Synthesis of novel side-chain triphenylamine polymers with azobenzene moieties via RAFT polymerization and investigation on their photoelectric properties

Article abstract of DOI:10.1002/pola.26167

Two novel and well-defined polymers, poly[6-(5-(diphenylamino)-2-((4- methoxyphenyl)diazenyl)phenoxy)hexyl methacrylate] (PDMMA) and poly[6-(4-((3-ethynylphenyl)diazenyl) phenoxy)hexyl methacrylate] (PDPMMA), which bear triphenylamine (TPA) incorporated t

Full text of DOI:10.1002/pola.26167

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Synthesis of Novel Side-Chain Triphenylamine Polymers with
Azobenzene Moieties via RAFT Polymerization and Investigation
on Their Photoelectric Properties
Yan Sun, Nianchen Zhou, Wei Zhang, Yaowen Li, Zhenping Cheng, Jian Zhu,
Zhengbiao Zhang, Xiulin Zhu
Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, College of Chemistry, Chemical Engineering
and Materials Science, Soochow University, Suzhou 215123, China
Correspondence to: N. Zhou (E-mail: nczhou@suda.edu.cn) or X. Zhu (E-mail: xlzhu@suda.edu.cn)
Received 15 March 2012; accepted 29 April 2012; published online
DOI: 10.1002/pola.26167
ABSTRACT: Two novel and well-defined polymers, poly[6-(5-(diphe-
after UV irradiation. The oxidation peak currents (IOX) of the
PDMMA and PDPMMA in CH2Cl2 were increased after UV irradia-
tion. The photoisomerization of the azobenzene moiety in PDMMA
had significant effect on the electrochemical behavior, compared
with that in PDPMMA. The changes of the hole mobility before
and after UV irradiation were very small for both polymers. The
HOMO energies (EHOMO, HOMO: the highest occupied molecular
orbital) of side chain moieties of TPA incorporated with cis-isomer
and trans-isomer of azobenzene in PDMMA and PDPMMA were
obtained by theoretical calculation, which are basically consistent
nylamino)-2-((4-methoxyphenyl)diazenyl)phenoxy)hexyl methacry-
late]
(PDMMA)
and
poly[6-(4-((3-ethynylphenyl)diazenyl)
phenoxy)hexyl methacrylate] (PDPMMA), which bear triphenyl-
amine (TPA) incorporated to azobenzene either directly
(PDMMA) or with an interval (PDPMMA) as pendant groups were
successfully prepared via reversible addition-fragmentation
chain transfer polymerization technique. The electrochemical
behaviors of PDPMMA and PDMMA were investigated by cyclic
voltammograms (CV) measurement. The hole mobilities of the
polymer films were determined by fitting the J-V (current-voltage)
curve into the space-charge-limited current method. The influence
of photoisomerization of the azobenzene moiety on the behaviors
of fluorescence emission, CV and hole mobilities of these two
polymers were studied. The fluorescent emission intensities of
these two polymers in CH2Cl2 were increased by about 100 times
C
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with the experimental results.
2012 Wiley Periodicals, Inc. J
Polym Sci Part A: Polym Chem 000: 000–000, 2012
KEYWORDS: azobenzene; fluorescence; hole mobility; living po-
lymerization; reversible addition fragmentation thermal proper-
ties chain transfer (RAFT); triphenylamine
INTRODUCTION Triphenylamine (TPA)-based compounds
were widely used as materials of photoconducter, organic
light-emitting diode, organic solar cell, and so on,1–4 because
TPA is an unique molecule possessing multiple properties,
such as redox, fluorescence, particularly good hole-transport.
The versatility of TPA compounds has attracted much atten-
tion in the field of polymer science. Over the past decade,
TPA-containing polymers with the fine photoelectric per-
formance have been successively reported, and various
attempts were made to improve the photoelectric properties
of these polymers.5–9 Kolb et al. firstly reported the hole
injecting polymer for an electroluminescent element by radi-
cal polymerization of a TPA-containing methacrylate mono-
mer.6 Zentel et al. prepared the block copolymers with TPA
and dopamine anchor as side-chain groups via reversible
addition-fragmentation chain transfer (RAFT) polymerization,
which were bounded to oxidic semiconductors like TiO2,
SnO2, and ZnO.7 Peter and Thelakkat synthesized bifunc-
tional polymers carrying tris(bipyridyl)ruthenium(II) and
TPA units as pendant groups via atom transfer radical poly-
merization method.8
Azobenzene chromophore has unique reversible photoiso-
merization property between the trans-to-cis isomers, where
large changes occur in its size, shape, and polarity.10,11 In
recent years, the studies on the versatility of azobenzene
derivatives as promising systems for various applications
have been developed.12–14 Most of these studies were
focused on the properties of the multi-functional compounds
modulated by the azobenzene photoisomerization. Aida and
coworkers succeeded in designing ‘‘light-driven molecular
scissors’’ by incorporating azobenzene with ferrocene unit.12
The scissors-like open-close motion was induced by photoi-
somerization of the azobenzene unit, due to the pivotal
motion of the connecting ferrocene unit. Sakai et al. reported
a drastic viscosity change of the azobenzene-modified cati-
onic surfactant under the trans-cis photoisomerization.13 Zhu
Additional Supporting Information may be found in the online version of this article.
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2012 Wiley Periodicals, Inc.
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and coworkers studied the influence of azobenzene photoiso-
merization on the electronic property of the azobenzene-
functionalized tetrathiafulvalene (TTF).15 The reversible
modulation of the electron-donating ability of the TTF moi-
ety was achieved by UV and visible light irradiation, respec-
tively. A few studies on the polymers carrying TPA and azo-
benzene group have also been reported.16,17 Fukuzumi and
coworkers synthesized poly[9,9-bis(4-diphenylaminophenyl)-
2,7-fluorene] which can act as an electroactive component
with long-lived charge-separated states, by functionalizing
with two moieties of 4-nitroazobenzene at both ends of the
polymer chain as withdrawing groups.16 Choi and coworkers
et al. synthesized four different polyoxetanes bearing 4-(N,N-
diphenyl) amino-40-azobenzene chromophores in the side
chain with short or long spacers to the main chain, and stud-
ied the reversible polarization gratings on thin films of those
polymers.17 So far, the investigation on the influence of the
azobenzene photoisomerization on the properties of poly-
mers containing azobenzene and TPA groups was rarely
reported. TPA unit has the good electrochemical behaviors
and hole-mobility due to the easy oxidizability of the nitro-
gen center. The photoelectric properties of the TPA moiety
can be affected by the photoisomerization of azobenzene
incorporated to TPA group. It is possible to modulate the
oxidizability of the nitrogen center of TPA by means of the
change of size, shape, and polarity of trans- or cis- azoben-
zene under UV/visible light irradiation. Herein, we designed
and prepared two dual functional methacrylate monomers
carrying TPA and azobenzene groups, 6-(5-(diphenylamino)-
2-((4-methoxyphenyl)diazenyl)phenoxy) hexyl methacrylate
(DMMA) and 6-(4-((3-ethynylphenyl)diazenyl)phenoxy)hexyl
methacrylate (DPMMA), and prepared their corresponding
polymers, poly[6-(5-(diphenylamino)-2-((4-methoxyphenyl)-
diazenyl)phenoxy)hexyl methacrylate] (PDMMA) and poly[6-
(4-((3-ethynylphenyl)diazenyl)phenoxy)hexyl methacrylate]
(PDPMMA), via RAFT polymerization. The effect of photoiso-
merization of azobenzene unit on the fluorescence emission,
electrochemical behaviors and the hole mobility of these two
polymers were investigated.
Synthesis
3-(Diphenylamino)phenol (compound (1)) and
4-(diphenylamino)phenol (compound (2))
The synthetic procedures of compounds (1) and (2) are pre-
sented in Scheme 1.19 The typical synthetic procedure of
compound (1) is as follows: A solution of 3-bromophenol
(8.6 g, 49 mmol), diphenylamine (8.45 g, 50 mmol), sodium
ter-butoxide (12.5 g, 130 mmol), tri-t-butylphosphine (0.5 g,
2.5 mmol) and palladium acetate (0.0050 g, 0.0074 mmol)
were mixed in 100-mL dry toluene in a 250-mL round
bottom flask under vigorous stirring. The resulted mixture
was refluxed for 30 min, and then was allowed to room tem-
perature. Then ethyl acetate (100 mL) was added. The mix-
ture was washed with deionized water (3 ꢁ 100 mL). After
being dried over anhydrous MgSO4 overnight, ethyl acetate
was evaporated under reduced pressure. The remaining mix-
ture was poured into 100-mL petroleum ether under stir-
ring. After filtration compound (1) was obtained as white
solid (10.2 g, yield 80.0%). 1H NMR (300 MHz, DMSO-d6),
d(tetramethylsilane, TMS, ppm): 9.34 (s, AOH), 7.29 (t, 4H,
ArH), 7.13–6.90 (m, 7H, ArH), 6.47-6.37 (m, 3H, ArH). The
yield of compound (2) is 75.5%. 1H NMR (300 MHz, DMSO-
d6), d (TMS, ppm): 9.42 (s, AOH), 7.42–7.17 (m, 4H, ArH),
6.91 (t, 8H, ArH), 6.75 (d, 2H, ArH).
6-(5-(Diphenylamino)-2-((4-methoxyphenyl)diazenyl)
phenoxy)hexyl methacrylate (DMMA (3)) and 6-(4-((3-
ethynylphenyl)diazenyl)phenoxy)hexyl methacrylate
(compound (4))
DMMA and compound (4) were prepared through diazotiza-
tion, reactions of azo-coupling, etherification, and acylation
procedures according to the method reported in the litera-
ture.20 1H NMR of DMMA (300 MHz, CDCl3), d (TMS, ppm):
7.57 (d, 1H, ArH), 7.35–7.15 (m, 7H, ArH), 7.10 (d, 2H, ArH),
6.92 (s, 3H, ArH), 6.86–6.65 (m, 4H, ArH), 6.09 (s, 1H,
C¼¼CH2), 5.54 (s, 1H, C¼¼CH2), 4.15 (s, 2H, ACH2A), 3.91 (s,
2H, ACH2A), 3.80 (s, 3H, AOCH3), 1.93 (s, 3H, ACH3), 1.70
(s, 4H, ACH2CH2A), 1.25 (s, 4H, ACH2CH2A). 1H NMR of
compound (4) (300 MHz, DMSO-d6), d (TMS, ppm): 7.98–
7.83 (m, 4H, ArH), 7.60 (m, 2H, ArH), 7.13 (d, 2H, ArH), 6.01
(s, 1H, C¼¼CH2), 5.66 (s, 1H, C¼¼CH2), 4.34 (s, 1H, ArCBH),
4.09–4.15 (m, 4H, AOCH2A), 1.87 (s, 3H, ACH3), 1.80–1.70
(m, 2H, ACH2A), 1.70–1.58 (m, 2H, ACH2A), 1.42 (m, 4H,
ACH2A).
EXPERIMENTAL SECTION
Materials
All chemicals were purchased from Shanghai Chemical Rea-
gent Co., Shanghai, China and used as received unless other-
wise stated. Tetrahydrofuran (THF), toluene and methylene
chloride were dried and distilled with standard methods
before use. 2,20-Azobis(isobutyronitrile) (AIBN, ꢀ98%) were
recrystallized twice from ethanol before use. Tetrabutylam-
monium fluoride (99%, J&K Chemical) was recrystallized
from ethyl acetate. Copper (I) bromide (CuBr, 98%) was
washed with acetic acid and acetone, and then dried in vac-
uum. 3-Ethynylaniline (ꢀ98%, Aldrich), sodium azide
(99.5%, Aldrich), sodium ter-butoxide (ꢀ98%; Acros), palla-
dium (II) acetate (J&K Chemical, 99%), diphenylamine, 3-
bromophenol, and tri-t-butylphosphine (J&K Chemical,
ꢀ99%, 10 wt % in hexane) were used as received. 2-Cyano-
prop-2-yl-1-dithionaphthalate (CPDN, 97%) was synthesized
according to the method as reported.18
4-(Diphenylamino)phenyl 6-azidohexanoate
(compound (5))
Compound (5) was synthesized through the acylation reac-
tion of compound (2) with 6-bromohexanoyl chloride and
then azide reaction. Detailed synthetic procedures were simi-
lar with those as reported.20 FTIR (KBr): cmax/cmꢂ1 2930,
2860, 2100, 1760, 1600, 1500, 1460, 750, and 700.
(4-(Diphenylamino)phenyl-6-(4-(3-((4-((6-(methacryloylox-
y)hexyl)oxy)phenyl) diazenyl)phenyl)-1,2,3-triazol-1-
yl)hexanoate) (DPMMA)
The mixture of compound (4) (2.2 g, 5.5 mmol), compound
(5) (2.0 g, 5.0 mmol), CuBr (0.0025 g, 0.500 mmol)
and N,N,N0,N00,N000-pentamethyldiethylenetriamine (PMDETA,
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SCHEME 1 The synthetic routes of DMMA, DPMMA, PDMMA and PDPMMA.
105 lL, 0.500 mmol) were dissolved in 10 mL THF in a 25-
mL single-necked flask. The reaction mixture was vigorously
stirred in deoxygenated system at room temperature for
24 h. After that, the content was evaporated in a reduced
pressure and purified by column chromatography (silica gel,
ethyl acetate/petroleum ether ¼ 1:8) to yield a yellow solid
(7.9 g, 90%). 1H NMR (300 MHz, CDCl3), d(TMS, ppm): 8.15
(s, 1H, AC2HN3A), 8.08–7.85 (m, 4H, ArH), 7.25 (d, 8H,
ArH), 7.22A6.91 (m, 10H, ArH), 6.11 (s, 1H, C¼¼CH2), 5.56 (s,
1H, C¼¼CH2), 4.18 (t, 2H, ACH2A), 4.06 (t, 2H, ACH2A), 1.95
(s, 3H, ACH3), 1.85 (t, 2H), 1.73 (t, 2H, ACH2A), 1.54 (d, 6H,
ACH2A), 1.21 (s, 8H, ACH2A).
dissolved in 2 mL of THF, and then poured into 400 mL of
methanol. The polymer (PDMMA) was obtained by filtration
and then dried in vacuum overnight at 40 C. The monomer
conversion is 74.0% as determined gravimetrically. The
RAFT polymerization procedure of DPMMA is similar with
those described above. The monomer conversion is 63.0%.
ꢃ
Characterization
1H NMR spectra were recorded on a Inova 300-MHz NMR
instrument, using deuterated chloroform (CDCl3) or deuter-
ated dimethyl sulfoxide (DMSO-d6) as the solvent, TMS as
the internal standard. The number-average molecular weight
(Mn,GPC) and polydispersity index value (PDI ¼ Mw/Mn) of
the polymers were determined with a Waters 1515 gel per-
meation chromatograph (GPC) equipped with a refractive
index detector, HR1(pore size: 100 Å, 100–5000 Da),
HR2(pore size: 500 Å, 500–20, 000 Da), and HR4 (pore size
10, 000 Å, 50–100, 000 Da) columns (7.8 ꢁ 300 mm, 5-lm
beads size) with a molecular weight range of 100–500 000
and calibrated with poly(methyl methacrylate) (PMMA) as
the standard sample. Tetrahydrofuran (THF) was used as the
eluent at a flow rate of 1.0 mL/min at 30 ꢃC. FTIR spectra
RAFT Polymerization
The typical procedure of RAFT polymerization is as follows:
DMMA (0.5 g, 0.8900 mmol), AIBN (1.46 mg, 0.0089 mmol),
and CPDN (8.13 mg, 0.0267 mmol) were dissolved in 1.5 mL
of toluene in a 5-mL ampoule tube. After purging with argon
for about 10 min to eliminate the oxygen, the ampoule tube
was sealed and placed in an oil bath held by a thermostat at
75 ꢃC to polymerize. After designed time interval, the tube
was cooled in ice water and then opened. The contents were
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TABLE 1 Macromolecular Characteristics and Photoisomerization Data of Polymers (PDMMA and PDPMMA)
Entry
[Monomer]0/[AIBN]0/[CPDN]0
Mn,th (g/mol)
Mn,GPC (g/mol)
Mw/Mn
ke sꢂ1
cis/trans
PDMMA
100/1/3
100/1/3
14200
16900
13600
18000
1.16
1.18
0.014
0.010
1.92
0.73
PDPMMA
Mn,th is the theoretical molecular weight.
Mn,GPC is the number average molecular weight measures by GPC using
The cis/trans is the cis/trans ratio of azobenzene in the photostationary
state on the basis of the p-p* absorbance.
PMMA as the standard.
M
w/Mn is molecular weight distribution measured by GPC. ke is the
first-order rate constant.
were recorded on a Nicolette-6700 FTIR spectrometer. Ultra-
violet visible (UV-vis) absorption spectra were determined
on a Hitachi U-3900 spectrophotometer at room tempera-
ture. The fluorescence emission spectra of the polymers
were obtained on a PerkinElmer LS-50B fluorescence spec-
trophotometer at room temperature. The spherical aggregate
size analysis was performed using dynamic light scattering
(DLS) measurements (Zetasizer Nano ZS: Malvern Instru-
ment, UK) at 20 ꢃC. The micellar solutions were filtered
through a 0.45-lm syringe filter before measurements. Cyclic
voltammograms (CV) were carried out with an electrochemi-
cal analyzer (CHI630B, Shanghai Chenghua instrument Co.
China) using C16H36F6NP (tetrabutylammonium hexafluoro-
phosphat) (0.1 M) as the supporting electrolyte and calomel
electrode as reference electrode at a scan rate of 0.1 V/s in
CH2Cl2. The measurement was conducted at room tempera-
ture under an oxygen-free atmosphere.
where e0 is the permittivity of free space, er is the dielectric
constant of the material, lh is the hole mobility, V is the
voltage drop across the device, and L is the thickness of
active layer.
RESULTS AND DISCUSSION
Preparation of PDMMA and PDPMMA via RAFT
Polymerizations
The synthetic routes of the polymers, PDMMA and PDPMMA,
are presented in Scheme 1. PDMMA and PDPMMA were syn-
thesized via RAFT polymerization of DMMA and DPMMA in
ꢃ
toluene at 75 C using CPDN as the RAFT agent and AIBN as
the initiator. The polymerization data of DMMA and DPMMA
were given in Table 1. It can be noted that the molecular
weights measured by GPC (Mn,GPC) of these two polymers
were very close to the corresponding theoretical values
(Mn,th). The GPC elution curves of PDMMA and PDPMMA
given in Figure 1 exhibited narrow and symmetric shapes
with relatively narrow molecular weight distributions (Mw/
Mn). Furthermore, the structures of PDMMA and PDPMMA
Fabrication and Characterization of Devices
for Hole Mobility
For device fabrication, indium tin oxide (ITO) coated glass
substrates were cleaned stepwise in semiconductor deter-
gent, deionized water, chloroform, acetone, and isopropanol
and then dried in a 110 ꢃC oven for 20 min, subsequently,
the substrates were treated with UV ozone for 5 min before
use. A thin layer (ca. 39 nm) of poly(ethylenedioxythio-
phene): polystyrenesulfonate (PEDOT: PSS, Baytron P VP AI
4083) was spin-coatꢃed (3500 rpm) onto the ITO substrates.
After baking at 125 C for 20 min in air, the substrates were
transferred to a N2-filled glovebox. The active layers (ꢄ90
nm) were spin-coated on the PEDOT: PSS layer from the
polymer solution in chloroform with weight concentration of
15 mg/mL before or after UV irradiation, respectively. The
devices were ready for measurement after thermal deposi-
tion of a 40 nm-thick film of Au at a pressure of 1 ꢁ 10ꢂ6
mbar. The effective layer area of one cell was 0.12 cm2. For
mobility measurements, the hole-only devices in a configura-
tion of ITO/PEDOT: PSS (39 nm)/polymer/Au (40 nm) were
fabricated. The current density-voltage (J-V) characteristics
were measured using a Keithley 2400 source meter. All
measurements were performed under ambient atmosphere
at room temperature. The hole mobility was determined by
fitting J-V curve into the space-charge-limited current (SCLC)
method21 based on the formula (1).
1
were also characterized by H NMR spectra as shown in Fig-
ure 2(A,B). The chemical shifts at d ¼ 8.14–8.06 ppm (a)
were attributed to the protons of the naphthaline groups (a)
in the end of polymer chains, which indicated that the RAFT
agent species were sucessfully attached to the chain ends of
PDMMA and PDPMMA, respectively. These results confirmed
FIGURE 1 GPC curves of PDMMA (Mn,GPC ¼ 13600 g/mol, Mw/
9
V2
h L3
Mn ¼ 1.16) and PDPMMA (Mn,GPC ¼ 18000 g/mol, Mw/Mn
¼
J ¼ ere0l
(1)
1.18).
8
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the absorption intensity of n-p* transition band correspond-
ing to cis-azobenzene gradually increased. The solutions of
PDMMA and PDPMMA reached the cis-rich stationary states
after 365 nm UV light irradiation for 5 and 8 min, respec-
tively. The cis/trans ratio in the photostationary state was
estimated from Figure 3(a,b) with the formula (A0-A )/A ,
1
1
where A and A0 are the p-p* absorbance of PDMMA and
1
PDPMMA at time infinite and time zero. The cis/trans ratio
of PDMMA and PDPMMA was 1.92 and 0.73, respectively.
The lower cis/trans ratio of PDPMMA may be ascribed a
higher trans-to-cis transition resistance existed in PDPMMA
with more flexible spacers.23 The kinetic plots of trans-to-cis
photoisomerization of PDMMA and PDPMMA were plotted in
Figure 3(c), which indicted the first-order kinetic plots of the
PDMMA and PDPMMA. Furthermore, the first-order rate con-
stant (ke) deduced from Figure 3(c) was determined by for-
mula (2), respectively.
A
1 ꢂ At
ln
¼ ꢂket
(2)
A
1 ꢂ A0
where A , A0, and At are absorbance of p-p* at time infinite,
1
time zero, and time t, respectively. The trans-to-cis photoiso-
merization rate constant (ke) of PDMMA and PDPMMA was
0.014 and 0.010 sꢂ1, respectively. The rate constant of
PDPMMA was lower than that of PDMMA. It may be inter-
preted as the higher degree of inter-chain entanglements
existed in PDPMMA with more flexible spacers resulting in a
higher trans-to-cis transition resistance compared with that
of PDMMA.23
Fluorescence Emission
Some researchers reported that the azobenzene-containing
polymers showed the light-driven or aggregation-induced flu-
orescence emission enhancement.24–26 In this system, when
PDMMA and PDPMMA solutions were excited by 395 nm
light corresponded to the n-p* transition band of azobenzene
group in PDMMA, almost no fluorescence emission was
observed (Figure 4). However, as PDMMA and PDPMMA sol-
utions were exposed to UV light of 365 nm (0.4 mW/cm2),
the fluorescence emission of PDMMA (centered at 560 nm)
and PDPMMA (centered at 506 nm) continually increased
with irradiation time, respectively. The PDMMA and
PDPMMA solutions before and after UV light irradiation
were examined by DLS to clarify the cause of the fluores-
cence enhancement. The results shown in Figure 5 indicated
that nano-scale aggregations were formed after irradiation
with 365 nm light. The aggregation size was 63 nm for
PDMMA and 92 nm for PDPMMA when reaching the photoi-
somerization equilibrium state. And the aggregation size
reached 79 and 113 nm for PDMMA and PDPMMA solution,
when prolonging the UV light irradiation time to 10 and 16
min, respectively. Normally, the azobenzene compound is
nonfluoresent due to the fast nonradiative relaxation process
originated from fast and high efficient tans-to-cis isomeriza-
tion for excited azobenzene.26 However, the aggregation will
hinder the excited azobenzene relaxation in a nonradiative
way, which resulted in the fluorescence emission.27 The
FIGURE 2 Typical 1H NMR specta of PDMMA (A, solvent
CDCl3, Mn,GPC ¼ 13600 g/mol, Mw/Mn ¼ 1.16) and PDPMMA (B,
solvent CDCl3, Mn,GPC ¼ 18000 g/mol, Mw/Mn ¼ 1.18) prepared
via RAFT polymerization using CPDN as RAFT agent and AIBN
as the initiator.
that the well-controlled PDMMA and PDPMMA were success-
fully prepared by RAFT polymerization.
Photoisomerization
PDMMA and PDPMMA exhibited good solubility in several
organic solvents, such as CHCl3, CH2Cl2, THF, and so on. The
UV-vis absorbance spectra of PDMMA and PDPMMA in
CH2Cl2 were examined, as shown in Figure 3(a,b), respec-
tively. The strong peak at 302 nm corresponds to the TPA
unit.22 Two absorption peaks (centered at 359 and 402 nm)
in Figure 3(a) were attributed to the transition bands of p-
p* and n-p* related to the groups of trans-azobenzene and
cis-azobenzene in PDMMA, while two absorption peaks (cen-
tered at 348 nm and 448 nm) in Figure 3(b) were attributed
to the transition bands of p-p* and n-p* related to the
groups of trans-azobenzene and cis-azobenzene in PDPMMA.
Upon 365 nm UV light irradiation (0.4 mW/cm2), the
absorption intensity of p-p* transition bands corresponding
to trans-azobenzene groups significantly decreased, and both
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FIGURE 3 UV-vis spectra of PDMMA (a) and PDPMMA (b) in CH2Cl2 (Cn ¼ 3 ꢁ 10ꢂ5 mol/L azo mesogens) at ambient temperature
as a function of 365 nm light irradiation with different time. (c) Kinetic plots of the trans-to-cis photoisomerization of azobenzene
upon UV irradiation for PDMMA and PDPMMA.
aggregates formation of the azobenzene-containing polymers
after UV irradiation should be attributed to the increasing
polarity induced by isomerization of azobenzene from its
planar rod-shaped trans-form to the nonplanar bent-shaped
cis-form, which resulted in a poor solubility of polymers in
non-polar CH2Cl2.28
Electrochemical Behavior
CV was used to investigate the electrochemical behavior of
polymer solutions before and after UV irradiation. Previous
studies demonstrated that the azobenzene moiety possesses
electrochemical activity.29 Fujishima and coworkers29
reported that the reduction potential of gold electrode modi-
fied with azobenzene functionalized SAMs was lower than
ꢂ0.6 V. When the scanning potential was much higher than
the reduction potential of azobenzene, it can avoid the elec-
trochemical reduction from azobenzene to hydroazoben-
zene.30 Thus, the positive scanning potential was employed
in our present work to avoid the reduction of azobenzene
group in polymer. Figure 6(a,b) showed the CV curves of
Furthermore, bathochromic shift can be seen from the maxi-
mum emission peaks of PDMMA (centered at 560 nm) and
PDPMMA (centered at 506 nm) in Figure 4. The cause of it
may be that the azobenzene group conjugates with the lone
electron pair of nitrogen in TPA moiety in case of PDMMA,
which leads to the increase of the conjugated length and
hence red-shift of the maximum fluorescence emission peak.
FIGURE 4 Fluorescence emission spectra of PDMMA (a) (Mn,GPC ¼ 13600 g/mol, Mw/Mn ¼ 1.16) and PDPMMA (b) (Mn,GPC ¼ 18000
g/mol, Mw/Mn ¼ 1.18) in CH2Cl2 (Cn ¼ 3 ꢁ 10ꢂ5 mol/L azo mesogens) at ambient temperature (kex ¼ 395 nm) under 365 nm light
irradiation (0.4 mW/cm2) with different time.
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FIGURE 5 DLS curves for the distributions of aggregation sizes of PDMMA (a) and PDPMMA (b).
PDMMA and PDPMMA in CH2Cl2 before and after UV irradia-
tion, respectively, and the corresponding data were summar-
ized in Table 2. For better comparison, the solutions used for
testing the CV before and after UV irradiation were also ana-
lyzed by the UV-vis absorption spectra as given in Supporting
Information Figure S1. The both peak currents at 0.8–1.0V
attributing to the TPA in PDMMA and PDPMMA increased af-
ter UV irradiation, which may be caused by the increased
concentration of TPA unit around the Pt electrode due to the
aggregation in solution after UV irradiation. The difference
between the CV curves of PDMMA and PDPMMA was
observed from Figure 6. The reversible redox activity of
PDPMMA was better than that of PDMMA. For PDMMA, the
absence of corresponding reduction peak at 0.8–1.0V indi-
cated poor activities of reversible reduction. The oxidation
peak potential (Eox) of PDMMA solution decreased from 0.97
to 0.87 V after UV light irradiation for 12 min, which could
indicate the reduction ability (that is, the electron-donating
ability) of TPA in PDMMA was increased after UV irradiation.
For PDMMA, the TPA can be well conjugated with trans-azo-
benzene group by adopting a planar structure before UV
irradiation, as a result, the lone electron pair of the nitrogen
atom of TPA was scattered to conjugated system. When the
trans-form of azobenzene groups translated to cis-form in
PDMMA after UV irradiation, the conjugation between TPA
and azobenzene was greatly weaken because the phenyl
rings of cis-form twisted around the plane of the nitrogen–
nitrogen double bond.31,32 That is, cis-form isomer could pre-
vent the electron density of the nitrogen atom of TPA scat-
tere as compared with that of trans-form, causing the
increase of electron-donating ability of TPA in PDMMA after
UV irradiation. In addition, a new pair of redox peaks at 0.2-
0.4V after UV irradiation [Fig. 6(a)] may be belonged to the
structures of quinone/hydroquinone derived from TPA unit
incorporated directly with azobenzene. The detailed reasons
need further study. The redox peak potentials (Eox and Ered
)
of TPA in PDPMMA had a little difference before and after
UV irradiation as shown in Figure 6(b), which may be attrib-
ute to the weak effect of photoisomerization of the azoben-
zene on the TPA in PDPMMA, where the TPA moiety was
separated from the azobenzene moiety by a flexible spacer.
To verify the electrochemical behavior of the TPA moieties in
the PDMMA and PDPMMA, the molecular orbital distribution
FIGURE 6 Cyclic voltammograms of PDMMA (a) (Mn,GPC ¼ 13600 g/mol, Mw/Mn ¼ 1.16) and PDPMMA (b) (Mn,GPC ¼ 18000 g/mol,
Mw/Mn ¼ 1.18) in CH2Cl2 (Cn ¼ 1 ꢁ 10ꢂ4 mol/L azo mesogens) containing n-Bu4NPF6 (0.1 M) before and after UV irradiation (0.4
mW/cm2).
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TABLE 2 The Data of Cyclic Voltammograms (CV) and Hole Mobility of Polymers (PDMMA
and PDPMMA)
Hole mobilityc
(cm2 Vꢂ1 Sꢂ1
)
Entry
EOX
V
IOX lA
EHOMO (eV)
Lb (nm)
PDMMA
0.97
0.87
1.00
1.00
ꢂ0.96
ꢂ1.19
ꢂ1.23
ꢂ1.80
ꢂ4.915
ꢂ4.905
ꢂ5.003
ꢂ5.006
89.5
83.0
96.5
83.0
4.83ꢁ10ꢂ6d
2.87ꢁ10ꢂ6e
3.66ꢁ10ꢂ6d
3.18ꢁ10ꢂ6e
PDMMA-UVa
PDPMMA
PDPMMA-UVa
a
The polymer solution are under UV irradiation (0.4 mW/cm2).
b
c
Thickness of the polymer film.
Hole mobility of these polymers in the BHJ blends was calculated via the SCLC method.
Spin coated from a 15 mg/mL chloroform solution.
d
e
Spin coated from a 15 mg/mL chloroform solution with UV irradiation to sufficiently reach a cis-rich sta-
tionary state.
calculations of the side-chain in the PDMMA and PDPMMA
were carried out by using quantum mechanical package
Gaussian 03, respectively. The optimum geometry and HOMO
energies (EHOMO) (HOMO: the highest occupied molecular or-
bital) of the TPA moiety incorporated with cis-isomer or
trans-isomer of azobenzene in the PDMMA and PDPMMA are
calculated by using the density functional theory (DFT) as
approximated by the B3LYP functional and employing the 6-
31G* basis set33 as shown in Figure 7 and Table 2. The elec-
tron-donating abilities of the TPA moieties in PDMMA and
PDPMMA before and after UV irradiation can be represented
by their oxidation peak potentials (Eoxs), which are related
to the corresponding HOMO energy levels (EHOMOs). The
both EHOMOs of cis- and trans-isomers in PDPMMA had a lit-
tle difference (ꢂ5.003 eV for the trans-isomer and ꢂ5.006
eV for the cis-isomer), and the corresponding Eoxs after and
before UV irradiation were both 0.96 V. The EHOMO (ꢂ4.905
eV) of the cis-isomer in PDMMA was a little higher than that
of the trans-isomer (ꢂ4.915 eV), and the Eox (0.87 V) after
UV irradiation was lower than that (0.97 V) before UV irradi-
ation. The HOMO energy levels obtained by theoretical calcu-
lation were basically consistent with corresponding oxidation
peak potentials (Eoxs) obtained by CV measurement.
Hole Mobility
Hole mobility is a key parameter to evaluate the carrier
transporting property of the hole transport material. TPA
unit has the good hole-mobility due to the easy oxidizability
of the nitrogen center, which can facilitate to transport posi-
tive charge via the radical cation species. The current-voltage
FIGURE 7 Optimized geometries for TPA moieties incorporated with cis-isomer and trans-isomer of azobenzene in the side-chain
of PDMMA and PDPMMA. (a) and (b): trans-isomer and cis-isomer in the side-chain of PDMMA; (c) and (d) trans-isomer and cis-
isomer in the side-chain of PDPMMA.
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REFERENCES AND NOTES
(J-V) curves of hole devices of the PDMMA and PDPMMA
before and after UV irradiation were given in Supporting In-
formation Figure S2. And the corresponding data of the hole
mobilities calculated by the SCLC method were presented in
Table 2. The testing samples were prepared from the poly-
mer solution (CHCl3) before or after UV irradiation by spin-
coating. The hole mobilities before UV irradiation were 4.83
ꢁ 10ꢂ6 cm2Vꢂ1 Sꢂ1 for PDMMA and 3.66 ꢁ 10ꢂ6 cm2Vꢂ1
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effect on the hole mobilities of both polymers. These results
may be ascribed to the restraint from the azobenzene moi-
eties on the hole transport of TPA.
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The financial support from the National Natural Science Foun-
dation of China (Nos. 21074082 and 21104052), the Special-
ized Research Fund for the Doctoral Program of Higher
Education contract grant (No.20103201110005), the Project of
Science and Technology Development Planning of Suzhou (Nos.
SYG201026 and SYG201111), the Project of International Coop-
eration of the Ministry of Science and Technology of China (No.
2011DFA50530), the Qing Lan Project, the Program of Innova-
tive Research Team of Soochow University, and the Project
Funded by the Priority Academic Program Development of
Jiangsu Higher Education Institutions (PAPD) are gratefully
acknowledged.
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