Photoinduced ring opening and closure reactions inchalcones monitored by time-resolved IR
spectroscopy
Masato Kondoh and Taka-aki Ishibashi*
Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba
1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571, Japan
ishibashi.takaaki.gn@u.tsukuba.ac.jp
Chalcones are naturally occurring compounds, which are
expected as an environmental-friendly photochromic material.
In this paper, we review nanosecond time-resolved infrared(TR-IR) study of the photoinduced ring-opening and -closure
reactions between trans-2-hydroxychalcone (Ct) and flav-3en-2-ol (F) in acetonitrile, which are crucial to understanding
the photochromic reactions in chalcones. TR-IR difference
spectra observed after photoexcitation of F showed a positive
band at 1632 cm−1, which was assignable to the C=O
stretching vibration of an enol form 2-hydroxychalcone; anintermediate of the photo-ring-opening reaction. From thetime dependence of the TR-IR spectra and vibrational analysis
by density functional theory (DFT) calculations, we proposethat the photo-ring opening reaction of F proceeds mainly through three different reaction paths.
In two of them, two kinds of enol form, single bond cis- and trans- enol intermediates are
generated right after the photoexcitation and then tautomerize into keto forms, cis- and transchalcones (Cc and Ct), respectively, while in the other path, the keto chalcones (Cc, Ct, or both)are produced directly from F without passing through the enol intermediates. In contrast, only
one intermediate was detected in TR-IR spectra of the photo-ring closure reaction from Ct to F.
On the basis of vibrational analysis by DFT calculations, the intermediate detected in the photoring-closure process was assigned to Cc hydrogen-bonding with solvent molecules.
Keyword: Time-resolved infrared spectroscopy, Photochromism, 2-hydroxychalcone, flav-3-en-2-ol
Masato Kondoh obtained his B.Sc. from Kyoto University in 2005, and completed his
Ph.D. at Kyoto University, in 2011. He worked as a postdoctoral fellow at Japan Atomic
Energy Agency from 2011 to 2014, and did at Osaka University from 2014 to 2015.
He has been working as an assistant professor at University of Tsukuba since 2015.
He is currently trying to detect conformational and intermolecular interaction dynamics
of molecules in solution and at interfaces.
Taka-aki Ishibashi received his B.S., M.S., and Ph.D. in chemistry from the Universityof Tokyo. After working in research and teaching at Kanagawa Academy of Scienceand Technology, the University of Tokyo, and Hiroshima University, he has been a full
professor at University of Tsukuba since 2012. His current research focuses on thedevelopment and applications of molecular spectroscopy for interfacial and solution
samples.

Acc. Mater. Surf. Res. 2023, Vol.8 No.1, 46-57.

46

Photoinduced ring opening and closure reactions inchalcones monitored by time-resolved IR spectroscopy
Masato Kondoh and Taka-aki Ishibashi*
Department of Chemistry, Graduate School of Pure and Applied Sciences, University ofTsukuba
1. Introduction

molecular structures than those of electronic

Certain organic compounds exhibit a reversible

absorption bands. This review describes ourtransformation between two states with different

recent application of TR-IR spectroscopy to acolorsphotochromicbya photochemical reaction. Suchsystembetween2-

phenomena are called photochromism. Organichydroxychalcone and flavylium. Outline of thisphotochromic

asreview is summarized in figure 1. Below, we will

photofunctional materials that can be applied infirst briefly introduce the photochromic system andphotonic devices such as optical sensors andthe reaction mechanism proposed by previous

switches.

molecular

UV/Vis studies. Then, we will present our TR-IR

mechanisms of photochromic reactions has longresults and discuss the mechanism of the

been a subject of interest to researchers,

photochromic reaction.

especiallycompounds

Thus,

in

expected

characterizing

materialscience.

reactionsofphotochemicalmolecules

are

have

been

To

date,

photochromic

extensively

studied,primarily by time-resolved electronic absorption
spectroscopy in the ultraviolet (UV)/visible (Vis)region. This method is powerful to detect transientintermediates of photochemical reactions andinvestigate their kinetics. However, we often havedifficulty in distinguishing co-existing chemical
species in electronic absorption spectra because
electronic absorption bands are in general broadin the solution phase. Besides, similar chemical
species exhibit the electronic bands which areoverlapped significantly.
To observe reaction intermediates that are

Figure 1. Outline of this review.

difficult to detect in electronic absorption spectra,
we have employed time-resolved infrared (TR-IR)
spectroscopy. Vibrational bands usually are
narrow and little overlapped as compared withelectronic absorption bands. Thus, the positions ofvibrational bands are more sensitive to the
Acc. Mater. Surf. Res. 2023, Vol.8 No.1, 46-57.

2. Photochromic reaction between 2hydroxychalcone and flavylium
Figure 2 shows the proposed mechanism of aphotochromic reaction of 2-hydroxychalcone in47

solutions.1-6 In the coloration process, a colorless

photoinduced ring-opening reaction from F to Ct isform, trans-2-hydroxychalcone (Ct) is converted toparticularly important when we consider applying

a ring-closed species, flav-3-en-2-ol (F) uponthis photochromic system to optical control

illumination with UV light. F is the photochromic

devices such as optical switches and memories.

intermediate of this photochromic reaction system,
and is then transformed into a colored form,

3.

flavylium cation (FH+) by a pH-dependent reaction

reactions monitored by time-resolved

in aqueous solutions. It is worth mentioning that

UV/Vis spectroscopy

the colored form species,

FH+

is the basic

Photo-ring-openingand-closure

3.1. Photo-ring opening reaction

structure of anthocyanins, which are an important

To date, the photo-ring-opening and -closure

family of natural plant pigment widely distributed

reactions between F and Ct in acetonitrile havein flowers, fruits, and vegetables.7-9 Anthocyanins

been

are widely used as natural coloring agents, andspectroscopy.1, 10, 11 Horiuchi et al. has studied theparticularly

food

UV/Visphoto-ring-opening reaction by nanosecond flash
photolysis.1 They detected a broad enhanced

expected to be safe for human health, and have aabsorption band due to an intermediate in the 400-

low environmental impact.2

680 nm region as well as that due to Ct in the 280this

400 nm region in the transient absorption spectra

photochromic reaction occurs within 1 second, theafter the photoexcitation of F. The temporal profile

thermal decoloration proceeds slowly over a few

of the intermediate band monitored at 530 nmweeks.1

thedecayed monotonically to the baseline with a time

decoloration is the back reaction from F to Ct.

constant of 31 μs. Since the intensity of the band

Interestingly,be

due to Ct monitored at 340 nm grew with the same

photochemically converted back to Ct. This

time constant, they attributed the intermediate to

Figure 2. Coloration and decoloration

Figure 3. Scheme of photoinduced ring-

schemes in the photochromic reaction

opening and -closure reactions between Ct

between trans-2-hydroxychalcone (Ct) andand F. The ring-closure reaction from Ct

Thecoloration

Thus,

time-resolved

the

the

additives.

byphotochromic systems between Ct and FH+ are

While

as

studiedprocess

rate-determining
however,

F

is

in

step
known

of
to

+

flavylium (FH ). Flav-3-en-2-ol (F) is theproceeds via cis-2-hydroxychalcone (Cc),photochromic intermediate, which can bewhile the ring-opening from F does via thethermally or photochemically back to Ct.

putative enol form intermediate.

Acc. Mater. Surf. Res. 2023, Vol.8 No.1, 46-57.

48

the precursor to Ct.

absorbance changes in the 400-750 nm region

The time constant they obtained was too large

following the photoexcitation of Ct´ in acetonitrile.

to assign the intermediate to the singlet excited

On a picosecond time range, they observed twostate of F. As one possible candidate, they

distinct signals. The initial signal was assigned toconsidered the triplet state of F. However, thisthe singlet excited state of Ct´, which evolved into

possibility

other

the relaxed state with a time constant of 2 ps. The

experiments showing that oxygen gave no effects

relaxed state was attributed to a twisted excitedon the decay time constant of the intermediate. As

state, the C–C=C–C dihedral angle of which wasanother candidate, they considered the groundabout 90°. The photochemical process after thestate enol form of 2-hydroxychalcone because thedecay of the relaxed state was traced byobserved decay time constant was comparable tonanosecond flash photolysis. They confirmed thatthose of the ground state enol forms of otherfollowing the decay of the relaxed state (~16 ns),

compounds such as 2-hydroxybenzaldehyde andcis-pyridinechalcone (Cc´) in the electronically

3-hydroxyflavone. This possibility was examinedground state is generated, and then transforms

by investigating the decay time constants of theinto hemiketal form (F´) with a time constant of 10

intermediate in acetonitrile in the presence of

ms.

was

excluded

by

their

protic molecules such as methanol, ethanol, andBelow, we describe our TR-IR results on thewater. It is known that protic molecules accelerate

photo-ring-opening and closure reactions. We

enol-ketotautomerization.12

Theyobserved

haveappliedthis

techniquetodetectsignificant acceleration of the decay time constantconformational dynamics including the formationby protic molecules. Thus, they suggested that theof intermediates, which are “silent” in electronicintermediate was the ground state enol form of 2-

absorption spectra.

hydroxychalcone. The reaction scheme theysuggested is shown in the lower part of figure 3.

4. Steady-state IR spectra
Figure 4a depicts steady-state IR spectra of Ct

3.2. Photo-ring-closure reaction

and F in acetonitrile at concentrations of 10 mM

In 1960s, Jurd studied the effect of light on theobtained in the C=O stretching wavenumber

equilibrium between 2-hydroxychalcones andregion. The IR spectrum of Ct exhibited vibrational

flavylium salts.3 He proposed that trans-form of 2-

bands at 1604 and 1664 cm−1, whereas that of Fhydroxychalcones photoisomerizes to cis-form,

contained a vibrational band at 1642 cm−1 and awhich cyclizes to the ring-closed form as shown inshoulder at 1608 cm−1. The vibrational bands of F

the upper part of figure 3. Since then, many

were much weaker than those of Ct in thestudies have been performed, and now theobserved wavenumber region. IR spectra of Ct

scheme is established as the general scheme forand F predicted by DFT calculations at B3LYP/6-

reactions of 2-hydroxychalcones to flavylium

31G(d) level are shown as sticks in figure 4b.

compounds.4-6, 13

Dotted curves in this figure represent the line
investigatedspectra convoluted with a Gaussian with a full

photoreaction of trans-pyridinechalcone (Ct´), an

width half-maximum (FWHM) of 10 cm−1. The

Morerecently,

Leydetetal.

analog of Ct, by femto- and nanosecond transientabsorption spectroscopy.11 They monitored theAcc. Mater. Surf. Res. 2023, Vol.8 No.1, 46-57.

49

calculated IR spectra well reproduced thewavenumbers and the relative intensities of the
bands. Referring to the calculated IR spectra, weassigned the observed vibrational bands asfollows. The 1604 cm−1 band was attributed to an
overlap of an in-phase stretching mode of C=O
and C=C groups, and a ring stretching mode of Ct.
The 1664 cm−1 band was did to an out-of-phase
one of the C=O and C=C groups (called a C=O
stretching mode in the following). The 1608 and
1642 cm−1 bands of F are assigned to a benzene
ring stretching mode and a stretching mode of the
C=C group in the chromene ring, respectively.

Figure 5. (a) Steady-state IR spectrum of F
(upper panel). TR-IR spectra observed after
photoexcitation of F (lower panel). (b) Timeprofiles of the TR-IR intensities at 1632 (red)−1

and 1664 (blue) cm .
profiles and TR-IR spectra, respectively. The
spectral resolution was set at 8 cm−1.
Figure 4. (a) Steady-state IR spectra of Ct(upper panel) and F (lower panel) in
acetonitrile. (b) IR spectra of Ct (upper
panel) and F (lower panel) predicted by DFT
calculations at the B3LYP/6-31G(d) level(sticks). The spectra convoluted with aGaussian with a full width half-maximum of

3-en-2-ol
We first investigated the photo-ring-opening
reaction from F by TR-IR spectroscopy.14 TR-IR
measured

The top trace is the steady-state IR spectrum of F,
while the other traces are TR-IR difference spectra,
each of which is the IR spectrum at a given delay
time minus that before the photoexcitation. TR-IR
difference spectra represent photoinduced IR
difference spectra represent photo-generated

5. Photo-ring-opening reaction from flav-werethe photoexcitation of F in acetonitrile at 266 nm.

absorbance changes. Thus, positive bands in the−1

10 cm (dotted curves).

spectraFigure 5a shows TR-IR spectra observed afterbyahome-build

apparatus, which was based on a dispersive
spectrometer with AC-coupled detection.15-18 The
time resolution of our system was approximately
30 ns and 350 ns for measurements of temporal
Acc. Mater. Surf. Res. 2023, Vol.8 No.1, 46-57.

species,

includingtransientintermediates,

whereas negative ones do the parent species.
The TR-IR difference spectrum at 0.0-0.8 μs
after

the

photoexcitation

showedpositivevibrational bands at 1604, 1632, and 1664 cm−1.
In the delay time of a few microseconds, the 1604
and 1632 cm−1 bands decreased in intensity,whereas the 1664 cm−1 band increased. The TRIR spectra showed no further changes at longer
delay times (>20 μs). The spectra at the long delay

50

times only showed positive vibrational bands at

One probable cause is water contamination to the1604 and 1664 cm−1, where Ct, the final productsample used in the TR-IR experiment. It is knownof the ring-opening reaction, has vibrational bands.

that the protic additives such as water increase theThis result indicates that the reaction from F to Ct

decay rate of the enol form species.1 We havealmosttheperformed an TR-IR experiment that examined an

photoexcitation. The 1632 cm−1 band is notable in

influence of adding water to the sample solution,

the

band

and confirmed that adding a small amount of water

disappeared at the long delay times. Thus, we(~0.5 volume % of acetonitrile) indeed accelerated

assigned this band to an intermediate in the photo-

the k2-kinetics.14 In contrast, the kinetics with thering opening reaction.

time constant of k1−1=0.5 μs was not clearlyfinishes

TR-IR

withindifference20μs

spectra.

after
This

The temporal profiles of the TR-IR banddetected in the previous flash photolysis study.

intensities at 1632 (red) and 1664 cm−1 (blue) are

Therefore, the observed time dependence of theshown in figure 5b. The band intensities at both

TR-IR spectra was not explained solely by thewavenumbers increased rapidly within the time

reaction scheme proposed in the previous flash

resolution of the apparatus and changed in a few

photolysis study.

microseconds to tens of microseconds time range.

We postulate that the two intermediates have

They were fitted by a function of A1exp(-

similar molecular structures for two reasons. First,k1t)+A2exp(-k2t)+C with common time constants ofthe double-exponential kinetics was observed on

0.5 and 11 μs; that is, the decay of the 1632 cm−1

the 1632 cm−1 band. This observation implies thatband due to the intermediate species and the riseboth intermediates have the vibrational band atof the 1664 cm−1 band were fitted with thethis wavenumber. Second, we did not detect any

common two time constants. This observation

TR-IR bands that are assignable exclusively toindicates that the 1632 cm−1 band originates fromone of the two intermediates. A clue to assigning

the precursor of the chemical species that has a

the two intermediates is found in a previous

vibrational band at 1664

cm−1.

At least, twoliterature on the photochemistry study of hydroxy-precursors must be assumed because we

4-pyridinechromene (F′).19 F’ has a pyridine ringobserved the kinetics with the two different time

instead of the phenyl ring of F. The study reported

constants.

that the photoexcitation of F′ generates two kinds

Then, what is the two intermediates? Theof enol forms, single bond cis- (s-cis-) and trans-

previous flash photolysis study has reported that(s-trans-) enol forms. The enol forms then undergo

following the photoexcitation of F in acetonitrile, an

keto-enol isomerization to produce cis- and trans-

enol form 2-hydroxychalcone intermediate isform chalcones, respectively. Referring to thisproduced and then transforms into Ct by keto-enol

literature, we suggest that the two intermediates

μs.1

detected in our TR-IR experiments were s-cis- andJudging from the value of the time constant, thes-trans-enol forms of 2-hydroxychalcone. The k1

kinetics with the time constant of k2−1=11 μs

anddetected in the TR-IR seems to correspond to thattautomerization processes from the s-cis-enol toobserved in the previous flash photolysis study.

Cc and from the s-trans-enol to Ct, respectively.

The time constant value obtained by the TR-IR

We assign the faster kinetics to the process fromwas smaller than that by the flash photolysis

s-cis-enol to Cc because the distance from the OH

isomerization with a time constant of 31

k2

kinetics

areattributed

to

the

study.

51
Acc. Mater. Surf. Res. 2023, Vol.8 No.1, 46-57.

to the C=O groups in s-cis-enol is shorter than thatin s-trans-enol. The reaction scheme that explains
the time-evolution of the TR-IR spectra is asfollows.

hν

F→

𝑘1

→
{ s‑cis-enol
𝑘2
s‑trans-enol →Cc
Ct

The validity of this reaction scheme was
examined by comparing the observed TR-IR
spectra with calculated IR spectra obtained by

Figure 6. (a) Optimized structures of 2-

DFT calculations. Figure 6a and 6b showhydroxychalcone obtained by the DFT

optimized structures and calculated IR spectra ofcalculations. (b) Calculated IR spectra

four related species, s-cis-enol, s-trans-enol, Cc,

predicted from the corresponding optimized

and Ct, respectively.

structures are shown in sticks. The number

The calculated spectra of the s-cis- and s-

with an asterisk in each panel is thetrans-enol forms show vibrational bands at

calculated wavenumber of the normal mode

1645 and 1640 cm−1, respectively. Both bands

mainly due to the C=O stretching coordinate.

are due to an out-of-phase stretching mode ofA scaling factor of 0.9613 was used to

C=O and C=C groups. (Hereafter they are

calibrate the calculated wavenumbers.

called simply a C=O stretching mode.) The
calculated wavenumbers are close to 1632 cm−1,
where the TR-IR band of the intermediates wasobserved. Thus, our assignment of the 1632 cm−1
band to both s-cis- and s-trans-enol forms isreaction path to the early photoproduct. The earlyphotoproduct should have a vibrational band at
1664 cm−1. Therefore, one possible assignment of
the early photoproduct is Ct. Cc also cannot be
excluded as a candidate of the early photoproduct.

consistent with the calculations.
The TR-IR bands at 1604, 1632, and 1664 cm−1

Asdiscussed

immediately

below,

a

DFT

after

the

calculation predicted that Cc can have the C=O

figure5a).

stretching mode at 1683 cm−1 when solvent effectConsidering the reaction scheme proposed above,is considered. The keto forms, Ct, Cc, or both,these TR-IR bands should be due to the enol

produced through the fast reaction path could

forms. However, the calculated spectra of neither

contribute to the 1664 cm−1 band observed rights-cis-enol nor s-trans-enol contain vibrational

after the photoexcitation. The modified reaction

bands at around 1664 cm–1. This result implies

scheme is as follows.

were

observedphotoexcitation

immediately

(red

lines

in

that early photoproducts exist other than the enol
forms, and indicates that the reaction scheme
above does not sufficiently explain the observed

keto forms (Ct, Cc, or both)
hν

F→

{

s‑cis-enol

𝑘1

→

𝑘2

s‑trans-enol →TR-IR spectra.

Cc
Ct

We modified the reaction scheme to remedy theinconsistency

byconsideringanadditional

Acc. Mater. Surf. Res. 2023, Vol.8 No.1, 46-57.

52

This modified scheme presumes that both Ct
and Cc have a vibrational band at around 1664
cm−1 because the three kinetics, fast rise, k1 decay,
and k2 decay were observed on the kinetic trace
at this wavenumber (blue line in figure 5b).
However, the calculation does not predict that Cc
has IR bands at around 1664 cm−1. Instead, theFigure 7. The second most stable structure

C=O stretching mode of Cc is predicted at 1625

of the Cc-CH3CN complex found in the DFT

cm−1 in the calculated spectrum (figure 6b).

calculations. The calculated IR spectrum

To resolve this inconsistency, we considered thesolvent effect. The low calculated wavenumber of
the C=O stretching mode of Cc(1625 cm−1)

from the structure is shown as stick.
bond has a C=O stretching band at 1683 cm−1. As

iswe expected, the wavenumber of the C=O

probably due to an intramolecular hydrogen bondstretching mode of Cc becomes close to 1664

between the OH and C=O groups. However, in

cm−1 when the intramolecular hydrogen bond isacetonitrile, this hydrogen bond could be broken

broken. This result allows us to explain the results

because the OH group can also form a hydrogenof the TR-IR experiment by the modified scheme.

bond with acetonitrile. Such a Cc-acetonitrile

Our observation suggests that the photo-ring-

complex would have the C=O mode at a

opening reaction process is not so simple aswavenumber higher than that of the C=O mode ofreportedinthethe isolated Cc. We examined this explanation bydemonstrating

thecalculating an IR spectrum of 1:1 complex

spectroscopy for studying the photochemistry of

between Cc and acetonitrile (Cc-CH3CN complex).

chalcones

previous

UV/Visusefulnessofstudy,
TR-IR

and related compound systems.

DFT calculations showed that the most stable

However, we have to mention that the proposed

structure of the Cc-CH3CN complex (not shown)reaction scheme relied on the assumption derived

has the intramolecular hydrogen bond (thefrom the DFT calculations: the C=O stretching

distance between H and O atoms, dHO is 1.56 Å),wavenumber of Ct and that of Cc are similar

whereas the second most stable structure does

values when the C=O group of Cc hydrogen-not (dHO is 3.35 Å).14 The complex without thebonds with acetonitrile molecules. One way tointramolecular hydrogen bond (the second mostvalidate the assumption is to obtain the IR

stable structure) has a larger Gibbs free energy

spectrum of Cc in acetonitrile. However, it could

than that with the intramolecular hydrogen bondnot be separated off from the TR-IR difference(the most stable structure) by 6.7 kJ mol−1.

spectra obtained in this section because the

However, the breaking of the intramolecular

photo-ring-openinghydrogenbecausecomplicated reaction involving several paths. In

conformations without intramolecular hydrogenthe next section, we discuss the IR spectrum of Cc

bonds may be preferable in terms of entropy.

by studying TR-IR spectra of the photo-ring-

bond

is

quite

possible

The second most stable structure and the IR

processisaratherclosure process from Ct to F.

spectrum calculated from it are shown in figure 7.
The calculated IR spectrum of the Cc-CH3CN

6. Photo-ring-closure reaction from trans-complex without the intramolecular hydrogen

2-hydrocychalcone

Acc. Mater. Surf. Res. 2023, Vol.8 No.1, 46-57.

53

We have investigated the photo-ring-closure
reaction from Ct to F by the TR-IR spectroscopy.18
The

investigationwas

expected

togive

information on the IR spectrum of Cc because it
has been experimentally shown that cis-form
chalcones are produced during photo-ring-closure
reactions from trans-form chalcones as shown in
the upper part of figure 2.4-6 The kinetics of thering-closure process

hasbeen studied bytransient absorption spectroscopy for a case fortrans-pyridinechalcone (Ct’). 11 As described in
section2.2,

cis-pyridinechalcone

(Cc´)

is

generated after the photoexcitation of Ct’, andthen decays into hemiketal form, hydroxy-4pyridinechromene (F´) with a time constant of 10
ms. The same reaction scheme is expected to

Figure 8. (a) Steady-state IR spectrum of Ct

hold for non-substitute trans-chalcone (Ct).

(upper panel). TR-IR spectra obtained after

Figure 8a shows TR-IR difference spectra

photoexcitation of Ct at each delay time

obtained after the photoexcitation of Ct in(lower panel). The black dotted curve in theacetonitrile at 355 nm. The top trace is the steady-lower panel is the IR difference spectrum

state IR spectrum of Ct, while the other traces are

between F and Ct. (b)Time profiles of the

TR-IR difference spectra at given delay times. The

TR-IR band intensities at 1604 (gray) andTR-IR difference spectrum at the delay time of

1664 (blue) cm .

−1

0.0–2.0 ms (red line) showed a large negative
signal at 1604 cm−1 with several small negative

the Ct minus F spectrum (dotted line), which wasand positive signals in 1630–1680 cm−1. The

obtained from the steady-state IR spectra of Ct

negative signals appeared at 1604 and 1664 cm−1,

and F shown in figure 4. These spectra were quite

which agreed well with the wavenumbers of thesimilar in shape, indicating that the photo-ring-vibrational bands of Ct observed in the steady-closure reaction of Ct to F almost finished at thisstate spectrum (top trace). Therefore, the negative

long delay time.

signals are mainly contributed from the vibrational

The TR-IR band intensities at 1604 and 1664

bands of Ct. The positive signals observed at

cm−1, where the negative signals due to theapparent wavenumbers of 1652 and 1676 cm−1

bleaching of Ct was observed, are plotted against

are due to a chemical species generated by thethe delay time in figure 8b as gray and blue lines,

photoexcitation of Ct.

respectively. At both wavenumbers, the negative

Amplitudes of the negative signals at 1604 andsignals immediately appeared, reflecting the1664 cm−1 grew more negative with time anddepletion of Ct upon the photoexcitation. Notably,reached almost a constant value at 40 ms after thethe amplitudes of the negative signals at bothphotoexcitation. In figure 7a, the TR-IR difference

wavenumbers became more negative during thespectrum at 40–42 ms (blue line) is comparedperiod from 1 to 40 ms. These temporal profiles

with

54
Acc. Mater. Surf. Res. 2023, Vol.8 No.1, 46-57.

were fitted by mono-exponential functions with acommon time constant of 10 ms. The observed
growth of the negative signals cannot be
understood by the negative contribution due to Ct
alone. If the negative signals were solely due to Ct,
their growth would mean that the reactant, Ct,
continued to decrease even when the excitation
light pulse is no longer present, but it is unlikely.
Instead,thisbehaviorcanbeexplained

adequately if we assume a positive contribution
from the photo-generated intermediate as follows.

Figure 9. The reconstructed IR spectrum of

The intermediate should be produced immediately

the intermediate species produced just after

after the photoexcitation and have vibrational

photoexcitation of Ct (upper panel). The IRbands at 1604 and 1664 cm−1. The positive

spectrum

contribution from the intermediate at theseobtained by the DFT calculation (the same

wavenumbers decayed with the depletion of thespectrum as that shown in figure 6) (lower

intermediate. In contrast, the negative contribution

panel).

from Ct remained after the decay of the positive
contribution, resulting in the growth of the negative
signals. The TR-IR spectra after the growth
showed no further changes. Therefore, theintermediate should be the precursor of the final
product, F. From the reaction scheme shown infigure 3, the precursor was assigned to Cc. The
time constant obtained in our TR-IR measurement
was in good agreement with that of Cc´ reported
intheprevious

study,11

supportingtheinterpretation that the observed intermediate is Cc.
The IR spectrum of Cc was estimated from the
TR-IR difference spectra and the steady-state IR
spectrum of Ct (Ct-spectrum). In the estimation,
Ct-spectrum was multiplied by an appropriate
constant, C, and added to the TR-IR difference
spectrum at 0.0-2.0 ms. C was determined so thatthe sum of the constant multiplied spectrum(C×Ct-spectrum) and the TR-IR spectrum at 40–
42 ms becomes similar to the steady-state
spectrum of F. The estimated spectrum is shownin the upper panel in figure 9, together with thecalculated

IR

spectrum

of

the

of

the

Cc-CH3CN

complexobtained by the DFT calculation (the same
spectrum in figure 7) in the lower panel. The
estimated spectrum showed two major bands at
1590–1620 cm−1 and 1650–1680 cm−1. The
observed band positions are close to 1612 and1683 cm−1, respectively, where the calculation of
the Cc-CH3CN complex predicts relatively strong
vibrational bands. This result is consistent with theassignment of the intermediate to Cc interactingwith solvent molecules.
In section 5, we suggested the reaction scheme
where Cc as well as Ct is produced after thephotoexcitation of F. This scheme, however,presumed that Cc has a vibrational band at around
1664 cm−1. The validity of the assumption was
experimentally supported by the results presented
in this section as we obtained experimental
evidence that the intermediate produced during
the photoreaction of Ct has an IR absorption bandat around 1664 cm−1. Therefore, the TR-IR resultson the photoreactions from F to Ct and from Ct to
F are consistent and support to each other.

Cc-CH3CN

complex55
Acc. Mater. Surf. Res. 2023, Vol.8 No.1, 46-57.

7. Summary and future prospective
This paper has reviewed our recent TR-IR
studies on the photo-ring-opening and -closure
reactions between Ct and F, which are importantto understand the photochromic reactions inchalcones. In the photo-ring-opening reaction, we
detected the TR-IR band at 1632 cm−1, which was
assigned to the C=O stretching modes of theintermediates, s-cis- and s-trans-enol forms of 2hydroxychalcone. The results of the TR-IR
measurements and the DFT calculations indicatedthat the photo-ring-opening proceeds throughthree different paths: the keto forms of 2hydroxychalcone are produced not only throughFigure 10. The overall reaction scheme forphoto-ring-opening (upper) and -closure(lower) processes between F and Ct.

the s-cis- and s-trans-enol intermediates but also

experiments of the ring-opening process in the

directly from F in the sub-microsecond time range.

millisecond time range. Second, experiments with

This

the

other solvents should be examined. Suchassumption that Cc has a vibrational band at

experiments can be effective to support ouraround 1664 cm−1 as a result of the interaction

conclusion, especially, concerning the formation ofwith solvent. The assumption was experimentally

Cc hydrogen bonding to the solvent. It might bevalidated by monitoring the TR-IR spectra of thealso important to examine solvent effects on the s-

photo-ring-closure process from Ct to F, during

cis and s-trans enol form intermediates by the DFT

which Cc is suggested to be produced as a

calculations for the full understanding of the

precursor to F. The overall reaction scheme

photoreactions of this system in solution. Third, it

proposed based on our TR-IR studies are

is attractive to investigate the ring-opening

summarized in figure 10.

process in the short time range (<0.1 μs) byreaction

scheme,however,had

To gain further evidence for the proposed

ultrafast TR-IR spectroscopy. Such investigation

scheme, we are now planning some additional

would provide information on the detailed kinetics

experiments and calculations in the future. First,of the direct path to the keto chalcones from F,

TR-IR experiments for the ring-opening process at

which probably takes place on a potential energy

long time delay are planned. From figure 10, we

surface in the electronic exited state.

can notice again that the same species, Cc

There are many photoresponsive molecules

hydrogen-bonding to the solvent, is produced boththatin the photo-ring-opening (upper part) and -photoreactions.

closure (lower part) processes. Considering themechanisms of such molecules is important for

TR-IR results of the ring-closure process, Cc

their applications as photofunctional materials. We

produced in the ring-opening process would be

believeback to F with the time constant of 10 ms. This

photochemicalback

spectra is useful to investigate the reaction

reactioncanbedetectedin TR-IR

Acc. Mater. Surf. Res. 2023, Vol.8 No.1, 46-57.

can

alter

that

their

opticalUnderstandingtime-resolved
reactions

properties
thereaction

observation

throughbyofvibrational

56

mechanisms, because it is possible to detect

3

L. Jurd, Tetrahedron. 1969, 25, 2367.

intermediates and reaction paths, which are “silent”

4

R. A. Mcclelland, S. Gedge, J. Am. Chem.

in electronic absorption spectra. We are currently

Soc. 1980, 102, 5838.

trying

5

to

apply

this

technique

to

other

photoreaction systems such as photoinduced

68, 755.

proton transfers and redox reactions.

6

We also note that our method that probes

R. Brouillard, J. Lang, Can. J. Chem. 1990,

J. Mendoza, N. Basilio, V. De Freitas, F. Pina,

Acs Omega. 2019, 4, 12058.

conformational dynamics longer than ~0.1 μs is

7complementary to ultrafast TR-IR spectroscopy.

Food Chem. 1997, 45, 304.

The

8

methodcanbesuitabletomonitor

H. Wang, G. H. Cao, R. L. Prior, J. Agric.

D. Costa, A. M. Galvao, R. E. Di Paolo, A.

photoreactions of large molecules such as

A. Freitas, J. C. Lima, F. H. Quina, A. L. Macanita,

proteins, where conformational changes distant

Tetrahedron. 2015, 71, 3157.

from a photosensor domain would be important for9their functions. Such changes are often “silent” in

H. Quina, J. Phys. Org. Chem. 2016, 29, 594.

electronic spectra and occur on long time scales,

10

microseconds to seconds. We expect that ourBallardini, V. Balzani, J. Am. Chem. Soc. 1997, 119,

method will be useful for such studies.

5556.

V. O. Silva, A. A. Freitas, A. L. Macanita, F.

F. Pina, M. J. Melo, M. Maestri, R.

11

Acknowledgement
We would like to thank Mr. Akira Sakuta and Ms.

Y. Leydet, P. Batat, G. Jonusauskas, S.

Denisov, J. C. Lima, A. J. Parola, N. D. Mcclenaghan,
F. Pina, J. Phys. Chem. A. 2013, 117, 4167.

Chinatsu Takizawa at University of Tsukuba for

12

their kind support in obtaining the TR-IR spectra

Grellmann, K. A. Zachariasse, Chem. Phys. 1994, 186,

presented here. We also thank Professor Misako

435.

Aida, Dr. Dai Akase, and Mr. Kazuki Okazawa at

13

F. Pina, J. Agric. Food Chem. 2014, 62, 6885.

Hiroshima University for providing the DFT

14

M. Kondoh, A. Sakuta, K. Okazawa, D.

calculation data. This study was supported in part

Akase, M. Aida, T. Ishibashi, J. Phys. Chem. B. 2019,

by JSPS KAKENHI Grant Numbers JP26104504,

123, 8499.

JP16H00821, JP18K14175, and JP19K05362.

15

The computation works were carried out in partPhys. Lett. 2001, 333, 271.

using Research Center for Computational Science

16

Okazaki, Japan (Project: 20-IMS-C175, 21-IMS-

H. Hamaguchi, Fast time-resolved mid-infrared

C203, 22-IMS-C220).

spectroscopy using grating spectrometers, John Wiley

J. S. Stephan, C. R. Rodriguez, K. H.

A. Yamakata, T. Ishibashi, H. Onishi, Chem.

M. Hashimoto, T. Yuzawa, C. Kato, K. Iwata,& Sons, Ltd, 2001.

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