Photochromic and fluorescence switching properties of oxidized triangle terarylenes in solution and in amorphous solid states

Article abstract of DOI:10.1039/c1jm12993g

Three hexatriene-type photochromic compounds 4,5-bis(2,4-dimethyl-5- phenylthiophene-S,S-dioxide-3-yl)-2-phenylthiazole, 1, 4,5-bis(2-methylbenzo[b] thiophene-S,S-dioxide-3-yl)-2-phenylthiazole, 2, and 1,2-bis(2,4-dimethyl-5- phenylthiophene-S,S-dioxide-3

Full text of DOI:10.1039/c1jm12993g

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PAPER
Photochromic and fluorescence switching properties of oxidized triangle
terarylenes in solution and in amorphous solid states†
*
Maki Taguchi, Tetsuya Nakagawa,‡ Takuya Nakashima and Tsuyoshi Kawai
Received 28th June 2011, Accepted 25th August 2011
DOI: 10.1039/c1jm12993g
Three hexatriene-type photochromic compounds 4,5-bis(2,4-dimethyl-5-phenylthiophene-S,S-dioxide-
3-yl)-2-phenylthiazole, 1, 4,5-bis(2-methylbenzo[b]thiophene-S,S-dioxide-3-yl)-2-phenylthiazole, 2,
and 1,2-bis(2,4-dimethyl-5-phenylthiophene-S,S-dioxide-3-yl)-3,3,4,4,5,5-hexafluorocyclopentene, 3,
are synthesized and their photochromic and fluorescence properties are studied, which are analogous to
photochromic molecules of triangle terarylene and diarylethene with oxidized aryl units containing S,S-
dioxide moieties. These compounds showed photochromic coloration reaction to form a ring-closed
form isomer having a cyclohexadiene backbone. Compound 2 showed backward cycloreversion
reaction with about 2.4% of photochemical quantum yield, whereas compounds 1 and 3 showed
markedly low cycloreversion quantum yields less than 0.1%. Their colored isomers exhibited green or
orange fluorescence with relatively high fluorescence quantum yields. Compound 1 also showed similar
fluorescence switching nature and fluorescence pattern formation in a stable amorphous film.
Intramolecular hydrogen bonding between S,S-dioxide units and methyl groups at photo-reactive
carbon atoms is discussed on the basis of molecular structure determined by X-ray single analysis and
quantum chemical calculation in the DFT method, which is regarded to contribute to the enhanced
emission nature and suppressed cycloreversion photoreactivity of the ring-closed form isomers.
compounds themselves have been studied,10 external fluo-
Introduction
rophores have been combined with them for extensive emission
properties, such as organic p-conjugated molecules,11 metal
complexes12 and nanocrystals.13 Such an external control of the
fluorescence emission with the photochromic units is usually
based on the photochromic modulation of electron or energy
transfer in the excited states. Structural distortion14 and molec-
ular aggregation15 upon the photochromic reactions have also
been applied for external control of fluorescence emission
properties. Recently, Prof. Kim and Prof. Ahn and their
coworkers have proposed oxidized diarylethene derivatives
having sulfone SO2 units. The tetra-oxidation of benzothienyl
units led to the enhancement of the fluorescent property in the
ring-closed form16 in comparison with that of non-oxidized
dibenzothienylethene whose fluorescence is detectable only at
low temperature (e.g. 77 K).10c Meanwhile, recent progress in
highly sensitive photochromic terarylenes highlights the photon-
quantitative photocyclization reaction, which has motivated us
to explore oxidized terarylenes with SO2 unit.17
Photochromic molecules have recently been extensively studied
as versatile photoswitching components for controlling various
chemical,1 electrical,2 electrochemical,3 biofunctional,4 mechan-
ical,5 and optical properties of molecular-based materials.6
Among various photochromic molecules, those based on hexa-
triene structures such as fluguides, diarylethenes and terarylenes
have been attracting considerable interest since some of them
exhibit kinetic persistency in the thermodynamically unstable
cyclohexadiene state and can be potential candidates for an
active material in future molecular memory systems.7–9 Espe-
cially, photo-responsive modulation of fluorescence emission
properties has been most extensively studied for developing
active materials for future 3-D ultrahigh density optical
recording and for ultra high resolution bio-imaging technology.
While intrinsic emission properties of these photochromic
Graduate School of Materials Science, Nara Institute of Science and
Technology, NAIST, 8916-5 Takayama, Ikoma, Nara, 630-0192, Japan.
E-mail: tkawai@ms.naist.jp
In the present paper, we report on the preparation and
photochromic properties of S,S-dioxide derivatives of triangle
terarylenes whose molecular structures are shown in Scheme 1.
Two precursor terarylenes are oxidized to form tetra-oxidized
terarylenes 1a and 2a and their photochromic and fluorescent
properties are studied in solution and also in amorphous states.
They exhibited relatively high fluorescence emission quantum
† Electronic supplementary information (ESI) available: Details of the
evaluation of quantum yields of photochemical reaction and additional
figures. See DOI: 10.1039/c1jm12993g
‡ Present address: Center for Organic Photonics and Electronics
Research, OPERA, Kyushu University, 744 Moto-oka, Nishi-ku,
Fukuoka, 819-0395, Japan.
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optical microscope (Olympus, BX51) was used for visible
observation of the amorphous films. Photoirradiation
was carried out with an ultra high-pressure Hg lamp (Ushio,
BA-H501, 1 kW) and a Xe short-arc lamp (Ushio, BA-X500,
500 W) as the exciting light sources. Light with appropriate
wavelength was obtained through optical filters and/or a mono-
chromator (Shimadzu, SPG-120S, 120 mm, f ¼ 3.5). Quantum
yields of ring-cyclization reactions (Foc) in solution were deter-
mined by using 1,2-bis(2-methylbenzo[b]thiophene-3-yl)perhex-
afluorocyclopentene (in hexane) as a reference whose Foc is 0.35
under UV light irradiation (l ¼ 313 nm) in hexane.18 Quantum
yields of ring-cycloreversion reactions (Fco) were determined by
using
1,2-bis(thiophen-2-yl)perhexafluorocyclopentene
(in
3-methylpentane) as a reference compound, whose Fco has been
reported to be 0.37 at 432 nm in 3-methylpentane.19
Syntheses
Precursors P1 and P2 were prepared according to the procedures
described in the previous literature.20,21 Precursors P3 and P4
were purchased from TCI. Precursors P1, P2 and P3 were treated
with 70% 3-chloroperbenzoic acid (m-CPBA, TCI, containing
30 wt% water) to afford compounds 1a, 2a and 3a, respectively,
as has been reported for compound 4a.16a
Scheme 1 Oxidation of photochromic precursors of P1–P4 to form 1a–
4a, respectively, and the photochromic reaction of 1a–4a.
yields in the ring-closed forms and significantly suppressed
photo-cycloreversion reaction quantum yields compared to those
of precursor terarylenes P1 and P2, which provide turn-on type
fluorescence switching nature of photochromic molecules
without external fluorophores. Oxidized diarylethene 3 is also
synthesized as a reference compound and its properties were
compared with a previous molecule 4.16a
4,5-Bis(2,4-dimethyl-5-phenylthiophene-S,S-dioxide-3-yl)-2-
phenylthiazole (1a). 4,5-Bis(2,4-dimethyl-5-phenylthiophen-3-
yl)-2-phenylthiazole (P1)20 (0.50 g, 0.94 mmol) and m-CPBA (2.3
g, 9.4 mmol) were dissolved in CH2Cl2 (100 mL). In the dark
room, the mixture was stirred at room temperature for 24 h, and
then washed with aqueous solution of NaHSO4 followed by the
extraction with CH2Cl2. The organic layer was dried with
MgSO4, filtered, and concentrated. The residue was purified by
silica gel column chromatography (hexane/ethyl acetate 3 : 1)
and recrystallized with toluene to afford 1a (0.19 g, yield 33%) as
Experimental
General
1
a colorless solid. Td > 140 ꢁC. H NMR (300 MHz, CDCl3/
1H NMR (300 MHz) and 13C NMR (75 MHz) spectra were
recorded on a JEOL JNM-AL-300 spectrometer. Mass spectra
were measured with mass spectrometers (for: JEOL JMS-700,
JEOL AccuTOF JMS-T100LC and PE biosystems Voyage DE-
STR, for FAB, ESI and MALDI-TOF MS, respectively).
Elemental analyses were performed with an elemental analyzer
(Perkin Elmer, 2400II CHNS/O). ATR-IR spectra were recorded
on a JASCO FT/IR-4200 spectrometer with ATR PRO410-S.
Separative HPLC was performed on a HPLC system (HITA-
CHI, LaChrom ELITE, L-2400 for detector, L-2130 for pump,
D-2500 for chromate-integrator) with a packed column (Nacalai
Tesque, COSMOSIL 55L-II, 20 ꢀ 250 mm). Absorption spectra
were studied with a spectrophotometer (JASCO V-670). Fluo-
rescence spectra were studied with a spectrofluorometer (JASCO
FP-6500). Fluorescence quantum yields were evaluated by using
an absolute PL quantum yield measurement system with an
integration sphere (Hamamatsu, C9920-02) for solutions and
a spectrofluorometer (JASCO FP-6300ST-Y ILF-533) for films.
Crystal structures were determined by using an X-ray crystal
structural analyzer (Rigaku VariMAX-RAPID). DSC
measurements were carried out using a differential scanning
calorimeter (Seiko, EXSTAR6000 DSC6200) with a liquid
nitrogen cooling system. The heating rate was 5 ꢁC minꢂ1. An
TMS): d (ppm) ¼ 7.99–8.02 (2H, d), 7.60–7.67 (4H, m), 7.47–7.55
(9H, m), 2.19–2.22 (6H, d), 1.98–2.04 (6H, d); 13C NMR
(75 MHz, CDCl3/TMS): d (ppm) ¼ 137.11, 136.34, 132.15,
131.56, 130.00, 129.76, 129.38, 129.24, 129.21, 129.14, 127.13,
126.76, 126.70, 126.45, 14.52, 9.10, 8.76; ATR-IR: peak wave-
number (cmꢂ1) ¼ 1292 (nas SO2), 1135 (ns SO2); FAB HRMS [M
+ H]+ (m/z), calcd for C33H28NO4S3+: 598.11, found: 598.11;
elemental analysis (%) calcd for C33H27NO4S3: C 66.31, H 4.55,
N 2.34; found: C 66.06, H 4.32, N 2.36.
4,5-Bis(2-methylbenzo[b]thiophene-S,S-dioxide-3-yl)-2-phenyl-
thiazole (2a). This compound was prepared by the same proce-
dure as had been used for 1a. From 4,5-bis(2-methylbenzo[b]
thiophen-3-yl)-2-phenylthiazole (P2)21 (0.10 g, 0.22 mmol) and
m-CPBA (0.33 g, 1.3 mmol), 2a (0.087 g, yield 79%) was obtained
ꢁ
1
as a colorless solid. Td > 160 C. H NMR (300 MHz, CDCl3/
TMS): d (ppm) ¼ 8.04–8.07 (2H, d), 7.77–7.83 (2H, t), 7.49–7.57
(8H, m), 7.37 (1H, s), 1.97–2.05 (6H, d); 13C NMR (75 MHz,
CDCl3/TMS): d (ppm) ¼ 170.78, 146.22, 139.88, 135.85, 135.65,
133.77, 133.63, 132.15, 131.72, 131.68, 130.24, 129.68, 129.42,
127.89, 126.84, 125.66, 123.98, 122.58, 122.13, 121.67, 8.59, 8.50;
ATR-IR: peak wavenumber (cmꢂ1) ¼ 1298–1163 (nas SO2), 1111
+
:
(ns SO2); FAB HRMS (m/z) [M + H]+ calcd for C27H20NO4S3
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518.05, found: 518.05; elemental analysis (%) calcd for
C27H19NO4S3: C 62.65, H 3.70, N 2.71; found: C 62.35, H 3.71,
N 2.66.
and 3b were confirmed by 1H NMR spectroscopy as well as that
of 4b (in CDCl3).22
A single crystal of colored form 1b was obtained by recrys-
tallization from a hexane/ethyl acetate solution. The crystal
structure of 1b was studied by X-ray crystal structural analysis.23
An ORTEP drawing of 1b in the crystal indicated a coplanar
cyclohexadiene-based structure as shown in Fig. 2. The twisting
angle between phenyl and thiazole rings is about 32ꢁ (S1–C1–C4–
C9) and the S and N atoms of the central thiazole group are close
to the hydrogen atoms in the central phenyl groups with shorter
distance than 0.27 nm, which is shorter than the sum of van der
Waals’ radii of each atom. The weak CH/N and CH/S hydrogen
bonding interactions seem to tether the central phenyl ring with
the thiazole ring, making the molecule have a coplanar structure.
Also these S and N atoms are regarded to interact with hydrogen
atoms in the two methyl groups at the 4th position of dioxidized
thiophenes with distance 0.271 nm for CH–S and 0.259 nm for
CH–N, also shorter than the sum of van der Waals’ radii of each
atom. The methyl groups at the reaction center carbon atoms are
out of the molecular plane and are close to the SO2 units.
Distances between the hydrogen and the oxygen are also shorter
than the sum of van der Waals’ radii of each atom. It suggests
weak CH/OS hydrogen bonding interactions between these
methyl groups and the oxygen atoms.
1,2-Bis(2,4-dimethyl-5-phenylthiophene-S,S-dioxide-3-yl)-
3,3,4,4,5,5-hexafluorocyclopentene (3a). This compound was
prepared by the same procedure as had been used for 1a.
From 1,2-bis(2,4-dimethyl-5-phenylthiophen-3-yl)-3,3,4,4,5,5-
hexafluorocyclopentene (P3, 0.10 g, 0.18 mmol) and m-CPBA
(0.45 g, 1.8 mmol), 3a (0.069 g, yield 62%) was obtained as
1
a colorless solid. Td > 100 ꢁC. H NMR (300 MHz, CDCl3/
TMS): d (ppm) ¼ 7.53–7.56 (4H, m), 7.46–7.49 (6H, m), 2.17–
2.26 (6H, d), 1.92–1.99 (6H, d); 13C NMR (75 MHz CDCl3/
TMS): d (ppm) ¼ 143.55, 142.32, 137.06, 136.66, 130.37, 129.11,
129.10, 126.09, 125.97, 124.62, 124.53, 14.30, 13.61, 9.96, 9.90,
9.57. ATR-IR peak wavenumber (cmꢂ1) ¼ 1305–1271 (nas SO2),
1136–1112 (ns SO2); FAB HRMS (m/z) [M + H]+ calcd for
C29H23F6O4S2+: 613.09, found: 613.09; elemental analysis (%)
calcd for C29H22F6O4S2: C 56.86, H 3.62; found: C 57.03, H 3.61.
Results and discussion
Colorless solutions of the open-ring isomers 1a, 2a and 3a in
2-methyltetrahydrofuran (2-MeTHF) were observed to turn
yellow or orange upon irradiation with UV light (l ¼ 313 nm).
As shown in Fig. 1, 1a, 2a and 3a showed no absorption band in
the visible wavelength range, while new absorption bands were
observed at 400–550 nm after irradiation with UV light. Char-
acteristic isosbestic points at 298 nm, 279 nm and 355 nm were
found for compounds 1, 2 and 3, respectively. These spectral
properties are similar to those of compound 4 and supported the
two-component photochromic reaction as illustrated in Scheme
1. The ring-closed form isomers 1b, 2b and 3b were prepared by
UV light irradiation to the solutions of 1a, 2a and 3a, and were
isolated from the colored solutions by HPLC (hexane/ethyl
acetate). Chemical structures of the ring-closed isomers 1b, 2b
Optical and photochromic properties of compounds 1, 2 and 3
are summarized in Table 1 with those of compound 4. The
absorption spectra of ring-closed forms 1b, 2b and 3b showed
Fig. 2 ORTEP drawings of 1b in crystal, showing 50% probability
displacement ellipsoids; (a) the front view and (b) the edge view.
Distances between the hydrogen of methyl groups at the reactive carbon
centers and the oxygen of SO2 units are 0.239 nm (for H6/O3) and 0.231
nm (for H17/O2), which are shorter than the sum of van der Waals’
radius of hydrogen and oxygen atoms (0.26 nm).
Fig. 1 Absorption spectral changes of compounds, (a) 1, (b) 2, (c) 3 and
(d) 4 in 2-MeTHF; open-forms (dashed lines), closed-forms (bold solid
lines) and P.S.S. under irradiation with 313 nm (dotted lines).
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Table 1 Absorption and photochromic properties of compounds 1–4 in
2-MeTHF
of fluorescence bands in these solutions. The colored solution of
2b exhibited intense green emission with fluorescence band at
543 nm. This emission band decreased upon irradiation with
visible light and again increased upon irradiation with UV light,
which indicates the reversible fluorescence switching capability.
The colored solutions of 1b and 3b showed orange and yellow
emission, respectively, whose emission maxima are 577 nm and
556 nm, respectively. Excitation spectra monitored at the emis-
sion maxima exhibited typical excitation bands corresponding to
the absorption spectra of the ring-closed form isomers. UV light
irradiation of the colorless solutions of 1a and 3a resulted in
progression of the fluorescence emission band with almost irre-
versible nature. Fig. 5 shows a typical example of stable fluo-
rescence emission intensity of compound 1b under continuous
irradiation with visible light (lex ¼ 432 nm), where faster decay of
emission from 2b is also provided for comparison. Fluorescence
quantum yields, Fem, of ring-closed isomers were evaluated to be
0.35, 0.37 and 0.03 for 1b, 2b and 3b, respectively. Fem of 4b was
roughly in agreement with the previously reported values in ethyl
acetate. Since the precursor diarylethene P4b gives weak emis-
sion at 77 K but not at room temperature,10c the Fem of 4b
indicates the marked enhancement of emission nature by
oxidation of S to SO2. Oxidized terarylenes 1b and 2b indicated
about 10 times larger fluorescence emission quantum yields than
those of oxidized diarylethenes 3b and 4b. 2b displayed a Fem of
0.45 in toluene, as summarized in Table S5†. Fluorescence decay
was studied for evaluating the fluorescence decay time constant
for these compounds, of which results are provided in Fig. S4†
and summarized also in Table 2. All compounds showed well-
defined single exponential decay and compounds 1b and 2b
showed notably longer lifetime than those of 3b and 4b. The
larger fluorescence quantum yields of 1b and 2b seem to originate
from the smaller non-radiative decay rate constant, knr, in
comparison with those of 3b and 4b. Although the origin of these
tendencies is not clear at present, we believe that the emission
efficiency might be predominantly controlled with the central
part of the molecules in this class of ring-fused p-conjugated
compounds. One may consider about suppressed vibrational
decay in the excited states of compounds 1b and 2b than those of
oxidized diarylethenes. Diarylethenes with the per-
fluorocyclopentene units may have the specific flexibility with the
flip motion on the central perfluorocyclopentene ring, which is
usually observed in the X-ray analyses of diarylethenes as the
l/nm
(3 [104 Mꢂ1 cmꢂ1])
a
apss
b
Foc
c
Fco
1a
1b
2a
2b
307 (2.28)
>0.99
—
0.23
(ꢃ0.01)
—
d
324 (4.20)
490 (1.71)
301 (1.83)
—
3 ꢀ 10ꢂ4
(ꢃ0.1 ꢀ 10ꢂ4
)
ꢄ0.80
—
0.46
(ꢃ0.01)
—
319 (2.80)
460 (2.03)
487 (1.96)
288 (0.687)
—
0.024
(ꢃ0.002)
3a
3b
>0.99
—
0.31
(ꢃ0.01)
—
d
423 (1.62)
—
4 ꢀ 10ꢂ5
(ꢃ0.3 ꢀ 10ꢂ5
—
0.036
)
4a
4b
309 (0.424)
400 (2.14)
ꢄ0.80
0.22e
—
—
(ꢃ0.002) 0.061e
a
apss: the cyclization conversion ratio at P.S.S. with the irradiation at 313
b
c
d
e
nm light. At 313 nm. At 432 nm. The samples were degassed. In
ethyl acetate.16d
a significant blue-shifted absorption band from those of
precursor molecules P1, P2 and P3 whose data are presented in
the ESI† (Table S1). This tendency of blue shift has been
reported for compound 4 and other oxidized diarylethenes,
which were reproduced here with TD-DFT calculations for
present molecules (see ESI†).16 The photochemical conversion
ratios from 1a to 1b and from 3a to 3b at the photostationary
states (P.S.S.) under irradiation at l ¼ 313 nm were estimated to
be almost 100% by using HPLC, which are mainly due to their
very low cycloreversion quantum yields.
Foc of 1a, 2a and 3a were 0.23, 0.46 and 0.31, respectively,
which are rather smaller than those of the precursor molecules,
0.58, 0.6 and 0.46 for P1, P2 and P3, respectively. This tendency
has already been suggested in the previous compound 4 which
shows Foc ¼ 0.22 and was less reactive than the precursor dia-
rylethene P4 (Foc ¼ 0.35).16d Upon irradiation with visible light,
compounds 2b and 4b were completely converted to 2a and 4a,
respectively. Fco of 2b and 4b were 0.024 and 0.036, respectively,
which are markedly smaller than those of the precursor mole-
cules P2 (Fco ¼ 0.45 (ref. 21)) and P4 (Fco ¼ 0.35 (ref. 18)). Fco
value of 4b in 2-MeTHF is similar to that in ethyl acetate solution
(Fco ¼ 0.061),16d which indicates no marked solvent-effect on the
photochemical cycloreversion reactivity. On the other hand, the
solutions of compounds 1b and 3b hardly showed photochromic
bleaching reaction even after irradiation with visible light for
several hours. Markedly low Fco values of 1b and 3b were
roughly estimated to be 3 ꢀ 10ꢂ4 and 4 ꢀ 10ꢂ5, respectively. For
evaluating such low Fco values, careful degassing of the sample
solution was desired. The extremely low Fco values make these
structural
disordering.24
Such
flexibility
of
the
compounds
1 and 3 effectively one-way photochromic
compounds. The extremely low photobleaching reactivity of
compound 1b is typically compared with that of 2b in Fig. 3.
Fluorescence spectra of compounds 1, 2, 3 and 4 in 2-MeTHF
are shown in Fig. 4 under excitation with visible light of appro-
priate wavelength for selective excitation of the ring-closed form
isomers. Continuous UV light irradiation resulted in the progress
Fig. 3 Absorbance changes at lmax of 1b (square line) and 2b (rhombus
line) in 2-MeTHF by continuous irradiation with visible light (l ¼ 432
nm; 500 W, through a monochromator).
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and 2a showed a very weak emission band at 452 nm (Fem
<
0.001) and 418 nm (Fem < 0.003), respectively. However, the
fluorescence emission of the open-ring form isomers was hard to
characterise precisely because of photochemical conversion
reaction during the fluorescence measurements. The fluorescence
intensities of the oxidized terarylenes, 1a and 2a, were lower than
those of the oxidized diarylethenes, 3a and 4a, which exhibited
a weak emission band at 358 nm (Fem ¼ 0.036) and 451 nm
(Fem ¼ 0.026), respectively. Compounds 1 and 2 can thus act as
fluorescence turn-ON molecules with quasi-irreversible and
reversible nature, which are worth to be noted as candidates for
future applications such as ultra-high resolution bio-imaging
with a PALM system, photo-patterned light emitting devices and
high density optical memory.
We here briefly discuss the conformation and intramolecular
hydrogen bonding in these molecules on the basis of quantum
chemical calculations. Fig. 6 shows the most stable structures of
1a and 1b optimized with DFT calculation at the B3LYP/6-31G
(d) level. The calculation for 1b well reproduced the structure
solved with the X-ray analysis presented in Fig. 2. For example,
the twisting angle between phenyl and thiazole rings in the
central part is ca. 8.7ꢁ in Fig. 6 and ca. 32ꢁ in Fig. 2. CH/OS
distances of 1b in Fig. 2 were 0.231 nm and 0.239 nm, and those
in Fig. 6 were 0.224 nm. The distance between the hydrogen and
oxygen was shorter than the sum of van der Waals’ radii of each
atom (0.26 nm) and indicates hydrogen bonding interaction
between the methyl groups and the SO2 units of side aryl groups
in the opposite side of the ring-closed form isomer. These CH/OS
hydrogen bonds may make the molecule rather rigid, which
might partly contribute to the improved emission nature of
oxidized ring-closed forms. It should also be noted that the
CH/OS distance was evaluated to be as long as 0.306 nm and
0.311 nm in the optimized structure of 1a (Fig. 6a) even though it
is in the quasi-C2 symmetric reactive conformation. That is, the
CH/OS hydrogen bonding seems to be invalid in the ring-open
form. These results suggest that the CH/OS hydrogen bonding
should be broken upon the photo-cycloreversion reaction and
excess heat is required for the photochemical reaction from 1b to
1a. As summarized in Table 3, a similar tendency of CH/OS
distance was also observed in compounds 2, 3 and 4. For all these
compounds, hence, the CH/OS hydrogen bonding can be
a possible origin of their suppressed cycloreversion reactivity,
although further theoretical and experimental studies are desired
for clear understanding. No such specific intramolecular inter-
action can be seen in corresponding precursor diarylethenes and
terarylenes.
Fig. 4 Fluorescence spectral changes of compounds (a) 1 (2.7 ꢀ 10ꢂ5
M), (b) 2 (2.7 ꢀ 10ꢂ5 M), (c) 3 (7.8 ꢀ 10ꢂ5 M) and (d) 4 (4.5 ꢀ 10ꢂ6 M) in
2-MeTHF upon UV light irradiation (313 nm); open-forms (dashed
lines), closed-forms (bold solid lines), and P.S.S. under irradiation at 313
nm (dotted lines). Excitation wavelengths were 490 nm, 460 nm, 423 nm
and 400 nm, for 1, 2, 3 and 4, respectively.
Fig. 5 Relative fluorescence intensity (I/I0) at 577 nm and 543 nm of 1b
(square line) and 2b (rhombus line) in 2-MeTHF, respectively, as
a function of irradiation time continuous irradiation with visible light
(l ¼ 432 nm; 0.33 mW cmꢂ2).
Table 2 Fluorescence properties of compounds 1b–4b in 2-MeTHF
a
lex/nm
lem/nm
Fem
semb/ns
kr/107 sꢂ1
knr/108 sꢂ1
1b
2b
3b
4b
490
460
423
400
577
543
556
478
0.35
0.37
0.03
0.03
4.5
4.3
0.37
0.07
7.8
8.6
8.1
4.3
1.4
1.5
26
140
a
Fluorescent quantum yields at room temperature; the sample
concentration of 3.5 ꢀ 10ꢂ6, 8.6 ꢀ 10ꢂ6, 3.4 ꢀ 10ꢂ6, and 2.1 ꢀ 10ꢂ6
M
for 1, 2, 3 and 4, respectively. Fluorescence decay profiles are
presented in Fig. S4†.
b
perfluorocyclopentene units in 3b and 4b is possible to contribute
to their knr values and suppressed Fem. Compounds 1b and 2b
have no such flexibility and even suppressed rotational motion
between phenyl and thiazole rings in the bridging unit tethered
with the CH/S and SH/N interactions might make the molecules
suitable for fluorescence emission. Considerably weaker fluo-
rescence was observed in solutions of their open form isomers. 1a
Fig. 6 Optimized molecular structures of 1a (a) and 1b (b) calculated by
Gaussian 03 at the B3LYP/6-31G(d) level. CH/OS interactions are
indicated by dotted lines.
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Table 3 CH/OS distance evaluated by DFT calculation
Table 4 Absorption and emission property of compound 1 amorphous
film
Compound
CH/OS distance/nm
a
b
lmax/nm
lex/nm
lem/nm
apss
Fem
1a
1b
2a
2b
3a
3b
4a
4b
0.306
0.224
0.315
0.222
0.390
0.226
0.506
0.223
0.311
0.224
0.310
0.222
0.382
0.225
0.504
0.224
1a
1b
307
331, 500
n.d.c
500
n.d.c
600
0.74
—
—
0.15
a
apss: the cyclization conversion ratio at 313 nm light irradiation was
estimated by comparing absorbance at P.S.S. with that of 1b.
Fluorescent quantum yields of the amorphous state (r.t.).
b
c
Fluorescence was unobserved at room temperature.
Fig. 8 shows absorption and fluorescence spectral changes of
a bulk amorphous film of compound 1, whose optical properties
are summarized in Table 4. The films were prepared by the spin-
coating method. Chloroform solutions of original and colored
forms of 1 (2.0 mg mLꢂ1) were cast onto quartz substrates (20 mm
ꢀ 20 mm, thickness ¼ 1 mm). These substrates were spun at
200 rpm for 10 s, then 1600 rpm for 30 s. The absorption spec-
trum of the original state of the 1a film showed no absorption
band in the visible range and also no diffusion tail of light-
scattering, indicating high transparency of the film. The trans-
parent film was stable at least for 3 month. The thicknesses of this
film was estimated to be about 70 nm, from the peak absorbance
at 307 nm by assuming that its extinction coefficient and volume
density are similar to those in solution phase and in crystals,
respectively.23 The 1a film turned to pale orange in color after UV
light irradiation, which shows progression of an absorption band
at 500 nm. A clear isosbestic point was observed at about 307 nm
supporting the two-component photochromic reaction also in
the amorphous state. This spectral change was effectively irre-
versible and no spectral change can be observed under visible
light irradiation. The maximum conversion ratio from 1a to 1b
was about 74%, which was evaluated by comparing absorbance
at 500 nm with that of a film prepared from pure 1b after
normalized at the isosbestic point. Since 1b showed almost no
photobleaching reaction, this maximum conversion must be
100% if all molecules would be uniformly photoreactive. It thus
suggests that about 26% of the molecules in the amorphous film
are not reactive for the photocyclization reaction. Such distri-
bution of reactivity in the amorphous film would be attributed to
fixed molecular conformations with different photo-reactivities.
Since the central hexatriene structure must be in the C2
symmetric conformation for the photocyclization reaction, some
of the molecules in non-C2 symmetric conformation might be
responsible for the decreased P.S.S. conversion ratio. Intermo-
lecular interaction such as excited state energy transfer and
Fig. 7 DSC curve of compound 1a.
The present compound 1 was observed to form a transparent
uniform film after the removal of solvent from the solution
similar to those of amorphous photochromic molecules.25 We
herein studied their thermal properties with DSC for evaluating
the glass transition temperature, Tg, which provides principal
information about the stability of the amorphous phase. In the
DSC profile of 1a presented in Fig. 7, a clear shift in the heat
capacity was observed at 80–100 ꢁC which is a characteristic
ꢁ
feature of glass transition. Tg of 1a was evaluated to be 85 C,
from the onset temperature of baseline shift. Below Tg, the
molecular motion and crystal growth seem to be frozen making
the amorphous state stable. Since the Tg is markedly higher than
the room temperature, compound 1a is expected to form a bulk
amorphous film which is stable at room temperature. Purified 1b
also formed a stable amorphous film with a Tg of 55 ꢁC, of which
DSC profile is presented in Fig. S5†. These results suggest that
terarylene derivatives having three aryl groups as backbone are
suitable for stable amorphous materials.21
Fig. 8 (a) Absorption and (b) fluorescence spectral changes of the
amorphous film of compound 1 upon UV irradiation (l ¼ 313 nm); open-
forms (dashed lines), closed-forms (bold solid lines), and P.S.S. under
irradiation at 313 nm (dotted lines).
Fig. 9 Photographs of the (a) visible and (b) emission pattern formed on
the amorphous film of 1 (image size: 2.2 mm ꢀ 3.4 mm).
17430 | J. Mater. Chem., 2011, 21, 17425–17432
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electron transfer might also be possible origins of the suppressed
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The emission band at 600 nm under excitation at 500 nm
continuously increased with the progressive conversion of 1a to
1b upon UV irradiation in the film (Fig. 8b). The emission
intensity at P.S.S. was completely maintained even after
continuous irradiation at 500 nm for 4 h. The colored film
showed an emission quantum yield of about 15% as also
summarized in Table 4.
Fig. 9 shows a typical example of optical patterns formed on
the amorphous film of 1a by irradiating with UV light (l ¼ 330–
385 nm) through a patterned photomask (Edmund Optics,
quartz test-target 1951 USAF). The irradiated area turned pale
orange in apparent color and emissive in orange. Compound 1
can afford a high contrast optical pattern on the amorphous film
of about 70 nm and its emission intensity is stable for continuous
excitation with visible light, which made it a candidate for future
write-once optical recording medium.
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We reported here three novel photochromic compounds 1, 2 and
3 with relatively high fluorescence efficiencies, Fem ¼ 0.34, 0.36,
and 0.036, respectively, in their colored isomers. The extremely
low photo-cycloreversion reactivity (Fco < 10ꢂ4) was found in the
compounds 1 and 3 having phenylthiophene-S,S-dioxide-3-yl
groups, whose photoisomerization reactions are almost
irreversible.
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The oxidized terarylene 1 forms a stable amorphous state with
relatively high glass transition temperature. Optical formation of
high contrast fluorescent patterning on the bulk amorphous film
was demonstrated, which has substantial stability for continuous
excitation with visible light. Present results demonstrate a char-
acteristic feature of oxidized terarylenes suitable for forming
amorphous materials with clear fluorescence turn-on switching
capability.
After submission of the present manuscript, similar
compounds were independently reported by Prof. Irie and
coworkers, which are based on oxidized diarylethenes having
ethyl groups at the reaction center carbon atoms and also
showing relatively large fluorescence quantum yield in the closed
form.26
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Acknowledgements
The authors thank Ms Nishikawa, Ms Nishiyama, Mr Asanoma
and Mr Katao in NAIST for their contribution to measurements
of HRMS spectra, X-ray crystal structural analyses and
elemental analyses. This work has been partly supported by
NAIST Advanced Research Network Project and also by The
Green Photonics Project on NAIST.
€
14 J. Karnbratt, M. Hammarson, S. Li, H. L. Anderson, B. Albinsson
ꢀ
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b ¼ 93.007(2) , g ¼ 90.0000 , V ¼ 2807.0(5) A , T ¼ 123.1 K, space
group P21/c (#14), Z ¼ 4, m(MoKa) ¼ 0.305 mmꢂ1, 22 307
reflections measured, 5129 independent reflections (Rint ¼ 0.068).
The final R1 values were 0.1002 (I > 2s(I)). The final wR(F2) values
were 0.2638 (all data). The goodness of fit on F2 was 1.072. CCDC
829809, Dcalc
¼
1.414 g . Crystallographic data for 3b:
cmꢂ3
ꢁ
C29H22F6O4S2, M ¼ 612.60, monoclinic, a ¼ 7.9909(3) A, b ¼
ꢁ
ꢁ
ꢁ
ꢁ
26.7793(10) A, c ¼ 13.2356(6) A, a ¼ 90.0000 , b ¼ 110.4300(12) ,
ꢁ
ꢁ
3
g ¼ 90.0000 , V ¼ 2654.17(18) A , T ¼ 123.1 K, space group P21/c
(#14), Z ¼ 4, m(MoKa) ¼ 0.279 mmꢂ1, 21 554 reflections measured,
4861 independent reflections (Rint ¼ 0.053). The final R1 values were
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The goodness of fit on F2 was 1.052. CCDC 829811, Dcalc ¼ 1.533 g
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1.93 (6H, d). For 2b: d (ppm) ¼ 8.93–8.95 (1H, d), 8.13–8.15 (2H, d),
7.93–8.00 (2H, qual), 7.78–7.84 (3H, m), 7.58–7.69 (5H, m), 1.912
(6H, d). For 3b: d (ppm) ¼ 7.61–7.65 (4H, m), 7.53–7.55 (6H, m),
2.35 (6H, s), 1.85 (6H, s).
cmꢂ3
.
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ꢁ
ꢁ
ꢁ
ꢁ
a ¼ 8.0132(8) A, b ¼ 17.357(2) A, c ¼ 20.210(2) A, a ¼ 90.0000 ,
17432 | J. Mater. Chem., 2011, 21, 17425–17432
This journal is ª The Royal Society of Chemistry 2011