Synthesis and photophysical properties of a photoelectrochromic polymer containing the bis(2-(4-Pyridiniumyl)thiazole) chromophore

Article abstract of DOI:10.1071/CH09099

A new polymer and small molecules containing the chromophore bis(2-(4-pyridiniumyl)thiazole) were synthesized. Their tetraphenylborate salts showed absorption spectral changes in the visible to near-infrared region accompanying a colour change from yellow

Full text of DOI:10.1071/CH09099

RESEARCH FRONT
Full Paper
CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2009, 62, 871–876
Synthesis and Photophysical Properties of a
Photoelectrochromic Polymer Containing the
bis(2-(4-Pyridiniumyl)thiazole) Chromophore
Toshihiko NagamuraA,B and Yasuhiro SotaA
ADepartment of Applied Chemistry, Faculty of Engineering, Kyushu University,
744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan.
BCorresponding author. Email: nagamura@cstf.kyushu-u.ac.jp
A new polymer and small molecules containing the chromophore bis(2-(4-pyridiniumyl)thiazole) were synthesized. Their
tetraphenylborate salts showed absorption spectral changes in the visible to near-infrared region accompanying a colour
change from yellow to green on steady photoirradiation. From electron spin resonance measurements and comparison with
Molecular Orbital PACkage (MOPAC) calculations, this was assigned to the formation of bis(2-(4-pyridiniumyl)thiazole)
radicals due to a photoinduced electron-transfer reaction from tetraphenylborate and decomposition of its oxidized form.
Transient absorption spectra corresponding to those of tetraphenylborate salts were observed for bromide salts on femto-
second laser excitation of the polymer in solid films and solutions. The fastest decay of transient absorption due to back
electron transfer was less than 1 picosecond.
Manuscript received: 17 January 2009.
Final version: 14 April 2009.
Introduction
trains in optical telecommunications with wavelength domain
multiplexing (WDM) need to be spatially converted to more
than 25 parallel signals. Ultrafast switching materials that work
in less than 25 picoseconds (ps) are thus essential for THz
communication or all-optical routing.
Novel materials, devices, and systems are required for much
faster data processing, much higher density of recording, or more
specific and efficient chemical sensing. Organic molecules have
many useful optical and electronic functions that can be easily
controlled by structure, substituent, or external fields. Molec-
ular interactions and organized molecular assemblies also can
afford much higher functions than isolated or randomly dis-
tributed molecules. Photons have many useful properties such as
wavelength, polarization, phase, ultrashort pulse, or parallel pro-
cessibility. Through strong interactions of molecules, polymers,
or molecular assemblies with photons, many exceptional prop-
erties of photons can be directly converted to changes in physical
properties of materials, such as fluorescence, absorption, refrac-
tive index, conductivity, or optical non-linearity. Excited-state
formation, photochromism, and photoinduced electron transfer
are some examples of these. Photon-mode recording or switch-
ing based on these changes can therefore achieve ultrafast mul-
tiple or three-dimensional recording and parallel processibility
with ultimate resolution at a molecular level. Molecular photon-
ics based on interactions of molecules and photons has many
advantages compared with electric or photoelectric switching,
heat-mode or magnetic recording.
Many types of ultrafast optical processing devices and semi-
conductors or organic materials have been studied so far.[1–18]
Single-shot demultiplexing of 1-THz light pulses was proposed
on the basis of transient bleaching of organic dye J-aggregate
films.[3–6] Single-walled carbon nanotubes and expanded por-
phyrin derivatives are also expected to be of use for ultrafast
optical switching in the NIR region.[7–14] All of them employ
transient bleaching of NIR absorption at the ground state. We
have been developing ion-pair charge-transfer complexes show-
ing transient absorption in the visible to NIR region due to
photoinduced electron transfer and reverse reactions between
redox-active ion-pairs.[15–19] Recently, we reported the synthe-
sis and photoresponses of a fluorene-containing redox polymer,
which showed new absorption up to 2000 nm by photoinduced
electron transfer from the tetraphenylborate anion in addition
to extremely fast photoinduced forward and thermal back elec-
tron transfer between iodide anion and fluorene-containing
redox polymer in the NIR region.[18,19] We also proposed all-
optical spatial light modulation and parallel switching based on
photoinduced complex refractive index changes in guided mode
geometry.[20–32] Both transient bleaching and absorption-type
materials can be used in this system.
Rapidly expanding volumes of information continuously
require the speed-up of switching and processing in optical
telecommunication systems. As silica-based optical fibres that
are employed worldwide in such systems show minimum trans-
mission loss at ∼1260–1675 nm, photoresponsive materials
need to show absorption or refractive index changes in this near-
infrared (NIR) region. As 40 GHz is the maximum speed of
electronic signal processing at present, terahertz (THz) pulse
In the present paper, the synthesis and photophysical prop-
erties of a new polymer and small molecules containing bis(2-
(4-pyridiniumyl)thiazole) are reported to achieve photoinduced
electrochromism in broad wavelength and time domains. We
© CSIRO 2009
10.1071/CH09099
0004-9425/09/080871

RESEARCH FRONT
872
T. Nagamura and Y. Sota
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
PEBTz2ϩ(BrϪ)2
ACN
Irradiation time
Before irradiation
90 s
10 s
20 s
40 s
180 s
360 s
Aerated
DMF
Ethanol
THF
Toluene
Cyclohexane
400
600
800
1000 1200 1400 1600
Wavelength [nm]
300
400
500
600
700
Wavelength [nm]
Fig. 2. Absorption spectral changes for tetraphenylborate (TPB−) salts of
4 (4.3 × 10−5 M) in THF before and during steady-state.
Fig. 1. Absorption and fluorescence spectra of 5 normalized by peak
intensity.
demonstrate that instantaneous expansion of the π-conjugated
system among four whole aromatic rings connected by carbon–
carbon single bonds is possible by the use of a photoinduced
electron-transfer reaction. We can also control the ‘off’response
in the visible and NIR region by back electron transfer in less
than 1 ps.
Results and Discussion
Absorption and fluorescence spectra normalized by peak
intensity are shown in Fig. 1 for the compound 5 in
N,N-dimethylformamide (DMF), acetonitrile (ACN), ethanol
(EtOH), tetrahydrofuran (THF), toluene (TOL), and cyclohex-
ane (CHX). Solvents effects on the absorption and fluores-
cence spectra were not large except for CHX as shown in
Fig. 1. The fluorescence quantum yield was relatively low
and not sensitive to the solvents, 0.051 in TOL, 0.087 in
ACN, and 0.029 in CHX. Compound 4 was soluble almost
only in polar solvents. Its fluorescence quantum yield was
higher, 0.16 in ACN and EtOH, 0.18 in DMF. The lower
fluorescence quantum yield of 5 might be due to the exis-
tence of more flexible ether bonds in the alkyl substituents
of 4,4ꢀ-diethyl-5,5ꢀ-di(N-alkyl-4-pyridiniumyl)-2,2ꢀ-bithiazole.
Easier molecular motion in alkyl substituents will accelerate
thermal deactivation of the excited singlet state of 4,4ꢀ-diethyl-
5,5ꢀ-di(N-alkyl-4-pyridiniumyl)-2,2ꢀ-bithiazole chromophores.
Because the chromophore of 5 has both cationic pyridinium
units and neutral thiazole units connected, solvent effects on the
stabilization of excited and ground states will work in opposite
directions, resulting in very small absorption shifts in solvents
with a wide range of polarity.
Before
After
Fig. 3. Photographs of THF solutions before and after photoirradiation.
salts of 4. These results indicated that radicals were formed and
the unpaired electron was distributed among the four aromatic
rings of 4. Based on these results, together with comparison with
Molecular Orbital PACkage (MOPAC) and Zerner’s Intermedi-
ate Neglect of Differential Overlap (ZINDO) calculations, new
peaks at 600 and 676 nm were assigned to the short-axis transi-
tion of photogenerated radicals of 4,4ꢀ-diethyl-5,5ꢀ-di(N-hexyl-
4-pyridiniumyl)-2,2ꢀ-bithiazole units. The absorption peaks at
1210 and 1438 nm were assigned to the long-axis transition
of the same radicals. Two peaks observed in each band are
most likely due to the vibrational structure. The molar extinc-
tion coefficient (ε) of radicals at 1438 nm was estimated to be
2.86 × 104 M−1 cm−1 from the decrease of the original absorp-
tion at 405 nm with ε = 3.90 × 104 M−1 cm−1. The colour of the
THF solution changed from yellow to green on photoirradiation
as shown in Fig. 3.
Absorption spectral changes for tetraphenylborate (TPB−)
salts of 4 (4.3 × 10−5 M) in THF before and during steady-state
photolysis are shown in Fig. 2. TPB− was selected because it is
known to work as a sacrificial electron donor owing to decom-
position of its oxidized form.[17] On photoirradiation, original
absorption at 405 nm decreased and new peaks at 600, 676, 1210,
and 1438 nm increased up to 40 s. After 40 s, absorbance at 600,
676, 1210, and 1438 nm gradually decreased with a concomi-
tant increase of a new peak at 745 nm. When air was introduced
after 6 min of irradiation, all peaks disappeared. Electron spin
resonance (ESR) spectra having hyperfine structure with at least
27 peaks were observed by steady photoirradiation of TPB−
Toelucidatetheoriginofthepeakat745 nmthatwasobserved
only at longer irradiation time and higher radical concentra-
tions, spectral changes of the sample irradiated for 30 min were

RESEARCH FRONT
Properties of a Photoelectrochromic Polymer
873
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0.15
0.10
0.05
0.00
Ϫ3.0 ps
0 ps
0.5 ps
2.5 ps
15 ps
Before irradiation
Excitation for 30 min
Stirring for 60 min
Dark for 60 min
Dark for 90 min
Dark for 570 min
Aerated
400
600
800
1000 1200 1400 1600
450
500
550
600
650
700
750
Wavelength [nm]
Wavelength [nm]
Fig. 4. Spectral changes of the sample irradiated for 30 min observed
during storage at room temperature in the dark.
Fig. 5. Time-resolved absorption spectra in the visible region for 5 cast
films on fs laser excitation at 400 nm.
observed during subsequent storage at room temperature in the
dark as shown in Fig. 4. Both visible and NIR absorption peaks
due to radicals were found to increase gradually during stor-
age with a concomitant decrease of absorption at 745 nm, as
shown in Fig. 4. These results strongly suggest an equilibrium
between radical dimers (or aggregates) and monomers depend-
ing on the total concentration, which decreased gradually in
the dark probably owing to radical coupling or reactions with
residual oxygen.
0.15
0.10
0.05
0.00
If halide ions such as bromide or iodide were used instead
of tetraphenylborate, no colour changes occurred on steady
photoirradiation. Meanwhile, transient absorption spectra were
observed in the visible to NIR region both in solutions and solid
films of polymer 5 on fs laser excitation. Fig. 5 shows transient
absorption spectra for solid films in the visible region. A part of
the transient absorption spectra decayed faster at ∼600–800 nm,
as shown in Fig. 5, which corresponded well with that observed
by steady photoirradiation shown in Fig. 2 and was assigned
to the short-axis transition of photogenerated radicals. This
thus indicates that there is another transient species, probably
the excited singlet state (S1) of bis(2-(4-pyridiniumyl)thiazole)
chromophore, in addition to photogenerated radicals. Analysis
of the time profile at 639 nm shown in Fig. 6 gave three com-
ponents: a fast one (lifetime of a fast decaying component of
transient absorption, τf = 0.83 ps, 29%), a slow one (τs = 25 ps,
17%), and an offset (54%). The offset component is most prob-
ably attributable to the S1 state judging from the fluorescence
lifetime mentioned below. The transient absorption spectrum in
the NIR region just after fs laser excitation of 5 in solid films
is shown in Fig. 7, which corresponds well with that in the NIR
region shown in Fig. 2. Schematic representation of photoreac-
tions in 4 or 5 is given in Fig. 8, in which dynamics of the excited
state, photoinduced electron transfer, and back electron transfer
mainly depend on the distance between cationic chromophores
and counter ions.
0
100
Time [ps]
200
300
Fig. 6. Time profile of transient absorption at 639 nm for 5 cast films on
fs laser excitation at 400 nm.
0.08
0.06
0.04
0.02
0.00
1000
1200
1400
1600
Wavelength [nm]
In addition to transient absorption spectra shown in Fig. 5,
bleaching with a negative absorption peak at 500 nm was clearly
observed for 5 in THF, which is most probably due to the flu-
orescence as shown in Fig. 1 at the steady-state. The decay at
639 nm was also not single-exponential in THF with τf = 4.6 ps
(19%), τs = 273 ps (41%), and an offset (40%). The decay due to
back electron transfer was much slower (approximately five to 10
times) in solution compared with that in solid films. This result
and higher fluorescence intensity was due to increased average
Fig. 7. Transient absorption spectrum in the near infrared region for 5 cast
films just after fs laser excitation at 400 nm.
distance between ion pairs in solution as reported previously for
a similar polymer containing 4,4ꢀ-bipyridinium units.
Fluorescence decay curves were measured in order to get
more information on the excited state. The results are shown

RESEARCH FRONT
874
T. Nagamura and Y. Sota
eϪ
DϪ
ϩ
ϩ
DϪ
Excited state
Photoinduced
electron transfer
C2H5
hν
N
S
ϩ
ϩ
N
N R
R
eϪ
S
N
DϪ
D.
C2H5
ϩ
•
ϩ
ϩ
DϪ
DϪ
Back electron transfer
Ground state
(dication)
Radical cation
Fig. 8. Schematic representation of photoreactions for the present system. The four groups in the chromophore are
schematically represented by ellipses.
2
in Fig. 9 for 4 in DMF and chloroform, which were single-
DMF
CHCl3
exponential with τf = 1.16 ns in DMF and double-exponential
1000
with τf = 0.32 ns (87%) and 1.29 ns (13%) in chloroform. Con-
4
tribution of excited singlet-singlet (S-S) annihilation to the
shorter lifetime component is excluded in dilute solutions. As
average distance is expected to be smaller in less polar sol-
2
100
vents such as chloroform than in polar solvents like DMF, the
shorter fluorescence lifetime component observed in chloro-
form is attributed to electron-transfer quenching by counter ions.
Meanwhile, the decay for polymer 5 was double-exponential
even in DMF, with τf = 0.62 ns (64%) and 2.46 ns (36%). The
solvent polarity ranging from toluene, THF to DMF hardly
affected the fluorescence decay of polymer 5. From these
results, the microenvironment around 4,4ꢀ-diethyl-5,5ꢀ-di(N-
alkyl-4-pyridiniumyl)-2,2ꢀ-bithiazole in the main chain and its
counter anions (bromide) is not directly affected by solvents,
but is most probably determined by the microenvironment of
poly(tetramethyleneoxide), in which ether oxygen will inter-
act with cationic units of chromophores in a similar manner
as coordination. The fluorescence decay of 5 became faster
in cast films, with τf = 0.26 ns (60%) and 1.57 ns (40%). This
result also strongly suggests decrease of the average distance
between ion pairs in solid films. Although local concentra-
tion of chromophores can be higher in solid films, contri-
bution of S-S annihilation seems to be very small because
of the above-mentioned microenvironmental effects on ion
pairs of 5,5ꢀ-di(N-hexyl-4-pyridiniumyl)-2,2ꢀ-bithiazole in the
polymer 5.
4
2
10
0
1
2
3
4
Time [ns]
Fig. 9. Fluorescence decay curves for 4 (1 × 10−6 M) in DMF and
chloroform excited at 400 nm.
photonics, which can fully utilize the superior properties of both
molecules and photons for information processing with ultimate
time or spatial resolution.
Experimental
Synthetic procedures for small molecules employed in the
present study are summarized in Scheme 1.
4,4ꢀ-Diethyl-2,2ꢀ-bithiazole 1
Conclusions
1-Bromo-2-butanone (2.1 mL, 25 mmol) was added to solu-
tion of dithiooxamide (1.53 g, 12.7 mmol) in dehydrated ethanol
under a nitrogen atmosphere according to the literature.[33] The
mixture was refluxed for 7 h. After cooling to room temperature
and keeping overnight in a refrigerator, red-brown crystals were
obtained. Recrystallization from methanol afforded 1 (1.48 g,
59%). δH (CDCl3, 400 MHz, TMS) 6.94 (s, 2H, thiazole ring
proton (Tz)), 2.85 (4H, m, –CH2–), 1.32 (6H, t, –CH3).
A new photoelectrochromic polymer was developed that showed
strong absorption changes in the visible to NIR wavelength
region due to photoinduced electron transfer from counter anion
to 5,5ꢀ-bis(4-pyridiniumyl)-2,2ꢀ-bithiazole groups in the main
chain. The lifetime of photogenerated radicals was controlled
over a very broad range: from infinity for tetraphenylborate salts
to 830 fs for bromide salts in solid films. From time-resolved
absorption and fluorescence spectroscopy, reaction mechanisms
and ultrafast dynamics were elucidated in solutions and in solid
films.This photoelectrochromic polymer can be applied to ultra-
fast all-optical modulation in the guided wave mode geometry
we have been studying with various photoresponsive materi-
als. The present results will contribute a great deal to molecular
5,5ꢀ-Dibromo-4,4ꢀ-diethyl-2,2ꢀ-bithiazole 2
Bromine (0.30 mL, 5.80 mmol) in chloroform (20 mL) was
added dropwise to 1 (0.51 g, 2.28 mmol) in chloroform (50 mL).
The mixture was refluxed for 4 h under a nitrogen atmosphere
and cooled to room temperature. The reaction mixture was

RESEARCH FRONT
Properties of a Photoelectrochromic Polymer
875
S
Br
S
N
N
S
C2H5OH
N
S
S
N
O
Br2, CHCl3
NH2
ϩ
H2N
Br
Br
S
1
2
ϩ C6H13
ϩ
C6H13
N
N
N
TPBϪ
NaTPB
CH3OH
Pd(PPh3)4
Toluene
N
S
S
N
Br ϪSn
BrϪ
N
S
S
N
C6H13Br
N
S
N
S
N
N
S
ϩ
TPBϪ
BrϪ
S
Br
N
ϩ
N
N
N
C6H13
ϩ
C6H13
3
4
Scheme 1. Structures of and synthetic procedures for compounds employed in the present study. TPB, tetraphenylborate.
CF3SO3Ϫ
CF3SO3Ϫ
O
(CF3SO2)2O
ϩ
ϩ
(CH2)4
O
O
O(CH2)
4 n
(7 mL)
BrϪ
EBTzP
in THF (3 mL)
N
BrϪ
N
ϩ
N
NaBr
S
S
Ϫ70ЊC, 3 h
0ЊC, 2 h
(CH2)4 O(CH2)4
ϩ
n
m
N
RT, 30 min
5
Scheme 2. Structure of and synthetic procedure for photoelectrochromic polymer 5.
washed with saturated sodium bisulfite, H2O, and brine. The
reaction mixture was dried with MgSO4 and concentrated under
reduced pressure to give orange solid 2 (0.54 g, 62.3%). δH
(CDCl3, 400 MHz, TMS) 2.78 (4H, m, –CH2–), 1.28 (6H, t,
–CH3).
Poly[4,4ꢀ-diethyl-5,5ꢀ-di(N-tetramethyleneoxide-
4-pyridiniumyl)-2,2ꢀ-bithiazole]dibromide 5
Photoelectrochromic polymer 5 was prepared according to
Scheme 2. Tetrahydrofuran (7 mL, 86.3 mmol) was polymerized
by the living-cation method at 0◦C using trifluoromethane-
sulfonic anhydride (115 µL) as an initiator for 35 min under
an argon atmosphere. After confirming the viscosity was high
enough, a solution of 3 (13.0 mg, 0.034 mmol) in THF (3 mL)
was dropped into the reaction mixture at −70◦C and was
stirred for 3 h, and for another 2 h at 0◦C. The rubbery prod-
uct was dissolved in chloroform and its ions exchanged with
an aqueous solution of sodium bromide to get 5 (2.36 g). The
number-average molecular weight (Mn) was evaluated by gel
permeation chromatography (GPC) on the basis of polystyrene
calibration. Two peaks were observed in GPC at M1 = 17400
and M2 = 34900. As a large excess of two living cations of
poly(tetramethylene oxide) will attack 3 at two pyridine nitro-
gen atoms, two kinds of polymers will be formed: reacted and
non-reacted. M2 and M1 most probably correspond to poly-
mers with quarternized chromophore 3 in the central part of
the main chain and poly(tetramethyleneoxide) homopolymers
without chromophore, respectively.
4,4ꢀ-Diethyl-5,5ꢀ-di(4-pyridyl)-2,2ꢀ-bithiazole 3
Trimethyl(4-pyridyl)tin (0.50 g, 2.07 mmol) and tetrakis
(triphenylphosphine)palladium(0) (0.20 g, 0.17 mmol) as a cata-
lyst were added to a solution of 2 (0.30 g, 0.79 mmol) in toluene
(30 mL). The reaction mixture was refluxed for 6.5 h under a
nitrogen atmosphere. Ammonium solution (1 M) was added to
the mixture after cooling to room temperature. The solution
was extracted with chloroform. The extracts were washed with
brine and dried with MgSO4, and concentrated under reduced
pressure. Recrystallization with ethanol afforded yellow solid 3
(0.19 g, 60.4%). δH (CDCl3, 400 MHz, TMS) 8.70 (4H, d, o-Py),
7.40 (4H, d, m-Py), 2.92 (4H, m, –CH2–), 1.39 (6H, t, –CH3).
4,4ꢀ-Diethyl-5,5ꢀ-di(N-hexyl-4-pyridiniumyl)-
2,2ꢀ-bithiazole Dibromide 4
Measurements
3 (39.0 mg, 0.10 mmol) in 1-bromohexane was refluxed for
14 h under a nitrogen atmosphere. The reaction mixture was
concentrated under reduced pressure and washed with hexane
to give orange solid 4 (62.5 mg, 89.8%). δH ([D4]methanol,
400 MHz, TMS) 9.01 (4H, d, o-Py+), 8.26 (4H, d, m-Py+),
4.64 (4H, m, Py+–CH2–), 3.10 (4H, m, Tz–CH2–), 2.06 (4H,
m, Py+CH2–CH2–), 1.39–1.47 (18H, m, TzCH2–CH3 and
Py+C2H4–(CH2)3–), 0.94 (6H, t, –CH3).
Absorption and fluorescence spectra were measured with a
Hitachi U-4100 spectrophotometer and F-4500 fluorescence
spectrophotometer, respectively. Transient absorption measure-
ments were made by a pump-probe method with fs white-
continuum generated by focussing fs 800-nm light into a
D2O/H2O mixture or carbon tetrachloride as a probe and fs
400-nm light as a pump.[16] An amplified Ti-sapphire laser
delivered pulses of ∼200 fs at a repetition rate of 10 Hz and

RESEARCH FRONT
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T. Nagamura and Y. Sota
3.4 mJ at 800 nm. The probe light was split by a beam splitter
into two beams, which were detected with a dual silicon photo-
diode array in the visible region or with two InGaAs photodiode
arrays in the NIR region.[16] Fluorescence lifetime was deter-
mined by a single-photon counting system with a streak camera
(Hamamatsu Photonics, C4334–01) on 1.5-ps laser excitation
at 400 nm. Fluorescence quantum yield was determined in sev-
eral solvents using quinine sulfate as a standard (fluorescence
quantum yield of a standard quinine sulphate, φST = 0.546 at
λex = 365 nm). Samples for steady-state photolysis were de-
aerated by the freeze–pump–thaw method. Steady photolysis
was made in films and in de-aerated solutions with a 150-W
Hg-Xe lamp through a band pass filter at 405 nm. ESR spectra
were measured with a Bruker EMX-plus spectrometer (100 kHz
field modulation) at room temperature in tetrahydrofuran solu-
tion after irradiation under vacuum with a 150-W Hg-Xe lamp
through UV and IR cut-off filters (UV-33 and IRA;AsahiTechno
Glass Co., Ltd).
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Acknowledgements
The authors thank Fuyuki Ito, Ryuji Matsumoto andTakayasu Nagai for their
partial contributions. The present study was partly supported by the Grant-
in-Aid for Scientific Research on Priority Areas ‘Strong Photon–Molecule
Coupling Fields’ (No. 20043027) from the Ministry of Education, Science,
Sports and Culture, Japan.
[21] K. Sasaki, T. Nagamura, Appl. Phys. Lett. 1997, 71, 434. doi:10.1063/
1.119571
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M. Hanack, Mol. Cryst. Liq. Cryst. 2001, 370, 97. doi:10.1080/
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