The photoorientation movement of a diarylethene-type chromophore

Article abstract of DOI:10.1021/ja002185f

We quantified coupled photoisomerization and photoorientation of a diarylethene-type chromophore in films of poly(methyl methacrylate). We found that the chromophore retains partial memory of its orientation during the A → B UV-induced isomerization and that the chromophore movement is thermalized after the A←B green light induced isomerization. The isomerization quantum yields are φ_AB/³⁶⁵ = 1.1 φ_AB/⁴⁰⁵ = 0.32, and φ_BA/⁵³² = 0.16, and the transitions at 365 (405) and 532 nm of the B isomer are oriented at an angle of 74.3 (61.3)° to each other. This relative orientation of the transition dipoles is rationalized by MOPAC molecular orbital calculations.

Full text of DOI:10.1021/ja002185f

12802
J. Am. Chem. Soc. 2000, 122, 12802-12805
The Photoorientation Movement of a Diarylethene-Type Chromophore
Hidekazu Ishitobi,† Zouheir Sekkat,*,†,‡ Masahiro Irie,§ and Satoshi Kawata†
Contribution from the Department of Applied Physics, Osaka UniVersity, Suita, Osaka 565-0871, Japan,
School of Science and Engineering, Al Akhawayn UniVersity in Ifrane, 53000 Ifrane, Morocco, and
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu UniVersity,
Higashiku, Fukuoka 812-8581, Japan
ReceiVed June 19, 2000. ReVised Manuscript ReceiVed October 20, 2000
Abstract: We quantified coupled photoisomerization and photoorientation of a diarylethene-type chromophore
in films of poly(methyl methacrylate). We found that the chromophore retains partial memory of its orientation
during the A f B UV-induced isomerization and that the chromophore movement is thermalized after the
365
AB
405
AB
ArB green light induced isomerization. The isomerization quantum yields are φ ) 1.1, φ ) 0.32, and
φ
532
BA
) 0.16, and the transitions at 365 (405) and 532 nm of the B isomer are oriented at an angle of 74.3
(61.3)° to each other. This relative orientation of the transition dipoles is rationalized by MOPAC molecular
orbital calculations.
Introduction
of poly(methyl methacrylate) (PMMA).11 In this paper, we will
discuss photoorientation by photoisomerization of a diarylethene-
type chromophore and how this chromophore moves upon
isomerization in films of PMMA. Diarylethenes are contrasted
to spiropyrans by the thermal stability of the B isomer, a feature
that brings about interesting photoorientation effects in spectrally
distinguishable photoisomers (vide infra). The order parameter
is independent of the irradiation light intensity at the photosta-
tionary state. The photochromism of diarylethenes has also been
extensively studied,2,12 and recently, we observed that when such
chromophores are isomerized by polarized light, the apparent
photoorientation is opposite for the UV versus the visible
absorption band of the chromophores,13 and we found that the
365- and 633-nm transitions of the B isomer of a spiropyran
chromophore are nearly perpendicular.11 We will show that
quantified photoorientation of diarylethenes reveals that the
closed form of such chromophores also exhibits perpendicular
UV and visible transition dipole moments.
In the past decade, the phenomena of light-induced orientation
of photochromic molecules in polymers have attracted much
attention due to requirements in the areas of optoelectronics,
photonics, and linear and nonlinear optics.1-5 Light can orient
the chromophores by photoisomerization via purely polarized
transitions and induces anisotropy and quadratic and cubic
optical nonlinearities. Inasmuch as optical ordering of photo-
isomerizable molecules is being intensively studied, its quan-
tification is needed to bridge independent studies in photochem-
istry and nonlinear optics.
Photoorientation by or without photoisomerization has been
known for a long time,6,7 and photoisomerization was studied
from a pure photochemical point of view.8-10 However, tools
to quantify photoisomerization-induced molecular orientation
were yet to be developed. Recently, we developed the analytical
theory that allows for the quantification of coupled photoi-
somerization and photoorientation in A T B photoisomerizable
systems where B is unknown, and we have used it to study
quantitatively both the photoisomerization and photoorientation
of photochromic spectrally distinguishable spiropyrans in films
Experimental Section
The structural formula of the diarylethene chromophore, e.g., 1,2-
dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl)ethene (Tokyo Kasei: B1536),
which we have studied and which is referred to in the text as DE, and
its photochemical isomer are shown in Figure 1, together with the UV-
visible spectra prior to and after UV light irradiation to the photosta-
tionary state. DE-type molecules have two photochemical isomers, A
and B, both of which are thermally stable at room temperature, and
UV and visible light irradiations produce photoreaction in both the A
f B and A r B and the A r B directions, respectively.
We investigated the dynamics of photoorientation of DE in films of
PMMA (Tg ) 110 °C, and molecular weight, Mω ) 50 000) by real-
time dichroism. Guest-host polymer films were prepared by spin-
casting from a 2-butanone solution on cleaned glass substrates. The
weight ratio of the chromophore relative to the polymer was 10%; a
loading ratio at which chromophore aggregation does not occur for
DE in PMMA. Differential scanning calorimetry measurements (not
* Corresponding author at Osaka University.
† Osaka University.
‡ Al Akhawayn University in Ifrane.
§ Kyushu University.
(1) Irie, M., Ed. Chem. ReV. 2000, 100 (5) (PMS Feature).
(2) Irie, M. in ref 1, pp 1685-1716 and references therein.
(3) Delaire, J. A.; Nakatani, K. in ref 1, pp 1817-1845 and references
therein.
(4) Sekkat, Z.; Knoll, W. In AdVances in Photochemistry; Neckers, D.
C., Volman, D. H., Von Bunau, G., Eds.; Wiley and Sons: New York,
1997: Vol. 22, pp 117-195. SPIE Proc. 1998, 164-185.
(5) Sekkat, Z.; Knoesen, A.; Lee, V. Y.; Miller, R. D.; Wood, J.; Knoll,
W. In Organic Thin Films; Frank, C., Ed.; ACS Symposium Series 695,
American Chemical Society: New York, 1998; pp 295-314.
(6) Neoport, B. S.; Stolbova, O. V. Opt. Spectrosc. 1961, 10, 146.
(7) Thulstrup, E. W.; Michl, J. J. Am. Chem. Soc. 1982, 104, 5594; 1986,
108, 671. Radziszewski, J. G.; Burkhalter, F. A.; Michl, J. J. Am. Chem.
Soc. 1987, 109, 61.
(8) Zimmerman, G.; Chow, L. Y.; Paik, U. Y. J. Am. Chem. Soc. 1958,
80, 3528.
(9) Fisher, E. J. Phys. Chem. 1967, 71, 3704.
(10) Rau, H.; Greiner, G.; Gauglitz, G.; Meier, H. J. Phys. Chem. 1990,
94, 6523.
(11) Ishitobi, H.; Sekkat, Z.; Kawata, S. Chem. Phys. Lett. 2000, 316,
578.
(12) Irie, M.; Mohri, M. J. Org. Chem. 1988, 53, 803.
(13) Ishitobi, H.; Sekkat, Z.; Kawata, S. Chem. Phys. Lett. 1999, 300,
421.
10.1021/ja002185f CCC: $19.00 © 2000 American Chemical Society
Published on Web 12/06/2000

Photoorientation of a Diarylethene-Type Chromophore
J. Am. Chem. Soc., Vol. 122, No. 51, 2000 12803
Figure 1. Absorption spectra of diarylethene before after UV irradiation to the photostationary state. The inset shows the chemical structure and
ring closing/opening isomerization of diarylethene. The 365- and 405-nm irradiation and 532-nm analysis wavelengths are indicated by arrows.
shown) that we have performed on both PMMA and the 10% DE/
PMMA, demonstrate that the Tg of PMMA decreases appreciably from
110 to 72 °C with the added DE solute. The DE dopants swell
significantly the polymer chains for a larger volume and, consequently,
are not closely packed by the PMMA matrix. Furthermore, if aggrega-
tion is present, it strongly distorts molecular absorption spectra due to
intermolecular interaction, and the spectrum of a 1 × 10-4 M CCl4
solution of DE is identical to that in the PMMA films both prior to
and after irradiation even though the chromophore concentration is much
smaller in this solution than in the DE/PMMA samples.12 Residual
solvent was removed from the films by heating for 1 h at 110 °C near
the polymer Tg, a temperature at which polymer segmental motion is
large enough to free possibly trapped solvent molecules. The films were
removed from the oven and allowed to slowly cool to room temperature.
Figure 2. Real-time evolution of the isotropic absorbance (left) and
the anisotropy (right) of DE in PMMA upon linearly polarized UV
irradiation for several irradiation intensities. Only the 365-nm UV
photoorientation is shown; 405-nm photoorientation showed similar
The film thickness was measured by atomic force microscopy and was
typically ∼0.77 µm. The films were irradiated by linearly polarized
UV light (365 ( 15 and 405 ( 5 nm; glass and interference filters
from Irie and Sigma Co., respectively) from a high-pressure mercury
dynamical behavior. The numbers from 1 to 5 indicate the value of
lamp (Nikon Co.) and polarized green light (532 nm) from a diode-
the irradiation intensity in units of einstein‚s-1‚cm-2 with the corre-
sponding sample absorbance A0′, value between parentheses, at the
irradiation wavelength, e.g., 365 nm. The moments of turning the
irradiation light on and off are indicated. Note that, after irradiation,
the isotropic absorbance is stable because the B isomer is thermally
stable, and the anisotropy relaxes due to molecular rotational diffusion.
pumped frequency-doubled Nd:YVO4 laser (Uniphase) to induce the
A T B and A r B photoreactions, respectively. In consideration of
the chromophore density (e.g., 2.37 × 1020 and 0.43 × 1020
molecules‚cm-3 for the A and B isomers prior to UV and green
photoorientation, respectively), the UV and green irradiation intensity
range imposes about 4 × 10-3-32 × 10-3 (for 365 nm) and 1.2 ×
10-2-8.5 × 10-2 (for 405 nm), and 0.55-8.2 incident photons per
superscript B if any stands for the B isomer, and the primed and
isomer per second, respectively. The samples were irradiated with
unprimed quantities refer to the irradiation and analysis wavelengths,
linearly polarized light while in situ transmittance measurements were
respectively.
performed with a probe light polarized either parallel or perpendicular
to the initial irradiating light polarization. The probe beam was
propagating perpendicular to the plane of the sample and linearly
Results and Discussion
polarized at 45° with respect to the plane of incidence of the irradiating
beam. The transmitted parallel and perpendicular components were
PMMA during and after linearly polarized UV irradiation for
separated by a Wollaston prism and detected separately. The probe
Figure 2 shows the time evolution of AB and ∆AB of DE/
different irradiation power values. The occurrence of anisotropy
is indicative of photoorientation of the chromophores. The green
beam was the green laser light (incident power ∼1.2 µW, and
∼1-mm-diameter spot), and photoorientation processes of the isomer
light-induced orientation of the chromophores shows a dynami-
cal behavior that reverses for long irradiation times (see Figure
3). In this experiment, the DE/PMMA samples were irradiated
by unpolarized UV light to the photostationary state, and linearly
polarized green irradiation followed. Even though diarylethenes
are robust chromophores, the reversal of the mean absorbance
observed at high UV irradiation intensities is due to the isomer’s
degradation. In fact, the colored intensity of DE decreases to
80% after 10 UV-visible irradiation cycles.12 To avoid possible
B are probed independently from those of isomer A. Dark conditions
were employed to avoid the influence of the room light on the
isomerization reaction.
AbsB| and AbsB were calculated from the amount of absorbed
parallel and perpendicular light, respectively, and the isotropic absor-
bance AB ) (Abs B + 2AbsB)/3, the anisotropy ∆AB ) AbsB- AbsB,
|
|
and the order parameter SB ) ∆AB/3AB of the B isomer’s orientational
distribution were deduced. SB was calculated at the steady-state of
photoorientation. For all of the notations of the paper, the sub- or

12804 J. Am. Chem. Soc., Vol. 122, No. 51, 2000
Ishitobi et al.
Figure 3. (a) Same as Figure 2, but for 532-nm analysis and irradiation. (b) Anisotropy observed by a UV-visible spectrophotometer after 546-
nm irradiation (∼1 mW/cm2). In (b), the time refers to the irradiation duration.
Figure 4. Slopes, (a) p(∆) and p(∆A), and (b) p(∆Ν) and p(∆AN), of the observed change of the early time evolution of the isotropic absorbance
and the anisotropy, respectively, on the irradiation intensity (UV for (a) and green for (b)). The full lines are linear fits. F′) 103(1 - 10-A′ )I′/A′;
0
0
0
and X′ ) F′A0′.
Table 1. Order Parameter SB at the Photostationary State of UV
Irradiation, and the Irradiation Intensity Expressed in Units of
einstein‚s-1‚cm-2
photodegradation complications after successive irradiation
cycles, each photoorientation experiment has been done on a
different previously nonirradiated sample, and for data analysis,
only the slopes and the maximum absorbances at high irradiation
intensities were considered.
I (× 1010) 365 nm 1.21
2.43
6.76
4.24
13.2
6.65
17.3
9.70
26.0
405nm 3.72
SB
365 nm 0.055 0.060
405 nm 0.027 0.026
0.058
0.024
0.064
0.021
0.062
0.021
The fitted slopes, p(∆) and p(∆A), of the early time evolution
of AB and ∆AB, respectively, showed a linear dependence on
the irradiation light intensity for both UV- (Figure 4a) and green
(Figure 4b) light-induced orientation. The solid lines in Figure
4a are linear theoretical fits by11
P2(cos ω) is given at the photostationnary state by11
2
SB ) - P2(cos ω)
(3)
13
p(∆) ) 1000I′(1 - 10-A0′)φA′ BꢀB
(1)
For DE in PMMA, the values of SB obtained at the photosta-
tionnary state of UV irradiation are given in Table 1. From these
values, it noteworthy that for different irradiation intensities and
wavelengths, the value of SB is fairly constant, e.g., ∼0.060
and ∼0.024 for 365- and 405-nm irradiation, respectively; a
feature that is theoretically rationalized by eq 3. The factor of
-2/13 is the constant of photoorientation by photoisomerization
and refers to the maximum orientation, e.g., the orientation
observed at the irradiation wavelength for systems either without
A r B thermal isomerization such as diarylethenes or where
the irradiation intensity is extrapolated to infinity, such as
azobenzenes and spiropyrans.
0
6
5
p(∆A) ) 1000I′(1 - 10-A0′)φA′ BP2(cos ω)P2ABꢀB (2)
0
where I′ is the incident flux of photons per centimeter squared,
0
A0′ is the sample absorbance prior to irradiation, φ′AB is the UV-
induced A f B photochemical quantum yield, PA2 B is a
parameter that characterizes the memory of the orientation of
the UV transition of the chromophore during the A f B
photoisomerization, P2(cos ω) is the second-order Legendre
polynomial of cos ω, and ω is the angle between the B isomer’s
irradiation, e.g., UV, and analysis, e.g., visible, transition
moments. For a system without A r B themal isomerization,
For the evaluation of the parameters above, ꢀB should be
known. We have determined ꢀB by the method of Fisher,9 which

Photoorientation of a Diarylethene-Type Chromophore
J. Am. Chem. Soc., Vol. 122, No. 51, 2000 12805
Table 2. Data of Coupled Photoisomerization and Photoorientation
of DEa
UV
532
φAB φBA P2(cos ωUV
)
ω
P2
A
fB
UV UV
λ/nm
ꢀA
ꢀB
532
365 4436 8332 1.10
405 2402 423 0.32
532 4574
-0.39
-0.015
74.3
61.3
-0.43
-1.04
0
0.16
a The extinction coefficients are expressed in units of L‚mol-1‚cm-1
.
holds not only for isotropic but also for anisotropic samples
when the isotropic absorbance is considered. For this determi-
nation, two different locations, which have exactly the same
absorbance value in the UV region of an all-cis DE/PMMA
sample, were irradiated by linearly polarized 365- and 405-nm
UV lights to the photostationnary state, respectively, and the
parallel and perpendicular spectra were measured on the entire
UV-visible spectrum by a spectrophotometer (model UV-1600,
Shimadzu) with two custom-built polarizers for the sample and
the reference. The obtained extinction coefficients are sum-
marized in Table 2 together with the photochemical quantum
yields and the parameters obtained from the photoorientation
experiments. The fitted slope of the isotropic absorbance of
Figure 4b of the green photoorientation experiments yielded
φ5B3A2. It is noteworthy that for systems without A r B thermal
isomerization, and if analysis is performed at the irradiation
wavelength, A r B photoorientation predictions suggest a slope
ratio p(∆A)/p(∆) equal to 1.2,11 and the slopes, calculated from
Figure 4b, of the green photoorientation of DE give a ratio of
p(∆A)/p(∆) of ∼1.1.
Figure 5. Drawing of the top view and transition moment vectors at
365 and 532 nm of the closed form of DE obtained by CNDO/S AM1
MOPAC molecular orbital calculations. The 532 nm is parallel to the
long axis of the molecule.
off as it does when isomerized from B to A by green irradiation.
Figure 3b shows that the green light-induced orientation
observed at both 532 and 365 nm disappears after all B forms
are isomerized to A forms; a feature which demonstrates that
isomer A is not oriented by green light-induced A r B
isomerization, and the observed anisotropy at both 532 and 365
nm is due to the orientation of only isomer B. If orientation
occurs in A at any time by green irradiation, some anisotropy
should remain at 365 nm after all B are isomerized to A. This
A
365rB532
behavior is theoretically rationalized by P2
∼ 0, since
the orientation of A is proportional to that of B through
To our knowledge, the quantum yields of coloring and
bleaching of DE have not yet been measured, but those of other
diarylethene derivatives have been determined. The values that
we found in PMMA by irradiation at 532 and 405 nm, e.g.,
A
365rB532 15
P2
.
The lack of orientation in A may be due to the
large amount of energy that needs to be dissipated during the
photochemical process induced by the 532- or 546-nm photon;
perhaps when the molecule is excited, it shakes strongly before
532
BA
405
AB
φ
∼ 0.16 and φ ∼ 0.32, respectively, are close to those,
365
532
365
532
546
BA
334
AB
it relaxes. ω ) 74.3 and ω ) 61.3° demonstrate that the
e.g., φ ∼ 0.14 and φ ∼ 0.27-0.33, found in CCl4 and
direction of the UV, e.g., 365 and 405 nm, and visible, e.g.,
532 nm, transitions of the B isomer, e.g., the closed form, are
oriented toward perpendicular directions and rationalize the
result of Figure 3b. This finding is reinforced by the result of
Figure 5 which shows that the calculated 365- and 532-nm
transitions, with 0.960 878 and 0.398 422 respective oscillator
strengths, of the closed form of DE are indeed perpendicular to
each other. The UV and visible transitions of the closed form
of DE have been calculated by using the CNDO/S (completely
neglected differential overlap/spectroscopy) with the associated
AM1 parametrization for geometry optimization which are
available with the MOPAC molecular orbital software.16
benzene solutions for other diarylethene derivatives that are
structurally related to DE.12 Even though little free volume
change is needed for DE ring opening and closing, the PMMA
matrix does not seem to hinder the isomerization movement of
the chromophore. Perhaps the large swelling of the polymer
chains, which is mirrored by the decrease of Tg with the added
DE solutes (vide infra), provides enough free volume for the
photoinduced movement of the chromophores. It is noteworthy
that polymer thin films present a distribution of free volumes
to solutes, and the film properties are averaged for chromophores
in different sites. Quantum yields depend on the excitation
wavelength, and when side reactions are present, they can be
larger than 1. Indeed, a single photon may lead to the isomer-
Conclusions
ization of more than one chromophore by side reaction isomer-
356
AB
Diarylethenes are oriented by photoisomerization via purely
polarized transitions. The movement of the chromophore has
been discussed for both UV- and green light-induced isomer-
ization. The quantum yields and the orientation parameters are
determined. The UV and visible transitions of the closed-form
isomer are perpendicular; a finding that is rationalized by
molecular orbital calculations. Quantified photoorientation
reveals the symmetry nature of molecular transitions of pho-
tochromic isomers.
ization. The value of 1.1 that we found for φ at 365 nm
might reflect the existence of an energy transfer between neigh-
boring open-form molecules to induce close ring isomerization,
a feature that may occur in polymer films with high chro-
mophore concentration. Other authors have also found a quan-
tum yield of 2 for the isomerization of another diarylethene
derivative in films of poly(vinyl butyral), and they too have
attributed it to an energy transfer from the open to the closed
ring form.14
A
365fB365
A405fB405
P2
∼ -0.4 and P2
∼ -1 show that the
Acknowledgment. This work is supported by the Japan
Society for the Promotion of Science under the Research for
the Future Program.
orientation of the UV transition dipole of the chromophore is
partially retained, e.g., not thermalized, upon isomerization from
A to B after UV irradiation, a feature which suggests that the
chromophore does not tumble indiscriminately before it cools
JA002185F
(15) Sekkat, Z.; Wood, J.; Knoll, W. J. Chem. Phys. 1995, 99, 17226.
(16) Kurtz, H. A.; Stewart, J. J. P.; Dieter, K. M. J. Comput. Chem.
1990, 11, 82.
(14) Tsujioka, T.; Kume, M.; Irie, M. J. Photochem. Photobiol. A: Chem.
1997, 104, 203.