Synthesis and isomerization of conjugated oligomers containing azoimidazole unit

Article abstract of DOI:10.1002/pola.24958

The Suzuki (for O1-O3) and Stille (for O4) coupling polymerization of 2-(phenylazo)imidazole bearing the benzyl protecting group at the 1-position gave conjugated oligomers. The transformation from the neutral imidazole in the conjugated oligomer O2, cons

Full text of DOI:10.1002/pola.24958

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ARTICLE
Synthesis and Isomerization of Conjugated Oligomers Containing
Azoimidazole Unit
Koji Takagi,1 Takato Isomura,1 Yohei Ito,1 Masanori Sakaida,1 Shusaku Nagano,2
Takahiro Seki2
1Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555,
Japan
2Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku,
Nagoya 464-8603, Japan
Correspondence to: K. Takagi (E-mail: takagi.koji@nitech.ac.jp)
Received 8 June 2011; accepted 16 August 2011; published online 9 September 2011
DOI: 10.1002/pola.24958
ABSTRACT: The Suzuki (for O1–O3) and Stille (for O4) coupling
polymerization of 2-(phenylazo)imidazole bearing the benzyl
protecting group at the 1-position gave conjugated oligomers.
The transformation from the neutral imidazole in the conju-
gated oligomer O2, consisted of the alternating 2,5-didecyl-1,4-
phenylene unit, to the cationic imidazolium salt O2S was per-
formed. Depending on the chemical structure of coupling part-
ners, the absorption maximum of conjugated oligomers
showed red shift or blue shift from that of the model com-
to the cis structure had a rough correlation with the maximum
absorption wavelength of materials. The trans-to-cis photoiso-
merization in the film state was sluggish. On the other hand,
the cis-to-trans thermal isomerization of the azoimidazole unit
was confirmed and the absorbance returned to the initial state
before the photoisomerization. The trans-to-cis photoisomeriza-
tion of the cationic conjugated oligomer O2S required large
energy, and the prolonged light irradiation might decompose
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the azoimidazole unit.
2011 Wiley Periodicals, Inc. J Polym
pound
M
with the benzene ring at the 4,5-positions. The
Sci Part A: Polym Chem 49: 4993–5000, 2011
absorption maximum wavelength of the cationic conjugated
oligomer O2S showed a blue shift from that of the neutral con-
jugated oligomer O2. The trans-to-cis photoisomerization of
the azoimidazole unit in conjugated oligomers was observed
by irradiating the light at 436 nm, and the conversion degree
KEYWORDS: azo polymers; conjugated polymers; isomer/iso-
merization; photochemistry; structure-property relations; UV-
vis spectroscopy
INTRODUCTION Imidazole derivatives have attracted increas-
ing attention as represented by the nonvolatile green solvent
(ionic liquid), the anhydrous proton mediator in the fuel cell,
the N-heterocyclic carbene, and so forth.1 The imidazole ring
is also an important building block of the naturally occurring
amino acid (histidine), the nucleobase (adenine and thy-
mine), and the biologically active agent (vitamin B12). From
the viewpoint of polymer chemistry, imidazole derivatives
play a key role not only as the polymerization catalyst but
also as the functional material.2 Yamamoto et al.3 prepared
conjugated polymers composed of (benz)imidazole and
showed interesting properties originated from the ampho-
teric nature, the metal binding affinity, and the redox activity
of imidazole. We have also investigated (benz)imidazole-con-
taining conjugated polymers to reveal the structure–property
relationship. The conjugated polymers having alternating im-
idazole and phenol units in the main chain could form an
intramolecular hydrogen bonding, and the absorption/emis-
sion wavelengths were susceptible to the surrounding
environment such as the solvent polarity.4 On the other
hand, the conjugated polymers having the phenol unit as the
side group, namely at the 2-position of imidazole, underwent
the excited state intramolecular proton transfer to observe
the fluorescence emission at the long wavelength region.5
The conjugated polymer having benzimidazole demonstrated
a pronounced peak shift compared with that of conjugated
polymer having imidazole to indicate that the p-conjuga-
ted system had an impact on the photoisomerization
behavior.6 In the course of our research, we became inter-
ested in the optical properties and the isomerization behav-
ior of conjugated polymers containing the ‘‘azoimidazole’’
unit.
Azoimidazole is one of the family of azo compounds and its
history goes back more than one century.7 The transforma-
tion to imidazolium salts, the coordination of metal ions, and
the formation of hydrogen bonding are expected, which
impart extra functions to the simple trans/cis isomerization
Additional Supporting Information may be found in the online version of this article.
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character of azobenzene. Sinha and coworkers8 have inten-
sively researched the preparation and isomerization of vari-
ous azoimidazole derivatives including transition metal com-
plexes. In contrast to low-molecular weight compounds,
however, azoimidazole-containing polymer has little atten-
tion. Fukuda et al.9 synthesized polymethacrylates bearing
cyano-substituted azoimidazole in the side chain for the
application to liquid crystalline polymer materials. To the
best of our knowledge, there is no report about conjugated
polymers and oligomers containing azoimidazole although
those containing azobenzene have been investigated so far.10
We herein describe the coupling polymerizations of 1-ben-
zyl-4,5-dibromo-2-(phenylazo)imidazole to compare the
ultraviolet (UV) spectra and examine the isomerization
behavior between the trans and cis structures.
extracted with CH2Cl2 and the combined organic layer was
dried over MgSO4. After evaporating solvents, the crude
product was recrystallized from CHCl3/hexane to obtain the
monomer 2 as a reddish-orange solid in 0.69 g (72% yield).
1
ꢀ
mp 155–156 C. H NMR (d, CDCl3) 8.04–7.90 (2H, m), 7.56–
7.44 (3H), 7.40–7.19 (5H), 5.68 (2H, s). 13C NMR (d, CDCl3)
152.9, 152.0, 135.1, 132.5, 129.4, 129.1, 128.5, 127.6, 123.6,
121.4, 108.5, 49.4. IR (cmꢁ1, ATR) 3031, 1588, 1495, 1474,
1455, 1444, 1409, 1359, 1301, 1244, 1151, 1071, 1008, 927,
765, 713, 681. Anal. Calcd. for C16H12Br2N4: C, 45.74; H,
2.88; N, 13.34%. Found: C, 45.71; H, 2.49; N, 13.21%.
Model Reaction
To a mixture of the monomer 2 (50 mg, 0.12 mmol) and
phenylboronic acid (29 mg, 0.24 mmol) in THF (2.5 mL),
Pd(PPh3)4 (4.2 mg, 3.6 lmol) and 2 M aqueous Na2CO3 (0.5
mL) were added. The mixture was heated to reflux for 24 h.
Then the solution was washed with saturated aqueous
NH4Cl and water. An aqueous phase was extracted with
CH2Cl2 and the combined organic layer was dried over
MgSO4. After evaporating solvents, the crude product was
purified by SiO2 column chromatography (CH2Cl2, Rf ¼ 0.4)
followed by the recrystallization from CH2Cl2/hexane to
obtain the model compound M as a bright orange solid in 37
mg (75% yield).
EXPERIMENTAL
Materials
All reactions were performed under dry nitrogen atmos-
phere. Dry tetrahydrofuran (THF) was purchased from Kanto
Chemical. Toluene was dried over CaH2 and distilled prior to
use. Tetrakis(triphenylphosphine)palladium(0) [Pd(PPh3)4]
was purchased from Sigma-Aldrich. Other commercially
available reagents were used as received. 4,5-Dibromoimida-
zole was synthesized according to the previous reports.11
1
ꢀ
mp 181–182 C. H NMR (d, CDCl3) 8.05–7.88 (2H, m), 7.68–
7.52 (2H), 7.52–7.33 (6H), 7.33–7.09 (8H), 7.02–6.89 (2H),
5.53 (2H, s). 13C NMR (d, CDCl3) 153.6, 152.1, 140.5, 137.2,
133.6, 132.9, 131.4, 130.9, 129.9, 129.4, 129.2, 128.7, 128.1,
127.8, 127.7, 127.4, 127.0, 123.4, 47.5. IR (cmꢁ1, ATR) 3061,
1602, 1503, 1442, 1409, 1353, 1323, 1298, 1222, 1185,
1135, 1070, 1029, 980, 916, 767, 720, 694, 643. Anal. Calcd.
for C28H22N4: C, 80.13; H, 5.35; N, 13.52%. Found: C, 79.82;
H, 5.35; N, 13.31%.
Monomer Synthesis
4,5-Dibromo-2-(phenylazo)imidazole (1)
Flask 1 containing aniline (0.50 mL, 5.5 mmol) was cooled
with an ice bath at 0 ꢀC, to which 2.9 M aqueous HCl (5.5
mL) and 2.6 M aqueous NaNO2 (2.2 mL) were added and
the reaction system was stirred for 2 h.12 A mixture of 4,5-
dibromoimidazole (1.3 g, 5.5 mmol) and 0.30 M aqueous
Na2CO3 (28 mL) was stirred at 0 ꢀC for 30 min in flask 2.
The solution in flask 1 was added dropwise into flask 2 at 0
ꢀC, and the reaction system was stirred for 24 h. The precipi-
tate was collected by filtration, washed with water, and dried
in vacuo. The solid was dissolved in CH2Cl2 and precipitated
into Et2O. After filtration, the filtrate was evaporated and the
crude product was purified by the recrystallization from
CHCl3 to obtain the monomer 1 as an orange solid in 1.3 g
(73% yield).
Polymerization (Typical Example)
To a mixture of the monomer 2 (605 mg, 1.44 mmol) and
2,20-(1,4-phenylene)bis(4,4,5,5-tetramethyl-1,3,2-dioxaboro-
lane)14 (475 mg, 1.44 mmol) in THF (30 mL), Pd(PPh3)4
(49.9 mg, 43.2 lmol) and 2 M aqueous Na2CO3 (6.00 mL)
were added. The mixture was heated to reflux for 76 h. Af-
ter pouring the mixture into water, an aqueous phase was
extracted with CH2Cl2. The combined organic phase was
dried over MgSO4. The concentrated solution was poured
into CH3OH and the precipitated product was collected to
obtain the conjugated oligomer O1 as an orange powder in
306 mg (63% yield).
mp 181–182 ꢀC. 1H nuclear magnetic resonance (NMR; d,
CDCl3) 7.96–7.79 (2H, m), 7.50 (3H). 13C NMR (d, DMSO-d6)
153.7, 151.6, 132.2, 129.7, 122.5. Infrared (IR; cmꢁ1, attenu-
ated total reflection [ATR]) 3848, 3729, 3063, 2850, 1585,
1513, 1464, 1446.4, 1428, 1392, 1362, 1304, 1175, 1038,
1006, 927, 793, 763, 708, 683. Anal. Calcd for C9H6Br2N4: C,
32.76; H, 1.83; N, 16.98%. Found: C, 32.49; H, 1.42; N,
17.00%.
Mn ¼ 2100, Mw/Mn ¼ 1.9. 1H NMR (d, CDCl3) 8.09–7.75
(2H), 7.75–7.32 (5H), 7.32–6.99 (4H), 6.99–6.42 (3H), 5.76–
5.15 (2H). IR (cmꢁ1, ATR) 3851, 3648, 3448, 3060, 3031,
2950, 2853, 2412, 2289, 1951, 1802, 1606, 1495, 1450,
1401, 1354, 1322, 1300, 1218, 1184, 1142, 1069, 1018,
1001, 979, 929, 850, 766, 723, 683. Other conjugated
oligomers O2–O4 were likewise synthesized by the Suzuki
or Stille coupling polymerizations, and the structure was
confirmed by the 1H NMR and IR spectra (see Supporting
Information). The number-averaged molecular weight was
1-Benzyl-4,5-dibromo-2-(phenylazo)imidazole (2)
To a flask containing monomer 1 (0.75 g, 2.3 mmol) in THF
(7.0 mL), NaH (55% in paraffin, 0.13 g, 2.9 mmol) was
added at 0 ꢀC.8(b),13 After stirring for 1 h, benzyl bromide
(0.32 mL, 2.7 ꢀmmol) was added and the reaction system was
heated at 60 C for 5 h. Then the solution was washed with
1 M aqueous NaOH and water. An aqueous phase was
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SCHEME 1 Synthetic route of monomers 1 and 2.
estimated as follows: O2 (Mn ¼ 1500, Mw/Mn ¼ 1.4),
O3 (Mn ¼ 2200, Mw/Mn ¼ 1.5), and O4 (Mn ¼ 1600, Mw/Mn
¼ 1.6).
isomerization were monitored by the change of molar extinc-
tion coefficient using a Hewlett-Packard 8452A diode array
UV spectrophotometer, which were performed in the yellow
ꢀ
room at 20 C.
Transformation into Imidazolium Salt
A solution of the neutral conjugated oligomer O2 (40 mg, 65
lmol) in CH3I (4.5 mL) was heated to reflux for 21 h.15 After
evaporating CH3I and drying in vacuo, the crude product was
dissolved in CH2Cl2 and poured into hexane to obtain the
cationic conjugated oligomer O2S as a reddish brown solid
in 60 mg (quantitative yield).
RESULTS AND DISCUSSION
Synthesis
2-(Phenylazo)imidazole derivatives 1 and 2 bearing the
bromo group at the 4,5-positions were prepared by the cou-
pling reaction of phenyldiazonium chloride with 4,5-dibro-
moimidazole followed by the benzylation of the 1-position
(Scheme 1). The model reaction between these monomers
and phenylboronic acid was primarily performed to check
the progress of the Suzuki coupling reaction. The coupling
reaction using a compound 1 was unsuccessful, on the other
hand, the corresponding reaction using a compound 2 bear-
ing the benzyl protecting group at the 1-position smoothly
proceeded to give the model compound M in good yield. The
polymerization of imidazole-based monomers bearing alkyl
and phenyl groups at the 2-position was successful as we
reported previously.4,5 Thus the N,N-chelate coordination of
the palladium ion to a compound 1 may be a plausible rea-
1H NMR (d, CDCl3) 8.17–7.32 (6H, Ar), 7.20–6.39 (6H, Ar),
6.16–5.09 (2H, CH2), 4.48–3.69 (3H, CH3), 2.93–1.85 (4H,
CH2), 1.48–0.96 (32H, C8H16), 0.96–0.66 (6H, CH3). IR (cmꢁ1
ATR) 3356, 2961, 2922, 2851, 1454.
,
Characterization
1H and 13C NMR spectra were recorded on a Bruker Avance
200 FT-NMR spectrometer in CDCl3 or DMSO-d6. IR spectra
were recorded on a Jasco FT-IR 460Plus spectrophotometer
in the ATR method. Melting points were determined on a
Yanaco micro melting point apparatus MP-500D. Elemental
analyses were performed on a Perkin Elmer PE2400II by the
CHN mode. Gel permeation chromatographic analyses were
carried out on a Tosoh DP-8020 pump using tandem TSK gel
a-3000 and a-5000 columns (DMF dissolvꢀing LiBr [0.5 g/L]
as an eluent, flow rate ¼ 1.0 mL/min, 40 C) equipped with
an UV detector (GL Science UV-620). Number-averaged mo-
lecular weight (Mn) and molecular weight distribution (Mw/
Mn) were determined on the basis of a calibration curve
made from standard polystyrene samples. UV spectra were
1
son for the recovery of the starting material. In the H NMR
spectrum of the model compound M, proton signals were
detected in the theoretical integral ratio suggesting that the
phenylazo group remained intact under the Suzuki coupling
condition. Likewise, the Suzuki coupling polymerization of
the monomer 2 with various diboronic acids afforded conju-
gated oligomers O1–O3 (Scheme 2). As the substituent at
the 4,5-positions of imidazole suffered from a large steric
hindrance, the number-averaged molecular weight of prod-
ucts were limited as we previously reported.4,5 The Stille
coupling reaction of the monomer 2 with 2,5-bis(tri-n-butyl-
stannyl)thiophene gave the conjugated oligomer O4 after
precipitating into hexane. These four neutral oligomers were
easily soluble in common organic solvents such as THF,
CH2Cl2, CHCl3, and toluene. Subsequently, the transformation
of the neutral imidazole in the conjugated oligomer O2 into
the imidazolium salt was carried out in refluxing CH3I by
reference to the procedure reported by Kawai and co-
workers15 (Scheme 3). To clearly disclose the influence of
quaternization on the trans-to-cis photoisomerization behav-
ior, O2 showing the largest change of the absorption
collected on
a Shimadzu UV-1650PC spectrophotometer
under dark condition. Thin films were obtained by spin coat-
ing an 1 wt % THF solution of oligomers onto a quartz
plate at 2000 rpm for 30 s using a spin coater (Kyowariken
K-359 S-1).
Isomerization
The light irradiation was performed with a 150 W Hg-Xe
lamp (UV Supercure-203, San-ei Electronic) passing through
appropriate optical filters (the combination of UV35 with
UVD36 [Toshiba] and Y44 with V44 [Toshiba] for 365 nm
and 436 nm lines, respectively). The photon energy was
evaluated using an Advantest optical power meter TQ8210.
The trans-to-cis photoisomerization and cis-to-trans thermal
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SCHEME 2 Suzuki and Stille coupling polymerizations using monomer 2.
spectrum by the light irradiation (vide infra) was selected as
the representative neutral material. After purification, the
cationic conjugated oligomer O2S was isolated as a reddish
brown solid. The relative integral ratio of benzyl/methyl pro-
ton signals was 2/3 to indicate the quantitative conversion
to the imidazolium salt. In contrast to the neutral conjugated
oligomer O2, the cationic conjugated oligomer O2S was solu-
ble in CH3CN reflecting the ionic structure.
length. The decyl chain in the conjugated oligomer O2 might
cause the steric repulsion between imidazole and benzene
rings to interrupt the p-electron delocalization. The absorp-
tion maximum of the conjugated oligomer O3 (432 nm)
showed a red shift owing to the highly planar character of
the fluorene unit. The interaction between the electron do-
nor (thiophene) and acceptor (imidazole) in the conjugated
oligomer O4 pushed the absorption maximum to the longest
Optical Properties
The UV spectra of the conjugated oligomers O1–O4 and the
model compound M in a THF solution were collected in yel-
ꢀ
low room at 20 C (Fig. 1). The absorption maximum wave-
lengths are summarized in Table 1. The absorption maxi-
mum of conjugated oligomers shifted to the longer
wavelength region compared with that of the model com-
pound M (412 nm), except for the conjugated oligomer O2
(402 nm), suggesting the elongation of effective conjugation
FIGURE 1 UV spectra of conjugated oligomers O1 (square), O2
(diamond), O3 (triangle), O4 (circle), and model compound M
(star) in THF (ca. 10ꢁ5 M; spectra were collected in yellow
room at 20 ꢀC).
SCHEME 3 Transformation of neutral conjugated oligomer O2
into cationic conjugated oligomer O2S.
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TABLE 1 Absorption Maximum Wavelengths (kmax), Molar Extinction Coefficients (e), and Decrease Ratio of e from Initial State
(%et–c) in Trans-to-Cis Photoisomerization of Conjugated Oligomers O1, O2, O3, O4, O2S, and Model Compound M
O1
O2
O3
O4
O2S
M
kmax (nm)a
e (L molꢁ1 cmꢁ1 b
424 (438)
402 (408)
432 (444)
14,100 (14,000)
35 (14)
442 (486)
17,400 (17,300)
25 (7)
382 (NA)
412
)
15,600 (15,000)
38 (19)
17,300 (10,900)
44 (30)
13,300 (5050)
19 (NA)
20,600 (14700)
46
c
%et–c
a
c
In THF (ca. 10ꢁ5 M relative to molecular weight of repeating unit).
In THF and calculated from equation: [(e at peak maximum in initial
state) ꢁ (e at peak maximum in photostationary state)]/(e at peak maxi-
mum in initial state) ꢂ 100. Numbers in parenthesis indicate values in
thin film.
Numbers in parenthesis indicate values in thin film.
b
At peak maximum in THF. Numbers in parenthesis indicate values at
436 nm.
wavelength region (442 nm). In the film state, all UV spectra
showed bathochromic shift from those in a THF solution
(Supporting Information Fig. S20). Especially, the shift width
of the conjugated oligomer O4 was largest (44 nm) to indi-
cate the strong p–p stacking interaction between oligomer
chains (Table 1).
gated oligomer O2 (402 nm; Fig. 2). This hypsochromic shift
might be ascribed to the steric hindrance of the methyl
group and/or the increase of dihedral angles between imid-
azole and benzene rings as Yamamoto and Kawai described
previously.3(b),16 The UV spectrum of the cationic conjugated
oligomer O2S measured in CH3CN had a peak maximum at
385 nm (not shown) indicating that the solvent character
had a negligible influence on the absorption wavelength.15(a)
On the other hand, the molar extinction coefficient of the cat-
ionic conjugated oligomer O2S was affected by the solvent
(13,300 L molꢁ1 cmꢁ1 in THF and 22,900 in CH3CN).
Subsequently, the influence of quaternization on the p-elec-
tron delocalization was investigated by comparing UV spec-
tra of the conjugated oligomers O2 and O2S. Kawai and
coworkers15(b) reported the modulation of p-electron deloc-
alization of aryleneethynylene-based conjugated polymers
having the imidazole unit by transforming into the imidazo-
lium salt. The absorption and emission maxima of the phe-
nyleneethynylene-type conjugated polymer showed a blue
shift on quaternization compared with the parent neutral
polymer, on the other hand, those of the thienyleneethyny-
lene-type conjugated polymer resulted in a red shift of spec-
tra that would have arisen from the coplanarization of the
main chain. In THF, the absorption maximum wavelength of
the cationic conjugated oligomer O2S (382 nm) demon-
strated a marked blue shift from that of the neutral conju-
Isomerization
The trans-to-cis photoisomerization of the azoimidazole unit
was investigated in a THF solution of the conjugated oligom-
ers O1–O4 and the model compound M in yellow room at
20 ꢀC. To optimize the irradiation wavelength, the photoiso-
merization of the conjugated oligomer O1 was examined
with the light at 365 and 436 nm (Fig. 3). The strong
absorption peak at 424 nm, attributed to the p–p* transition
band of the trans structure, was gradually decreased on irra-
diating the light of 436 nm with the isosbestic points at 370
and 520 nm. The weak absorption around 540 nm, attrib-
uted to the n–p* transition band of the cis structure, was
increased. The photon energy required to reach the photo-
stationary state was 480 mJ and the molar extinction coeffi-
cient at the maximum peak was decreased to 62% of the ini-
tial state. This spectral change clearly indicates that the
azoimidazole unit in the conjugated oligomer O1 underwent
the trans-to-cis photoisomerization. The absorption maxi-
mum shifted to the shorter wavelength region, which could
be explained by the decrease of effective conjugation length
with increasing the cis content. On the other hand, the irra-
diation with the light at 365 nm brought about the smaller
change of the spectra, namely, the molar extinction coeffi-
cient at the maximum peak was decreased to 78% of the ini-
tial state with the larger photon energy (565 mJ). Accord-
ingly, the trans-to-cis photoisomerization of the conjugated
oligomer O1 smoothly occurred using the light of 436 nm
due to the stronger absorption at 436 nm than at 365 nm.
The trans-to-cis photoisomerization of the azoimidazole unit
included in other conjugated oligomers O2–O4 and the
model compound M were likewise performed by irradiating
the light at 436 nm. The change of UV spectra is shown in
Supporting Information Figure S21 and the results are sum-
marized in Table 1. All samples showed the decrease of the
FIGURE 2 UV spectra of neutral conjugated oligomer O2
(circle) and cationic conjugated oligomer O2S (square) in THF
(ca. 10ꢁ5 M; spectra were collected in yellow room at 20 ꢀC).
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FIGURE 3 Change of UV spectra of conjugated oligomer O1 in THF on irradiation at (a) 436 nm and (b) 365 nm (spectra were col-
lected in yellow room at 20 ꢀC).
absorption peak along with a blue shift of maximum wave-
length. The isosbestic points were observed at 360/488 nm
and 380/526 nm for the conjugated oligomers O2 and O3,
respectively. The decrease ratio of the molar extinction coef-
ficient, which generally reflects the conversion ratio to the
cis structure, had a rough correlation with the maximum
wavelength as Masuda and coworkers10(f) previously
reported. The trans-to-cis photoisomerization in the film
state was rather sluggish because the mobility of oligomer
chains would be lower compared with that in the solution
state and/or the photon energy is absorbed only by mole-
cules on the film surface.
Subsequently, the cis-to-trans thermal isomerization of the
azoimidazole unit was monitored in THF while keeping solu-
tions in yellow room at 20 ꢀC. Four conjugated oligomers
O1–O4 and the model compound M in the photostationary
state obtained after the irradiation (436 nm) was used. The
representative UV spectrum of the conjugated oligomer O1
is shown in Figure 4. Other UV spectra are shown in Sup-
porting Information Figure S22 and the half-life periods are
summarized in Table 2. In general, the half-life period of a
material having the longer conjugation length became
shorter. The spectra returned to the initial state before the
photoisomerization in 1 day to suggest the reversible photo-
induced and thermal-induced trans/cis isomerization proc-
esses of the conjugated oligomers containing the azoimida-
zole unit.
The photoisomerization experiment was finally carried out
for the conjugated oligomer O2S by irradiating the light at
436 nm in THF [Fig. 5(a)] because the UV light irradiation at
365 nm might destroy the imidazolium ring as the previous
reports pointed out.17 Similar to the neutral conjugated
oligomer O2, the strong absorption peak at 382 nm ascribed
to the p–p* transition band of the trans structure was gradu-
ally decreased and blue shifted on irradiating the light. The
isosbestic points were observed at 347 and 490 nm. These
results may indicate that the azoimidazole unit in the
TABLE 2 Half-life Periods (T1/2) in Cis-to-Trans Thermal
Isomerization of Conjugated Oligomers O1–O4 and Model
Compound M in THF
O1
20
O2
60
O3
25
O4
15
M
T1/2 (min)a
70
FIGURE 4 Change of UV spectra of conjugated oligomer O1 in
THF after reaching photostationary state (spectra were col-
lected in yellow room at 20 ꢀC).
a
At which molar extinction coefficient at maximum peak recovers 50%
of decrease.
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FIGURE 5 Trans-to-cis photoisomerization of cationic conjugated oligomer O2S in (a) THF and (b) CH3CN on irradiation at 436 nm
(spectra were collected in yellow room at 20 ꢀC).
conjugated oligomer O2S underwent the trans-to-cis photoi-
somerization. When the total photon energy over 1000 mJ
was applied, however, the UV spectrum changed without
isosbestic points to imply the decomposition of the azoimi-
dazole unit (Supporting Information Fig. S23). The similar
photoisomerization was carried out in CH3CN [Fig. 5(b)]
and the decomposition of the azoimidazole unit was again
observed when the total photon energy exceeded 2500 mJ.
Accordingly, Figure 5 shows the change of UV spectra
before the decomposition starts. The maximum peak inten-
sities were decreased to 59% (in CH3CN) and 81% (in
THF) of the initial state, which could be explained by the
molar extinction coefficients in these solvents (vide supra).
The azoimidazole unit underwent the significant trans-to-
cis photoisomerization by absorbing the larger photon
energy in CH3CN. The longer distance between the imidazo-
lium cation and the iodo anion separated by the polar
CH3CN molecule might be another reason for the smooth
photoisomerization.
toisomerization of the cationic conjugated oligomer required
the larger photon energy and the excess light irradiation
brought about the decomposition of the azoimidazole unit.
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