Photochemical reaction of p-hydroxycinnamic-thiophenyl ester in the microcrystalline state

Article abstract of DOI:10.1021/jp909850r

We have studied the photochromic reaction of p-hydroxycinnamic-thiophenyl ester in the microcrystalline state. We attributed the fluorescence spectral evolution of the mierocrystal, under UV irradiation, to the photoinduced trans-to-cis isomerization, The photocyclic behavior of the chromophore was demonstrated by cis-to-trans back reaction under a subsequent visible light irradiation. In addition, the [2 + 2] topochemical photocyclodimer was observed as another photoproduct. It is considered that the cooperative photoisomerization is initiated at the local lattice distortion and free spaces around the [2 + 2] cyclodimer near the crystal surface, and the photoisomerization induces larger lattice deformation and further photoisomerization in the interior of the crystal.

Full text of DOI:10.1021/jp909850r

J. Phys. Chem. B 2010, 114, 14233–14240
14233
Photochemical Reaction of p-hydroxycinnamic-thiophenyl Ester in the
Microcrystalline State†
Anwar Usman,*,‡ Tsuyoshi Asahi,*,§ Teruki Sugiyama,| Hiroshi Masuhara,*,‡,§,|
Norimitsu Tohnai,⊥ and Mikiji Miyata⊥
Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung UniVersity,
Hsinchu 30010, Taiwan, Department of Applied Physics, Osaka UniVersity, Yamadaoka 2-1, Suita,
Osaka 565-0871, Japan, Graduate School of Materials Science, Nara Institute of Science and Technology,
8916-5 Takayama, Ikoma 630-0192, Japan, and Department of Material and Life Science, DiVision of
AdVanced Science and Biotechnology, Graduate School of Engineering, Osaka UniVersity,
Yamadaoka 2-1, Suita, Osaka 565-0871, Japan
ReceiVed: October 14, 2009; ReVised Manuscript ReceiVed: January 26, 2010
We have studied the photochromic reaction of p-hydroxycinnamic-thiophenyl ester in the microcrystalline
state. We attributed the fluorescence spectral evolution of the microcrystal, under UV irradiation, to the
photoinduced trans-to-cis isomerization. The photocyclic behavior of the chromophore was demonstrated by
cis-to-trans back reaction under a subsequent visible light irradiation. In addition, the [2 + 2] topochemical
photocyclodimer was observed as another photoproduct. It is considered that the cooperative photoisomerization
is initiated at the local lattice distortion and free spaces around the [2 + 2] cyclodimer near the crystal surface,
and the photoisomerization induces larger lattice deformation and further photoisomerization in the interior
of the crystal.
1. Introduction
SCHEME 1: Chemical Structure of
p-Hydroxycinnamic-thiophenyl Ester (PCT)
Photochromic molecules, whose reversible two isomeric
structures with different physical properties in absorption,
emission, and vibrational spectra can be controlled by external
perturbation light, have been attracting great interest from the
viewpoints of both fundamental scientific bases, such as
elucidation of the photochemical reactions and their potential
applications into optical memories and devices.1–3 The studies
on a variety of the photochromic reactions have revealed that
the photoreaction dynamics is dependent on the polarity and
viscosity of the solvent medium, and can be strongly affected
when the medium is changed from solution to polymer networks
or viscous surrounding matrixes.4,5
(Scheme 1) has been probably the most attractive to reproduce
the electronic transition and Stokes shift of the PYP chromophore,
since its deprotonated form shows a massive absorption and
emission spectral shift depending on the solvent polarity.17 In
the solutions, however, the deprotonated PCT does not show
photoinduced isomerization in a significant amount.10–12,17 On
the other hand, we have recently found that protonated PCT
shows photoisomerization in solid microcrystal, but the reaction
mechanism remains to be fully understood.20 It is generally
accepted that the photochemical trans-cis isomerization process
involves large-amplitude motions of a rotation around a mo-
lecular axis. Therefore, such a large molecular rearrangement
in the PCT crystal lattice is rarely observed because of limited
free volume and restricted molecular motions compared to those
if the compound is in the solution. Moreover, it should be
emphasized that this finding is in contrast to other well-known
solid state photochromic reactions, which involve either small
molecular movements in bulk crystals, such as photoinduced
ring-closing and ring-opening reactions for diarylethenes2,21,22
and bicycle-pedal motion for salicylideneanilines,3,23–25 or large
molecular motions on the crystal surface for the trans-to-cis
isomerization of azobenzene.26 Important work dealing with
intermolecular free volume in the crystal lattice is the introduc-
tion of a bulky group into the aromatic rings of salicylidene-
anilines, expecting that the bulky group opens a space allowing
the photochromic reaction.24 Another intriguing observation of
the solid state photochromism is cooperative photochemical
In nature, photochromic reactions of chromophores bound
to proteins belonging to photoreceptor families are key processes
in determining the photochemical properties and biological
functions of the proteins.6 For instance, the signal transduction
of photoactive yellow protein (PYP), a photoreceptor protein
for the negative phototactic response of the bacterium Halor-
hodosphira halophila, bound covalently to p-hydroxycinnamic
thioester, involves a photocycle consisting of the trans-to-cis
isomerization of the chromophore and proton transfer from and
restoring back to the protein pocket.7,8 Photoexcitation dynamics
of free molecules related to the PYP chromophore has also been
systematically explored by using femtosecond transient spec-
troscopies in efforts to elucidate the intrinsic photochromic
reaction of the PYP chromophore.9–19 Among the free chro-
mophores studied, p-hydroxycinnamic-thiophenyl ester (PCT)
† Part of the “Michael R. Wasielewski Festschrift”.
* Corresponding authors. E-mail: usman@faculty.nctu.edu.tw (A.U.);
asahi@ap.eng.osaka-u.ac.jp (T.A.); masuhara@masuhara.jp (H.M.).
‡ National Chiao Tung University.
§ Department of Applied Physics, Osaka University.
| Nara Institute of Science and Technology.
⊥ Graduate School of Engineering, Osaka University.
10.1021/jp909850r  2010 American Chemical Society
Published on Web 03/11/2010

14234 J. Phys. Chem. B, Vol. 114, No. 45, 2010
Usman et al.
Figure 1. ORTEP drawing of two independent PCT molecules in the asymmetric unit showing 50% probability displacement ellipsoids.
reaction for spirooxazines and spiropyrans induced by high light
intensity excitation, where some free volume in the crystal lattice
can be generated by the photochemical reactions of the plural
transient species in the high density formation of excited
states. The generated free volume allows photoisomerization
of the spirooxazines and spiropyrans leading to trans-planar
photomerocyanines.27–29
In the present paper, we describe that the protonated PCT in
the solid crystal shows a new type of solid state photochromic
reaction, involving [2 + 2] topochemical cyclodimerization and
trans-cis isomerization, in which the formation of the [2 + 2]
cyclodimer provides dynamic free volume in the crystal lattice.
This interpretation is evidenced by experimental data concerning
X-ray diffraction, NMR spectroscopy, and diffuse reflectance
in addition to the fluorescence and infrared spectroscopies. We
also demonstrated that, by analyzing the photoexcitation dynam-
ics upon different intensity of light excitation, the cooperative
photochemical reaction model is applicable for the PCT crystal.
2.3. Fluorescence, Diffuse Reflectance, and IR Spec-
troscopies. Steady state fluorescence, diffuse reflectance, and
IR spectroscopies have been employed to monitor the photo-
chromic reactions of PCT microcrystal. The fluorescence spectra
of a single microcrystal were measured on an inverted micro-
scope (Olympus IL70) with a halogen lamp as a light irradiation
source. The wavelength and power of the light were selected
using a dichroic mirror and a neutral density filter, respectively.
Diffuse reflectance was recorded on a single beam spectropho-
tometer (Hitachi F-4500) equipped with a Xe lamp. In this
measurement, PCT crystal was mixed with sodium chloride
(NaCl) with a 1:125 weight ratio, and was crushed into powder.
The diffuse reflectance spectra of PCT before and after light
irradiation were referred to the spectrum of pure NaCl. IR
spectra of PCT microcrystals before and after UV irradiation
were measured by the ATR method on a FTIR spectrophotom-
eter (Horiba FT720).
3. Results
2. Materials and Methods
3.1. PCT Structure. PCT crystallizes in the monoclinic
P21/n space group [a ) 16.564(1), b ) 7.514(1), c ) 20.842(1)
Å, ꢀ ) 99.496(1)°, Z ) 8, V ) 2558.5(1) Å3]. The crystal
packing consists of two crystallographically independent PCT
molecules, A and B, in the asymmetric unit, as shown in Figure
1. Intermolecular OH · · ·O bonds between the hydroxyl and
carbonyl oxygen atom interconnect 2-fold rotation-symmetry-
related molecules into columns along the crystallographic b axis.
The molecules are arranged in antiparallel configurations related
by inversion symmetry. The center-to-center distance between
the ethylenic CdC bonds is 4.08, 4.38, 5.43, and 5.67 Å with
the angle between the translation axis and the ethylenic CdC
bond being 62, 65, 64, and 38°, respectively (see Figure 2).
There are four intermolecular weak interactions involving both
the centroids of phenol and phenyl moieties with H · · ·π
distances being in the range 2.6-2.9 Å. A considerable static
free volume or void in the tight crystal packing is not observed.
The crystal structure of PCT single crystal after long-time UV
irradiation, however, cannot be solved. We found that the
photoreaction is not observed in all over the bulk crystal,
but only for the surface layer of a few µm, which is much
shorter than the crystal dimensions. We also found that the
photoexcitation resulted in some fractures on the surfaces of
the crystal.
2.1. Materials. PCT was synthesized and purified following
the procedures reported by Duran et al.30 The chemical structure
of the PCT was confirmed using UV-vis absorption, 1H NMR
spectrum, and melting point measurement, where the data were
in agreement with those reported earlier.10,30 Bulk single crystals
of PCT were obtained from slow evaporation of dichlo-
romethane (CH2Cl2) solution, whereas needle-like microcrystals
(2-15 µm in width and up to 100 µm in length) were grown
from evaporation of the same solution on a quartz substrate.
After complete drying, the bulk and microcrystals were obtained
and used for measurements.
2.2. X-ray Diffraction and Structure Determination. X-ray
diffraction data of the bulk single crystal were collected using
a Rigaku R-AXIS RAPID X-ray diffractometer with two-
dimensional area detector and graphite monochromated Cu KR
radiation (λ ) 1.541 87 Å). Lattice parameters were obtained
by a least-squares analysis from the reflections for three
oscillation images. Molecular and crystal structures of the PCT
were solved by direct methods and refined by a full matrix least-
squares procedure. All non-H atoms were refined anisotropically.
The H atoms were attached geometrically in idealized positions,
and were refined isotropically. Details on X-ray structural
determination, refinement, atomic coordinates, bond lengths, and
bond angles are provided in the Supporting Information.

Photochemical Reaction of PCT
J. Phys. Chem. B, Vol. 114, No. 45, 2010 14235
3.3. Diffuse Reflectance and Infrared Spectra. Upon UV
irradiation, the diffuse reflectance spectrum of the crystal shows
a red-shift from 398 to 410 nm, a decrease in the intensity, and
an increase of the shoulder above 430 nm which extends to
around 500 nm (see Figure 6). The reverse photoinduced
process, upon subsequent visible light irradiation, is revealed
by the reshift from 410 to 398 nm, the reincrease in the intensity,
and the decrease of the shoulder above 430 nm. We also
demonstrate the alternation of the diffuse reflectance spectrum
by several cycles of alternating UV and visible irradiation with
intensity degradation of the 398 nm band at each cycle (see
Figure 7), and we note that the initial spectrum was not
reproduced. This supports evidently the similar observation on
the presence of reversible and irreversible photoprocesses shown
by the fluorescence spectroscopy.
The IR spectrum in the fingerprint region before and after
UV irradiation is shown in Figure 8 (top panel). In the different
spectrum between before and after the irradiation (Figure 8,
bottom panel), we observed bleaching vibrational bands at 1050,
1273, 1291, 1322, 1572, and 1639 cm-1, shifting bands from
1169, 1435, 1513, 1595, and 1658 cm-1 to 1178, 1448, 1517,
1590, and 1672 cm-1, respectively, and new bands at 973, 994,
1283, and 1613 cm-1. In addition, some localized vibrational
modes at 1067, 1108, and 1476 cm-1 remain unchanged.
Figure 2. The inversion-symmetry related PCT molecules in antipar-
allel (head-to-tail) arrangement with various ethylenic CdC distances.
All H atoms have been omitted for the sake of clarity.
4. Discussion
4.1. Molecular Structure of PCT. First, we must assume
that the molecular structures, molecular arrangement, intermo-
lecular OH· · ·O bonds, and CH· · ·π weak interactions of PCT
in the microcrystal are identical to those in the bulk crystal.
The presence of two PCT structures in the crystal in general
could be addressed by the stabilization by van der Waals force,
intermolecular interactions, and H-bonds in the densely packed
crystal lattice. The carbonyl oxygen atom of the two PCT
molecules is twisted out of the thiocinnamate plane with a
C7dC8sC9dO2 torsion angle of 8.2° (molecule A) and 12.0°
(molecule B). In many crystal structures of hydroxycinnamic
acids, the CdO group is twisted out of the aromatic plane by
4-6°,32,33 and the torsion angle increases to be around 11° when
the π-electron density is increased either by ionization of the
carboxylic group or by introducing an electron-rich substituent
in the cinnamate moiety.34 Therefore, we suggest that the
twisting CdO group is the typical feature of the cinnamic
derivative due to repulsion between the localized π-electrons
at the ethylenic CdC and at the CdO group. In consistency
with the feature of hydroxycinnamic acids in the crystals, the
CdO group of PYP chromophore is similarly rotated 11.2°.35,36
We recall that the PCT molecules in the crystal packing are
in the antiparallel configuration with the center-to-center distance
between the ethylenic CdC bonds being 4.08, 4.38, 5.43, and
5.67 Å. It is known that the two neighboring trans ethylenic
CdC bonds in the crystal lattice in parallel, antiparallel, or even
crisscrossed arrangement with the distances between the centers
in the range of 3.5-4.2 Å are favorable to form a [2 + 2]
cyclodimer upon photoirradiation, as shown for trans-cinnamic
or cinnamamide derivatives.37–41 Thus, we should consider that
there is a [2 + 2] photodimerization taking place in the PCT
crystal lattice; especially the molecules with a center-to-center
distance as short as 4.08 Å meets the criterion.37 In cases where
the distances between the centers are above the limit, such a [2
+ 2] photodimerization will fail to proceed.
3.2. Fluorescence Spectra of PCT Microcrystal. We
considered the absorption coefficient of the crystal at 405 nm
to be the same as that of the CH2Cl2 solution (ε405 ) 1500 mol-1
L cm-1). With the molecular density of PCT crystal being 5.19
× 10-3 mol cm-3, we can calculate the light penetration depth
of the 405 nm irradiation to be 1.2 µm. We observed that the
emission color of the PCT microcrystal is visibly changed from
blue to greenish yellow upon continuous UV irradiation,20,31 and
the emission spectrum is red-shifted as displayed in Figure 3.
Emission with a maximum at 490 nm from the trans-PCT shows
the intensity increase, in early time irradiation, without any
changes in peak position and spectral shape. The intensity of
this emission band then gradually decreases following the rise
of a new broad emission with a peak at 564 nm, and the latter
band continuously increases up to the saturated intensity with
the irradiation time. The two emission bands are strongly
overlapped without any isoemissive point. The temporal profiles
of the emission intensity at 490 and 564 nm upon 8 mW UV
irradiation are depicted in Figure 4A. The emission intensities
were globally fitted with a two-exponential decay function with
time constants of 18.8 ( 0.7 and 140 ( 1 s-1. The same global
fit procedure to the data at lower irradiation power resulted in
longer time constants, i.e., 75 ( 1 and 151 ( 1 s-1 at 4 mW
and 104 ( 1 and 150 ( 1 s-1 at 2 mW. Figure 4B shows the
emission at 490 nm at different irradiation powers. This result
indicates clearly that the temporal profile is dependent on the
irradiation power but not linearly correlated with each other,
suggesting that the photoexcitation dynamics of PCT in the
crystal lattice is power-dependent and may involve many
transient and photoproduct states, which will be addressed in
section 4. The emission intensity at 490 nm can be partially
recovered by a subsequent visible light irradiation. We dem-
onstrate partial recovery of the emission intensity by over 10
cycles of alternating UV and visible irradiation. However, each
cycle shows degradation, which tends to saturate at a high
number of cycles (Figure 5).
4.2. Photoinduced [2 + 2] Cyclodimerization. In order to
quantify the [2 + 2] photodimerization, we first evaluate the
emission properties of PCT microcrystal. In the excited state,

14236 J. Phys. Chem. B, Vol. 114, No. 45, 2010
Usman et al.
Figure 3. Fluorescence spectra of a single microcrystal of PCT under UV irradiation (λ ) 405 nm, 8 mW) at 1.6 (a), 4.8 (b), 16 (c), 64 (d) 100
(e), 150 (f), 200 (g), 250 (h), 300 (i), and 400 s (j).
Figure 5. Emission intensity at 605 nm against the number of
subsequent UV and visible irradiations; the rectangular and circular
data points are the intensity after UV (λ ) 405 nm) and visible
irradiations (λ ) 546 nm), respectively.
The formation of such a [2 + 2] cyclodimer was detected
1
using H NMR spectroscopy for the PCT microcrystals after
long-time UV irradiation and then dissolved in CDCl3. Indeed,
the 1H NMR spectrum contains the [2 + 2] cyclodimer (around
10%) and trans PCT. The chemical shifts of [2 + 2] cyclobutane
were in the range 4.00-4.70 ppm, as shown in Figure 9, and
were in accordance with the assignment of the other photocy-
clodimerization in crystal lattice reported by Hasegawa et al.38
Figure 4. (A) Time-dependent emission intensity at 490 and 564 nm
under 8 mW 405 nm light irradiation. The lines are calculated fits with
The chemical shifts assigning the protons of phenol and
two-exponential decay functions. (B) Time-dependent emission intensity
thiophenyl of the [2 + 2] cyclodimer were nearly overlapped
at 490 nm at different irradiation power (black, 8 mW; green, 4 mW;
with those of the trans PCT at 6.81-6.87, 6.92-6.96, and
cyan, 2 mW).
7.21-7.31 ppm. This suggests that either the amount of cis
before any conformational changes take place, a PCT molecule
is considered to loosen or break its intermolecular OH · · ·O
bonds and CH· · ·π weak interactions with neighboring mol-
ecules in the crystal. This loosening of intermolecular interac-
tions results in an enhancement of emission efficiency, by which
the emission intensity at 490 nm is enhanced in the early time
irradiation. The new emission at 564 nm, however, must be
associated to the emission of distinctly different species, that
is, a structure converted from the trans configuration. This
emissive species should not be the [2 + 2] cyclodimer which
absorbs in the far UV region. The backward reaction by visible
light irradiation also indicates that the species is a reversible
photoproduct state. The degradation of the emission intensity
upon alternating UV and visible irradiation (Figure 5) indicates
a reduction of the conversion from the trans form to the
reversible photoproduct state in each photocycle. This is
indicative of branched relaxation pathways, and one of the
pathways should be the irreversible [2 + 2] cyclodimer.
isomer is much smaller than the [2 + 2] cyclodimer or a twisted
or a cis isomer relaxes into the trans form if the trans
configuration is energetically favorable, similar to its parent
molecule, p-hydroxycinnamic acid,42 because the steric con-
straints and mutual interactions between the [2 + 2] cyclodimer
and the formations of a twisted or a cis isomer become weaker
upon dissolution.
4.3. trans-cis Photoisomerization. The bleaching bands in
the IR spectrum (Figure 8) must be related to the vibrational
modes of trans-PCT, and the new bands should be attributed to
those of the photoproducts. Part of the new bands should be
assigned to the vibrations of the [2 + 2] cyclodimer. However,
the marker modes of its [2 + 2] cyclobutane vibrations are
expected to appear at lower frequency positions below the
fingerprint region, and they should be very weak due to either
the low quantum yield or low IR extinction absorption coef-
ficient. An indirect marker band is the frequency upshift of the
carbonyl νCdO mode at 1672 cm-1
.

Photochemical Reaction of PCT
J. Phys. Chem. B, Vol. 114, No. 45, 2010 14237
Figure 8. IR spectrum of PCT before (black) and after long time UV
irradiations (λ ) 405 nm) (red) (top panel) and the difference IR
spectrum between before and after irradiations for 750 s (bottom panel).
Figure 6. (A) UV-vis diffuse reflectance of powdered PCT before
(a) and at 30 (b), 60 (c), 90 (d), 150 (e), and 180 min (f) under low
intensity UV irradiation (λ ) 400 nm). (B) UV-vis diffuse reflectance
of the sample after 180 min of UV irradiation (a) and then at 30 (b),
60 (c), 90 (d), 150 (e), and 270 min (f) under low intensity visible (λ
) 480 nm) irradiation. The arrows indicate the rise and decrease of
the diffuse reflectance.
Figure 9. 1H NMR spectra (270 MHz) of PCT after UV irradiations
for 750 s showing the presence of the [2 + 2] photocyclodimer with
the assignment of [2 + 2] cyclobutane protons. Coupling constant: JAA′
) 7.02 Hz, JBB′ ) 7.02 Hz.
intermediate of the PYP chromophore is located at 1599
cm-1 44–48
The bleach of the ethylenic mode from the trans
.
δC7HdC8H(Bu) at 1291 cm-1 and a new mode of cis νC3sO
+ δC7HdC8H(A1) at 1283 cm-1 are also in agreement with a
recent IR study on trans p-hydroxycinnamic acid, which has a
related pair of bleaching/new bands at 1294/1288 cm-1 upon
trans-to-cis isomerization.46 In the cis intermediate PYP chro-
mophore, the δC7HdC8H(A1) mode has been assigned to the
band at 1286 cm-1 47,48
.
A frequency shift of the carbonyl νCdO mode from 1656
cm-1 to a broadband at 1672 cm-1 can also provide a direct
insight into the trans-to-cis isomerization, since by twisting or
flipping of the thioester bond the carbonyl group has less
substantial couplings with other nuclear vibrations, resulting in
higher frequency position of the νCdO mode. A similar
frequency upshift of this mode was observed from 1631 to 1653
cm-1 in PYP,44–48 and from 1700 to 1722 cm-1 in p-hydroxy-
cinnamic acid methyl ester, upon trans-to-cis photoisomeriza-
tion.49
In addition to those isomerization marker modes, the fre-
quency positions of the phenolic moiety17,44–46 are also slightly
shifted from 1169, 1435, 1513, and 1595 cm-1 to 1178, 1448,
1517, and 1590 cm-1, respectively, due to most probably
couplings with minor contributions of other nuclear motions,
although the phenolic moiety is structurally unchanged. On the
other hand, localized vibrational modes of the thiophenyl
moiety17 at 1067, 1108, and 1476 cm-1 should remain un-
changed. These findings provide evidence that the vibrational
motions of these two moieties are not significantly altered upon
the conformational changes in the crystal lattice.
Figure 7. Diffuse reflectance intensity at the 398 nm band (upper data)
and at the 468 nm shoulder (bottom data) against the number of
subsequent UV and visible irradiations; the filled and open data points
are the intensity of the bands after UV (λ ) 400 nm) and visible (λ )
480 nm) irradiations, respectively.
Important marker bands are the bleaching band at 1050 cm-1
together with the new bands at 973 and 995 cm-1. These bands
have been assigned to the specific γC7HdC8H + νC8sC9 mode
of the trans and cis form of thiocinnamic ester, respectively.17,18,43
We note that the trans-to-cis conversion of the PYP chromphore
clearly exhibits similar bleaching/new bands of the γC7HdC8H
+ νC8sC9 mode of the chromophore at 1058/994 cm-1 44–48
.
This finding may indicate that the UV photoirradiation also
induces trans-to-cis isomerization, and that the reversible product
state, mentioned in section 4.2, is the cis PCT isomer. Further
support for the trans-to-cis isomerization is the bleaching/new
bands at 1639/1613 cm-1, which have been assigned to mainly
the ethylenic νC7dC8 bond of trans and cis configuration,
respectively, and the similar mode of the protonated cis

14238 J. Phys. Chem. B, Vol. 114, No. 45, 2010
Usman et al.
Figure 10. Schematic diagram of photoexcitation dynamics of PCT microcrystal.
4.4. Photochromic Reaction of PCT in the Microcrystal-
line State. Here, we propose that photochromic reaction of PCT
microcrystal involves the branched photoexcitation dynamics
as discussed above. As shown schematically in Figure 10, we
consider that photoexcitation of the PCT molecules could induce
loosening of intermolecular interactions or an excited state
proton shift of PCT*, leading to a charge translocation along
the π-conjugation. This charge translocation, which affects the
characters of the CdC double and CsC single bonds, has been
suggested to be the early intramolecular process for the
isomerization.6,14,50,51 The PCT in the excited state then relaxes
into branched pathways; a photoisomerization by flipping the
carbonyl group or a [2 + 2] cyclodimerization by coupling the
two adjacent CdC moieties. The quantum yield of the two
relaxation pathways is dependent on irradiation power. A higher
irradiation power induces a higher ratio between the emission
maximum intensity at 490 nm and the saturated intensity at 564
nm (Figure 4B). The increase in the two emission intensities
can be ascribed to the increase in the concentration of the excited
states of trans and cis PCT in the crystal, respectively. Under
the higher intensity of light irradiation, more photons are sent
on to the small area of the crystal. We suggest that one photon
excites a single molecule, giving rise to generation of defects
in surrounding molecules, and the other photons induce more
defects within the small area, making a local distortion in the
crystal lattice. In the distorted lattice, then, coupling between
the photoexcited, transient, and ground state PCT or confor-
mational changes of an excited PCT take place. Thus, all of
the processes result in a nonlinear relationship between the
temporal profiles of the emission spectra and the number of
irradiated photons. Such a nonlinear relationship has also been
reported previously for the crystals of spirooxazines and
spiropyrans under fs pulse excitation.27–29 We should consider
that the molecular couplings in the distorted lattice also result
in local heating, which induces larger lattice distortion and
modifies the center-to-center distance between the ethylenic
CdC bonds. Therefore, it is reasonable that the higher power
intensity induces larger lattice distortion in the tight crystal
packing, leading to a more favorable condition for the [2 + 2]
photocyclodimerization rather than for the photoisomerization.
Although a detailed description of the reaction coordinates
of the photoisomerization is still an open question, the formation
of the [2 + 2] cyclodimer is a clear indication that the
photoactive sites are around the ethylenic CdC and two adjacent
CsC bonds. Indeed, the cis PCT is produced after a large
molecular motion, which requires some spaces. The isomeriza-
tion should be prohibited in the densely packed crystal, even
though this reaction proceeds via a volume-conserving rotation
of the carbonyl group by the hula twist mechanism,52,53 such as
the conformational motions of the protonated p-hydroxycin-
namic derivatives in solution phase.54
One possible explanation for the isomerization of PCT in the
crystal lattice is a cooperative photochemical reaction model.
In this context, the continuous light irradiation generates local
lattice distortion and intermolecular couplings as mentioned
above, resulting in dynamic free spaces, which allow the
surrounding excited PCT molecules to isomerize cooperatively.
This should be considered as an extension of our previous
cooperative reaction model, where the photoexcited and plural
transient species of spiropyrans or spirooxazines generated
densely by intense femtosecond light in the crystal lattice are
proposed to interact and couple with each other, creating a free
volume, and move cooperatively into trans-planar or near-planar
photomerocyanines within nanosecond time scales.27–29,55 We
should note that, although the intermolecular couplings forming
the [2 + 2] cyclodimer is the topochemical reaction with
minimal disruption of the crystal lattice, the formation of the
[2 + 2] cyclodimer provides dynamic free spaces, since the
two antiparallel PCT molecules form a cyclobutane ring with
the C-C distance turning from around 4 Å apart into within
1.6 Å. The free space allows the closest neighboring excited
PCT to accomplish the volume-conserving rotation followed
by such rotation of the next neighboring excited PCT. However,
the photoisomerization must be initiated near the crystal surface,
and the isomerization in turn increases more lattice deformation
and induces further photoisomerizations. Since the [2 + 2]
cyclodimers are formed periodically, the photoisomerization is
further enhanced toward the interior of the crystal. This is in
contrast to the trans-to-cis isomerization of azobenzene which
only occurs on the crystal surface with a low yield.26 The
photodynamics of trans-cis isomerization of PCT in the crystal
is even different from that of salicylideneanilines with a bulky
group substituent introduced into the aromatic rings, where the
bulky group acts as a space opener for static free volume in the
crystal lattice.3,24
The temporal profiles of the emission at 490 and 564 nm are
well fitted with the two-exponential decay function, except for
the two data points at early time irradiation, as shown in Figure
4A. This indicates that the rate of the emission increase at 490
nm is not constant. However, it is useful to note that, from the

Photochemical Reaction of PCT
J. Phys. Chem. B, Vol. 114, No. 45, 2010 14239
photoexcitation dynamics proposed in Figure 10, the concentra-
tion of the excited states of trans and cis PCT in the crystal
also follows the two-exponential decay functions, pointing out
that the photodynamic model is relevant, although we could
not calculate the rate constant of each reaction.
intermolecular coupling, leading to the modification of the
center-to-center distance between the ethylenic CdC bonds
favorable for the [2 + 2] photocyclodimer.
Acknowledgment. Financial support provided by the Japan
Society for the Promotion of Science (JSPS) for a research
fellowship in Osaka University to A.U., by the Grant-in-Aid
for Scientific Research (KAKENHI No. 19049011) to T.A., by
the Grant-in-Aid for Scientific Research (KAKENHI No.
18106002) to H.M., by the MOE-ATU Project (National Chiao
Tung University) of the Ministry of Education, Taiwan, to H.M.,
and by the National Science Council of Taiwan (No. 0970027441)
to H.M. is gratefully acknowledged.
In the above proposed photochromic reaction, the cis pho-
toproduct state undergoes a backward reaction to the trans form
upon visible light irradiation. We believe that both forward and
backward photoisomerizations proceed under the condition in
which the crystal lattice is distorted, since the original lattice
must have been broken by the first UV irradiation. This is in
contrast to the uniform photochromism,2,23 photocyclodimer-
ization,41 or photopolymerization,39,56 in which these topochemi-
cal reactions occur in a single-crystal-to-single-crystal trans-
formation with the original crystal lattice being almost unchanged
throughout the photoreaction.57 For the PCT crystal, the lattice
deformation, the dynamic voids, and the coexistences of two
or more structures in its single bulk crystal after UV irradiation
are the reasons for the fractures on the crystal surface and for
the difficulty to solve its crystal structure. The powder X-ray
diffraction pattern for PCT crystals after UV irradiation,
however, does not show a new diffraction peak compared to
that before irradiation, regardless of some differences in
diffraction peak intensities (data not shown). This finding is an
indication that the cis isomer does not form a new crystal lattice.
The formation of the cis isomer in the crystal, to some extent,
is important, since there are only a few examples of chro-
mophores showing photochromism in the crystalline state due
to the fact that the chromophores have unstable photoproduct
isomers.2,22,27–29,55
Supporting Information Available: Detailed information
on X-ray structural determination, bond lengths, bond angles,
intermolecular interactions, and an X-ray crystallographic file
in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
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