Photochemistry of 2′-hydroxychalcone. One-way cis-trans photoisomerization induced by adiabatic intramolecular hydrogen atom transfer

Article abstract of DOI:10.1021/jp0133081

Intramolecular hydrogen atom transfer and novel one-way photoisomerization of 2′-hydroxychalcone (2HC) were investigated by quantum yield measurements, transient absorption spectroscopy, and semiempirical calculations. The trans isomer of 2HC (trans-2HC) did not give the cis isomer, while cis-2HC underwent one-way isomerization to give the trans-2HC. The triplet-triplet absorption spectra of cis- and trans-2HC were similar and were assigned to the tautomer produced via the intramolecular hydrogen atom transfer in the excited triplet state. The triplet energies of the normal form and the tautomer form of trans-2HC were estimated by quenching experiments to be 229 and 177.5 kJ mol^-1, respectively. Heats of formation, the frontier molecular orbital coefficients, and optimized structures of trans-2HC and cis-2HC in the ground, the excited singlet, and the excited triplet states were calculated by semiempirical (MOPAC93 PM3/4CI) calculations. These calculations agree well with the experimental observations. Based on the experimental observations and theoretical considerations, a novel mechanism and the potential energy surface for the one-way photoisomerization of 2HC were proposed. The photoisomerization of 2HC was induced by hydrogen atom transfer in the excited triplet state. Thus, 2HC can be viewed as a molecule in which the mode of isomerization around the carbon-carbon double bond can be controlled by the remote intramolecular hydrogen bonding. Solvent affected the triplet lifetime without changing the mode of isomerization. On prolonged irradiation, trans-2HC gave flavanone by very low quantum yield (Φ = 1.9 × 10^-3 in benzene). The cyclization reaction to form flavanone was studied and the quantum yields were determined.

Full text of DOI:10.1021/jp0133081

2766
J. Phys. Chem. A 2002, 106, 2766-2776
Photochemistry of 2′-Hydroxychalcone. One-Way Cis-Trans Photoisomerization Induced
by Adiabatic Intramolecular Hydrogen Atom Transfer
Yasuo Norikane,† Hiroki Itoh,‡ and Tatsuo Arai*,†
Department of Chemistry, UniVersity of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan, and
Department of Material and Biological Chemistry, Faculty of Science, Yamagata UniVersity, Yamagata,
Yamagata 990-8560, Japan
ReceiVed: August 27, 2001
Intramolecular hydrogen atom transfer and novel one-way photoisomerization of 2′-hydroxychalcone (2HC)
were investigated by quantum yield measurements, transient absorption spectroscopy, and semiempirical
calculations. The trans isomer of 2HC (trans-2HC) did not give the cis isomer, while cis-2HC underwent
one-way isomerization to give the trans-2HC. The triplet-triplet absorption spectra of cis- and trans-2HC
were similar and were assigned to the tautomer produced via the intramolecular hydrogen atom transfer in
the excited triplet state. The triplet energies of the normal form and the tautomer form of trans-2HC were
estimated by quenching experiments to be 229 and 177.5 kJ mol-1, respectively. Heats of formation, the
frontier molecular orbital coefficients, and optimized structures of trans-2HC and cis-2HC in the ground, the
excited singlet, and the excited triplet states were calculated by semiempirical (MOPAC93 PM3/4CI)
calculations. These calculations agree well with the experimental observations. Based on the experimental
observations and theoretical considerations, a novel mechanism and the potential energy surface for the one-
way photoisomerization of 2HC were proposed. The photoisomerization of 2HC was induced by hydrogen
atom transfer in the excited triplet state. Thus, 2HC can be viewed as a molecule in which the mode of
isomerization around the carbon-carbon double bond can be controlled by the remote intramolecular hydrogen
bonding. Solvent affected the triplet lifetime without changing the mode of isomerization. On prolonged
irradiation, trans-2HC gave flavanone by very low quantum yield (Φ ) 1.9 × 10-3 in benzene). The cyclization
reaction to form flavanone was studied and the quantum yields were determined.
Introduction
isomer.3,6 However, in the case of 1b, a mutual isomerization
between cis and trans isomers was observed. In addition, cis-
1b underwent hydrogen atom transfer through the intramolecular
hydrogen bonding in the excited singlet state.5 Recently, we
have come to find a novel photochromic dye14 (2), which
exhibited the color change between red (E-2) and green (Z-2)
Much attention has been paid to the effect of hydrogen
bonding on the photochemical behaviors of ethylenic com-
pounds.1-14 Lewis and Arai et al. have studied the effect of
hydrogen bonding on the photoisomerization behavior of olefins
such as 2-[2-(2-pyridyl)ethenyl]indole3,6 (1a) and 2-[2-(2-
on photoirradiation. The isomer ratio at the photostationary state
could be controlled by selecting the exciting wavelength
although the quantum yields of isomerizations showed almost
one-way character. The photostability and the anomalous
bathochromic shift of the absorption spectra of E-2 were due
to the intramolecular hydrogen bonding. In these compounds,
it seems that the isomerization process competes with the
deactivation via intramolecular hydrogen bond in the excited
states.
It has been well recognized that various types of molecules,
which form intramolecular hydrogen bonding, exhibit the
intramolecular hydrogen atom transfer in the excited singlet
state.15-18 Although the intramolecular hydrogen atom transfer
in the excited singlet state has been studied extensively, the
hydrogen atom transfer in the triplet state has scarcely been
reported.19-24 Methyl salicylate (MS) is a well-known com-
pyrrolyl)ethenyl]quinoline5,7 (1b). In these compounds, only the
cis isomer forms intramolecular hydrogen bonding as revealed
1
by H NMR spectroscopy. On photoirradiation trans-1a gave
cis-1a, while cis-1a was stable and did not give any trans
* To whom correspondence should be addressed.
† Department of Chemistry, University of Tsukuba.
‡
Department of Material and Biological Chemistry, Yamagata Univer-
sity.
10.1021/jp0133081 CCC: $22.00 © 2002 American Chemical Society
Published on Web 02/26/2002

Photochemistry of 2′-Hydroxychalcone
J. Phys. Chem. A, Vol. 106, No. 11, 2002 2767
pound for the study of the intramolecular hydrogen atom transfer
in the excited singlet state.17 MS exhibits the fluorescence with
a large Stokes shift and the emitting species is assigned to the
tautomer form. We have investigated the behavior of MS in
the excited triplet state by means of laser-flash photolysis19 and
revealed that MS undergoes the intramolecular hydrogen atom
transfer in the triplet state to give the tautomer. We have
extended our study of hydrogen atom transfer in the triplet state
to N-salicylideneaniline24 (SA), which has been well-known as
photochromic and thermochromic compound. SA gave the
tautomer in the excited triplet state as revealed by the observa-
tion of triplet tautomer by means of transient absorption
spectroscopy. There are some difficulties in the study of the
hydrogen atom transfer in the excited triplet state because of
the limited method for the observation such as phosphorescence
and transient absorption spectroscopies, while fluorescence
spectroscopies are commonly applied to study the behavior of
the excited singlet state in various atmospheres.
We should refer to using the term intramolecular hydrogen
atom transfer instead of intramolecular proton transfer. In the
study of methyl salicylate, the structure of the tautomer has been
drawn as a “zwitterionic” structure since Weller had found the
dual emission in the fluorescence spectrum of MS. However,
recent extensive investigations on the structure of MS and
related 2-hydroxy-benzoyl compounds have indicated that the
final form has no zwitterion character.25-28 Thus, this reaction
can be viewed as tautomerization or intramolecular hydrogen
atom transfer where the atomic bonds and electrons are
redistributed in the time domain of the reaction.
tautomer fluorescence15 and of the cyclization reactions,15,31,32
not much information about the behavior of 2′-hydroxychalcone
in the excited states have been known. In addition, behavior of
2′-hydroxychalcone in the excited triplet manifold has not been
reported, and the photochemical behavior of cis isomer of 2HC
(cis-2HC) has been unknown. We have found that 2HC
underwent one-way cis-trans isomerization and have reported
preliminary results as communications.13
In the present paper, we report the photochemistry of trans-
and cis-2′-hydroxychalcone studied by means of the quantum
yield measurements, laser-flash photolysis, and semiempirical
(MOPAC93 PM3/4CI) calculations. Our main interest is the
effect of intramolecular hydrogen bonding on the photoisomer-
ization behavior of 2HC where the intramolecular hydrogen
bond exists in both trans and cis isomers, in contrast to the
compounds where only cis isomers are capable to form the
hydrogen bond such as 1 and 2. 2HC underwent intramolecular
hydrogen atom transfer in the excited triplet state as revealed
by the observation of the tautomer triplet. 2HC exhibited one-
way isomerization only from the cis isomer to the trans isomer.
This one-way isomerization is induced by the adiabatic hydrogen
atom transfer in the triplet state. The potential energy surface
of the isomerization and the hydrogen atom transfer are
discussed. The cyclization reaction to form flavanone is also
studied.
Results
Ground-State Properties. Both trans- and cis-2HC form
O-H:O intramolecular hydrogen bonding. The hydroxyl proton
signals were observed by 1H NMR in chroloform-d at 12.8 and
12.3 ppm for trans- and cis-2HC, respectively. These downfield
shifts were indicative of the intramolecular hydrogen bonding.
The semiempirical calculations indicated that the hydrogen-
bonded conformers are the most stable for both trans- and cis-
2HC. The optimized structures of the two isomers by PM3
calculation indicate that they have planar structure so that there
are conjugation between styryl and benzoyl moieties. The
rotation around the single bond between the carbonyl R-carbon
and the benzene ring which possesses the hydroxyl group in
trans-2HC was studied by using PM3 calculation. Two rota-
tional isomers around the single bond, trans-s-trans and trans-
s-cis forms can be equilibrated with each other. However, the
heat of formation (∆Hf) calculated for trans-s-trans and trans-
s-cis forms indicated that the trans-s-cis form is more stable
than the trans-s-trans form. These results are in good agreement
with the previous study by the AM1 calculation.33
2′-Hydroxychalcone (2HC) has an intramolecular hydrogen
bonding between the carbonyl oxygen and the hydroxy group.
It should be noted that both cis and trans isomer form the
intramolecular hydrogen bond, while only cis isomers form the
intramolecular hydrogen bond in 1 and 2. It was previously
reported that the trans isomer of 2HC (trans-2HC) did not
UV and Fluorescence Spectra. The absorption spectra of
trans-2HC in various solvents are shown in Figure 1. trans-
2HC has its absorption maxima around 315 nm and a shoulder
about 350 nm in benzene. The shoulder becomes a peak in
cyclohexane (λmax ) 352 nm). The electronic transition corre-
sponding to the peak and the shoulder are both likely to have
the π,π* character because they have comparatively large molar
extinction coefficients (ꢀ ∼ 104 M-1 cm-1). The PM3 calculation
(C.I. ) 4) supported that the S1 state of trans-2HC has π,π*
character. Not much shift of the absorption maximum was
observed by changing the solvents. Similarly, trans-3b (Figure
1) exhibits its maximum at 304 nm with π,π* character and
very weak absorption band from 345 to 400 nm with n,π*
character in benzene.
undergo trans-cis isomerization on photoirradiation,29 while it
has been known that chalcone undergoes the mutual isomer-
ization.30 Chou et al. reported that trans-2HC exhibited very
weak fluorescence maximum at 635 nm in nonpolar solvent.15
The emitting species with a very large Stokes shift has been
assigned to the tautomer form of trans-2HC produced via
hydrogen atom transfer in the excited singlet state. The
photocyclization reactions from trans-2HC to flavanone31,32 and
3-hydroxyflavone15 have been reported. Chou et al. reported
that 3-hydroxyflavone was formed with 355 nm laser irradiation
of trans-2HC in aerated n-hexane.15 Matsushima et al. reported
that the photoirradiation of trans-2HC gave flavanone and they
proposed that the formation of the tautomer via intramolecular
hydrogen atom transfer is necessary for the cyclization.31,32 The
quantum efficiencies for these cyclization processes seem to
be very small. Due to the very low quantum yield of the
The molar extinction coefficient of cis-2HC is smaller than
that of trans-2HC, indicating that there should be a distortion
of conjugation due to the steric hindrance between the carbonyl
group and the benzene ring as implied by the PM3 calculation.

2768 J. Phys. Chem. A, Vol. 106, No. 11, 2002
Norikane et al.
Figure 1. Absorption spectra of trans-2HC in benzene (PhH),
cyclohexane (CH), methanol (MeOH), cis-2HC in benzene, and trans-
3b in benzene.
TABLE 1: Triplet Lifetimes (τT), Quantum Yields of
Dissappearance of trans-2HC (ΦDis), and Quantum Yields of
Formation of Flavanone (ΦFla) in Various Solvents under Ar
and Oxygen
Figure 2. Transient absorption spectra of trans-2HC observed on
excitation at 308 nm in benzene (a) and methanol (b). Time indicates
the delay time after laser pulse.
Φdis
Φfla
a
a
solvent
τT/ns Ar
O2
Ar
O2
1Φ0
3Φ0
benzene
hexane
acetonitrile
1200 0.0020 0.0017 0.0019 0.0016 0.0016 0.0003
1100 0.0014 0.0012 0.0012 0.0010 0.0010 0.0002
420 0.0103 0.0063 0.0102 0.0056 0.0053 0.0049
ethyl acetate 210 0.0029 0.0016 0.0018 0.0009 0.0018 0.0009
methanol 140 0.0030 0.0023 0.0021 0.0004 0.0001 0.0020
methanol-d1 150 0.0037 0.0027 0.0021 0.0005 0.0001 0.0020
Ar
O
2
a ΦflaAr ) 1Φ0 + 3Φ0. 1Φ0 ) (τT
Φ
O2 - τT ΦflaAr)/(τTAr - τTO2).
fla
It has been reported that trans-2HC exhibits fluorescence at
635 nm.15 The large Stokes shifted fluorescence is attributed to
the formation of the tautomer in the excited singlet state. The
quantum yield of the emission is very low (∼10-5). No
fluorescence has been detected in the region of the normal
emission (450-500 nm). It must be due to the ultrafast
intramolecular hydrogen atom transfer and/or ultrafast deactiva-
tion processes such as intersystem crossing and internal conver-
sion through the hydrogen bonding.
Figure 3. Transient absorption spectra observed benzil-sensitized laser
excitation of trans-2HC in benzene. The excitation wavelength was
425 nm.
Photoirradiation. trans-2HC did not isomerize to the cis
isomer on photoirradiation at 366 or 300 nm in benzene, hexane,
acetonitrile, or methanol. On prolonged irradiation, trans-2HC
cyclized to give flavanone instead of forming the cis isomer.
The quantum yields of disappearance of trans-2HC (Φdis) and
formation of flavanone (Φfla) are listed in Table 1. There are
no distinguishable differences among the quantum yields Φfla
in nonpolar solvents (1.9 × 10-3 in benzene and 1.2 × 10-3 in
hexane), protic polar solvents (2.1 × 10-3 in methanol), and
aprotic polar solvent (1.8 × 10-3 in ethyl acetate), except for
relatively high yield in acetonitrile (1.02 × 10-2). Thus, Φdis
and Φfla values were very low in all solvents examined except
for acetonitrile. In all solvents, molecular oxygen decreased the
quantum yields. The Φdis values are in fair agreement with the
of isomerization of 2HC is smaller than that of two-way
isomerizing chalcones 3a-d. These results indicate that the
intramolecular hydrogen bonding influences the photoisomer-
ization mechanism of 2HC and imply that the character of the
electronic state of trans-2HC might be different from those of
3a-d.
Transient Absorption Spectra of trans-2HC. The transient
absorption spectra of trans-2HC on excitation at 308 nm in
benzene and methanol are shown in Figure 2 and their observed
lifetimes are listed in Table 1. In benzene, trans-2HC gave the
transient absorption spectrum peaked at 410 and ca. 600 nm.
The both bands have the same lifetime (1.2 µs under argon)
and were assigned to the same transient. The triplet to triplet
(T-T) absorption spectrum observed on benzil-sensitized
excitation of trans-2HC in benzene is shown in Figure 3. These
transient spectra, observed on direct and sensitized excitation,
exhibited similar spectral profile especially at 600 nm and were
both quenched by oxygen with the rate constant of 2.4 × 109
M-1 s-1. Thus, both transients observed were assigned to the
triplet state.
Φfla values indicating that consumption of trans-2HC does not
lead to the formation of cis-2HC but flavanone.
On the other hand, cis-2HC isomerized to form the trans-
2HC on photoirradiation at 366 nm in benzene, acetonitrile,
and methanol. The quantum yield of the photoisomerization
from cis-2HC to trans-2HC was 0.05 on irradiation at 366 nm
in benzene under Ar atmosphere. In contrast, 3a-d isomerized
mutually between trans and cis isomers on photoirradiation. The
quantum yield for cis-to-trans isomerization of chalcone in
cyclohexane was reported to be 0.2330 which is almost five times
as large as that of 2HC. It is noteworthy that the quantum yield
Similar T-T absorption spectra were observed in all solvents
examined. However, there was a considerably large solvent
dependence on the lifetime of the triplet transients (Table 1).
In methanol, for example, the T-T spectra was observed at

Photochemistry of 2′-Hydroxychalcone
J. Phys. Chem. A, Vol. 106, No. 11, 2002 2769
calculated to be 6.0 kJ mol-1 by eq 2.39
390 nm and around 550-600 nm. Both bands decayed with
the lifetime of 140 ns, which was almost 1/9 of the lifetime in
benzene. The shortest lifetime (78 ns) was observed in pyridine.
Attempts to fit the correlation between the observed triplet
lifetime and various physical parameter of solvents, such as
dielectric constants (ꢀ), acidity (pKa), viscosity (η) and empirical
value for solvatochromizm (ET(30)),34 were unsuccessful. Those
plots did not exhibit any correlation between solvent properties
and observed triplet lifetimes. Thus, there should be other factor-
(s) which govern the deactivation process from the observed
triplet state. These solvent effects on the triplet lifetime of trans-
2HC are discussed in the later section.
exp(-∆Ea/RT)
dif1 + exp(-∆Ea/RT)
kq ) k
(2)
Thus, the triplet energy of trans-2HC is 6 kJ mol-1 higher than
the triplet state of benzil. Since the triplet energy of benzil is
223 kJ mol-1 40
,
we can estimate the triplet energy of trans-
2HC to be 229 kJ mol-1
.
Triplet Energy of the Relaxed Triplet of trans-2HC
Observed by Transient Absorption Spectra. The triplet energy
of the transient observed on laser-flash photolysis was estimated
by the similar quenching experiments. In benzene, the T-T
absorption spectra of trans-2HC on benzil sensitization by 425
nm laser excitation were quenched by several quenchers (Q)
with varying triplet energies (eqs 3, 4 and 5).
The observed triplet state would be a planar trans triplet (3t*),
a planar cis triplet (3c*), or a perpendicular triplet (3p*) in which
the carbon-carbon double bond is twisted by 90°. The planar
cis triplet (3c*) is precluded because no cis isomer is detected
as a photoproduct, since the planar cis triplet should deactivate
to give the ground-state cis isomer as a photoproduct. The
perpendicular triplet is also excluded since it must deactivate
to the ground state to give the trans and the cis isomers in a
certain ratio. The observed triplet was quenched by oxygen with
kr
9
3(trans-2HC)*
8 3(trans-2HC′)*
(3)
(4)
(5)
3(trans-2HC′)* + Q
9
kq′8 trans-2HC + 3Q*
O2
a rate constant of kq ) 2.4 × 109 M-1 s-1, which is nearly
3(trans-2HC′)*
9
kd′8 trans-2HC
1/9 of the diffusion controlled rate constant in benzene (kdif
)
(2-3) × 1010 M-1 s-1 35
) and strongly indicates that the triplet
The concentration of trans-2HC (10 mM) was carefully
chosen so that the sensitization of trans-2HC by benzil was
achieved immediately after the laser pulse. The lifetime of the
observed triplet is decreased with increasing the concentration
of anthracene as a quencher. A reciprocal plot of the triplet
lifetime of 2HC vs the concentration of anthracene gave the
straight line (eq 6)
is the planar geometry. In addition, the triplet lifetime of trans-
2HC (1.2 µs in benzene) is much longer than those of chalcone
(3a), 2′-methoxychalcone (3b), 3′- (3c), and 4′-hydroxychalcone
(3d); their triplet lifetimes are ∼20 ns and have been assigned
to the perpendicular triplet state.36,37 In addition, the absorption
maxima of these perpendicular triplets were around 430 nm and
no T-T absorption was observed around 600 nm. Thus, the
observed triplet for trans-2HC was assigned to a planar triplet.
However, there are still two possible forms for the planar triplet;
the normal form or the tautomer form. Taking account of 3b as
a model compound for the normal form of trans-2HC, the most
stable conformer of 3b in the excited triplet state should be the
perpendicular form as mentioned above. Therefore, from T-T
absorption, triplet lifetimes, and quenching rate constant by
oxygen we may assign the observed triplet to the triplet state
of enol tautomer (3(trans-2HC′)*). To make clear the assignment
of the triplet state which was observed by the transient
absorption, the triplet excitation energy of the normal form of
trans-2HC and that of the observed triplet were estimated.
Details of the estimations are described in the following sections.
kobs ) kq′[Q] + kd′
(6)
and the quenching rate constant by anthracene was determined.
Thus, anthracene quenches the observed triplet of 2HC with
the rate constant of kq′ ) 1.7 × 109 M-1 s-1. The process (eq
4) is slightly endothermic as calculated to be 2.5 kJ mol-1 from
eq 2. Since the triplet energy of anthracene is 178 kJ mol-1 40
,
the triplet energy of the observed transient is estimated to be
175.5 kJ mol-1
The similar quenching experiment was carried out using
pyrene (ET ) 203 kJ mol-1 40
instead of anthracene as a
.
)
quencher in benzene. Pyrene did not quench the triplet state of
2HC up to the concentration of pyrene at 3.8 × 10-3 M.
Therefore, the triplet energy of the observed triplet is much
lower than that of pyrene and is lower than that of anthracene
Triplet Excitation Energy of trans-2HC. To estimate the
triplet excitation energies of trans-2HC, triplet quenching
experiments were carried out in benzene, since we could not
observe the phosphorescence of trans-2HC in glass forming
solvents at 77 K. On laser excitation of benzil at 425 nm, a
T-T absorption spectrum of benzil was observed peaked at 490
nm.
The pseudo-first-order decay rate constant (kobs) of the benzil
triplet increased with increasing trans-2HC concentration. The
slope of eq 1
by 2.5 kJ mol-1
.
If the observed triplet is assigned to the triplet state of the
starting geometry, that is, trans-2HC, these quenching experi-
ments should give similar triplet energies. However, the energy
transfer from benzil triplet to trans-2HC gave the triplet energy
of 229 kJ mol-1, while the energy transfer from the triplet state
of 2HC to anthracene gave the triplet energy of 175.5 kJ mol-1
.
Therefore, the observation of the long-lived planar triplet and
the difference in triplet energies from quenching experiments
strongly indicate the formation of the tautomer (trans-2HC′)
in the triplet state.
kobs ) kq[trans-2HC] + kd
(1)
gives the rate constant (kq ) 5.0 × 108 M-1 s-1) for the triplet
energy transfer from the benzil triplet to trans-2HC. The value
of kq ) 5.0 × 108 M-1 s-1 is lower than the diffusion controlled
rate constant (kdif ) 6.3 × 109 M-1 s-1),38 and indicates that
the quenching process of benzil triplet by trans-2HC should
be endothermic. The difference of the triplet energies (∆Ea)
between triplet sensitizer (benzil) and quencher (trans-2HC) is
Transient Absorption Spectra of cis-2HC. As mentioned
above, cis-2HC exhibited the one-way photoisomerization to
trans-2HC. Absorbance from 270 to 430 nm increased upon
photoirradiation and finally the spectra becomes identical to that
of trans-2HC. Figure 4 shows the transient absorption spectra
observed on excitation of cis-2HC in benzene by a 308 nm
laser pulse under Ar atmosphere. The transient spectra peaked

2770 J. Phys. Chem. A, Vol. 106, No. 11, 2002
Norikane et al.
Figure 4. Transient absorption spectra of cis-2HC observed on
excitation at 308 nm in benzene.
at 610 nm and was assigned to the excited triplet state. The
lifetime of the triplet was 1.2 µs. The permanent absorption in
the shorter wavelength region (λ < 450 nm) is due to the ground
state of trans-2HC as a photoproduct. The T-T absorption
spectrum is very similar to that of trans-2HC with the absorption
maxima at 610 nm and the lifetime of 1.2 µs (Figure 2a), which
has been assigned to the triplet state of the tautomer of trans-
2HC (3(trans-2HC′)*). Therefore, we can assign the observed
T-T absorption spectra for cis-2HC to the tautomer of trans-
2HC (3(trans-2HC′)*). Thus, based on the photochemical
studies and laser-flash photolysis experiments, we can propose
that the tautomer of trans-2HC (3(trans-2HC′)*) is formed on
direct irradiation of cis-2HC and the twist of the carbon-carbon
double bond takes place from 3(cis-2HC′)* to 3(trans-2HC′)*.
Quantum Yield for Formation of 3(trans-2HC′)*. The
Figure 5. Numbering of atoms for the calculation of trans-2HC (a)
and cis-2HC (b). Underlined numbers indicate hydrogen atoms.
Heats of Formation (∆Hf). The estimated heats of formation
(∆Hf) of each optimized conformation of 2HC in the ground,
the excited singlet, and the excited triplet states of trans-2HC,
trans-2HC′, cis-2HC, and cis-2HC′ forms are shown in Figure
6. In the ground state, the normal forms (cis- and trans-2HC)
are more stable than the corresponding tautomer forms (trans-
2HC′ and cis-2HC′). The differences of ∆Hf between the normal
and the tautomer forms are ca. 55 kJ mol-1 for both cis and
trans isomers. These values are lower than those of salicylic
acid18 (78-98 kJ mol-1) and methyl salicylate17 (74 kJ mol-1),
while higher than those of o-hydroxyacetophenone17 (47.4 kJ
mol-1). In the excited state of planar trans and cis forms of
2HC, all the ∆Hf values of the tautomers are lower than those
of corresponding normal forms. These results are indicative of
the occurrence of the intramolecular hydrogen transfer in the
triplet state as well as the singlet state.
1
quantum yield of intersystem crossing from (trans-2HC)* to
3(trans-2HC′)* in benzene on 308 nm excitation was 0.09,
which was measured by comparing the intensities of T-T
absorption of trans-2HC and anthracene as a standard. The
1
3
quantum yield of intersystem crossing from (cis-2HC)* to -
(trans-2HC′)* was 0.06, which was determined by comparing
the intensities of T-T absorption of trans-2HC and cis-2HC
on 308 nm excitation, since the transients observed on excitation
of trans-2HC and cis-2HC are the same triplet (3(trans-2HC′)*).
The quantum yield of intersystem crossing of cis-2HC (0.06)
is very close to the quantum yield of cis-trans photoisomer-
ization (0.05). These results indicate that the photoisomerization
proceeds in the excited triplet state.
Bond Orders. Table 4 shows the bond orders of each
optimized conformation of trans-2HC and trans-2HC′ in the
ground, the excited singlet, and the excited triplet states. It is
noteworthy that the bond orders of CdC double bond (C3-C4)
increase by tautomerization in the excited states. In the normal
forms, the values of the bond order are 1.136 and 1.301 for
1
3(trans-2HC)* and (trans-2HC)*, respectively. However, in
the tautomer forms, the bond orders are 1.478 and 1.511 for
1
3(trans-2HC′)* and (trans-2HC′)*, respectively. Generally, a
Theoretical Considerations. Optimized Structures. Num-
bering of atoms that were used in the calculation is shown in
Figure 5. These numbers are used for the expression of the
structure and bond orders.
bond order of a CdC double bond decreases in the excited states
and it leads to isomerization. In the present case, the increase
of the bond orders by tautomerization indicates that the twisting
3
of the C3-C4 bond in (trans-2HC′)* might have a higher
Molecular structures of each conformation of 2HC (trans-
2HC, trans-2HC′, cis-2HC, and cis-2HC′) in the ground, the
excited singlet, and the excited triplet states were optimized by
MOPAC93 (PM3) calculation. Bond lengths, bond angles, and
dihedral angles are listed in Tables 2 (trans-2HC and trans-
2HC′) and 3 (cis-2HC, and cis-2HC′). As mentioned above,
the stable geometries of both trans-2HC and cis-2HC in the
ground state are planar. The planarity holds for each electronic
state of trans-2HC, trans-2HC′, and cis-2HC. However, 3(cis-
2HC)* was not stable and could not be optimized. This indicates
energy barrier than that in 3(trans-2HC)*. Another notable result
is concerned with the bond order of the single bond between
the phenyl and carbonyl groups (C2-C5 bond). In the ground
state, it increases from 0.986 to 1.485 by tautomerization, while
it increases slightly in the excited triplet and singlet states. These
values indicate that the C2-C5 bond still has a single bond
character in the excited states, rather than a double bond
character.
Frontier Molecular Orbitals and CI Contributions. Fron-
tier molecular orbitals and the weight of each electronic
configuration derived from the PM3 calculation are shown in
Figure 7. It is noteworthy that in the LUMO, the phase of C2-
C5 bond has a bonding character in either trans-2HC or cis-
2HC, while it has an antibonding character in either trans-2HC′
or cis-2HC′. Those LUMO orbitals reflect the decrease in the
3
that there is no potential minimum in (cis-2HC)*. All the
electronic states of cis-2HC′ were not planar in geometry. The
single bond between the olefinic carbon and the benzene ring
(C4-C12) is twisted by ca. 30° and the single bond between the
carbonyl R-carbon and the olefinic carbon (C2-C3) is twisted
by 25-44°.

Photochemistry of 2′-Hydroxychalcone
J. Phys. Chem. A, Vol. 106, No. 11, 2002 2771
TABLE 2: Bond Lengths, Bond Angles, and Dihedral Angles of Each Conformer of trans-2HC Calculated by PM3/CI4
trans-2HC
trans-2HC′
3(trans-2HC)*
3(trans-2HC′)*
1(trans-2HC)*
1(trans-2HC′)*
Bond Length
1.235
O1-C2
C2-C3
C3-C4
C4-C12
C2-C5
C5-C6
C6-O11
O11-H29
O1-H29
1.232
1.474
1.343
1.456
1.482
1.413
1.352
0.963
1.792
1.340
1.460
1.346
1.455
1.390
1.471
1.240
1.763
0.968
1.350
1.414
1.365
1.451
1.457
1.483
1.237
1.793
0.961
1.236
1.473
1.396
1.395
1.476
1.410
1.354
0.961
1.809
1.354
1.415
1.380
1.439
1.422
1.483
1.233
1.788
0.963
1.455
1.447
1.362
1.484
1.412
1.353
0.963
1.795
Bond Angles
122.3
121.7
C2-C3-C4
C3-C4-C12
C3-C2-C5
123.4
122.5
118.7
125.1
122.2
122.4
125.1
122.2
121.3
123.3
122.0
118.7
124.9
122.2
122.6
118.6
Dihedral Angles
-0.5
O1-C2-C3-C4
C2-C3-C4-C12
O1-C2-C5-C6
C3-C4-C12-C13
C5-C6-O11-H29
C3-C2-O1-H29
2.2
-179.7
-0.7
-179.0
0.6
0.4
179.7
0.1
-0.1
179.8
0.1
1.9
-178.0
-2.5
178.7
0.4
-1.1
180.0
0.0
-177.2
0.3
-179.9
179.9
-0.3
-179.8
179.5
179.8
179.7
-179.7
TABLE 3: Bond Lengths, Bond Angles, and Dihedral Angles of Each Conformer of cis-2HC Calculated by PM3/CI4
cis-2HC
cis-2HC′
3(cis-2HC)*a
3(cis-2HC′)*
1(cis-2HC)*
1(cis-2HC′)*
Bond Length
O1-C2
C2-C3
C3-C4
C4-C12
C2-C5
C5-C6
C6-O11
O11-H29
O1-H29
1.234
1.472
1.344
1.451
1.487
1.414
1.352
0.962
1.781
1.339
1.464
1.340
1.455
1.388
1.470
1.240
1.765
0.970
1.348
1.428
1.352
1.454
1.456
1.485
1.237
1.785
0.962
1.236
1.467
1.403
1.389
1.491
1.403
1.357
0.958
1.801
1.350
1.447
1.341
1.456
1.427
1.479
1.239
1.777
0.967
Bond Angles
C2-C3-C4
C3-C4-C12
C3-C2-C5
132.7
134.1
118.4
131.2
130.9
121.2
132.6
131.8
120.8
133.2
132.6
118.3
131.7
130.0
120.2
Dihedral Angles
O1-C2-C3-C4
C2-C3-C4-C12
O1-C2-C5-C6
C3-C4-C12-C13
C5-C6-O11-H29
C3-C2-O1-H29
5.8
0.4
-0.3
-176.9
0.5
43.7
0.4
-4.0
149.3
24.5
1.9
-8.1
148.9
5.1
-0.0
0.6
178.0
0.9
43.1
0.1
-3.9
146.3
180.0
-175.1
-179.6
a 3(cis-2HC)* was not stable and could not be optimized.
bond order of C2-C5 bond of the excited-state tautomers. The
results of CI contributions indicate that the main electronic
transitions occurs from HOMO to LUMO. These molecular
orbitals and CI contributions indicate that both S1 and T1 have
π,π* character, since the n orbital is calculated in lower energy
level than HOMO by 1.713 eV for trans-2HC.
the singlet state and subsequent intersystem crossing (eqs 7, 8,
and 9),
hV
trans-2HC 98 1(trans-2HC)*
(7)
hydrogen atom
transfer
1(trans-2HC)*
8 1(trans-2HC′)*
(8)
(9)
Discussion
1(trans-2HC′)* ISC8 3(trans-2HC′)*
Intramolecular Hydrogen Atom Transfer of trans-2HC.
Laser flash photolysis of trans-2HC gave the T-T absorption
spectra with maxima at 410 and 610 nm. The quenching
experiments indicated the formation of the triplet tautomer
(3(trans-2HC′)*) with the triplet energy of 175.5 kJ mol-1 over
the ground state, while the triplet energy of the normal form
or via intersystem crossing followed by intramolecular hydrogen
atom transfer in the triplet state (eqs 7, 10, and 11).
1(trans-2HC)* ISC8 3(trans-2HC)*
(10)
(11)
hydrogen atom
transfer
was 229 kJ mol-1
.
3(trans-2HC)*
8 3(trans-2HC′)*
On direct irradiation, the excited singlet state of trans-2HC
follows two possible pathways to form the triplet tautomer
(3(trans-2HC′)*); via intramolecular hydrogen atom transfer in
Chou et al. reported that the weak fluorescence at 635 nm should
be attributed to the emission from trans-2HC′.15 It indicates

2772 J. Phys. Chem. A, Vol. 106, No. 11, 2002
Norikane et al.
tautomer (3(trans-2HC′)*) forms in the excited triplet state
because there was no temperature or solvent dependence upon
the T-T absorption spectra. Although a blue shift of the T-T
spectra was observed with increasing the polarity of solvent, it
is not attributable to the shift of the equilibrium in the triplet
state because the same decay rate constant was observed at any
monitoring wavelength in each solvent. In addition, the spectra
did not exhibit temperature dependence in methanol (180-324
K) or in benzene (280-323 K). Thus, the blue shift of the T-T
absorption spectra by solvent is attributed to the stabilization
of the triplet tautomer (3(trans-2HC′)*). Therefore, the T-T
spectra was assigned to one species i.e., the triplet tautomer
(3(trans-2HC′)*). The heats of formation (∆Hf) derived from
PM3 calculations support the formation of 3(trans-2HC′)*. As
can be seen in Figure 6, trans-2HC′ is the most stable conformer
in the excited triplet state. The difference of ∆Hf value between
3(trans-2HC)* and 3(trans-2HC′)* is 17.5 kJ mol-1 indicating
that the hydrogen atom transfer proceeds adiabatically.
Attempts to observe the ground-state tautomer (trans-2HC′)
were unsuccessful. It was not detected by transient absorption
spectroscopy. However, the results are in good agreement with
the previous studies of O:H-O intramolecular hydrogen bonded
systems. In methyl salicylate (MS), the ground-state energy gap
between the normal and the tautomer forms is as high as 74 kJ
mol-1 which indicates the ground-state tautomer is very
unstable.17 Furthermore, some groups have pointed out that there
is no potential minimum at the ground-state tautomer in O:H-O
intramolecular hydrogen bonded systems.17,18 Although it is still
a controversial point whether there is a minimum or not, there
is no doubt that the ground-state tautomer is very short-lived.
Since the triplet tautomer (3(trans-2HC′)*) has a longer lifetime
than that of the ground-state tautomer, the ground-state tautomer
could not be detected by the transient absorption.
Photoisomerization Mechanism. From quantum yield mea-
surements, laser flash photolysis and PM3 calculations, a novel
mechanism for one-way photoisomerization of 2HC as shown
in Figure 8 is proposed. On direct excitation cis-2HC gives the
triplet tautomer of cis-2HC (3(cis-2HC′)*) either via intramo-
lecular hydrogen atom transfer in the singlet state and subsequent
intersystem crossing (eqs 12, 13, and 14),
Figure 6. Heats of formation (∆Hf) of 2HC in the ground, the excited
triplet and the excited singlet states calculated by PM3. Each ∆Hf value
is calculated by using totally optimized molecular geometries in each
state.
that on direct irradiation, the triplet state of the tautomer form
(3(trans-2HC′)*) is produced by the hydrogen atom transfer in
1
the excited singlet state to give (trans-2HC′)* followed by
intersystem crossing (eqs 7, 8, and 9). However, since the rates
of intersystem crossings of carbonyl compounds are usually very
fast (kISC ∼ 1010 s-1) and this process can compete with the
intramolecular hydrogen atom transfer, the intersystem crossing
1
might take place from the normal form (trans-2HC)* (eqs 7,
10, and 11).
On triplet sensitization, the triplet state of the normal form
(3(trans-2HC)*) is initially formed. However, according to the
transient absorption spectra and the quenching experiments, the
triplet observed by the laser-flash photolysis is assigned to
3(trans-2HC′)*. These results indicate that the intramolecular
hydrogen atom transfer occurs so fast in the excited triplet state
that the normal form (3(trans-2HC)*) could not be detected upon
sensitized excitation.
hV
cis-2HC 98 1(cis-2HC)*
(12)
hydrogen atom
transfer
1(cis-2HC)*
8 1(cis-2HC′)*
(13)
(14)
Intramolecular Hydrogen Atom Transfer in the Excited
Triplet State. As mentioned in the Introduction, there are a lot
of extensive studies of the intramolecular hydrogen atom transfer
in the excited singlet state for various type of compounds.15-18
However, only a few molecules have been reported about the
intramolecular hydrogen atom transfer in the excited triplet
state.19-24 For example, there is an equilibrium between the
normal and the tautomer form in the triplet state of 1-(acylami-
no)anthraquinone derivatives22 which possess O:H-N intramo-
lecular hydrogen bond. The equilibrium depends on substituents
and solvents. In O:H-O intramolecular hydrogen bonded
system, 3-hydroxyflavone23 and methyl salicylate19 have been
reported. We have estimated the triplet energies of the normal
and the tautomer forms of methyl salicylate (MS) and concluded
that MS gives the tautomer form in the excited triplet state in
benzene at room temperature.19 No evidence of the establishment
of the equilibrium between the normal and the tautomer form
has been reported in O:H-O intramolecular hydrogen bonded
systems. In trans-2HC, it is concluded that there is no
equilibrium between the normal (3(trans-2HC)*) and the
1(cis-2HC′)* ISC8 3(cis-2HC′)*
or via intersystem crossing followed by intramolecular hydrogen
atom transfer in the triplet state (eqs 12, 15, and 16).
1(cis-2HC)* ISC8 3(cis-2HC)*
(15)
hydrogen atom
transfer
3(cis-2HC)*
8 3(cis-2HC′)*
(16)
3
It should be noted that (cis-2HC)* was not stable and could
not be optimized in the PM3 calculation, indicative of very fast
hydrogen atom transfer in eq 16. The triplet tautomer of cis-
2HC (3(cis-2HC′)*) isomerizes adiabatically to give the triplet
tautomer of trans-2HC (3(trans-2HC′)*) (eq 17),
3(cis-2HC′)* isomerization8 3(trans-2HC′)*
which has sufficient lifetime to be detected by the nanosecond
(17)

Photochemistry of 2′-Hydroxychalcone
J. Phys. Chem. A, Vol. 106, No. 11, 2002 2773
TABLE 4: Bond Orders of Each Conformer of trans-2HC Calculated by PM3/CI4
trans-2HC
trans-2HC′
3(trans-2HC)*
3(trans-2HC′)*
1(trans-2HC)*
1(trans-2HC′)*
O1-C2
C2-C3
C3-C4
C4-C12
C2-C5
C5-C6
C6-O11
O11-H29
O1-H29
1.786
0.990
1.833
1.044
0.986
1.331
1.118
0.874
0.043
1.173
1.049
1.802
1.051
1.485
1.025
1.712
0.041
0.845
1.699
1.092
1.136
1.553
0.981
1.332
1.117
0.872
0.045
1.108
1.339
1.478
1.133
1.081
0.987
1.730
0.034
0.889
1.705
1.054
1.301
1.366
0.985
1.341
1.116
0.876
0.042
1.105
1.257
1.511
1.127
1.165
1.016
1.692
0.053
0.853
Figure 7. Frontier molecular orbitals, its energies (eV), and descriptions of the excited triplet and singlet states for trans-2HC (a), trans-2HC′ (b),
cis-2HC (c), and cis-2HC′ (d) as optimized by PM3/CI4. Geometry optimized in the ground state using the PM3 Hamiltonian.
laser flash photolysis. The triplet tautomer of trans-2HC
(3(trans-2HC′)*) is more stable than the perpendicular triplet
state (3p′*); deactivation from the triplet state takes place only
the mode of isomerization as well as the potential energy
surfaces of isomerization are well understood by the effect of
aryl group on the triplet energies of planar and perpendicular
geometries. For example, the triplet energies of trans isomers
of one-way isomerizing olefins such as 2-anthrylethenes were
estimated to be ca. 180 kJ mol-1. Thus, in the excited triplet
state of 2HC, the tautomer of cis-2HC (3(cis-2HC′)*) formed
by the intramolecular hydrogen atom transfer undergoes twisting
around the double bond to give the perpendicular triplet state
3
from (trans-2HC′)*. If the perpendicular triplet state (3p′*) is
more stable than 3(trans-2HC′)* or there is an equilibrium
3
3
3
between p′* and (trans-2HC′)*, the deactivation from p′*
takes place to give the mixture of trans-2HC and cis-2HC.
However, the excited state trans-2HC does not give the cis
isomer, while the excited state cis-2HC gives the trans-2HC.
Thus, the deactivation of triplet state does not take place from
the perpendicular conformation (3p′*).
3
(3p′*), but p′* is no longer the funnel for the deactivation to
the ground state state and the deactivation takes place solely at
Such one-way properties from 3(cis-2HC′)* to 3(trans-2HC′)*
can be explained by the very low triplet energies of these
tautomers. These tautomers have conjugation of tetraenone
structure and therefore the planar trans triplet should be the most
stable conformation in the triplet state. Actually, the estimated
triplet energy of trans-2HC′ is 175.5 kJ mol-1 and is much
lower than the triplet energy of trans-2HC (229 kJ mol-1).
Photochemical isomerization of arylethenes41 are reported and
3(trans-2HC′)*.
Solvent Effect on the Lifetime of 3(trans-2HC′)*. As shown
in Table 1, the lifetime of the triplet (τT) observed on excitation
of trans-2HC varies with solvents. It is longer in nonpolar
solvents such as benzene, hexane, and methylcyclohexane, while
shorter in protic or aprotic polar solvents. As described above,
the plot of the ln(1/τT) against dielectric constant, ET(30), or
other parameters of solvents did not exhibit any correlation.

2774 J. Phys. Chem. A, Vol. 106, No. 11, 2002
Norikane et al.
Figure 8. Potential energy surfaces of intramolecular hydrogen atom transfer and isomerization around double bond of 2HC. (a) Calculated from
absorption spectra. (b) Calculated from fluorescence spectra in ref 15.
SCHEME 1
perpendicular triplet state compared to the planar triplet state
and finally gives ca. 50 ns in the olefin where 3p* is more stable
3
than t* as observed for stilbene and alkylstyrenes.41
Taking into account the above discussion, the solvent effect
on the triplet lifetime of 2HC seems to indicate the following.
3
3
In benzene (trans-2HC′)* is more stable than p′* and the
3
deactivation from the triplet state takes place from (trans-
2HC′)* to give one-way cis-trans isomerization, while in more
3
3
polar protic solvent p′* becomes to equilibrate with (trans-
2HC′)* (Scheme 1) and the deactivation may take place not
only from 3(trans-2HC′)*, but also from 3p′*, to exhibit mutual
cis-trans isomerization (two-way isomerization). However, we
observed only one-way cis-trans isomerization in all solvents.
3
3
Therefore, p′* cannot be a candidate for the X*.
Generally, since a spin-orbit coupling is hardly enhanced by
solute-solvent interaction, these solvent effects on the triplet
lifetime cannot be explained by the increase of the intersystem
crossing rate constant. Thus, we can assume that there should
be another triplet state (3X*) which is in equilibrium with the
For the latter case, a lengthening or a breaking of the
intramolecular hydrogen bond is required to give the twisted
conformer (3q*) (Scheme 1). In nonpolar solvents such as
3
benzene and cyclohexane, q* cannot be stabilized and the
equilibrium between the planar 3(trans-2HC′)* and the twisted
3
observed triplet. As candidates for X*, two types of twisted
3q* shifts to 3(trans-2HC′)*. In protic and aprotic polar solvents,
forms are considered: the perpendicular form around the double
bond of the styryl group (3p′*) and that of the quinoid ring (3q*)
(Scheme 1).
3
on the other hand, q* can be stabilized by interaction with
solvent molecules and the equilibrium may be established. The
equilibrium constant should depend on the solvent properties.
3q* should have a shorter triplet lifetime because its ground
state (1q) is unstable and S0-T1 energy gap is very small. Thus,
For the former candidate, one can discuss the triplet lifetime
and the mode of isomerization from the triplet state. Generally,
when an olefin exhibits two-way isomerization in the triplet
state, the deactivation usually takes place from the perpendicular
3
the intersystem crossing should be faster from q* than from
3(trans-2HC′)* and the observed triplet lifetime becomes shorter
in polar solvents. Deactivation from 3q* yield rotational isomers
of the starting compound trans-2HC, but no stable photochemi-
cal product is formed by this mechanism. This equilibrium
model can explain the solvent effect on the triplet lifetime and
the absence of trans-cis isomerization. Therefore, the 3X* can
3
triplet state (3p*), where p* are often equilibrated with the
planar trans triplet state (3t*) with the equilibrium constant Ktp
()[3p*]/[3t*]). Then the triplet lifetime can be described by using
the Ktp value and the decay rate constant from 3t* (kdt as assumed
3
to be ca. 2 × 104 M-1 s-1) and p* (kdp as assumed to be 2 ×
107 M-1 s-1) as shown in eq 18.
3
be assigned to the twisted conformer q*. The twisting of the
1/τT ) kdt/(1 + Ktp) + kdpKtp/(1 + Ktp)
(18)
quinoid ring may be explained by the phase of the frontier
molecular orbitals (Figure 7a and b). In the excited state (either
singlet or triplet) of the normal form (1(trans-2HC)* or 3(trans-
2HC)*), orbitals on both C2 and C5 atoms are in the same phase.
Thus, the C2-C5 bond has bonding character indicative of no
twisting of the quinoid ring, which takes place in the excited
states. On the other hand, in the tautomer form, the C2-C5 bond
3
Since the decay rate constant from p* is about 103 times
3
faster than that of t*, one-way isomerization can be achieved
in the case that Ktp , 10-3. In that case, the triplet lifetime τT
should be at least longer than 10 µs as previously observed.
The τT becomes shorter when the equilibrium is shifted to the

Photochemistry of 2′-Hydroxychalcone
J. Phys. Chem. A, Vol. 106, No. 11, 2002 2775
SCHEME 2
ization was induced by the intramolecular hydrogen atom
transfer in the excited triplet state. There was a large solvent
dependence on the lifetimes of the triplet tautomer of trans-
2HC. The solvent effect was discussed in terms of the fast
equilibrium between the isomers. The present findings of the
one-way isomerization of 2HC indicate that the mode of
carbon-carbon double bond can be controlled by the remote
intramolecular hydrogen bonding.
Experimental Section
Materials. The solvents benzene, cyclohexane, hexane,
acetonitrile, ethyl acetate, and methanol are spectroscopic grade
(Dojin Chem. Co. or Kanto Chem. Co.) and were used as
received. Benzil was purchased from Wako Chem. Co. and
recrystallized four times from hexane. Anthracene was pur-
chased from Wako Chem. Co. and recrystallized from ethanol
and benzene.
trans-2′-Hydroxychalcone, trans-2′-methoxychalcone, trans-
3′-hydroxychalcone, and trans-4′-hydroxychalcone were pre-
pared by Aldol condensation between benzaldehyde and cor-
responding acetophenone. Chalcone was purchased from Tokyo
Chem. Industry Co. and recrystallized from ethanol.
has antibonding character in the excited states, while it has
bonding character in the ground state. This indicates that the
twisting of the quinoid ring may take place in the excited state
of the tautomer form. Therefore, the equilibration between the
tautomer triplet and the conformer triplet (3q*) can be supported
by the PM3 calculation for the frontier molecular orbitals.
In any solvent, the above-mentioned novel solvent effect on
the triplet lifetime was observed without affecting the mode of
isomerization in the triplet state.
Cyclization Mechanism. Photoirradiation of trans-2HC
yielded a cyclization product, flavanone. As can be seen in Table
1, the quantum yields were very low. The Φdis values are in
fair agreement with the Φfla values indicating that consumption
of trans-2HC leads to the formation of flavanone. The effect
of molecular oxygen on the quantum yields seems to indicate
that the cyclization takes place both in the excited singlet and
triplet states. On this point, the contributions of the spin
trans-2′-Acetoxychalcone. Acetic anhydride (35 mL) was
added to a solution of trans-2′-hydroxychalcone (3.1 g, 13.8
mmol) in 150 mL of pyridine. The mixture was stirred at room
temperature for 12 h. The reaction mixture was poured into ice
water, and extracted with ether. The organic layer was washed
with saturated sodium carbonate and 1 N hydrochloric acid. The
mixture was separated by column chromatography with silica
1
gel to give 3.5 g of trans-2′-acetoxychalcone (96%). H NMR
3
multiplicity on the cyclization (1Φ0 and Φ0) were roughly
(200 MHz, CDCl3) δ(ppm): 2.24 (3H, s), 7.17 (1H, d, J )
16.0 Hz), 7.18 (1H, d, d, J ) 1.0 Hz, 8,0 Hz), 7.31-7.45 (4H,
m), 7.51-7.60 (3H, m), 7.59 (1H, d, J ) 16.0 Hz), 7.71 (1H,
d, d, J ) 1.8 Hz, 7.6 Hz).
estimated by eqs 19 and 20:
ΦflaAr ) 1Φ0 + 3Φ0
(19)
(20)
cis-2′-Acetoxychalcone. trans-2′-Acetoxychalcone (3.5 g)
was dissolved in 1000 mL of benzene and irradiated for 15 h
by 400 W high pressure mercury lamp. The deep UV light was
eliminated by passing the light beam through 10 mm path length
of acetone solution. Nitrogen gas was passed during the
photoirradiation. The photoirradiation gave approximately 1:1
mixture of trans- and cis-2′-acetoxychalcone. The mixture was
separated by HPLC and 620 mg of cis-2′-acetoxychalcone was
1Φ0 ) (τT
Φ Φ )
O2 - τT Ar)/(τTAr - τT
Ar
O2
O2
fla fla
where τT is the triplet lifetime and the superscripts (Ar and O2)
indicate under argon and oxygen atmosphere, respectively. The
1
3
calculated values of Φ0 and Φ0 are listed in Table 1. In
nonpolar solvents, the cyclization mainly takes place in the
excited singlet state. In aprotic polar solvents (ethyl acetate and
acetonitrile), the reaction proceeds equally both in the excited
singlet and triplet state, while the triplet state is the main
pathway in methanol. The cyclization reaction was previously
reported by Matsushima et al.31,32 They pointed out that the
intramolecular hydrogen atom transfer is required for the
cyclization (Scheme 2). However, the proposed mechanism does
not explain the larger quantum yield in acetonitrile and the spin
multiplicity dependence. Another possibility is that the efficiency
of the cyclization might depend on the distribution of rotamers
in the ground state.
1
obtained. H NMR (200 MHz, CDCl3) δ(ppm): 2.32 (3H, s),
6,46 (1H, d, J ) 12.6 Hz), 6.97 (1H, d, J ) 12.6 Hz), 7.09
(1H, d, d, J ) 1.2 Hz, 8.1 Hz), 7.19-7.26 (4H, m), 7.40-7.53
(3H, m), 7.78 (1H, d, d, J ) 1.6 Hz, 7.8 Hz).
cis-2′-Hydroxychalcone. cis-2′-Acetoxychalcone (620 mg)
was dissolved in 150 mL of methanol and refluxed for 26 h.
The solvent was removed and mixture was separated by column
chromatography with silica gel to give 100 mg of cis-2′-
hydroxychalcone (19%). 1H- NMR (200 MHz, CDCl3) δ-
(ppm): 6.63 (1H, d, J ) 12.8 Hz), 6.82 (1H, d, d, d, J ) 1.2
Hz, 7.2 Hz, 9.2 Hz), 6.99 (1H, d, d, J ) 1.0 Hz, 8.4 Hz), 7.02
(1H, d, J ) 12.8 Hz), 7.27-7.29 (3H, m), 7.36-7.49 (3H, m),
7.76 (1H, d, d, J ) 1.6 Hz, 8.0 Hz), 12.25 (1H, s).
Quantum Yields of Isomerization and Quantum Yields
of Formation of Flavanone. The solutions were purged with
argon or oxygen gas and irradiated with 400 W high-pressure
mercury lamp. Glass filters (Toshiba U360 and L35) were used
to obtain a monochromatic 366 nm light. The isomer yields
were determined by HPLC using internal standard. Irradiation
time was controlled so that the product yields were within 5%.
For the estimation of the light quanta absorbed by the sample
solutions, potassium ferroxalate was used.
Conclusion
The photochemical properties of 2HC were studied by
quantum yield measurements, laser flash photolysis, and semiem-
pirical (PM3/CI4) calculations. trans-2HC does not give any
cis-2HC, but undergoes very inefficient cyclization to form
flavanones. While cis-2HC undergoes one-way photoisomer-
ization to give trans-2HC. On excitation of trans-2HC and cis-
2HC, the tautomer form of trans-2HC in the excited triplet state
was observed by laser-flash photolysis. The assignment of the
transient absorption was made by the quenching experiments
and semiempirical calculations. The one-way cis-trans isomer-

2776 J. Phys. Chem. A, Vol. 106, No. 11, 2002
Norikane et al.
(3) Arai, T.; Iwasaki, T.; Tokumaru, K. Chem. Lett. 1993, 691.
(4) Lewis, F. D.; Yoon, B. A. J. Org. Chem. 1994, 59, 2537.
(5) Arai, T.; Moriyama, M.; Tokumaru, K. J. Am. Chem. Soc. 1994,
Quantum Yields of Disappearance of trans-2HC. The
sample solutions were irradiated by the same optical system as
described above. The consumption of trans-2HC was estimated
by measuring of the absorbance at 350 and 366 nm. Irradiation
time was controlled so that the isomer yields were within 5%.
Potassium ferroxalate was used as actinometer.
116, 3171.
(6) Lewis, F. D.; Yoon, B. A.; Arai, T.; Iwasaki, T.; Tokumaru, K. J.
Am. Chem. Soc. 1995, 117, 3029.
(7) Arai, T.; Obi, M.; Iwasaki, T.; Tokumaru, K.; Lewis, F. D. J.
Photochem. Photobiol. A: Chem. 1996, 96, 65.
Transient Absorption Spectroscopy. Laser flash photolyses
were performed with 308-nm pulses (XeCl, 10 ns fwhm) from
an excimer laser (Lambda Physik LPX-100) or with 425 nm
pulses (Stilbene 3, 10 ns fwhm) from an excimer laser-pumped
dye laser (Lambda Physik FL-3002). A pulsed xenon arc
(Wacom, KXL-151, 150 W) was used as a monitoring light
source. The detailed setup was described previously.35 Some
of the sample solutions were deaerated by bubbling argon.
Quantum Yields of Intersystem Crossing. The quantum
yields of intersystem crossing (ΦISC) were determined by laser
flash photolysis comparing the absorbance (∆O.D.) of the triplet
state of sample and standard compound generated by direct
excitation and sensitized excitation. The value of quantum yield
of intersystem crossing was estimated by eq 21.
(8) Arai, T.; Maeda, Y. Chem. Lett. 1997, 335.
(9) Arai, T.; Hozumi, Y.; Takahashi, O.; Fujimori, K. J. Photochem.
Photobiol. A: Chem. 1997, 104, 85.
(10) Obi, M.; Sakuragi, H.; Arai, T. Chem. Lett. 1998, 169.
(11) Arai, T.; Hozumi, Y. Chem. Lett. 1998, 1153.
(12) Yang, Y.; Arai, T. Tetrahedron Lett. 1998, 39, 2617.
(13) Arai, T.; Norikane, Y. Chem. Lett. 1997, 339. Norikane, Y.; Itoh,
H.; Arai, T. Chem. Lett. 2000, 1094.
(14) Arai, T.; Ikegami, M. Chem. Lett. 1999, 965.
(15) Chou, P.-T.; Martinez, M. L.; Cooper, W. C. J. Am. Chem. Soc.
1992, 114, 4943.
(16) Formosinho, S. J.; Arnaut, L. G. J. Photochem. Photobiol. A: Chem.
1993, 75, 21.
(17) Catala´n, J.; Palomar, J.; de Paz, J. L. G. J. Phys. Chem. A 1997,
101, 7914.
(18) Maheshwari, S.; Chowdhury, A.; Sathyamurthy, N.; Mishra, H.;
Tripathi, H. B.; Panda, M.; Chandrasekhar, J. J. Phys. Chem. A 1999, 103,
6257.
(19) Norikane, Y.; Arai, T. Chem. Lett. 1999, 909.
(20) Catala´n J.; D´ıaz, C. J. Phys. Chem. A 1998, 102, 323.
(21) Al-Soufi, W.; Grellmann, K. H.; and Nickel, B. J. Phys. Chem.
1991, 95, 10503.
sens
∆O.D.dir
∆O.D.std
ΦISC ) Φ
(21)
std ∆O.D.std
∆O.D.sens
dir
(22) Nagaoka, S.; Yamamoto, S.; Mukai, M. J. Photochem. Photobiol.
A: Chem. 1997, 105, 29.
(23) Tokumura, K.; Kurauchi, M.; Yagata, N.; Itoh, M. Chem. Phys.
Lett. 1996, 258, 495.
where Φstd is the quantum yield of intersystem crossing of the
standard. The superscripts “dir” and “sens” indicate direct and
sensitized excitation, respectively. The subscript “std.” means
standard.
Calculations. The semiempirical calculations were performed
using CAChe MOPAC ver. 94.10 on a Macintosh G3 with
parameters in ref 42. The energy parameters, the weight of each
electronic configuration, and the MO coefficients were calcu-
lated through the semiempirical PM3 method. In the calculation,
totally optimized molecular geometries in each electronic state
or those in the ground state were used.
(24) Suzuki, T.; Kaneko, Y.; Arai, T. Chem. Lett. 2000, 756.
(25) Nagaoka, S.; Nagashima, U. Chem. Phys. 1989, 136, 153.
(26) Nagaoka, S.; Hirota, N.; Sumitani, M.; Yoshihara, K.; Lipczynska-
Kochany, E.; Iwamura, H. J. Am. Chem. Soc. 1984, 106, 6913.
(27) Law, K.-Y.; Shoham, J. J. Phys. Chem. 1994, 98, 3114.
(28) For detailed discussion, see: Herek, J. L.; Pedersen, S.; Ban˜ares,
L.; Zewail, A. H. J. Chem. Phys. 1992, 97, 9046.
(29) Miquel, J. F. Compt. Rend. 1962, 254, 4479.
(30) Nicodem, D. E.; Matos, J. A. D. G. J. Photochem. 1981, 15, 193.
(31) Matsushima, R.; Hirao, I. Bull. Chem. Soc. Jpn. 1980, 53, 518.
(32) Matsushima, R.; Kageyama, H. J. Chem. Soc., Perkin Trans. 2 1985,
6, 743.
(33) Yamin, L. J.; Blanco, J. M.; Ferretti, F. H. J. Mol. Struct.
THEOCHEM 1997, 390, 209.
(34) Reichardt, C. SolVents and SolVent Effects in Organic Chemistry;
Acknowledgment. This work was supported by a Grant-
in-Aid for Scientific Research (No. 10440166) and a Grant-in-
Aid for Scientific Research on Priority Areas (A) (No. 10146103)
from the Ministry of Education, Science, Sports, and Culture,
Japan, by Research Foundation for Opto-Science and Technol-
ogy, and by the Asahi Glass Foundation.
VCH Publishers: New York, 1988.
(35) Gijzeman, O. L. J.; Kaufman, F.; Porter, G. J. Chem. Soc., Faraday
Trans. 2 1973, 69, 708. Gorner, H.; Schulte-Frohlinde, D. J. Phys. Chem.
1981, 85, 1835. Salitiel, J.; Atwater, W. AdV. Photochem. 1988, 14, 1.
(36) Caldwell, R. A.; Singh, M. J. Am. Chem. Soc. 1983, 105, 5139.
(37) Caldwell, R. A. Pure Appl. Chem. 1984, 56, 1167.
(38) In the experimental condition, benzil triplets were quenched by
anthracene with the rate constant of 6.3 × 109 M-1 s-1, and therefore, the
diffusion controlled rate constant (kdif) was estimated to be 6.3 × 109 M-1
s-1
.
References and Notes
(39) Sandros, K., Acta Chem. Scand., Ser. A 1976, 30, 761.
(40) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochem-
istry; Marcel Dekker: New York, 1993.
(1) Eenkhoorn, J. A.; de Silva, S. O.; Snieckus, V. Can. J. Chem. 1973,
51, 792.
(2) Lewis, F. D.; Stern, C. L.; Yoon, B. A. J. Am. Chem. Soc. 1992,
114, 3131.
(41) Arai, T.; Tokumaru, K. Chem. ReV. 1993, 93, 23.
(42) Stewart, J. P. J. Comput. Chem. 1989, 10, 209.