Individualized optically induced orientation of photochemical isomers

Article abstract of DOI:10.1016/S0009-2614(98)01390-6

We present evidence of individualized optically induced orientation in photoisomers of diarylethene and spiropyran photochromic chromophores. The light-induced orientation studied on the entire UV-vis spectrum of the chromophores shows spectral dependencies which inform about the relative orientation of the directions of the ultra-violet and visible photochemical transitions of the chromophores.

Full text of DOI:10.1016/S0009-2614(98)01390-6

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5 February 1999
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Chemical Physics Letters 300 1999 421–428
Individualized optically induced orientation of photochemical
isomers
)
Hidekazu Ishitobi, Zouheir Sekkat , Satoshi Kawata
Department of Applied Physics, Osaka UniÕersity, Suita, Osaka 565-0871, Japan
Received 28 September 1998; in final form 16 November 1998
Abstract
We present evidence of individualized optically induced orientation in photoisomers of diarylethene and spiropyran
photochromic chromophores. The light-induced orientation studied on the entire UV–vis spectrum of the chromophores
shows spectral dependencies which inform about the relative orientation of the directions of the ultra-violet and visible
photochemical transitions of the chromophores. q 1999 Elsevier Science B.V. All rights reserved.
The growing interest that has arisen in photosensi-
tive chromophores containing polymeric films in the
past few years has been driven by requirements in
the areas of optoelectronics, photonics and optical
signal processing. Such materials have been used for
holography, optical data storage, integrated optics,
molecular switching, all-optical modulation and sec-
oriented photoisomers of diarylethene and spiropyran
derivatives in thin films of a poly-methyl-methacry-
Ž
.
late PMMA polymer. Such photochromic chro-
mophores have been extensively studied, not only
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x
from a photochemical point of view 4–6 , but also
for use in near-field and three-dimensional optical
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data storage 7–9 . Besides the technological interest
into the photoisomerization of diarylethene and
spiropyran derivatives in polymeric thin films, stud-
ies of the optical orientation in such systems will
improve the comprehension of the reorientation pro-
cess of photochromic chromophores within isomer-
ization reactions. Indeed, in contrast to azobenzene
derivatives, one of the two photochemical isomers of
diaryletehene- and spiropyran-type chromophores can
be spectrally distinguished, a feature which allows
for the individualization of the photo-induced orien-
tation processes in the photo- and thermal isomeriza-
tion reactions. In this Letter, we separate the optical
orientation processes of both photoisomers of each
of the diarylethene and spiropyran chromophores
studied, by using the polarized UV–vis spectroscopy
technique, and by taking advantage of the natural
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ond- and third-order nonlinear optical effects 1 .
Reorientation of photo-excited chromophores allevi-
ates centrosymmetry and isotropy, thereby inducing
anisotropy and quadratic and cubic optical nonlinear-
ities. For fundamental understanding of the reorienta-
tion process itself and its correlation with the micro-
cogent environment of the chromophore, much of the
optically induced reorientation studies have focused
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on azo dye containing materials 2 . However, de-
spite this activity, studies of light-induced orientation
of chromophores other than azobenzene derivatives
and azobenzene-type molecules have rarely been re-
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ported 3 . In this Letter we individualize optically
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Corresponding author. Fax: q81 6 6879 7876
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
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PII: S0009-2614 98 01390-6

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422
H. Ishitobi et al.rChemical Physics Letters 300 1999 421–428
DE and SP derivatives has been studied extensively
for a long time, and spectroscopic characterization of
DE- and SP-type molecules is performed by observ-
ing a low-energy band in the visible region of the
absorption spectrum and a high-energy band in the
UV region. Both of the A and B isomers of DE and
SP can absorb UV light and simultaneously induce
the A™B and A§B photoisomerizations. When
either of DE or SP is irradiated with visible light,
only the B isomer can appreciably absorb light and
induce the A§B photoisomerization. In other
words, the A and B forms of the chromophores
studied and the related photochemical and photo-
physical processes can be individualized inasmuch as
the spectral features of the photochemical isomers
can be distinguished.
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Fig. 1. Chemical structures and isomerization of top diarylethene
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and bottom spiropyran isomers.
spectral differences exhibited by the photoisomers in
We have studied the photoisomerization and the
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the UV–vis region vide infra . In particular, it will
be shown that the apparent optically induced orienta-
tion depends on the UV and visible photochemical
transition bands.
photo-induced orientation in films of PMMA glass
transition temperature, Tg s1058C, and molecular
weight, MWs40 000 doped with DE or SP.
.
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 1.7% for SP and 10% for
DE to ensure an absorbance that is large enough in
the visible region of the spectrum. Residual solvent
was removed from the films by heating for 1 h at
1108C. The films were removed from the oven and
allowed to slowly cool to room temperature. Irradia-
The structure formula of 1,2-dicyano-1,2-bis-
2,4,5-trimethyl-3-thienyl ethene, and 6-nitro-
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X
X
X
X
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1 ,3 ,3 -trimethylspiro 2H-1-benzopyrane-2,2 -indoline
that we studied, referred to in the text as DE and SP,
respectively, and their photochemical isomers are
shown in Fig. 1. The DE and SP chromophores have
two photochemical isomers, a stable isomer and a
thermally unstable isomer, namely the open- and
closed-ring forms for DE, and the spiropyran and
photomerocyanine for SP. The stable and thermally
unstable isomers are henceforth referred to in the
text as the A and B isomers, respectively. Light
irradiation produces photoreaction in both the A™B
and A§B directions, and the thermal reaction pro-
ceeds in the A§B direction. In contrast to the
colored photomerocyanine form which usually fades
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tion was performed by linearly polarized UV 365"
15 nm; glass filter, UV-D33S from Irie and green
546"5 nm; interference filter, VPF-25C-10-50-
54610 from Sigma lights from a high-pressure mer-
.
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.
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cury lamp Nikon to induce the AlB and A§B
photoreactions, respectively. A UV–vis spectrometer
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Model UV-1600 from Shimadzu was used to record
linearly polarized spectra of the film samples. The
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after several minutes at room temperature 5,9 , the
colored closed-ring form of DE is stable for more
polarized spectra were recorded for probe light lin-
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early polarized either parallel Abs5 or perpendicu-
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than 3 months at 808C 6 . The photochromism of
lar AbsH to the polarization of the irradiation light.
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a diarylethene and b spiropyran before 1 and after 2, 3 linearly
Fig. 2. UV–vis absorption spectra of PMMA films containing:
polarized UV irradiation. The probe light was also linearly polarized, and spectra were obtained for both parallel, Abs5 , and perpendicular,
Ž .
AbsH , orientations. The insets in a are expanded views of both the UV and visible absorption of the diarylethene chromophore. Identical
spectra were obtained for both Abs5 and AbsH prior to UV irradiation. For both of the diarylethene and spiropyran chromophores,
inasmuch as the OHB and OR processes occur with AbsH)Abs5 within the irradiation wavelength band, note the inversion of the sign of
the dichroism between the UV and visible bands of the spectra.

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H. Ishitobi et al.rChemical Physics Letters 300 1999 421–428
423
The spectra were recorded after the end of the
irradiation. Dark conditions were employed to avoid
the influence of the room light on the isomerization
reaction.
Photoisomerization of both DE and SP clearly
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occur in films of PMMA. Spectra not shown taken
before, and after several amounts of UV and green
irradiation exhibit shape changes and isosbestic points

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424
H. Ishitobi et al.rChemical Physics Letters 300 1999 421–428
at about 329, 377 and 429 nm for DE and 317, 339
and, less pronounced, 440 nm for SP, clearly demon-
strate the forth A™B and back A§B photoisomer-
ization reactions. The isosbestic points shifted slightly
when film samples are irradiated with green vs. UV
lights. We could notice no spectral changes during
the A§B thermal back reaction of the DE chro-
the UV and visible absorptions of the DE chro-
mophore. These spectra were obtained 30 s after
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polarized UV irradiation irradiation dose: 78
2
.
mJrcm . It is clear that Abs5 and AbsH are differ-
ent, in other words the irradiated samples show
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anisotropic dichroic absorbance upon polarized UV
irradiation. Identical spectra were recorded for Abs5
and AbsH prior to UV irradiation, demonstrating
that the samples were in-plane isotropic at that time.
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mophore over 24 h, e.g. the closed-ring form the B
.
isomer of the DE chromophore is stable at room
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. Ž
temperature for this time period in agreement with
The order parameter S, Abs5 yAbsH r Abs5 q
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.
.
the finding of Irie et al. 6 vide infra . The colored
2AbsH , and the dichroic ratio R, Abs5rAbsH
,
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photomerocyanine isomer the B isomer of the SP
achieved under polarized irradiation are summarized
in Table 1 for all the experiments described in this
Letter. It is particularly remarkable from Fig. 2 that
Abs5 is higher than AbsH in the visible band where
the absorption of the A form is negligibly small and
only the B form exhibits an appreciable absorption.
This result was further confirmed by real-time
dichroism experiments we performed. This clearly
shows that when the DE and SP chromophores un-
dergo the AlB photoreaction under polarized UV
light, the B isomer is formed with its Õisible transi-
tion dipole moment parallel to the UV polarization.
AbsH is higher than Abs5 in the UV band; a feature
which shows that the UV transition dipole moment
of the DE and SP chromophores is perpendicular to
the UV polarization after cycles of the AlB pho-
toisomerization. This shows that the direction of the
UV transition of the mixture of the A and B isomers
and the Õisible transition of the B isomer of the
chromophore persists for several minutes at room
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temperature. A biexponential fading A§B thermal
.
recovery of this form at room temperature was
y1
Ž .
observed with rate constants k of k1 s0.00125 s
and k2 s0.00009 sy1 and weighting factors ampli-
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tudes 0.5157 for k1 and 0.4017 for k2 in reasonable
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agreement with previous results 5,9 . Although the
polymer free volume, the free volume distribution, or
both, could influence the fading rate constants, in the
absence of aggregation, the biexponential behavior
is, however, usually attributed to the dual form of the
photomerocyanine, e.g. the quinonic and zwitterionic
forms. The assessment of these photo- and thermal
isomerization features is necessary for the light-in-
duced orientation studies reported in this Letter.
Fig. 2 shows the dichroic spectra observed in
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.
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films of DErPMMA Fig. 2a and SPrPMMA Fig.
.
2b . The insets in Fig. 2a are expanded views of both
Table 1
Order parameter, S, and dichroic ratio, R, of diarylethene- and spiropyran-containing PMMA films achieved under polarized UV and green
light irradiation
2
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.
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.
Irradiation wavelength nm and dose mJrcm
Order parameter, S
Dichroic ratio, R
Chromophore
Diarylethene
Spiropyran
345
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.
Analysis wavelength nm
360
520
570
365 and 78
y0.03
0.93
q0.07
1.22
y0.07
0.81
q0.10
1.32
365 and 401
y0.04
0.90
q0.05
1.16
q0.07
1.22
546 and 117a, and 72b
q0.01a
y0.06a
y0.04b
1.03a
0.83a
0.89b
S and R are defined in the text.