Figure 5. Computational simulation of photoelectron spectra. (A) Variation of eBE calculated along the geodesically interpolated ring-opening
reaction path connecting the FC state, S2/S1 MECI, S1/S0 MECI, cZc, cZt, and tZt forms of HT. Ionization from S0−S2 to D0−D4 is considered.
Photoionization cross-sections estimated from the norm of Dyson orbitals for each ionization channel are superimposed. The value on thehorizontal axis, shown in a logarithmic scale, is the total arc length for nuclei to move along this reaction path. Molecular structures in FC, S2/S1
MECI, S1/S0 MECI and the S0 minimum are illustrated on top with the bond length (Å). (B) Photoelectron spectra simulated based on trajectorysurface hopping calculations by Polyak et al.8 Both CHD and HT forming channels are included, and experimental temporal (48 fs) and energy(0.12 eV) resolutions are incorporated. (C) Experimental time energy map obtained by elimination of the ground-state bleach signal from the data
shown in Figure 2A.
E

https://doi.org/10.1021/jacs.1c01896
J. Am. Chem. Soc. XXXX, XXX, XXX−XXXJournal of the American Chemical Society

pubs.acs.org/JACS

ArticleFigure 6. Photoelectron spectra due to transitions from S1 state to diﬀerent cationic states simulated based on trajectory surface hopping
calculations by Polyak et al.8 Experimental temporal (48 fs) and energy (0.12 eV) resolutions are incorporated.

transitions can clearly be seen in the panel for D0−S1. As
described earlier, one puzzling feature concerning theexperimentally observed D1 ← S1* band is that it appearedwith a greater intensity than expected (Figure 2). Since theleading electron conﬁgurations for S1* and D1 are respectively[(10b)2(12a)1(11b)1] and [(10b)1(12a)2(11b)0] (see Table1), the D1 ← S1* ionization transition is expected to be very
weak; Figure 5A also predicts a low intensity for this transitionin accordance with this expectation. However, examination of
Figure 5B reveals that the spectral simulation based on the fulldimensional surface hopping trajectory calculations reproduces
the strong appearance of the D1 ← S1* band. What is thereason for this diﬀerence? In order to ﬁnd the origin of thisenhanced ionization intensity, we examined the D1 ← S1*
photoionization cross-section along the full-dimensional
nuclear trajectories in the S1* state. We found that the D1← S1* cross-section estimated from the Dyson norm is only
0.1 for the FC geometry, while it is 0.3 on average for an
ensemble of trajectories spreading out from the FC geometry.
Therefore, the higher intensity of the D1 ← S1* band isascribed to a non-Condon eﬀect due to variation of theelectronic wave function caused by nuclear motions
perpendicular to the ring-opening reaction coordinate.
Meanwhile, the leading electron conﬁgurations for S2** andD0 are [(10b)2(12a)0(11b)2] + [(10b)1(12a)2(11b)1] and[(10b)2(12a)1(11b)0], respectively, in the ground-state equilibrium geometry; therefore, photoionization from S2** to D0
is unfavorable for this geometry. However, the leading electron
conﬁgurations for S1** and D0 change to[(11a) 2 (11b) 2 (12a) 0 ] + [(11a) 2 (11b) 0 (12a) 2 ] and[(11a)2(11b)1(12a)0] and/or [(11a)2(11b)0(12a)1] for theequilibrium geometry of S1**, respectively. This enables astrong D0 ← S1** ionization transition. Examination of Figure
5A reveals that the D0 ← S1** band is overlapping with the D1← S1** band in the reaction coordinate range from 2 to 7 Å.
These are basically transitions to the cationic 12A and 12B
states. As seen in Figure 3A, the potential energy curves forS1*(21A) and D0(12B) are nearly parallel to each other;
therefore, the D0 ← S1** band exhibits a relatively constanteBE value.
Although the accuracy of the computational simulations
presented in Figure 5B is among the highest reported, closecomparison between the computational and experimental
results reveals some discrepancies. First of all, the reaction
time is slightly longer for the computational results, which
points to small inaccuracies in the calculated potential energy
surfaces. Second, the photoemission intensity for the excitedionization channels in the range of small displacement along
the reaction path. The ﬁgure indicates a steep increase in theeBE value for the D0 ← S1* band, followed by a rather
constant eBE value for D0 ← S1** and a large increase in eBE
upon formation of the ground-state HT. These features are inexcellent agreement with the experimental results shown in
Figure 2. The eBE values calculated using the XMS-CASPT2
level of theory were highly accurate and in quantitative
agreement with the experimental values; however, we foundthat the simulated spectra were being shifted by −0.4 eV withrespect to the experimental ones. Therefore, we adjusted thesimulated spectra by +0.4 eV for closer comparison with theexperimental results. In order to relate the eBE values and themolecular structures, the calculated structures at the criticalpoints along the reaction path are also shown in Figure 5A.
The CHD molecule has two CC double bonds in the FC
geometry, while all carbon−carbon bonds except the
dissociating C5−C6 become almost the same length in the
structures for S2/S1 and S1/S0 CoIn. The equilibrium
structures of HT isomers have three CC double bonds.
Greater structural change occurs on the ground-state surface ofHT. The actual nuclear motions are not restricted to thisgeodesically interpolated path; therefore, the map shown in
Figure 5A only describes the principal part of the multidimensional dynamics. Nonetheless, the one-dimensional analysis in
Figure 5A provided highly valuable insights into the reaction
and photoionization dynamics.
A clear limitation of the results shown in Figure 5A is thatthey do not include a nonreactive internal conversion path to
the S0 state of CHD or predict the reaction time. Therefore, inthe second approach, we performed a thorough dynamical
simulation of the experimental results by a combination of the
full-dimensional surface hopping calculations performed by
Polyak et al.8 and our own calculations of the ionization
dynamics at the XMS-CASPT2 level of theory. The spectrathus simulated are shown in Figure 5B, including both
pathways to form CHD and HT. The highest cationic state
considered in this simulation is D4, and the simulated spectra
are shifted by 0.4 eV as mentioned earlier. The experimental
temporal (48 fs) and energy (0.12 eV) resolutions wereincorporated. The simulated spectra are in excellent agreementwith the experimental results shown in Figure 5C.
For closer examination of the photoelectron spectra, we
present photoemission time−energy proﬁles for individual
photoionization channels from the S1 state to ﬁve cationicstates of CHD in Figure 6. In these panels, S1* and S1** are
not diﬀerentiated; however, the D0 ← S1* and D0 ← S1**F

https://doi.org/10.1021/jacs.1c01896
J. Am. Chem. Soc. XXXX, XXX, XXX−XXXJournal of the American Chemical Society

pubs.acs.org/JACS

Articlestates of CHD appears slightly stronger in the computational
results, which suggests small inaccuracies in the ionization

Figure 8. Representative trajectory leading to CHD. (A) Snapshots ofDyson orbitals for ionization from current electronic state to D0. (B)Electronic energies along nuclear trajectory, where current state ismarked by purple circles.

Figure 7. Representative trajectory leading to HT. (A) Snapshots ofDyson orbitals for ionization from current electronic state to D0. (B)Electronic energies along nuclear trajectory, where current state ismarked by purple circles.

cross-sections obtained from the Dyson norms. Third, thecalculated photoemission bands are slightly shifted in energy
from the experimental ones, as described earlier.
To understand the electronic and structural dynamics moreclearly, Figures 7 and 8 present the time-evolution of the
Dyson orbital, which can be regarded as an electron holecreated by photoionization, and molecular geometry along thetwo nuclear trajectories leading to HT and CHD products,
obtained using full-dimensional surface hopping calculations.
In Figure 7A, it can be seen that the Dyson orbital associatedwith photoionization from S1* at delay times of 0 and 20 fs is a
11b singly occupied molecular orbital. However, the 12acharacter with a node between the C2 and C3 carbon atoms
emerges at 32 fs, and the Dyson orbital exhibits frustrated
ﬂickering until 50 fs when the 11b character dominates in the
ground state of HT. In Figure 8A, the Dyson orbitals at 0 and
20 fs also have an 11b character, while at 27 fs it has a 12acharacter. The Dyson orbital at 55 fs is clearly 12a in the
ground state of CHD. The important contributions of both the
11b and 12a orbitals seen here are consistent with Figure 3B.
Using these Dyson orbitals, we will further discuss the notionof doubly excited state characters. The Dyson orbitals shownin Figures 7 and 9 clearly indicate that the S1 and S0 electronic
wave functions in the avoided crossing region have equal 11b
and 12a characters. A linear combination of 11b and 12a

Figure 9. (a) 11b and 12a molecular orbitals (as in Figure 1). (b)Linear combination of 11b and 12a orbitals with equal amplitudes.

orbitals with equal weights creates the orbital patterns 12a +11b and 12a − 11b, as shown in Figure 9, and these are clearlyseen at delay times of 32 and 39 fs in Figure 7A. When we takethese orbitals as the basis functions, the following relationshipis obtained:[(12a + 11b)1(12a − 11b)1]= [(12a)2 (11b)0 ] − [(12a)0 (11b)2 ]Thus, the same electronic state can be expressed either with asingly excited electronic character with one basis (left) or witha doubly excited electronic character with the other basis(right). The aforementioned notion of a doubly excited state isbased on the latter standpoint; it should be kept in mind thatthe description of the electronic wave function depends on theG

https://doi.org/10.1021/jacs.1c01896
J. Am. Chem. Soc. XXXX, XXX, XXX−XXXJournal of the American Chemical Society

pubs.acs.org/JACS

Articlenot exhibit any oscillation. An intense pump pulse cangenerally induce impulsive Raman scattering to create
vibrational coherence in the ground state of the reactant;however, the oscillatory features observed in the eBE region of
9.7−10.4 eV are well separated from the photoelectron bandsof cold CHD (Figure 2A). Therefore, the observed beat signals
are clearly of the vibrational coherence in the products (hotCHD or HT). We performed Fourier transform of these time
proﬁles for the temporal region indicated with blue dots in
Figure 10A, and we found the main quantum beat component
was centered at 270 cm−1 in the unit of wavenumbers (seeFigure 10B). We have measured similar data for a shorterpump wavelength of 250 nm and obtained essentially the same
oscillatory features albeit with slightly smaller amplitudes (seeSupporting Information section S4).
The deﬁnitive assignment of these quantum beats is diﬃcult
at this point. As shown in Figure 11, the photoelectron spectra

basis functions. The interesting features in Figures 7B and 8B
are that the electronic energies for both CHD and HT

Figure 10. Vibrational coherence in reaction products. (A) Time
proﬁle of photoelectron intensities integrated over eBE regions of
7.0−8.0, 8.2−8.7, and 9.7−10.4 eV. Black lines are the experimental
data, and red lines are the population obtained by the analysis
described in section S3 of the Supporting Information. It is noted thatthe phase of the oscillations is shifted by π between the traces of 7.0−
8.0 and 9.7−10.4 eV. The trace for 8.2−8.7 eV is for thephotoelectron band center of the ground-state CHD, for which no
oscillatory component is identiﬁed. (B) Fourier transform ofoscillatory components in D0 ← S0 and D1 ← S0 transitions of thereaction products. Fourier components were obtained from theresidues shown with blue dots in (A). A rectangular window function
was used in the Fourier transform. (C) Comparison of the observed
vibrational quantum beat and computed photoelectron intensities for
CHD and HT products in selected eBE regions of 7.0−8.0 eV (solidline) and 9.7−10.4 eV (dashed line).

Figure 11. Photoelectron spectra of CHD and HT. (Top two panels)
Experimental photoelectron spectra of CHD and HT(tZt)26,34
measured at room temperature along with the band positions
predicted using XMS-CASPT2 quantum chemical calculations. The
calculated positions were not shifted. (Middle two panels) Photoelectron spectra of CHD and HT produced by the ring-opening
reaction of CHD calculated using XMS-CASPT2 surface hopping
calculations. The calculated spectra were shifted by +0.4 eV tocompare with the experimental result. (Bottom panel) Experimental
photoelectron spectrum of reaction products. Solid and dashed lines
show the experimental results with pump pulses of 267 and 252 nm,
respectively. The negative signals in the experimental spectrum due tothe ground-state bleach of CHD were compensated by addition of thephotoelectron spectrum of cold CHD with an estimated pumping
eﬃciency.

products vary between 1 and 3 eV, indicating that these
trajectories do not sample the potential region near the
ground-state equilibrium geometries of the products.
Returning to the photoelectron spectra in Figure 2A, it canbe seen that the photoelectron signals from the nascent
reaction products exhibit sinusoidal intensity modulation.
These are vibrational coherence in the reaction products
created by this ballistic ring-opening reaction. Figure 10A
shows the time proﬁles of these photoelectron signal intensities
integrated over selected eBE regions. The signals in the 7.0−
8.0 and 9.7−10.4 eV regions exhibit clear intensity oscillations.
Furthermore, the phase of the oscillation diﬀers by π between
these two regions. For comparison, the signal integrated for
8.2−8.7 eV is of the ground-state bleach of CHD, and it doesof hot CHD and HT computed based on the aforementioned
full-dimensional trajectory calculations and ionization crosssections were quite similar to each other and their spectral
separation was diﬃcult. Alternatively, we examined thecomputed photoelectron spectra for CHD and HT andextracted the oscillatory features as shown in Figure 10C.
Both of the calculated photoelectron spectra for CHD and HT
products exhibit oscillatory features, although the amplitude
was greater for HT. Therefore, the experimentally observed
vibrational coherence is most likely of the HT product;H

https://doi.org/10.1021/jacs.1c01896
J. Am. Chem. Soc. XXXX, XXX, XXX−XXXJournal of the American Chemical Society

pubs.acs.org/JACS

single harmonic (14ω: 57 nm, 21.7 eV) was selected with a gratingbased time-preserving monochromator. The pump pulses weregenerated using FFWM in Ar gas using the fundamental (ω, 0.5mJ) and 2ω (0.3 mJ) pulses.24 The output pulses were reﬂected andmonochromatized using multilayer mirrors designed for 3ω. The
optical path length of the 3ω pulses was controlled using a translation
stage with 5 nm resolution. The temporal width and energy of thepump pulses were sub-30 fs and 800 nJ/pulse, respectively. The crosscorrelation time between the pump and probe pulses was determined
to be 48 fs using nonresonant ionization of Xe.
CHD vapor seeded in He carrier was injected into a photoelectron
spectrometer through a pinhole (⌀0.1 mm) at a stagnation pressure of
0.06 MPa at room temperature. The eKE distribution was measured
using a magnetic bottle time-of-ﬂight (TOF) spectrometer.38 The
photoelectrons traveled through a 1300 mm long ﬂight tube weredetected using a microchannel plate detector (⌀42 mm) at the end of
the ﬂight tube. A retardation voltage of −6.0 V was applied to theﬂight tube in order to reject low-energy electrons produced by onecolor two-photon ionization of CHD with the pump pulses. Theenergy calibration of the photoelectron spectrometer was performed
using the 2P3/2 and 2P1/2 peaks in the photoelectron spectrum of Xe.
The energy resolution was estimated to be 0.12 eV. The pressure in
the photoionization chamber and TOF analyzer was 4.0 × 10−5 and<1.0 × 10−7 Torr, respectively, during the measurements. Further
information on the experimental setup is described in the Supporting
Information section S1.
Computational Simulation. The ring-opening reaction of CHD
proceeds by breaking its C2 symmetry. However, the potential energy
curves calculated under C2 symmetry provide information regarding
the correlation of the electronic states between CHD and HT. The
eﬀective potential energy curves for the ring-opening reaction of CHD
were computed using the Roos atomic natural orbital (ANO) basis
set39 contracted to 4s3p2d functions for carbon and 2s1p functions
for hydrogen using the ab initio quantum chemistry software
MOLPRO.40,41 For neutral CHD, static electron correlation was
treated at the complete-active-space self-consistent ﬁeld (CASSCF)
level of theory42,43 with an active space composed of six electrons
distributed among six orbitals, i.e., two π and two π* orbitals in
addition to one σ and one σ* orbital initially localized at the breaking
C5−C6 bond. The CASSCF orbitals were averaged over the states ofinterest (11A, 11B, and 21A) with equal weights. Dynamic electron
correlation was then taken into account at the level of the extended
multistate CAS second-order perturbation theory (XMSCASPT2)44,45 with a level shift of 0.3 hartree adopted to avoid
intruder state problems.46 The geometry of CHD was optimized in
the 11A and 21A states under C2 symmetry constraints. Along the pathlinearly connecting the two optimized geometries, the three neutral
states were evaluated using the multistate multireference (MS-MR)
scheme of XMS-CASPT2. A number of cationic states accessible fromthe neutral states by a 21.7 eV photon were also obtained using thesingle-state single-reference (SS-SR) scheme of XMS-CASPT2 with alarger active space of thirty-one electrons in seventeen (fourteen σ,
two π, and one π*) orbitals. The results thus obtained are presented
in Figure 3 and Table 1.
The ionization energies (vertical eBEs) for three isomers of coldHT, shown in Figure 11, were computed in a similar manner. The
geometry optimization of neutral HT in the 11A state was conductedat the CASPT2 level with an active space consisting of six electrons insix (three π and three π*) orbitals. The resultant equilibriumgeometries of the cZc, cZt, and tZt isomers were of C2, C1, and C2v
symmetry, respectively. Cationic states reachable from the 11A statefor each isomer were evaluated using the SS-SR scheme of XMSCASPT2 with an active space of thirty-one electrons in seventeen(thirteen σ, three π, and one π*) orbitals.
For simulations of photoelectron spectra shown in Figures 5−8, weevaluated the electronic structures of CHD and HT in a similarmanner as that used by Polyak et al.8 For singlet (doublet) cases, the
XMS-CASPT2/cc-pVDZ calculations with an active space of eight(seven) electrons in eight orbitals were performed including the S0−
S2 (D0−D4) states with the BAGEL47 software. An energy shift of 0.5

however, quantum mechanical analysis is necessary for more
deﬁnitive assignment in the future. An ultrafast electron
diﬀraction study on this system21 indicated an oscillatory
feature of the diﬀraction signal of HT with an interval of 0.25
and 0.29 ps. The period of the vibrational quantum beat shownin Figure 10 is about 100 fs and ascribed to a diﬀerent type ofvibrational motion.
Vibrational coherence in the photochemical reaction
product has been observed for the cis−trans photoisomerization of rhodopsin by Mathies and co-workers using stimulated
Raman spectroscopy30−32 and by Miller and co-workers usingtransient grating spectroscopy.33 The product vibrational
coherence observed in the ring-opening reaction of CHD is
attributed to strong electronic coupling and an extremely short
reaction time, which are similar to the case of rhodopsin.

■

SUMMARY■

EXPERIMENTAL SECTION

ArticleThe present study ﬁrmly established the reaction pathway
mediated by the doubly excited state for the paradigmatic ringopening reaction of 1,3-cyclohexadiene and also uncovered its
extremely short reaction time (68 ± 7 fs). Our estimate of thereaction time is based on the decay of the excited state of
CHD, which corresponds to formation of the cZc form of HT.
Formation of the tZt involves rotational isomerization aroundC−C single bonds, and it will be slightly delayed. The
ultrashort reaction time is comparable with the cis−transphotoisomerization of retinal in rhodopsin for which productvibrational coherence has been observed.30−33 The computedeBE map along the geodesically interpolated reaction pathway
reproduced the general features of the experimental photoelectron spectra well, and the full-dimensional computational
simulation of the photoelectron spectra based on the XMSCASPT2 surface hopping dynamics calculation reproduced theexperimental results semiquantitatively. Rigorous assignmentof the observed vibrational quantum beat was diﬃcult in theframework of this study, and it awaits more thorough quantum
mechanical analysis of nuclear motions. At this point, we
speculate that it is most likely of HT products. We estimatedthe photoelectron spectra of hot CHD and HT from the
trajectory data and photoionization cross-sections andconcluded that clear diﬀerentiation of the CHD and HT
products is not feasible with photoelectron spectroscopy. For
this reason, reliable experimental evaluation of the CHD/HT
branching ratio in this reaction was diﬃcult in the present
study. Estimation of the branching ratio is expected to bepossible with diﬀraction experiments.35 The time-resolution of
EUV photoelectron spectroscopy can be further improved byshortening the driving laser pulse duration36 and the use of atime-compensated EUV monochromator;37 the cross-correlation time of the laser system in the present study was
predominantly determined by the temporal width of an EUV
pulse. It is of great interest to explore various photochemical
reactions using EUV-TRPES with a higher temporal resolution.

Experimental Setup. A one-box 1 kHz Ti:sapphire regenerative
ampliﬁer (35 fs, 800 nm, 1 kHz, 6 mJ) was used as a driving laser for
nonlinear optical processes to generate the DUV pump (267.5 nm)
and EUV probe (57.1 nm) pulses. The probe pulses were produced
using HHG in Kr gas with the second harmonic (2ω, 0.29 mJ) of theTi:sapphire laser as a driving pulse.22 The 2ω laser pulses were
produced with a 0.3 mm thick β-barium borate crystal and focusedusing a quartz lens (f = 500 mm) into a Kr gas cell. The 14th orderI

https://doi.org/10.1021/jacs.1c01896
J. Am. Chem. Soc. XXXX, XXX, XXX−XXXJournal of the American Chemical Society

pubs.acs.org/JACS

Roland Mitrić − Institut für Physikalische und Theoretische
Chemie, Universität Würzburg, Würzburg 97074, Germany;
orcid.org/0000-0002-4941-0436

hartree was introduced. The photoionization cross-section was
estimated from the norm of the Dyson orbitals, i.e., the overlap
between CASPT2 wave functions for the initial (Si) and the ﬁnal (Df)states, by neglecting the dependence on the electronic continuum.
Dyson orbitals were computed with the help of a recent
implementation by one of the authors (A.H.) in a development
version of BAGEL. The molecular orbitals in the Si and Df wave
functions diﬀer due to orbital relaxation and are therefore nonorthogonal. The Dyson orbitals between Slater determinants built
from nonorthogonal orbitals were evaluated as previously described.48
The calculated energy structures of the cation are slightly diﬀerent
between Figures 3 and 5 owing to the diﬀerence in the active spaceconsidered in the calculations.
The minimum energy path that led to diﬀerent isomers of HT in
the ring-opening reaction of CHD was calculated at the XMSCASPT2/cc-pVDZ level of theory. The reaction path from the FC
region in S1 through S2/S1 and S1/S0 MECIs to cZc, cZt, and tZt-HT
was considered. The stationary points and MECIs were optimized,49
and the intermediate segments joining these points were calculated bygeodesic interpolation using a previously presented algorithm.29 The
time-resolved photoelectron spectra were calculated using previously
obtained trajectory data (geometries and current electronic states).8
The eBE values and Dyson norms were computed for ionization fromeach Si to ﬁve Df states along each trajectory with a time interval of 1
fs. The contributions from all trajectories were summarized in a 2D
map and convoluted with the instrumental function for comparison
with the experimental data.

■

Complete contact information is available at:https://pubs.acs.org/10.1021/jacs.1c01896
Funding

This research was supported by JSPS KAKENHI Grant
15H05753, Mitsubishi Foundation, and the Japan-Belgium
Research Cooperative Program between JSPS and F.R.S.FNRS Grant JSBP120192201.
Notes

The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS
The authors thank Dr. I Polyak and Professor P. J. Knowles forgenerously making the results of their surface hopping
trajectory calculations available for comparison with ourexperimental results. M.K. gratefully acknowledges fruitful
discussions with Professor F. Remacle and Dr. B. Mignolet on
electronic structure computations.

■

REFERENCES(1) Deb, S.; Weber, P. M. The ultrafast pathway of photon-induced
electrocyclic ring-opening reactions: The case of 1,3-cyclohexadiene.
Annu. Rev. Phys. Chem. 2011, 62, 19−39.
(2) Woodward, R. B.; Hoﬀmann, R. The Conservation of Orbital
Symmetry; Verlag Chemie, 1969; Vol. 8, pp 781−932.
(3) Garavelli, M.; Page, C. S.; Celani, P.; Olivucci, M.; Schmid, W.
E.; Trushin, S. A.; Fuss, W. Reaction path of a sub-200 fs
photochemical electrocyclic reaction. J. Phys. Chem. A 2001, 105(18), 4458−4469.
(4) Tamura, H.; Nanbu, S.; Ishida, T.; Nakamura, H. Ab initio
nonadiabatic quantum dynamics of cyclohexadiene/hexatriene ultrafast photoisomerization. J. Chem. Phys. 2006, 124 (8), 084313.
(5) Schalk, O.; Geng, T.; Thompson, T.; Baluyot, N.; Thomas, R.
D.; Tapavicza, E.; Hansson, T. Cyclohexadiene revisited: A timeresolved photoelectron spectroscopy and ab initio study. J. Phys.
Chem. A 2016, 120 (15), 2320−2329.
(6) Lei, Y. B.; Wu, H. Y.; Zheng, X. L.; Zhai, G. H.; Zhu, C. Y.
Photo-induced 1,3-cyclohexadiene ring opening reaction: Ab initio
on-the-fly nonadiabatic molecular dynamics simulation. J. Photochem.
Photobiol. A 2016, 317, 39−49.
(7) Ohta, A.; Kobayashi, O.; Danielache, S. O.; Nanbu, S.
Nonadiabatic ab initio molecular dynamics of photoisomerization
reaction between 1,3-cyclohexadiene and 1,3,5-cis-hexatriene. Chem.
Phys. 2015, 459, 45−53.
(8) Polyak, I.; Hutton, L.; Crespo-Otero, R.; Barbatti, M.; Knowles,
P. J. Ultrafast photoinduced dynamics of 1,3-cyclohexadiene using
XMS-CASPT2 surface hopping. J. Chem. Theory Comput. 2019, 15(7), 3929−3940.
(9) van der Lugt, W. T. A. M.; Oosterhoff, L. J. Quantum-chemicalinterpretation of photo-induced electrocyclic reactions. Chem.
Commun. 1968, No. 20, 1235−1236.
(10) Nenov, A.; Kölle, P.; Robb, M. A.; de Vivie-Riedle, R. Beyond
the van der Lugt/Oosterhoff model: When the conical intersection
seam and the S1 minimum energy path do not cross. J. Org. Chem.
2010, 75 (1), 123−129.
(11) Kosma, K.; Trushin, S. A.; Fuß, W.; Schmid, W. E.
Cyclohexadiene ring opening observed with 13 fs resolution:
Coherent oscillations confirm the reaction path. Phys. Chem. Chem.
Phys. 2009, 11 (1), 172−181.
(12) Fuß, W.; Schmid, W. E.; Trushin, S. A. Time-resolved
dissociative intense-laser field ionization for probing dynamics:
Femtosecond photochemical ring opening of 1,3-cyclohexadiene. J.
Chem. Phys. 2000, 112 (19), 8347−8362.

ASSOCIATED CONTENT* Supporting Information
sı

The Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/jacs.1c01896.
Further information on experimental methods, analysisof the TRPES data, results of TRPEI experiments, andcomputational simulations (PDF)■

ArticleAUTHOR INFORMATION

Corresponding Author

Toshinori Suzuki − Department of Chemistry, Graduate
School of Science, Kyoto University, Kyoto 606-8502, Japan;
orcid.org/0000-0002-4603-9168; Email: suzuki@
kuchem.kyoto-u.ac.jp
Authors

Shutaro Karashima − Department of Chemistry, Graduate
School of Science, Kyoto University, Kyoto 606-8502, Japan
Alexander Humeniuk − Institut für Physikalische undTheoretische Chemie, Universität Würzburg, Würzburg
97074, Germany
Ryuta Uenishi − Department of Chemistry, Graduate Schoolof Science, Kyoto University, Kyoto 606-8502, Japan
Takuya Horio − Department of Chemistry, Graduate Schoolof Science, Kyoto University, Kyoto 606-8502, Japan; Present
Address: T.H.: Department of Chemistry, Faculty of
Science, Kyushu University, 744 Motooka, Nishi-ku,
Fukuoka 819-0395, Japan.
Manabu Kanno − Department of Chemistry, Graduate Schoolof Science, Tohoku University, Sendai 980-8578, Japan;
orcid.org/0000-0003-4701-906X
Tetsuro Ohta − Department of Chemistry, Graduate School of
Science, Kyoto University, Kyoto 606-8502, Japan
Junichi Nishitani − Department of Chemistry, Graduate
School of Science, Kyoto University, Kyoto 606-8502, JapanJ

https://doi.org/10.1021/jacs.1c01896
J. Am. Chem. Soc. XXXX, XXX, XXX−XXXJournal of the American Chemical Society

pubs.acs.org/JACS(13) Mori, T.; Kato, S. Dynamic electron correlation effect onconical intersections in photochemical ring-opening reaction of
cyclohexadiene: MS-CASPT2 study. Chem. Phys. Lett. 2009, 476(1−3), 97−100.
(14) Schönborn, J. B.; Sielk, J.; Hartke, B. Photochemical ringopening of cyclohexadiene: Quantum wavepacket dynamics on a
global ab initio potential energy surface. J. Phys. Chem. A 2010, 114(12), 4036−4044.
(15) Tudorovskaya, M.; Minns, R. S.; Kirrander, A. Effects of probe
energy and competing pathways on time-resolved photoelectron
spectroscopy: The ring-opening of 1,3-cyclohexadiene. Phys. Chem.
Chem. Phys. 2018, 20 (26), 17714−17726.
(16) Filatov, M.; Min, S. K.; Kim, K. S. Non-adiabatic dynamics ofring opening in cyclohexa-1,3-diene described by an ensemble
density-functional theory method. Mol. Phys. 2019, 117 (9−12),1128−1141.
(17) Minitti, M. P.; Budarz, J. M.; Kirrander, A.; Robinson, J. S.;
Ratner, D.; Lane, T. J.; Zhu, D.; Glownia, J. M.; Kozina, M.; Lemke,
H. T.; Sikorski, M.; Feng, Y.; Nelson, S.; Saita, K.; Stankus, B.;
Northey, T.; Hastings, J. B.; Weber, P. M. Imaging molecular motion:
Femtosecond X-ray scattering of an electrocyclic chemical reaction.
Phys. Rev. Lett. 2015, 114 (25), 255501.
(18) Adachi, S.; Sato, M.; Suzuki, T. Direct observation of groundstate product formation in a 1,3-cyclohexadiene ring-opening reaction.
J. Phys. Chem. Lett. 2015, 6 (3), 343−346.
(19) Pemberton, C. C.; Zhang, Y.; Saita, K.; Kirrander, A.; Weber, P.
M. From the (1B) spectroscopic state to the photochemical productof the ultrafast ring-opening of 1,3-cyclohexadiene: A spectral
observation of the complete reaction path. J. Phys. Chem. A 2015,
119 (33), 8832−8845.
(20) Attar, A. R.; Bhattacherjee, A.; Pemmaraju, C. D.; Schnorr, K.;
Closser, K. D.; Prendergast, D.; Leone, S. R. Femtosecond X-ray
spectroscopy of an electrocyclic ring-opening reaction. Science 2017,356 (6333), 54−59.
(21) Wolf, T. J. A.; Sanchez, D. M.; Yang, J.; Parrish, R. M.; Nunes,
J. P. F.; Centurion, M.; Coffee, R.; Cryan, J. P.; Guhr, M.; Hegazy, K.;
Kirrander, A.; Li, R. K.; Ruddock, J.; Shen, X.; Vecchione, T.;
Weathersby, S. P.; Weber, P. M.; Wilkin, K.; Yong, H.; Zheng, Q.;
Wang, X. J.; Minitti, M. P.; Martinez, T. J. The photochemical ringopening of 1,3-cyclohexadiene imaged by ultrafast electron diffraction.
Nat. Chem. 2019, 11 (6), 504−509.
(22) Nishitani, J.; Yamamoto, Y.; West, C. W.; Karashima, S.;
Suzuki, T. Binding energy of solvated electrons and retrieval of true
UV photoelectron spectra of liquids. Sci. Adv. 2019, 5 (8), eaaw6896.
(23) Pathak, S.; Ibele, L. M.; Boll, R.; Callegari, C.; Demidovich, A.;
Erk, B.; Feifel, R.; Forbes, R.; Di Fraia, M.; Giannessi, L.; Hansen, C.
S.; Holland, D. M. P.; Ingle, R. A.; Mason, R.; Plekan, O.; Prince, K.
C.; Rouzee, A.; Squibb, R. J.; Tross, J.; Ashfold, M. N. R.; Curchod, B.
F. E.; Rolles, D. Tracking the ultraviolet-induced photochemistry of
thiophenone during and after ultrafast ring opening. Nat. Chem. 2020,
12 (9), 795−800.
(24) Fuji, T.; Horio, T.; Suzuki, T. Generation of 12 fs deepultraviolet pulses by four-wave mixing through filamentation in neongas. Opt. Lett. 2007, 32 (17), 2481−2483.
(25) Honda, Y.; Shida, T.; Nakatsuji, H. Excitation spectra of cation
and anion radicals of several unsaturated hydrocarbons: Symmetry
adapted cluster-configuration interaction theoretical study. J. Phys.
Chem. A 2012, 116 (48), 11833−11845.
(26) Kimura, K.; Katsumata, S.; Achiba, Y.; Yamazaki, T.; Iwata, S.
Handbook of HeI Photoelectron Spectra of Fundamental Organic
Molecules; Japan Scientiﬁc Societies Press, 1981; p 68.
(27) Merchán, M.; Serrano-Andrés, L.; Slater, L. S.; Roos, B. O.;
McDiarmid, R.; Xing, X. Electronic spectra of 1,4-cyclohexadiene and1,3-cyclohexadiene: A combined experimental and theoreticalinvestigation. J. Phys. Chem. A 1999, 103 (28), 5468−5476.
(28) McDiarmid, R.; Sabljić, A.; Doering, J. P. Valence transitions in
1,3-cyclopentadiene, 1,3-cyclohexadiene, and 1,3-cycloheptadiene. J.
Chem. Phys. 1985, 83 (5), 2147−2152.

Article(29) Zhu, X.; Thompson, K. C.; Martinez, T. J. Geodesic
interpolation for reaction pathways. J. Chem. Phys. 2019, 150 (16),164103.
(30) Schoenlein, R. W.; Peteanu, L. A.; Mathies, R. A.; Shank, C. V.
The first step in vision: femtosecond isomerization of rhodopsin.
Science 1991, 254 (5030), 412−415.
(31) Kukura, P.; McCamant, D. W.; Yoon, S.; Wandschneider, D. B.;
Mathies, R. A. Structural observation of the primary isomerization invision with femtosecond-stimulated Raman. Science 2005, 310 (5750),
1006−1009.
(32) Schnedermann, C.; Yang, X.; Liebel, M.; Spillane, K. M.;
Lugtenburg, J.; Fernández, I.; Valentini, A.; Schapiro, I.; Olivucci, M.;
Kukura, P.; Mathies, R. A. Evidence for a vibrational phase-dependentisotope effect on the photochemistry of vision. Nat. Chem. 2018, 10(4), 449−455.
(33) Johnson, P. J. M.; Halpin, A.; Morizumi, T.; Prokhorenko, V. I.;
Ernst, O. P.; Miller, R. J. D. Local vibrational coherences drive theprimary photochemistry of vision. Nat. Chem. 2015, 7 (12), 980−986.
(34) Beez, M.; Bieri, G.; Bock, H.; Heilbronner, E. The ionization
potentials of butadiene, hexatriene, and their methyl derivatives:
Evidence for through space interaction between double-bond πorbitals and non-bonded pseudo-π orbitals of methyl groups? Helv.
Chim. Acta 1973, 56 (3), 1028−1046.
(35) Ruddock, J. M.; Yong, H. W.; Stankus, B.; Du, W. P.; Goff, N.;
Chang, Y.; Odate, A.; Carrascosa, A. M.; Bellshaw, D.; Zotev, N.;
Liang, M. N.; Carbajo, S.; Koglin, J.; Robinson, J. S.; Boutet, S.;
Kirrander, A.; Minitti, M. P.; Weber, P. M. A deep UV trigger for
ground-state ring-opening dynamics of 1,3-cyclohexadiene. Sci. Adv.
2019, 5 (9), eaax6625.
(36) Cheng, Y. C.; Lu, C. H.; Lin, Y. Y.; Kung, A. H.
Supercontinuum generation in a multi-plate medium. Opt. Express
2016, 24 (7), 7224−7231.
(37) Poletto, L.; Villoresi, P.; Benedetti, E.; Ferrari, F.; Stagira, S.;
Sansone, G.; Nisoli, M. Intense femtosecond extreme ultraviolet
pulses by using a time-delay-compensated monochromator. Opt. Lett.
2007, 32 (19), 2897−2899.
(38) Kruit, P.; Read, F. H. Magnetic-field parallelizer for 2π electronspectrometer and electron-image magnifier. J. Phys. E: Sci. Instrum.
1983, 16 (4), 313−324.
(39) Widmark, P.-O.; Malmqvist, P.-Å.; Roos, B. O. Density matrix
averaged atomic natural orbital (ANO) basis sets for correlated
molecular wave functions. Theor. Chim. Acta 1990, 77, 291−306.
(40) Werner, H.-J.; Knowles, P. J.; Knizia, G.; Manby, F. R.; Schütz,
M. Molpro: A general-purpose quantum chemistry program package.
Wiley Interdisp. Rev. Comput. Mol. Sci. 2012, 2 (2), 242−253.
(41) Werner, H.-J.; Knowles, P. J.; Knizia, G.; Manby, F. R.; Schütz,
M.; Celani, P.; Korona, T.; Lindh, R.; Mitrushenkov, A.; Rauhut, G.;
Shamasundar, K. R.; Adler, T. B.; Amos, R. D.; Bernhardsson, A.;
Berning, A.; Cooper, D. L.; Deegan, M. J. O.; Dobbyn, A. J.; Eckert,
F.; Goll, E.; Hampel, C.; Hesselmann, A.; Hetzer, G.; Hrenar, T.;
Jansen, G.; Köppl, C.; Liu, Y.; Lloyd, A. W.; Mata, R. A.; May, A. J.;
McNicholas, S. J.; Meyer, W.; Mura, M. E.; Nicklass, A.; O’Neill, D.
P.; Palmieri, P.; Peng, D.; Pﬂüger, K.; Pitzer, R.; Reiher, M.; Shiozaki,T.; Stoll, H.; Stone, A. J.; Tarroni, R.; Thorsteinsson, T.; Wang, M.
MOLPRO, version 2012.1; Cardiﬀ, U.K., 2012.
(42) Werner, H. J.; Knowles, P. J. A second order multiconfiguration
SCF procedure with optimum convergence. J. Chem. Phys. 1985, 82(11), 5053−5063.
(43) Knowles, P. J.; Werner, H.-J. An efficient second-order MC
SCF method for long configuration expansion. Chem. Phys. Lett. 1985,
115 (3), 259−267.
(44) Werner, H.-J. Third-order multireference perturbation theory
The CASPT3 method. Mol. Phys. 1996, 89 (2), 645−661.
(45) Shiozaki, T.; Győ rffy, W.; Celani, P.; Werner, H.-J.
Communication: Extended multi-state complete active spacesecond-order perturbation theory: Energy and nuclear gradients. J.
Chem. Phys. 2011, 135 (8), 081106.
K

https://doi.org/10.1021/jacs.1c01896
J. Am. Chem. Soc. XXXX, XXX, XXX−XXXJournal of the American Chemical Society

pubs.acs.org/JACS

Article(46) Roos, B. O.; Andersson, K. Multiconfigurational perturbation
theory with level shift the Cr2 potential revisited. Chem. Phys. Lett.
1995, 245 (2−3), 215−223.
(47) BAGEL, Brilliantly Advanced General Electronic-structure
Library. http://www.nubakery.org (accessed May 7, 2021), under the
GNU General Public License.
(48) Humeniuk, A.; Wohlgemuth, M.; Suzuki, T.; Mitrić, R. Timeresolved photoelectron imaging spectra from non-adiabatic molecular
dynamics simulations. J. Chem. Phys. 2013, 139 (13), 134104.
(49) Bearpark, M. J.; Robb, M. A.; Schlegel, H. B. A direct method
for the location of the lowest energy point on a potential surface
crossing. Chem. Phys. Lett. 1994, 223 (3), 269−274.

Lhttps://doi.org/10.1021/jacs.1c01896
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX