Phenylene ethynylene azobenzenes with symmetrical peripheral chromophores: Synthesis, optical properties and photoisomerization behaviors study

Article abstract of DOI:10.1016/j.dyepig.2011.06.006

A series of phenylene ethynylene azobenzenes bearing different symmetrical peripheral chromophores were synthesized by Sonogashira coupling and characterized by UV-vis absorption, fluorescence emission, cyclic voltammetry and cis-trans photoisomerization.

Full text of DOI:10.1016/j.dyepig.2011.06.006

Dyes and Pigments 92 (2011) 626e632
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Phenylene ethynylene azobenzenes with symmetrical peripheral chromophores:
Synthesis, optical properties and photoisomerization behaviors study
*
Rui Liu, Qi Xiao, Yuhao Li, Hongbin Chen, Zhangwei Yan, Hongjun Zhu
Department of Applied Chemistry, College of Science, Nanjing University of Technology, Nanjing 210009, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of phenylene ethynylene azobenzenes bearing different symmetrical peripheral chromophores were
synthesized by Sonogashira coupling and characterized by UVevis absorption, fluorescence emission, cyclic
Received 21 March 2011
Received in revised form
5 June 2011
Accepted 7 June 2011
Available online 14 June 2011
voltammetry and cisetrans photoisomerization. All compounds exhibit strong 1
p
,
p
* absorption bands in the
-electron donating chromophore attached on the
para-position of phenyl group. When excited at the 1p
* band, the compounds exhibit violet to blue
emission (363 w 430 nm), which is attributed to a 1
* emission. All of these compounds show reversible
transecis isomerization in dichloromethane solution, indicating that the -conjugated phenylene ethynylene
UV region, which pronouncedly blue-shifts when the
p
,p
p,p
p
Keywords:
Azobenzene
backbone does not prevent the occurrence of the photochemical processes of the azobenzene center. The
photoisomerization properties were influenced by the peripheral chromophores in the conjugated system
and the steric interactions. In addition, the kinetic investigation of the photoisomerization process was
carried out.
Phenylene ethynylene
Chromophores
Photoisomerization
Optical property
Synthesis
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
switching device candidates, which indicate that the oligo(phenylene
ethynylene)s (OPEs) attached to an azobenzene moiety have a strong
Photochromic compounds have attracted a great interest in recent
years due to their intriguing spectroscopic properties and potential
applications in photoresponsive materials, non-linear optics, micro-
electronics and optical data storage [1e6]. Among these, azobenzene
derivatives have been extensively investigated and gained enormous
interest. As photoactive compounds, they can undergo reversible
transformation from the thermodynamically stable trans form to the
cis form by irradiation with UV or visible light. The back cis-to-trans
reaction can be achieved by illumination with UV/vis light of
a different wavelength or appears by thermal relaxation. This shift in
structure causes a significant change in colour, refractive index,
dielectric constant, dipole moment, etc., making azobenzenes
potentially attractive in the realms of photoresponsive materials and
non-linear optics [7e14].
impact on its photoisomerization behavior. When they are combined
together to form more complex systems, the present work under-
scores that their synergistic effects must be considered.
The reported work are quite intriguing, however, the study on
azobenzene-OPE derivatives, especially the effect of various chro-
mophores on their photophysics is still limited. In this context, the
development of new phenylene ethynylene azobenzene derivatives
coupled with various chromophores, also the further study of the
photophysical properties, photoisomerization behaviors and struc-
tureeproperty relationships are feasible, challengeable, and valuable.
To remedy this deficiency, our groups have designed and synthesized
a series of new phenylene ethynylene azobenzenes derivatives with
symmetrical peripheral chromophores (Scheme 1). Compound 1a
has been reported in the literature [7] and was synthesized as
a reference. Their photophysical properties and photoisomerization
behaviors have been investigated with the aim of understanding the
structureephysical property relationships and developing potential
photoresponsive materials.
Zeitouny and co-workers [8] have synthesized a series of ethynyl-
bearing azobenzene compounds with suitably-designed peripheral
groups. All of them exhibit reversible trans-to-cis isomerization in
cyclohexane solution showing that
p-delocalized appended frag-
ments do not prevent the occurrence of the photochemical and
thermal processes of the azoalkene center. Tour and co-workers
[6,7,12] have also developed a class of novel oligo(phenylene ethy-
nylene) azobenzene derivatives as potential molecular electronic
2. Experimental
2.1. Materials
Bis(triphenylphosphine)palladium (II) dichloride was purchased
from ABCRChemicalLtd. Phenylacetylene(2a) and 4-ethynylbiphenyl
* Corresponding author. Tel.: þ86 25 83172358; fax: þ86 25 83587428.
E-mail address: zhuhjnjut@hotmail.com (H. Zhu).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.06.006

R. Liu et al. / Dyes and Pigments 92 (2011) 626e632
627
Scheme 1. Synthesis of phenylene ethynylene azobenzene compounds (1a e 1g).
(2f) were purchased from SigmaeAldrich Company. Other reagents
were purchased from Sinopharm Chemical Reagent Co. Ltd. and were
used without any further purification. Tetrahydrofuran (THF) and
N,N-diisopropylethylamine (DIEA) were distilled under N2 over
sodium benzophenone ketyl. Silica gel chromatography was carried
out on silica gel (200e300 mesh). (E)-bis-(4-iodo-phenyl)-diazene(3)
[15] and arylacetylide derivative 2b [16,17], 2c [18,19], 2e [20,21], 2g
[22] were synthesized according to literature methods.
anhydrous MgSO4 and evaporated to dryness. Then the crude
product was purified by column chromatography (silica gel,
hexane/CH2Cl2 ¼ 6:1, v/v) to give the desired product.
2.3.2. (E)-bis-(4-phenylethynyl-phenyl) diazene (1a)
Yield was 40.0% as orange solid; m.p. decomposes at 245 ꢁC
(Lit. [7]: 246 ꢁC). 1H NMR (CDCl3, 500 MHz):
d
ppm 7.93
(d, J ¼ 8.6 Hz, 4H), 7.68 (d, J ¼ 8.7 Hz, 4H), 7.56e7.59 (m, 4H),
7.36e7.39 (m, 6H). 13C NMR (CDCl3, 300 MHz):
ppm 152.2, 132.8,
132.3, 129.1, 128.9, 125.5, 123.5, 123.3, 92.8, 89.9. HRMS calcd for
28H18N2: 382.1470. Found: 382.1465. Anal. calcd. (%) for C28H18N2:
d
2.2. Measurements
C
Melting points were measured on an X-4 microscope electro-
thermal apparatus (Taike China) and were uncorrected. 1H NMR
and 13C NMR spectra were recorded on a Bruker AV-500 spectrom-
eter at 500 MHz or a Bruker AV-300 spectrometer at 300 MHz using
CDCl3 or DMSO-d6 as the solvent, with tetramethylsilane as internal
standard. The elemental analyses were performed with a Vario El III
elemental analyzer. Optical absorption spectra were obtained using
an HP-8453 UV/vis/near-IR Spectrophotometer (Agilent). PL spectra
were carried out on an LS-55 spectrofluorometer (PerkineElmer).
High resolution mass (HRMS) analyses were performed at a Bruker
BioTof III mass spectrometer (Bruker). The UV light for irradiation is
generated by a 500 W mercury lamp cooled by a circulated cooling
water system with two different light filters centered at 365 nm
(P ¼ 0.35 W cmꢀ2) and 254 nm (P ¼ 0.22 W cmꢀ2) respectively. Azo-
benzene photoisomerization experiments were carried out using
a 1 cm quartz cuvette to measure absorbance changes immediately
after the sample solution in cuvette had been subject to UV light. The
electrochemical experiments were carried out using a CHI 660C
C, 87.93; H, 4.74; N, 7.32. Found: C, 87.88; H, 4.78; N, 7.30.
2.3.3. (E)-bis-(4-(4-diphenylamino phenylethynyl) phenyl)
diazene (1b)
Yield was 32% as orange solid; m.p. decomposes at 202 ꢁC. 1H
NMR (DMSO-d6, 500 MHz):
d
ppm 7.98 (d, J ¼ 8.5 Hz, 4H), 7.67
(d, J ¼ 8.9 Hz, 4H), 7.42 (d, J ¼ 8.5 Hz, 4H), 7.27e7.34 (m, 8H), 7.09
(d, J ¼ 8.7 Hz, 8H), 6.98e7.05 (m, 4H), 6.89 (d, J ¼ 8.7 Hz, 4H). 13C
NMR (DMSO-d6, 300 MHz):
d ppm 151.2, 151.0, 146.2, 138.5, 137.6,
132.7, 132.3, 129.9, 129.7, 126.4, 126.0, 125.2, 125.0, 124.4, 124.2,
123.0, 121.6, 120.8, 94.2, 89.2. HRMS calcd for C52H36N4: 716.2940.
Found: 716.2937. Anal. calcd. (%) for C52H36N4: C, 87.12; H, 5.06; N,
7.82. Found: C, 87.08; H, 5.08; N, 7.84.
2.3.4. (E)-bis-(4-(4-carbazol-9-yl phenylethynyl) phenyl)
diazene (1c)
Yield was 35% as orange solid; m.p. decomposes at 257 ꢁC. 1H
NMR (CDCl3, 500 MHz):
d
ppm 8.14 (d, J ¼ 7.0 Hz, 4H), 7.95
electrochemistry workstation (CHI USA).
A
standard one-
(d, J ¼ 6.5 Hz, 4H), 7.89 (d, J ¼ 7.0 Hz, 4H), 7.72 (d, J ¼ 7.5 Hz, 4H),
7.68 (d, J ¼ 7.5 Hz, 4H), 7.61 (d, J ¼ 7.8 Hz, 4H), 7.41e7.48 (m, 4H),
compartment three-electrode cell was used with a Pt electrode
as the working electrode, a Pt wire as the counter electrode and a
Ag/Agþ electrode (Ag in 0.1 M AgNO3 solution, from CHI, Inc.) as the
reference electrode. TBAP (Tetrabutylammonium perchlorate, 0.1 M)
was used as the supporting electrolyte and the scan rate was
7.28e7.33 (m, 4H). 13C NMR (CDCl3, 300 MHz):
d ppm 151.9, 151.7,
140.5,138.4,138.1,137.9,133.2,133.1,132.5,132.2,126.8,126.1,124.5,
123.6, 123.1, 122.4, 121.8, 120.4, 120.3, 109.7, 101.8, 91.4, 90.0. HRMS
calcd for C52H32N4: 712.2627. Found: 712.2631. Anal. calcd. (%) for
100 mV sꢀ1
.
C52H32N4: C, 87.62; H, 4.52; N, 7.86. Found: C, 87.88; H, 4.58; N, 7.84.
2.3. Synthesis
2.3.5. (E)-bis(4-(fluoren-2-yl ethynyl) phenyl) diazene (1d)
Yield was 38% as orange solid; m.p. decomposes at 260 ꢁC. 1H
2.3.1. General procedure for synthesis of compound 1ae1g
NMR (CDCl3, 500 MHz):
d
ppm 7.92 (d, J ¼ 8.5 Hz, 4H), 7.88
A
mixture of compound
3
(0.43 g, 1 mmol, 1.0 equiv.),
(d, J ¼ 8.5 Hz, 2H), 7.81 (d, J ¼ 7.5 Hz, 2H), 7.78 (d, J ¼ 8.0 Hz, 2H),
7.75 (d, J ¼ 8.0 Hz, 4H), 7.68 (d, J ¼ 7.5 Hz, 2H), 7.58 (d, J ¼ 8.0 Hz,
2H), 7.38e7.42 (m, 2H), 7.31e7.35 (m, 2H), 3.93 (s, 4H). 13C NMR
bis(triphenylphosphine) palladium (II) dichloride (0.035 g, 0.5 mol
%), CuI (0.01 g, 0.5 mol%), DIEA (0.20 g, 1.6 mmol, 1.6 equiv.), aryla-
cetylide derivative (1 mmol, 1.0 equiv.) in anhydrous THF (30 mL)
was stirred for 24 h under a nitrogen atmosphere at 60 ꢁC in the
absence of light. The mixture was then quenched with water,
extracted with CH2Cl2 (3 ꢂ 30 mL), washed by brine, dried over
(DMSO-d6, 300 MHz):
d ppm 152.1, 151.8, 143.6, 143.4, 131.4, 128.9,
127.7, 126.9,125.2,120.7, 120.4, 89.7, 86.8. HRMS calcd for C42H26N2:
558.2096. Found: 558.2089. Anal. calcd. (%) for C42H26N2: C, 90.29;
H, 4.69; N, 5.01. Found: C, 90.18; H, 4.78; N, 5.14.

628
R. Liu et al. / Dyes and Pigments 92 (2011) 626e632
2.3.6. (E)-bis(4-(thiophen-2-yl ethynyl) phenyl) diazene (1e)
Yield was 36% as orange solid; m.p. decomposes at230 ꢁC.1H NMR
(DMSO-d6, 500 MHz):
d
ppm 8.01 (d, J ¼ 8.7 Hz, 4H), 7.79
(d, J ¼ 8.5 Hz, 4H), 7.48e7.52 (m, 4H), 7.15e7.18 (m, 2H). 13C NMR
(DMSO-d6, 500 MHz):
d ppm 151.2, 151.1, 138.4, 135.9, 133.4, 133.3,
133.2, 132.6, 132.3, 131.8, 131.5, 129.6, 129.5, 128.0, 127.8, 125.0, 124.4,
123.2, 123.0, 122.8, 122.6, 121.3, 92.4, 91.8, 86.5, 85.5. HRMS calcd for
C
C
24H14N2S2: 394.0598. Found: 394.0588. Anal. calcd. (%) for
24H14N2S2: C, 73.07; H, 3.58; N, 7.10. Found: C, 73.08; H, 3.68; N, 7.04.
2.3.7. (E)-bis(4-(4-phenyl phenylethynyl) phenyl) diazene (1f)
Yield was 53% as yellow solid; m.p. decomposes at 199 ꢁC.1H NMR
(DMSO-d6, 500 MHz):
(d, J ¼ 7.0 Hz, 4H), 7.66e7.74 (m, 12H), 7.48e7.52 (m, 4H), 7.39e7.42
(m, 2H). 13C NMR (DMSO-d6, 500 MHz):
ppm 151.1, 141.4, 138.8,
d
ppm 7.99 (d, J ¼ 6.8 Hz, 4H), 7.76
d
138.4, 137.7, 132.9, 129.1, 128.0 127.0, 126.6, 124.4, 121.9, 119.3, 95.3,
88.9, 82.8. HRMS calcd for C40H26N2: 534.2096. Found: 534.2091.
Anal. calcd. (%) for C40H26N2: C, 89.86; H, 4.90; N, 5.24. Found: C,
89.88; H, 4.88; N, 5.23.
Fig. 1. UVeVis absorption spectra of 1ae1g in CH2Cl2 solution (10ꢀ5 mol/L).
charge transfer (ICT) transitions originated from the diphenylamine
moiety. Compared with the ground state absorption of the core
structure compound 4,40-diethynylazobenzene [8], these derivatives
(1ae1g) exhibit a dramatic increase of absorption intensity and
a marked shift to lower energy of the intense pep* transition band.
Compared with 1a and 1e (Fig. 1), 1c, 1d, 1f and 1g exhibit more
structured absorption bands in the range of 300w375 nm, which
2.3.8. (E)-bis(4-(naphthalen-2-yl ethynyl) phenyl) diazene (1g)
Yield was 48% as yellow solid; m.p. decomposes at 173 ꢁC. 1H
NMR (DMSO-d6, 500 MHz):
7.94e8.01 (m, 10H), 7.65e7.68 (m, 4H), 7.63e7.64 (m, 2H), 7.58e7.62
(m, 4H). 13C NMR (DMSO-d6, 500 MHz):
ppm 151.0, 138.4, 133.0,
d ppm 8.30 (s, 1H), 8.23 (s, 1H),
d
132.9, 132.5, 132.4, 132.3, 131.1, 128.5, 128.3, 128.0, 127.9, 127.8, 127.7,
127.6, 127.1, 127.0, 126.8, 124.3, 119.5, 117.6, 95.3, 90.0, 83.8, 82.9.
HRMS calcd for C36H22N2: 482.1783. Found: 482.1779. Anal. calcd. (%)
for C36H22N2: C, 89.60; H, 4.60; N, 5.81. Found: C, 89.88; H, 4.68; N,
5.84.
may arise from the
chromophores. The
p
e
p
* transitions feature of the
p-conjugated
pep* transitions appearing are influenced
significantly by the nature of the peripheral chromophores.
Compared with 1a, the electron donating substituents appended on
the para-position of phenyl group, such as NPh2 and carbazolyl, cause
3. Results and discussions
a pronounced blue-shift of this transition. The p-electron donating
substituents such as fluorenyl, biphenyl and naphthyl, instead of the
phenyl group, also induced a significant blue-shift compared to 1a.
3.1. Synthesis and characterization
However, it is noted that in the case of 1e, the
heterocycle substituent thienyl causes a red-shift of 8 nm compared
to 1a. Considering the -electron delocalization and the presence in
cis-isomers of strained conformations, the electron donating ability
of the substituent and the existence of cis-isomers should both
influence the ground state absorption properties of these azo
compounds. Optical band gaps (EgOpt) determined from the absorp-
tion edge of the solution spectra are given in Table 1, which varies
from 2.41 eV in 1b to 3.11 eV in 1g.
p-electron donating
Scheme 1 outlines the synthesis of the phenylene ethynylene
azobenzene derivatives with symmetrical peripheral chromo-
phores. The Pd/Cu-catalyzed Sonogashira coupling reactions yiel-
ded the desired final products in 32e53% yields. All the products
are air-stable and soluble in CH2Cl2, CHCl3, tetrahydrofuran,
dimethylformamide (DMF) and dimethyl sulphoxide (DMSO) in
varying degrees. 1H NMR spectra, 13C NMR spectra, HRMS and
element analysis on 1ae1g confirmed the proposed structures. The
NMR data for all of the compounds are consistent with the assigned
structures and previously reported compound of this type [7,8,12].
p
3.3. Emission
3.2. UVevis absorption
The emission characteristics of compounds 1ae1g in CH2Cl2 at
room temperature were investigated, and their normalized emis-
sion spectra at a concentration of 1 ꢂ10ꢀ7 mol/L are illustrated in
The UVevis absorption spectra of 1ae1g in CH2Cl2 solution at
room temperature are presented in Fig. 1, the band of absorption
maxima and molar extinction coefficients for each compound are
compiled in Table 1. The absorption obeys Lambert-Beer’s law in the
concentration range studied (1 ꢂ10ꢀ6 w 1 ꢂ10ꢀ4 mol/L), suggesting
no dimerization or oligomerization occurs within this concentration
range. As shown in Fig. 1, all the compounds exhibit intense
absorption bands between 330 and 390 nm, which are assigned to
Table 1
Photophysical propertiesof 1ae1g recorded in CH2Cl2 solution.a
b
Opt
Compound
lAbsmax/nm
3
/103
lEmmax/nm
Stokes
shift
(nm)
Fem
Eg
L molꢀ1
(eV)
cmꢀ1
pe
p
* electronic transitions. Meanwhile, the characteristic azo-
benzene bands, namely the ne electronic transition bands
between 400 and 500 nm, are no longer distinguishable and over-
lapped by the * bands. In accord with the previous work on
azobenzene systems [3,8,10,13], the azobenzene central unit is
involved in extended -conjugation all the way to the terminal units,
p*
1a
1b
1c
1d
1e
1f
385
344
371
375
393
334
333
54.8
42.9
51.2
37.8
18.6
17.5
51.6
426
378
404
401
430
371
363
41
34
33
26
37
37
30
0.02
0.06
0.15
0.13
0.27
0.19
0.06
2.79
2.41
2.76
2.76
2.65
3.05
3.11
pep
p
1g
which are known to promote low-lying charge transfer absorption
bands. In the case of 1b, an additional broad moderate band at
425 nm was observed, which can be attributed to the intramolecular
a
Measured in degassed CH2Cl2 solutions at 298 K.
9,10-Diphenyanthracene as reference(FF ¼ 0.90 in cyclohexane).
b

R. Liu et al. / Dyes and Pigments 92 (2011) 626e632
629
Table 2
Electrochemistry data of compound 1ae1g.a
LUMOb
(eV)
HOMOb
(eV)
peak
onset
peak
onset
Compound
Eox
Eox
Ered
(V)
Ered
(V)
(V)
(V)
1a
1b
1c
1d
1e
1f
0.03
0.04
0.09
0.04
0.08
0.09
0.11
ꢀ0.31
ꢀ0.29
ꢀ0.24
ꢀ0.40
ꢀ0.21
ꢀ0.20
ꢀ0.19
ꢀ1.63
ꢀ1.64
ꢀ1.61
ꢀ1.59
ꢀ1.61
ꢀ1.65
ꢀ1.65
ꢀ1.35
ꢀ1.41
ꢀ1.35
ꢀ1.37
ꢀ1.35
ꢀ1.39
ꢀ1.33
3.05
2.99
3.05
3.03
3.05
3.01
3.07
5.84
5.40
5.81
5.79
5.70
6.06
6.18
1g
a
All potentials vs SCE reference.
EA (LUMO) ¼ Ered
b
onset
þ 4.4 eV; the HOMO energies are estimated according to
the optical band gap and the LUMO energy values from electrochemical experi-
mental results.
using 9,10-diphenylanthracene (0.90 in cyclohexane) as reference,
which were found in the order of 1e > 1f > 1c > 1d > 1g > 1b > 1a.
When the conjugation length of the phenylacetylide chromophores
extends, the corresponding emission quantum yield increases.
Notably, compound 1e exhibits the highest emission quantum yield
in this series, we tentatively propose that the quantum yields were
influenced by their trans-cis inter-conversions.
It is well known that the azobenzene moiety does not show any
fluorescence because azobenzene is characterized by a shallow
minima for singlet and triplet levels of both pep* and nep* electronic
configuration. As a consequence, the non-irradiative processes,
which include transecis inter-conversion, are very fast and compete
efficiently with radiative process [8,9]. The azobenzene unit has
a strong impact on the photoluminescence of all the compounds. By
considering the weak emission and low quantum yields of these
compounds, we propose that the emission of the substituted
peripheral chromophores is partly reabsorbed by the strong azo-
benzene absorption band, and the fluorescent excited state of the
chromophoric moiety is quenched by the azobenzene unit, which is
in line with the properties of azobenzenes reported in literatures
[3,4,6,8e10].
Fig. 2. Emission spectra of 1ae1g in CH2Cl2 solution (10ꢀ7 mol/L).
Fig. 2. As shown in Fig. 2 and Table 1, excitation of these compounds
at their respective absorption band maximum results in weak violet
to blue luminescence, corresponding to a Stokes shift of 26 w
41 nm. The relatively small Stokes shift and structured emission
spectra suggest that observed emission from these compounds are
originated from pep* singlet excited state. In contrast, the emission
energies (lmax, nm) of 1ae1g are influenced by the nature of the
ancillary substituents. Compared with 1a, the appended electron
donating substituents NPh2 and carbazolyl, cause a blue-shift of the
emission maxima. Meanwhile, when the
p-electron donating
substituents attached instead of the phenyl group, such as fluorenyl
in 1c, biphenyl in 1f and naphthyl in 1g, these compounds also
show a pronounced blue-shift of the emission band compared to
1a. Except for 1e, the band of absorption maxima of all the other
compounds exhibit an obvious blue-shift compared to 1a, which
increase in the order of 1e > 1a > 1c > 1d > 1b > 1f > 1g. This trend
is consistent with that observed for the absorption bands in the
UVevis absorption spectra. This suggests that the emitting state for
1ae1g could possibly be dominated by the electron donating
ability of the substituent and the existence of strained conforma-
tions from cis-isomers. Additionally, the emission quantum yields
of these compounds are relatively low in the range of 0.02w0.27
3.4. Electrochemical properties
The electrochemical behaviors of 1ae1g were investigated by
cyclic voltammetry (CV) in dry DMF solutions at 298 K (Fig. 3). In
general, the cyclic voltammograms exhibit one reversible reduction
couple with E1/2 in the range of ꢀ1.59 to ꢀ1.65 V versus Cp2Feþ/0
,
Fig. 4. Time evolution of the absorption spectra of trans-1a in CH2Cl2 solution
(10ꢀ5 mol/L) under 365 nm light irradiation, until PSS is reached in 60 min. Inset: Back
reaction observed over time under 254 nm light irradiation.
Fig. 3. Cyclic voltammetry of 1ae1g in DMF solution.

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R. Liu et al. / Dyes and Pigments 92 (2011) 626e632
3.5. Photoisomerization studies
The Phenylene ethynylene azobenzenes 1ae1g were subjected
to photoisomerization experiments and monitored using UVevis
spectroscopy with the aim of probing the occurrence of cisetrans or
transecis isomerization in dichloromethane solution. Before the
optical measurement, the sample solutions were kept in dark for
several days at room temperature and at elevated temperature,
about 60 ꢁC, for several hours, to ensure that all of the azobenzene
units were in trans form. This process could be monitored by
observing the absorption peak increasing of trans form
pep*
transition until the absorbance reached maximum value.
a
Preliminary tests showed that after more than ten cycles of UV light
induced transecis and cisetrans isomerization cycles, the peak
values and positions were still maintained without noticeable
change. The initial trans spectrum is fully recovered showing
complete reversibility.
Irradiation of 1a solution at 365 nm with UV light results in
photoisomerization from the trans- to the cis-isomer, as evidenced
in Fig. 4, by a decrease in absorbance at about 385 nm and an
increase at about 290 nm. The isosbestic points at 346 and 443 nm
indicated that the azobenzene isomerization was the only process
and was a reversible photoreaction. A photostationary state (PSS)
is reached within 60 min of irradiation, whereas by changing the
irradiation wavelength (254 nm), the spectral profile of the
starting trans-samples is fully recovered in just 10 min. Full
reversibility of this cis to trans process can also be observed by
keeping the solution in the dark for two hours, and similar pho-
toisomerizations are observed for a large number of azobenzenes
[4,8e10,13]. Compared to the absorption spectra of 4,40-dieth-
ynylazobenzene reported by Zeitouny group [8], the characteristic
azobenzene bands are not obvious in 1a because the azobenzene
Fig. 5. Time evolution of the absorption spectra of trans-1b in CH2Cl2 solution
(10ꢀ5 mol/L) under 365 nm light irradiation, until PSS is reached in 60 min. Inset: Back
reaction observed over time under 254 nm light irradiation.
which originates from the reduction of the azobenzene moiety [7],
and does not show obvious dependence on different peripheral
chromophores. During the process of anodic scan, all compounds
were observed an irreversible oxidation wave at 0.03e0.11 V, which
can be assigned to the
p-electron conjugated chromophore
segments. The electro-donating groups on the para-position of the
phenyl group shift the oxidation waves to more positive direction.
For example, the electron-donating group NPh2 and carbazolyl shift
the oxidation wave to 0.04 and 0.09 V, while this oxidation occurs at
0.03 for compound 1a which has no substituent on its phenyl unit.
The conjugation length of the phenyl acetylene chromophore
central unit is involved in extended OPEs
p-conjugated system,
segments also influence the oxidation. The
p-electron donating
which could promote low-lying and intense charge transfer
absorption bands.
substituents such as fluorenyl, biphenyl, naphthyl and thienyl,
instead of the phenyl group, shift the oxidation waves to more
positive direction significantly. Electron affinities (lowest unoccu-
pied molecular orbital, LUMO) were estimated from the onset of the
reduction wave (EA ¼ Eredonset þ 4.4) by using an SCE energy level of
4.4 eV versus a vacuum [23e27]. All eight compounds have electron
affinities between 2.99 and 3.07 eV (below vacuum), revealing low
energy barriers to electron injection from the cathode. Compared to
1a, the appended electron donating substituents NPh2 and carba-
zolyl make the band gaps decreased. Meanwhile, when the
The photoisomerization spectrum of 1b, as another example, is
illustrated in Fig. 5. The process exhibits similar trend as 1a and
prompts reversibility, showing that the forward and back isomeri-
zation processes also occurred in 1b. However, it is noted that the
irradiation of 1b with UV light results in the increase of the ICT
absorbance at 425 nm, and a strong decrease of the pep* absor-
bance accompanied by a blue-shift of the maximum from 344 nm to
335 nm, due to the formation of the cis isomer. The sharp isosbestic
points can also be observed clearly, indicating that the only process
and reversible photoreaction. When the photoisomerization
experiments carried out with other phenylene ethynylene azo-
benzenes (1ce1g), the effects of the irradiation are nearly the same
and only some fine distinctions can be observed (Fig. S1eS5). The all
observed reversibility of the photochemical process goes along
with: (1) the observation of clean isosbestic points; (2) the shorter
irradiation times needed to accomplish the back photoreaction;
(3) the occurrence of thermal back-reaction [10].
p
-electron donating substituents attached on the acetylene unit
instead of the phenyl group, such as fluorenyl in 1c and thienyl in 1e,
the band gaps also showa pronounced decrease compared to 1a. For
biphenyl in 1f and naphthyl in 1g, the band gaps between HOMO
and LUMO increase compared to 1a, which are tentatively assigned
to the influence of the existence of cis-isomers to their electron
structure properties. Detailed data for the CV and energy level
parameters are listed in Table 2.
Table 3
Photoisomerization data of compound 1ae1g.a
Compound
Absmax at PSS under 365 nm
C365 (trans-isomer)/%
Absmax at PSS under 254 nm
C254 (trans-isomer)/%
k1/10ꢀ6 sꢀ1
k2/10ꢀ6 sꢀ1
1a
1b
1c
1d
1e
1f
0.42
0.53
0.45
0.35
0.41
0.36
0.53
81.6
65.7
87.1
90.0
92.4
76.3
86.5
0.58
0.82
0.59
0.40
0.49
0.46
0.58
93.7
91.0
94.9
98.1
98.2
97.8
97.4
281.3
383.3
233.7
131.2
91.2
3021.6
1160.0
8423.7
4727.5
10023.3
1135.3
1575.6
332.8
285.8
1g
a
Apparent first-order rate constant k1 for transecis photoisomerization under 365 nm irradiation, first-order rate constant k2 for cisetrans photoisomerization under
254 nm irradiation, the trans-isomer content at the PSS under 365 nm irradiation (C365), and the trans-isomer content at the PSS under 254 nm irradiation (C254).

R. Liu et al. / Dyes and Pigments 92 (2011) 626e632
631
Fig. 6. Changes in the trans-fractions as a function of irradiation time for trans-to-cis
Fig. 8. Changes in the trans-fractions as a function of time for cis-to-trans isomeriza-
photoisomerization in CH2Cl2 solution (10ꢀ5 mol/L).
tion in CH2Cl2 solution (10ꢀ5 mol/L).
The apparent first-order rate constant k1 for trans-cis photo-
isomerization under 365 nm irradiation is determined from a plot
of (1 ꢀ APSS(365)/A0)ln[(A0 ꢀ APSS(365))/(A(t,365) ꢀ APSS(365))]wt, where
A(t,365) is the absorption maximum at time t under 365 nm irradiation
(Fig. 7 and Table 3). It is evident from the comparison of the first order
plots shown in Fig. 7 that the photoisomerization rate is affected by
the phenylene ethynylene moieties, which follows the trend of
1b > 1f > 1g > 1a > 1c > 1d > 1e. We would presume that the
observed decrease in the photoisomerization yield and slow photo-
isomerization rate can be attributed to the constrained molecular
motion of azobenzene in the conjugated system and the steric
interactions caused by the increased conjugation effect. This inter-
pretation is based on the findingsand the interpretationof the similar
behavior of OPE-azobenzene derivatives reported by Tour group [6].
Whileusing254 nm lightforirradiationfrom thePSS obtained,the
content of the trans-isomer is greatly enhanced. The content of cis-
isomerat thePSSunder 254 nmirradiation (C254) for1ae1gwere also
calculated by (A0 ꢀ A(t,254))/A0, where A0 and A(t,254) are the absorption
maxima of the unirradiated solution and of thesolution at time t upon
irradiation at 254 nm. The changes in the trans-fractions as a function
of irradiation time for cis-to-trans photoisomerization are illustrated
in Fig. 8. The apparent first-order rate constant k2 for cisetrans
isomerizations were determined by a plot of ln[(A0 ꢀ APSS(254))/
(A0 ꢀ A(t,254))]wt (Fig. 9), which are also listed in Table 3.
Under the experimental conditions used, the contentofcis-isomer
at the PSS was calculated by (A0eA(t,365))/A0, where A0 and A(t,365) are
the absorption maxima of the unirradiated solution and of the
solution at time t under 365 nm irradiation, respectively [4,28]. The
corresponding content of trans-isomer at the PSS under 365 nm
irradiation (C365) for 1ae1g are listed in Table 3, and the changes in
the trans-fractions as a function of irradiation time for trans-to-cis
photoisomerization are plotted in Fig. 6. As can be seen in Fig. 6 and
Table 3, the content of trans-isomer decreases with the time evolu-
tion under 365 nm irradiation. The result clearly indicates that with
different peripheral chromophores attached on the azobenzene, the
photoisomerization yield at the PSS varies in different grade. The
photoresponsive behavior of the azobenzene appended with
peripheral chromophores is dependent on their structures, which is
consistent with previous work on azobenzene reported by Mullen
group [4]. Notably, the content of trans-isomer for 1b decreased to
65.7% at the PSS, whereas the content of trans-isomer for 1e just
reached to 92.4% at the PSS. The electron donating ability of the
substituent and the existence of cis-isomers could both influence the
photoresponsive behaviors of these azo compounds, which may arise
from the electron delocalization in the p-conjugated system as well
as the presence of strained conformations.
Fig. 7. The first kinetic reaction relation of the trans-to-cis photoisomerization in
Fig. 9. The first kinetic reaction relation of the cis-to-trans isomerization in CH2Cl2
CH2Cl2 under 365 nm light irradiation.
under 254 nm light irradiation.

632
R. Liu et al. / Dyes and Pigments 92 (2011) 626e632
[2] Okano K, Tsutsumi O, Shishido A, Ikeda T. Azotolane liquid-crystalline poly-
As seen in Fig. 8, the content of trans-isomer for 1ae1g increased
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obviously with the time evolution under 254 nm irradiation, which
is analogous to the transecis isomerization and follows the trend of
1b > 1a > 1c > 1g > 1f > 1d > 1e. It is important to note that the
reaction rate of the cisetrans isomerization is much faster than that
of the transecis photoisomerization induced by UV irradiation. As
seen the first-order rate constant k2 for cisetrans isomerization in
Table 3, the thienyl (1e) and carbazyl (1c) substituted derivative
have notable recovery data compared to the other compounds.
These rather anomalous fast recovery rates can be attributed to cis-
isomers trapped in a strained conformation, and the peripheral
chromophores incorporated with phenylene ethynylene lead to
some photoinduced electron-transfer reactions. It seems that the
photoisomerization of the azobenzene derivatives is more or less
dependent on their structures which need more thorough study in
their electrochemical properties and electronic structures.
Furthermore, The steric effect has only a slight influence on the rate
of the transecis photoisomerization and the reverse photoreaction
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4. Conclusions
In summary, a series of phenylene ethynylene azobenzenes with
symmetrical peripheral chromophores were synthesized and
characterized by UVevis absorption, fluorescence emission, cyclic
voltammetry and photoisomerization studies. The UV absorption
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donating chromophore causes a pronounced blue-shift in both UV
absorption and emission spectra. The fluorescent excited state of the
phenylene ethynylene chromophoric moiety is partly quenched by
the azobenzene unit. Furthermore, the electrochemical properties,
the cisetrans photoisomerization behaviors and first-order rate
constants for the photoisomerization were investigated. The
p
-conjugated phenylene ethynylene backbone does not prevent the
occurrence of the photochemical processes of the azobenzene
center. The photoisomerization properties were influenced by the
peripheral chromophores in the conjugated system and the steric
interactions caused by the increased conjugation effect. We hope
these compounds would contribute to the application of photo-
responsive materials, and also serve as a model system for investi-
gating structureeproperty relationships with respect to the
nonlinear optical properties of phenylene ethynylene azobenzenes.
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Photophysical properties of 1,3,6,8-tetrakis(arylethynyl)pyrenes with donor
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Acknowledgment
[22] Hwang J-J, Tour JM. Combinatorial synthesis of oligo(phenylene ethynylene)s.
Tetrahedron 2002;58(52):10387e405.
[23] Liu S, Wang Q, Jiang P, Liu R, Song G, Zhu H, et al. The photo- and electro-
chemical properties and electronic structures of conjugated diphenylan-
thrazolines. Dyes Pigments 2010;85:51e6.
[24] Promarak V, Pankvuang A, Ruchirawat S. Synthesis and characterization of
novel N-carbazole end-capped oligothiophene-fluorenes. Tetrahedron Lett
2007;48(7):1151e4.
The authors acknowledge the National High Technology
Research and Development Program (“863” Program) of China
(2009AA032901), the Postgraduate Innovation Fund of Jiangsu
Province (2009, CX09B_141Z) and the Doctor Thesis Innovation
Fund of Nanjing University of Technology (2008, BSCX200812) for
financial support.
[25] Promarak V, Saengsuwan S, Jungsuttiwong S. Synthesis and characterization
of N-carbazole end-capped oligofluorenes. Tetrahedron Lett 2007;48(1):
89e93.
Appendix. Supplementary information
[26] Promarak V, Ichikawa M, Sudyoadsuk T. Synthesis of electrochemically and
thermally stable amorphous hole-transporting carbazole dendronized fluo-
rene. Synth Met 2007;157(1):17e22.
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.dyepig.2011.06.006.
[27] Promarak V, Punkvuang A, Jungsuaiwong S. Synthesis, optical, electrochemical,
and thermal properties of
a,a
0-bis(9,9-bis-n-hexylfluorenyl)-substituted
oligothiophenes. Tetrahedron Lett 2007;48(21):3661e5.
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azobenzene moieties compartmentalized in hydrophobic microdomains in
a microphase structure of amphiphilic polyelectrolytes. Macromolecules 1992;
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