Fluorine-substituted azobenzene destabilizes polar form of optically switchable fulgimide unit in copolymer system

Article abstract of DOI:10.1002/pola.23926

Fulgimide and various size and electronic nature of substituents on the terminal position of azobenzene in the pendant homo/copolymethacrylates were synthesized. Differential scanning calorimetry analysis indicates the homopolymer possessing C-form fulgimide unit exhibited higher Tm than that of E-form of the homopolymer and revealed C-form is highly ordered. Thermal stability suggests azobenzene homopolymers with electron donating substituents have high thermal stability than electron withdrawing substituents. Polarized optical microscopy observation disclosed homopolymers viz., NI, CY, FL, ME, and T-ME exhibited liquid crystalline mesophases between their T_m and T_i. Optical properties of homo/copolymers were investigated by UV-vis and fluorescence spectroscopy. UV-vis spectroscopy displayed C-form fulgimide absorption in F-co-FL around 482 nm which is around 40 nm lesser than C-form of substituted azobenzene copolymers. Similarly, fluorescence pattern of F-co-FL by UV irradiation exhibited emission intensity slowly increased to certain level then decreases with two new emissions at 430 and 480 nm attributed to terminal position of fluorine atom on azobenzene destabilizes polar form (C-form) fulgimide unit in the copolymer.

Full text of DOI:10.1002/pola.23926

Fluorine-Substituted Azobenzene Destabilizes Polar Form of Optically
Switchable Fulgimide Unit in Copolymer System
SARAVANAN CHINNUSAMY, KANNAN PALANINATHAN
Department of Chemistry, Anna University, Chennai, Tamilnadu 600 025, India
Received 25 October 2009; accepted 7 January 2010
DOI: 10.1002/pola.23926
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Fulgimide and various size and electronic nature of
substituents on the terminal position of azobenzene in the
pendant homo/copolymethacrylates were synthesized. Differ-
ential scanning calorimetry analysis indicates the homopoly-
mer possessing C-form fulgimide unit exhibited higher Tm
than that of E-form of the homopolymer and revealed C-form
is highly ordered. Thermal stability suggests azobenzene
homopolymers with electron donating substituents have high
thermal stability than electron withdrawing substituents. Polar-
ized optical microscopy observation disclosed homopolymers
viz., NI, CY, FL, ME, and T-ME exhibited liquid crystalline
mesophases between their Tm and Ti. Optical properties of
homo/copolymers were investigated by UV–vis and fluores-
cence spectroscopy. UV–vis spectroscopy displayed C-form ful-
gimide absorption in F-co-FL around 482 nm which is around
40 nm lesser than C-form of substituted azobenzene copoly-
mers. Similarly, fluorescence pattern of F-co-FL by UV irradia-
tion exhibited emission intensity slowly increased to certain
level then decreases with two new emissions at 430 and 480
nm attributed to terminal position of fluorine atom on azoben-
zene destabilizes polar form (C-form) fulgimide unit in the co-
C
V
polymer.
2010 Wiley Periodicals, Inc. J Polym Sci Part A:
Polym Chem 48: 1565–1578, 2010
KEYWORDS: azobenzene;
azopolymer; copolymer; donor-
acceptor; fulgimide; fluorescence quench; fluoropolymers;
photochemistry; photoisomerization; structure-property rela-
tions; thin films
INTRODUCTION Photochromic molecules often carry consid-
erable attention as molecular switches and as control ele-
ment in molecular devices.1 Recently, many advances have
been made in the development of organic photochromic com-
pounds and their derivatives such as diarylethene,2,3 fulgides/
fulgimide,4,5 azobenzene,6 and spiropyran.7 Photochromic ful-
gides have been synthesized with aromatic heterocyclic com-
pounds such as furan, thiophene, pyrrole, indole, and thiazole
ring and their properties described.8–10 Among them, indolyl-
based fulgides are known for their high thermal stability and
enhanced resistance to photochemical fatigue.11 In particular,
fluorine-substituted indolylfulgide meet many of the require-
ments for optical memory media including high resistance to
photochemical degradation and efficient quantum yield.12
Newly discovered fluorine-substituted indolylfulgide displayed
enhanced photochemical stability compared to nonfluorine
containing analogues and exhibited strong bathochromic shift
in the cyclizable Z-form.13 Wolak et al.14 have synthesized a
series of fluorine-substituted indolylfulgimides using nitrogen
atom as a linker to connect substituted phenyl ring of increas-
ing cumulative electron withdrawing ability.
density data storage, and nonlinear optics.15,16 Ho et al.17
have demonstrated azobenzene with electron-donor and
electron-acceptor substituents as the best candidate in terms
of level and stability of photoinduced birefringence. Azoben-
zene-substituted polythiophenes have revealed a novel re-
versible dual photochromic behavior, where trans–cis–trans
isomerization of azobenzene in the side chain leads to
change of UV–visible absorption band on both side chain and
main chain polymers.18 Jin et al.19 established fluorescence
modulation in azobenzene-substituted triphenylpyrazoline
derivative and indicated intramolecular electronic energy
transfer from pyrazoline moiety to azobenzene and degree of
energy transfer was determined by content of cis-isomer of
azobenzene moiety. Substituent present in the terminal posi-
tion of azobenzene can alter trans–cis–trans isomerization
behavior upon irradiation with UV/Vis light.20 Nitro group
present in the terminal position of azobenzene stabilizes po-
lar form of fulgimide unit in copolymethacrylate system.21
Further, to investigate the effect of substituent present in
azobenzene unit on polar form of fulgimide unit in copoly-
methacrylate system, an attempt has been made to synthe-
size various substituted azobenzenes and fulgimide contain-
ing copolymethacrylates. Optical properties of homo/
copolymers were examined with respect to substituent on
Azobenzene derivatives were widely investigated as promis-
ing molecule as switching element for microelectronics, high-
Additional Supporting Information may be found in the online version of this article. Correspondence to: K. Palaninathan (E-mail: pakannan@
annauniv.edu)
C
V
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 1565–1578 (2010) 2010 Wiley Periodicals, Inc.
FLUORINE-SUBSTITUTED AZOBENZENE IN COPOLYMER SYSTEM, CHINNUSAMY AND PALANINATHAN
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
the azobenzene unit. Surprisingly, the fluorine-substituted
azobenzene destabilizes polar form fulgimide unit in the co-
polymer system upon irradiation with 360 nm UV light and
discussed.
10-(4-(Phenyldiazenyl)phenoxy)decan-1-ol (20)
A suspension of anhydrous K2CO3, pinch of KI, and com-
pound 13 (0.02 mol, 3.96 g) in dry DMF (80 mL) are stirred
at 90 ꢀC for 1 h. 10-Bromo-1-decanol (0.024 mol, 4.78 mL)
was added drop wise to the reaction mixture and maintained
at that temperature for 48 h. Reaction mixture was poured
into ice/water mixture, precipitated solid filtered, and dried.
Crude product was further purified by column chromatogra-
phy (silica gel, chloroform). Eluent was evaporated to give
yellow colored solid (Yield 68%).
EXPERIMENTAL
Materials
4-Flouroaniline, 4-chloroaniline, 4-cyanoaniline, 4-nitroani-
line, 4-methoxyaniline, and propargil alcohol (Sigma–Aldrich,
Bangalore) were used as supplied. Aniline, methanol, ethanol,
phenol, THF, diethyl ether, chloroform, triethylamine, N,N-
dimethylformamide (SRL, India), and other solvents were
purified following reported procedures.22 Diethyl succinate,
acetone, t-butyl alcohol, potassium tert-butoxide, dimethyl
sulfate, sodium hydride, sodium hydroxide, methacrylic acid,
hydroquinone, and 2,20-azobisisobutyronitrile (AIBN) (Merck,
Germany) were used as received. 1,10-Decanediol, hexame-
thyldisilazane (HMDS), and 4-hydroxyacetanilide (Fluka,
Switzerland) were used as supplied. The compounds 13, 14,
15, and 18 were prepared by the reported procedures.23
The monomers (f, ni, cy, t-me), homopolymers (NY, CY, T-
ME), and copolymers (F-co-CN, F-co-NI, F-co-T-ME) were
synthesized by adopting the similar procedure reported
recently by us.21,24
FTIR (KBr, cmꢁ1): 3364 (AOH), 2942, 2872 (CH, aliphatic),
1533 (aromatic asym str), 1252 (PhAOACH2A), 1236
(CAOAC). 1H NMR (300 MHz, CDCl3) d: 7.82–7.89 (m, 4H,
ArAN¼¼NA), 7.52 (d, 2H, Ar), 7.21 (m, 1H, Ar), 6.94 (d, 2H,
Ar), 3.95 (t, 2H, ArAOACH2A), 3.58 (t, 2H, ACH2AOH), 1.15–
1.68 (m, 16H, ACH2A(CH2)8ACH2A). 13C NMR (CDCl3) d
(ppm): 162.8, 156.3, 147.8, 147.2, 125.1, 123.7, 113.6 (ArAC),
68.4, 63.2 (AOACH2), 33.5, 29.5, 28.8, 27.6, (aliphatic).
10-Hydroxydecyloxy-4-phenylazo-40-flurobenzene (21)
Compound 21 was prepared from compound 14 by using
similar procedure as that of 20, (Yield 62%).
FTIR (KBr cmꢁ1): 3361 (AOH), 2941, 2875 (CH, aliphatic),
1536 (aromatic asym str), 1261 (PhAOACH2A), 1241
(CAOAC), and 843 (CAH out of plane). 1H NMR (300 MHz,
CDCl3) d: 7.87 (d, 4H, Ar), 7.14 (d, 2H, Ar), 6.98 (d, 2H, Ar),
3.91 (t, 2H, ArAOACH2A), 3.56 (t, 2H, ACH2AOH), 1.32–
1.82 (m, 16H, ACH2A (CH2)8ACH2A). 13C NMR (300 MHz,
CDCl3) d: 165.3, 162.8, 149.5, 146.8, 124.7, 124.5, 114.8
(ArAC), 68.5, 63.3 (AOACH2), 32.5, 29.6, 29.3, 26.9, 25.8
(aliphatic).
Instrumentation
Infrared spectra were obtained on a Bruker IFS 66V Fourier
transform spectrometer using KBr pellet. High-resolution 1H
and 13C NMR spectra were recorded on a Bruker 300MHz
spectrometer in CDCl3 with TMS as an internal standard.
Weight-average molecular weight (Mw), number-average mo-
lecular weight (Mn), and polydispersity (Mw/Mn) of polymers
were obtained on a PL-GPC model 210 chromatograph using
DMF as eluent at 25 ꢀC and calibrated with polystyrene
standard. Thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC) of polymers were carried out on
10-Hydroxydecyloxy-4-phenylazo-40-chlorobenzene (22)
Compound 22 was prepared from compound 15 by using
similar procedure as that of 20, (Yield 63%).
FTIR (KBr, cmꢁ1): 3361 (AOH), 2941, 2872 (CH, aliphatic),
1537 (aromatic asym str), 1264 (PhAOACH2A), 1245
(CAOAC), and 844 (CAH out of plane). 1H NMR (300 MHz,
CDCl3) d: 7.83 (d, 4H, Ar), 7.18 (m, 4H, Ar), 6.96 (d, 2H, Ar),
3.98 (t, 2H, ArAOACH2A), 3.56 (t, 2H, ACH2AOH), 1.30–1.79
(m, 16H, ACH2A(CH2)8ACH2A). 13C NMR (300 MHz, CDCl3)
d: 165.1, 162.6, 149.2, 146.4, 124.8, 124.7, 114.5 (ArAC),
68.5, 63.3 (AOACH2), 32.5, 28.9, 28.3, 26.9, 25.4 (aliphatic).
a
Seiko thermal analyzer (SSC/5200H) under nitrogen
ꢀ
atmosphere. Measurementꢁw1 as performed up to 800 C at a
heating rate of 10 ꢀC min for TGA and up to 400 ꢀC at a
heating rate of 5 ꢀC minꢁ1 for DSC analysis. Texture of
homo/copolymer samples were studied using a Euromex
polarizing optical microscope equipped with
a Linkem
HFS91 heating stage and a TP-93 temperature programmer.
Absorbance spectra of polymers in chloroform solution and
in thin film were measured on a Shimadzu UV-260 spectro-
photometer. Photoisomerization between trans and cis-form
of azobenzene unit, 6p-electrocyclization reaction of fulgi-
mide unit were carried out using a 500-W high-pressure Hg
lamp (Ushio SX-UI 5000) equipped with a cut filter (Sigma,
UTVAF-35U) for UV irradiation (365 nm) and with a cut fil-
ter (Sigma, DIF-GRE) for visible light (560 nm). Fluorescence
spectra of polymers were measured on a PerkinElmer LS-45
fluorescence spectrofluorimeter with excitation near and
absorption maximum of azobenzene and fulgimide moieties
using 5 nm resolutions for both excitation and emission.
Change of fluorescence switching cycles of fulgimide unit
was carried out using similar filter of UV/Vis light.
10-Hydroxydecyloxy-4-phenylazo-40-methoxybenzene
(25)
Compound 25 was also prepared from compound 18 by
using similar procedure as that of 20, (Yield 66%).
FTIR (KBr, cmꢁ1): 3352 (AOH), 2940, 2871 (CH, aliphatic),
1312 (ArAOCH3), 1533 (aromatic asym str), 1266
(PhAOACH2A), 1243 (CAOAC), and 838 (CAH out of
plane). 1H NMR (300 MHz, CDCl3) d: 7.85 (d, 4H, Ar), 6.99
(d, 2H, Ar), 4.02 (t, 2H, ArAOACH2A), 3.88 (s, 3H,
ArAOCH3), 3.38 (t, 2H, ACH2AOH), 1.32–1.85 (m, 16H,
ACH2A(CH2)8ACH2A). 13C NMR (300 MHz, CDCl3) d: 163.1,
161.2, 148.4, 146.1, 125.1, 124.2, 124.6, 115.4 (ArAC), 68.7,
63.6 (AOACH2), 32.6, 28.5, 27.6, 26.3, 25.2, 24.8 (aliphatic).
1566
INTERSCIENCE.WILEY.COM/JOURNAL/JPOLA

ARTICLE
Synthesis of Monomers hy, fl, cl, and me
1632 (AC¼¼CH2), 847 (p-substituted aromatic). 1H NMR
(300 MHz, CDCl3) d: 7.86 (d, 4H, Ar), 7.08 (d, 4H, Ar), 6.10
(s, 1H, AC¼¼CH2), 5.53 (s, 1H, AC¼¼CH2), 4.12 (t, 2H,
ArAOACH2), 3.88 (s, 3H, ArAOCH3), 3.63 (t, 2H,
ACH2AOA(C¼¼O)A), 1.91 (s, 3H, CH2¼¼CACH3), 1.32–1.84
(m, 16H, ACH2ACH2ACH2A). 13C NMR (500 MHz, CDCl3) d:
167.6 (carbonyl), 161.3, 155.2, 146.4, 146.6, 136.2, 127.9,
125.6, 124.2, 123.6, 115.7 (aromatic), 68.8, 64.6
(AOACH2A), 31.5, 29.6, 28.4, 26.7, 19.5, 17.7 (aliphatic).
The monomers were synthesized from the corresponding
compounds by using similar procedure. Typical procedure
for the synthesis of hy is as follows: Compound 20 (1.55 g,
4.4 mmol) and triethylamine (1.16 mL, 8.0 mmol) were dis-
solved in dry THF under nitrogen atmosphere and stirred
around 5–10 ꢀC. Methacryloylchloride (0.78 mL, 8.0 mmol)
in dry THF was added drop wise over a period of 30 min,
and the mixture was warmed to room temperature and
stirred for another 12 h. Precipitated amine hydrochloride
was filtered and solvent evaporated under vacuum. Crude
product was dissolved in ethyl acetate, washed with water
(4 ꢂ 100 mL), brine solution (3 ꢂ 100 mL), and dried over
anhydrous sodium sulfate. The solvent was removed under
vacuum; then, the residue was column chromatographed
(silica-gel, hexane and ethyl acetate (9:1 v/v) to give yellow
colored solid of hy.
Synthesis of Homopolymers HY, FL, CL, and ME
The homopolymers were synthesized by a free radical solu-
tion addition polymerization technique from the correspond-
ing monomers using AIBN as an initiator in THF at 60 ꢀC.
Typical procedure for the synthesis of polymer HY is as fol-
lows: The monomer hy (400 mg, 0.95 mmol) and AIBN (2
wt %) were dissolved in dry THF and a gentle steam of
nitrogen purged into the solution. The solution was kept in
an oil bath at 60 ꢀC for 48 h. Then, the solution was cooled
and poured into excess of hexane to precipitate the product.
Crude polymer thus obtained was reprecipitated twice using
hy
(Yield: 92%). FTIR (KBr, cmꢁ1): 3006 (ACH3), 2928, 2846
(CAH, aliphatic), 1112 (ArAOACH2), 1743 (ester carbonyl),
1632 (AC¼¼CH2), 843 (p-substituted aromatic). 1H NMR
(300 MHz, CDCl3) d: 7.83–7.86 (m, 4H, Ar), 7.49 (d, 2H, Ar),
7.25 (m, 1H, Ar), 6.92 (d, 2H, Ar), 6.10 (s, 1H, AC¼¼CH2),
5.53 (s, 1H, AC¼¼CH2), 4.08 (t, 2H, ArAOACH2), 3.94 (t, 2H,
ACH2AOA (C¼¼O) A), 1.91 (s, 3H, CH2¼¼CACH3), 1.33–1.82
(m, 16H, ACH2ACH2ACH2A). 13C NMR (500 MHz, CDCl3) d:
167.6 (carbonyl), 161.2, 153.4, 147.5, 146.9, 136.1, 130.2,
129.4, 125.2, 123.2, 114.2 (aromatic), 68.5, 64.4
(AOACH2A), 30.4, 29.2, 28.4, 26.4, 19.3, 18.7 (aliphatic).
ꢀ
chloroform and hexane. Purified polymer was dried at 45 C
under vacuum for 48 h to afford orange colored powder.
HY
(Yield 71%). FTIR (KBr, cmꢁ1): 2922 and 2853 (methylene
chain), 1741 (AC¼¼O), 1114 (CAOAC), 855 (p-substituted
aromatic). 1H NMR (300 MHz, CDCl3) d: 7.83 (s, 2H, Ar),
6.96–7.01 (m, 3H, Ar), 3.92 (s, 4H, AOACH2A), 0.82–1.67
(m, 16H, ACH2A (CH2)8ACH2A). 13C NMR (300 MHz, CDCl3)
d: 166.6 (carbonyl), 156.4, 147.9, 146.8, 136.4, 127.5, 124.9,
116.9 (aromatic), 65.3 (AOACH2A), 32.7, 28.9, 27.5, 26.9,
17.2 (aliphatic).
fl
(Yield: 86%). FTIR (KBr, cmꢁ1): 3005 (ACH3), 2928, 2844
(CAH, aliphatic), 1109 (ArAOACH2), 1742 (ester carbonyl),
1632 (AC¼¼CH2), 844 (p-substituted aromatic). 1H NMR
(300 MHz, CDCl3) d: 7.87 (d, 2H, Ar), 7.14 (d, 4H, Ar), 6.98
(d, 2H, Ar), 6.10 (s, 1H, AC¼¼CH2), 5.53 (s, 1H, AC¼¼CH2),
4.06 (t, 2H, ArAOACH2), 3.92 (t, 2H, ACH2AOA(C¼¼O)A),
1.93 (s, 3H, CH2¼¼CACH3), 1.36–1.86 (m, 16H,
ACH2ACH2ACH2A). 13C NMR (500 MHz, CDCl3) d: 167.5
(carbonyl), 162.3, 155.6, 146.1, 146.2, 136.2, 128.4, 124.9,
123.2, 115.1 (aromatic), 68.6, 64.4 (AOACH2A), 30.5, 29.2,
28.4, 26.3, 19.3, 18.8 (aliphatic).
FL
(Yield 68%). FTIR (KBr, cmꢁ1): 2925 and 2851 (methylene
chain), 1743 (AC¼¼O), 1118 (CAOAC), 857 (p-substituted
aromatic). 1H NMR (300 MHz, CDCl3) d: 7.83 (s, 4H, Ar),
7.11 (s, 2H, Ar), 6.92 (s, 2H, Ar), 3.93 (s, 4H, AOACH2A),
0.87–1.75 (m, 16H, ACH2A (CH2)8ACH2A). 13C NMR (300
MHz, CDCl3) d: 165.6 (carbonyl), 159.8, 156.4, 148.1, 146.5,
135.9, 127.5, 118.7 (aromatic), 65.6 (AOACH2A), 33.8, 29.7,
28.2, 26.4, 18.4 (aliphatic).
cl
(Yield: 89%). FTIR (KBr, cmꢁ1): 3005 (ACH3), 2927, 2844
(CAH, aliphatic), 1109 (ArAOACH2), 1743 (ester carbonyl),
1631 (AC¼¼CH2), 844 (p-substituted aromatic). 1H NMR
(300 MHz, CDCl3) d: 7.85 (d, 2H, Ar), 7.13 (d, 4H, Ar), 6.97
(d, 2H, Ar), 6.10 (s, 1H, AC¼¼CH2), 5.53 (s, 1H, AC¼¼CH2),
4.06 (t, 2H, ArAOACH2), 3.92 (t, 2H, ACH2AOA(C¼¼O)A),
1.91 (s, 3H, CH2¼¼CACH3), 1.35–1.85 (m, 16H,
ACH2ACH2ACH2A). 13C NMR (500 MHz, CDCl3) d: 167.5
(carbonyl), 161.1, 155.1, 146.3, 146.5, 136.3, 128.6, 124.9,
123.5, 115.6 (aromatic), 68.6, 64.4 (AOACH2A), 30.5, 29.4,
28.2, 26.5, 19.2, 18.6 (aliphatic).
CL
(Yield 65%). FTIR (KBr, cmꢁ1): 2926 and 2853 (methylene
chain), 1742 (AC¼¼O), 1118 (CAOAC), 857 (p-substituted
aromatic). 1H NMR (300 MHz, CDCl3) d: 7.81 (s, 4H, Ar),
7.08 (s, 2H, Ar), 6.91 (s, 2H, Ar), 3.93 (s, 4H, AOACH2A),
0.87–1.75 (m, 16H, ACH2A (CH2)8ACH2A). 13C NMR (300
MHz, CDCl3) d: 165.7 (carbonyl), 159.6, 156.4, 148.4, 146.3,
135.9, 127.6, 119.2 (aromatic), 65.8 (AOACH2A), 32.9, 29.6,
28.2, 27.5, 26.6, 17.9 (aliphatic).
ME
(Yield: 67%) FTIR (KBr, cmꢁ1): 2925 (methylene chain),
1
me
1724 (AC¼¼O), 1244 cmꢁ1 (PhAOACH2A). H NMR (CDCl3):
(Yield: 78%). FTIR (KBr, cmꢁ1): 3010 (ACH3), 2938, 2851
(CAH, aliphatic), 1113 (ArAOACH2), 1746 (ester carbonyl),
d ¼ 7.85–7.89 (m, 4H, ArAN¼¼NA), 6.96–7.01 (m, 4H, aro-
matic proton), 3.87 (s, 3H, AOCH3), 1.29–1.59 (m, 19H,
FLUORINE-SUBSTITUTED AZOBENZENE IN COPOLYMER SYSTEM, CHINNUSAMY AND PALANINATHAN
1567

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
aliphatic). 13C NMR (CDCl3, ppm), d ¼ 167.6, 161.5, 147.0,
138.53, 132.90, 127.94, 127.00, 124.42, 123.62, 121.65,
119.55, 115.27, 114.91, 109.12, 63.07, 32.80, 29.70, 27.04,
22.72, 10.72.
146.9, 136.59, 125.1, 124.3, 114.6.
Synthesis of Copolymers F-co-HY, F-co-FL, F-co-CL,
and F-co-ME
Preparation of Film
Copolymers were prepared by adopting the similar proce-
dure for the synthesis of HY with feed molar ratio 1:1 of the
corresponding comonomers.
Quartz slides (25 mm ꢂ 25 mm ꢂ 1.5 mm) were cleaned
with water, acetone, and chloroform in an ultrasonic bath for
30 min and dried in a hot air oven at 60 ꢀC for 3 h. The
homo/copolymers were dissolved in THF, filtered with 0.45-
nm syringe filters, and then spin coated at 1000 rpm on to
sonicated quartz slide and annealed around their Tg to yield
dry amorphous film with good optical quality.
F-co-HY
Feed ratio: (f: 400 mg, 0.66 mmol; hy: 277 mg, 0.66 mmol,
Yield: 63%). FTIR (KBr, cmꢁ1): 3028 (ACH3), 2936 and
2878 (methylene chain), 1744 (A(C¼¼O)OA), 1741, 1701
(AC¼¼O), 1636 (ACH¼¼CAC¼¼O), 1610 (AC¼¼CA (CH3)2),
RESULTS AND DISCUSSION
1
1716 (NAC¼¼O), H NMR (300 MHz, CDCl3) d: 7.39–7.92 (m,
Synthesis
11H, ArACH¼¼C, Ar), 6.81–7.23 (m, 7H, Ar), 3.34–3.82 (m,
11H, ArAOACH2A, ACH2AC¼¼O, ANA(CH3)A), 2.36 (s, 3H,
ArACH3), 0.82–1.99 (m, 48H, aliphatic protons). 13C NMR
(300 MHz, CDCl3) d: 168.8, 167.5, 167.7 (carbonyl), 158.9,
152.7, 133.6, 124.4, 123.9, 122.5, 120.6, 116.7, 115.1, 111.1
(aromatic), 87.9, 67.3, (AOACH2), 33.8, 33.6, 31.7, 29.1, 28.4,
23.4, 15.3, 14.6 (aliphatic).
Various substituted azobenzene containing polymethacrylates
were synthesized (HY, FL, CL, CY, NI, ME, T-ME) by free radi-
cal addition polymerization technique in THF solution using
ꢀ
AIBN as an initiator at 60 C. Effect of size and electronic na-
ture of substituent on trans–cis–trans photoisomerization of
azobenzene unit were investigated. To study the effect of
substituent of azobenzene on the polar form (C-form) fulgi-
mide unit, substituted azobenzene monomers were copoly-
merized with fulgimide containing monomer with 1:1 molar
ratio to get seven different copolymethacrylates (F-co-HY, F-
co-FL, F-co-CL, F-co-CN, F-co-NI, F-co-ME, F-co-T-ME), under
similar reaction condition. Homo and copolymers were pre-
pared by following synthetic routes as shown in Schemes 1–
3, respectively.
F-co-FL
Feed ratio: (f: 400 mg, 0.66 mmol; n: 289 mg, 0.66 mmol,
Yield: 61%). FT IR (KBr, cmꢁ1): 3025 (ACH3), 2931 and
2874 (methylene chain), 1743 (A(C¼¼O)OA), 1742, 1701
(AC¼¼O), 1632 (ACH¼¼CAC¼¼O), 1612 (AC¼¼CA(CH3)2),
1
1716 (NAC¼¼O), H NMR (300 MHz, CDCl3) d: 7.43–7.98 (m,
11H, ArACH¼¼C, Ar), 6.84–7.46 (m, 6H, Ar), 3.34–3.92 (m,
11H, ArAOACH2A, ACH2AC¼¼O, ANA (CH3)A), 2.35 (s, 3H,
ArACH3), 0.81–2.01 (m, 48H, aliphatic protons). 13C NMR
(300 MHz, CDCl3) d: 168.5, 167.3, 166.9 (carbonyl), 157.8,
153.7, 136.8, 125.8, 124.7, 123.9, 120.7, 116.9, 115.5, 110.8
(aromatic), 86.7, 67.8, (AOACH2), 33.5, 32.6, 31.3, 28.4, 28.6,
25.1, 19.3, 16.2, 15.6, 13.8 (aliphatic).
Introduction of decamethylene spacer between the back
bone and photochromic unit enhanced solubility of corre-
sponding homo/copolymers in common organic solvents
such as chloroform, dichloromethane, DMF, and THF. Number
average molecular weights of homo/copolymers were tabu-
lated in Tables 2 and 3, respectively. Chemical structure of
synthesized homo/copolymers was confirmed by FTIR, 1H
and 13C NMR spectral techniques. Characteristic absorption
band observed at 1634 cmꢁ1 in FTIR spectra corresponding
to methacrylate double bond of monomer was completely
disappeared after polymerization. Similarly, chemical shift at
5.54 ppm and 6.10 ppm assigned to vinyl protons of each
F-co-CL
Feed ratio: (f: 400 mg, 0.66 mmol; n: 301 mg, 0.66 mmol,
Yield: 65%). FTIR (KBr, cmꢁ1): 3022 (ACH3), 2929 and
2871 (methylene chain), 1742 (A(C¼¼O)OA), 1742, 1705
(AC¼¼O), 1633 (ACH¼¼CAC¼¼O), 1612 (AC¼¼CA (CH3)2),
1
1715 (NAC¼¼O), H NMR (300 MHz, CDCl3) d: 7.42–7.92 (m,
1
monomer vanished after polymerization in the H NMR spec-
11H, ArACH¼¼C, Ar), 6.81–7.46 (m, 6H, Ar), 3.34–3.93 (m,
11H, ArAOACH2A, ACH2AC¼¼O, ANA(CH3)A), 2.31 (s, 3H,
ArACH3), 0.84–2.05 (m, 48H, aliphatic protons). 13C NMR
(300 MHz, CDCl3) d: 168.5, 167.8, 166.5 (carbonyl), 156.9,
154.8, 136.6, 126.2, 125.4, 123.2, 122.7, 120.9, 116.5, 115.7,
110.8 (aromatic), 86.5, 67.4, (AOACH2), 34.2, 33.6, 32.2,
31.7, 28.4, 28.5, 25.4, 19.5, 16.6, 15.8, 13.5 (aliphatic).
tra.21 At the same time, an additional chemical shift of
ACH2A was observed at 0.98 ppm in 1H NMR spectra of
homo/copolymers. Chemical shift at 125.3 ppm in 13C NMR
spectrum corresponding to vinyl carbon is shifted to ali-
phatic region in homo/copolymers after polymerization.
Methylene carbon adjacent to ether and ester linkage in the
monomer resonated at 68.4 ppm and 55.8 ppm, respectively,
and retained chemical shift values in the original position af-
ter polymerization. In addition, other chemical shift repre-
senting monomer was broadened in polymer to confirm the
monomeric units completely involved in the polymerization
reaction.
F-co-ME
(f: 400 mg, 0.67 mmol; me: 286 mg, 0.67 mmol, yield: 61%)
FTIR (KBr pellet), m: 1748 (C¼¼OO), 3024 (ACH3), 2925
(methylene
chain),
1734,
1693
(AC¼¼O),
1637
(ACH¼¼CAC¼¼O), 1609 (AC¼¼CA(CH3)2), 1714 (NAC¼¼O),
1724 (AC¼¼O), 1244 cmꢁ1 (PhAOACH2A). 1H NMR (CDCl3,
ppm), d ¼ 7.89–6.9 (m, 17H, Ar, AArACH¼¼C), 3.93–3.56 (m,
11H, AOACH2A, ¼¼NACH3), 0.87–2.12 (m, 48H, aliphatic).
13C NMR (CDCl3, ppm), d ¼ 168.61, 167.95, 158.90, 153.97,
Thermal Property of Homo/Copolymers
Thermal stability of homo/copolymers was examined using
TGA. The thermal stability was studied with respect to
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ARTICLE
SCHEME 1 Synthesis of HY, FL,
CL, CY, NI, ME, and T-ME.
substituent on the terminal position and compared. They dis-
played 5% weight loss (decomposition temperature, Td) in
nitrogen around 306 ꢀC and 50% weight loss observed
beyond 392 ꢀC indicating their good thermal stability. TGA
thermograms of homo/copolymers were displayed in Sup-
porting Information Figures S1, S2, and S3, and their decom-
position temperatures were depicted in Table 1.
nature of methoxy group, it pushes the electron toward azo
group and causes elimination of nitrogen molecule at high
temperature. In T-ME, the triazole ring cleaved at 306 ꢀC,
attributed to cleavage of triazole ring sooner than azo group,
T-ME depicted three stage decompositions. The copolymer
indicated similar kind of thermal stability as that of homo-
polymer and revealed no effect on substituent in the termi-
nal position of azobenzene. Char yield was 17–26% for
homo/copolymers, measured at 600 ꢀC. The homopolymer
T-ME renders high percentage of char yield than the other
polymers ascribed to more number of nitrogen atoms pres-
ent in the triazole ring.
Thermal stability of homopolymers decreases as follows: ME
> FL > CL > HY > CY > F ꢃ NI > T-ME and copolymers
as well obeying similar trend: F-co-ME > F-co-FL > F-co-CL
> F-co-HY > F-co-CY > F-co-NY > F-co-T-ME. Except F and
T-ME, TGA thermograms of homopolymers exhibited two
stage decomposition, first one occurred in the range of 306–
DSC experiments were conducted during second heating at 5
ꢀC/min scanning rate and thermograms of homo/copolymers
are traced in Figures 1 and 2, respectively. Except F, CL, and
HY, homopolymers exhibited two endothermic transitions
between 96 and 233 ꢀC. Phase transition temperatures of
homo/copolymers are listed in Tables 2 and 3, respectively.
ꢀ
ꢀ
366 C and second one is in the range of 392–448 C corre-
sponding to pyrolytic cleavage of azo group in pendant and
ester linkage in the back bone, respectively. The homopoly-
mer F underwent single stage decomposition. Comparing the
azobenzene containing homopolymers, NI displayed first
ꢀ
ꢀ
stage decomposition at 336 C, which is around 25 C lower
than the other polymers. It reveals that strong electron with-
drawing nature of nitro-group pull the electrons toward it
and remove the nitrogen molecule easily from azo group.
The ME shows first decomposition at 366 ꢀC, which is
around 30 ꢀC higher than NI. Because of electron donating
In addition to these two endothermic transitions, each azo-
benzene containing homopolymer exhibited one small hump
just below the low temperature endothermic transition cor-
responding to glass transition (Tg). Among the homopoly-
mers, T-ME demonstrated broad Tg at 68 ꢀC ascribed to
dipole–dipole interaction between the triazole rings.
FLUORINE-SUBSTITUTED AZOBENZENE IN COPOLYMER SYSTEM, CHINNUSAMY AND PALANINATHAN
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 2 Synthesis of F.
SCHEME 3 Synthesis of F-co-HY, F-co-FL, F-co-CL, F-co-CN, F-co-NI, F-co-ME, and F-co-T-ME.
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ARTICLE
TABLE 1 TGA data of F, NY, CY, FL, CL, HY, ME, T-ME, F-co-NI,
F-co-CY, F-co-FL, F-co-CL, F-co-HY, F-co-ME, and F-co-T-ME
Weight Loss
Corresponding to (ꢀC)
Char Yield
Polymer
5%
50%
at 600 ꢀC (%)
F
336
335
351
361
360
359
366
306
331
336
339
329
327
342
309
408
430
448
423
429
431
436
396
415
434
443
422
431
439
392
20
21
19
18
20
19
18
22
17
19
18
16
15
18
21
NY
CY
FL
CL
HY
ME
T-ME
F-co-NY
F-co-CY
F-co-FL
F-co-CL
F-co-HY
F-co-ME
F-co-T-ME
FIGURE 2 DSC thermograms of F-co-HY, F-co-CL, F-co-FL, F-co-
CN, F-co-NI, F-co-ME, and F-co-T-ME.
textures of liquid crystalline phases were observed in
between their Tm and Ti (DT ¼ Ti ꢁ Tm) in POM experiment
and disclosed that except F, CL, and HY, remaining homo-
polymers exhibited stable mesophases. Further, it is found
that T-ME exhibits very large mesophase stability due to
dipole–dipole interaction between triazole rings. In DSC anal-
ysis, T-ME displayed a small endothermic transition before
Polymers exhibiting liquid crystallinity in DSC analysis estab-
lishes Tm in the range of 96–152 ꢀC and Ti in the range of
128–233 ꢀC. DSC analysis of F indicates polymer possessing
C-form (polar form) fulgimide shows higher Tm than E-form
(nonpolar form) revealed that C-form fulgimide in F is highly
ordered than E-form due to planner conformations.24 The
ꢀ
Ti at 216 C confirms one more mesophase and supported in
POM analysis.23 POM images of NI, CY, FL, ME, and T-ME
were shown in Supporting Information Figures S4 and S5,
respectively. Among the liquid crystalline homopolymers,
electron withdrawing substituent displayed longer DT than
the other polymers (Table 2). In general, Tm and Ti values of
azobenzene containing liquid crystalline polymers are
strongly influenced by substituent present in the terminal
position.25 Decomposition temperatures observed in DSC are
in accordance with decomposition temperatures obtained in
TGA.ꢀNI and MEꢀshow decomposition in TGA thermogram at
335 C and 366 C due to cleavage of azo group, respectively.
Decomposition temperatures observed from DSC also con-
firmed the effect of substituent on the thermal stability of
polymers. Copolymers did not render any liquid crystalline
property due to introduction of nonplanar E-form fulgimide
unit. Upon introduction of E-form fulgimide unit in the
copolymeric system, orderly arrangement of azobenzene
mesogen during heating is completely disturbed. But, copoly-
mers displayed glass transition temperature in the range of
63–83 ꢀC due to the existence of equal amount of E-form
fulgimide and liquid crystalline azobenzene units. Albeit,
azobenzene moiety increases liquid crystallinity which is
insufficient to provide liquid crystalline phase in the copoly-
mer. As confirmed from DSC, homopolymer F did not render
mesophase in POM analysis. POM observation revealed
FIGURE 1 DSC thermograms of F, HY, FL, CL, CY, NI, ME, and
T-ME.
FLUORINE-SUBSTITUTED AZOBENZENE IN COPOLYMER SYSTEM, CHINNUSAMY AND PALANINATHAN
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 2 Thermal Properties, Molecular Weights, and Absorption Maxima of F, NY, CY, FL, CL, HY, ME, and T-ME
Thermal Properties ( ꢀC)
kmax (nm)
kmax (nm)
Homo Polymer
Isomers
Tg
Tm
Ti
Td
DT
Mn
Mw/Mn
(Chloroform)
(Thin Film)
F
E
–
219
237
102
–
–
335
–
–
–
129,564
1.38
396
399
C
–
–
528
521
NI
trans
cis
55
–
161
–
335
–
59
145,672
136,542
130,485
134,232
139,928
135,672
145,672
1.23
1.19
1.18
1.21
1.26
1.32
1.23
379
348
–
–
300, 473
366
302, 481
354
CY
FL
trans
cis
51
–
109
–
156
–
353
–
47
32
260, 453
350
261,453
331
trans
cis
49
–
96
–
128
–
362
–
–
–
–
–
–
250, 440
351
244, 442
329
CL
trans
cis
54
–
152
–
–
359
–
–
256, 438
350
241, 439
333
HY
ME
T-ME
trans
cis
56
–
123
–
–
360
–
–
251, 442
361
239, 443
340
trans
cis
62
102
142
365
40
314, 447
360
253, 447
334
trans
cis
68
–
107
–
233
–
303
–
126
–
313, 447
248, 455
TABLE 3 Thermal Properties, Molecular Weights, and Absorption Maxima of F-co-NI, F-co-CY, F-co-FL, F-co-CL, F-co-HY, F-co-ME,
and F-co-T-ME
Thermal Properties( ꢀC )
kmax (nm)
Co-Polymer
Isomers
Tg
Tm
Ti
Td
Mn
Mw/Mn
(Chloroform)
F-co-NI
E/trans
cis
75
–
236
–
–
–
–
–
–
–
–
–
–
331
–
89,240
1.04
380
–
C
–
–
–
523
368
–
F-co-CY
F-co-FL
E/trans
cis
72
–
232
–
335
–
71,312
58,612
56,492
61,826
64,240
89,240
1.41
1.03
1.05
1.11
1.01
1.04
C
–
–
–
520
353
–
E/trans
cis
83
–
233
–
337
–
C
–
–
–
482
358
F-co-CL
F-co-HY
F-co-ME
F-co-T-ME
E/trans
cis
79
–
228
–
328
–
–
–
C
–
–
–
523
352
–
E/trans
cis
63
–
186
–
321
–
–
–
–
–
–
–
–
–
C
–
–
–
525
361
–
E/trans
cis
69
–
237
–
340
–
C
–
–
–
516
364
–
E/trans
cis
79
–
236
–
307
–
C
–
–
–
524
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ARTICLE
TABLE 4 POM Data and Types of Mesophases of ME, FL, CY,
NI, and T-ME
for NI, CY, FL, ME, and T-ME in chloroform solution upon
irradiation with 360 nm UV light as a function of time is
depicted in Supporting Information Figures S6A, S7A, S8A,
S9A, and S10A, respectively. A photostationary state was
attained as cis-isomer predominated after a suitable irradia-
tion period. Amount of cis-fraction at photostationary state
can be calculated using the reported procedure.25 The cis-
fractions for NI, CY, FL, CL, HY, ME, and T-ME at photosta-
tionary state are 0.31, 0.69, 0.87, 0.83, 0.79, 0.85, and 0.84,
respectively. Time required for attaining photostationary
state for FL, CL, HY, ME, and T-ME in solution is around 30
s, but NI and CY took around 75 s. It was observed that NI
required more time to attain photostationary state and
exhibited less amount of cis-fraction in stationary state
ascribed to electron withdrawing nature of nitro group hin-
dered trans–cis isomerization of azobenzene unit, but elec-
tron donating nature of methoxy group enhances photoiso-
merization reaction. Thus, revealed the nature of substituent
present on the terminal position of azobenzene units play
vital role in reversible trans–cis isomerization. After attaining
photostationary state, the resultant solution of polymer was
placed in dark environment; absorption band in the range of
350–379 nm regained (up to 93–95% of the initial value)
slowly due to slow thermal cis–trans back isomerization of
azobenzene unit at ambient temperature in dark. In an
another experiment, after attaining photostationary state, the
resultant polymer solution was irradiated with 560-nm visi-
ble light causes cis–trans back isomerization rapidly com-
pared to ambient temperature in dark. Even though the click
chemistry assisted triazole ring is directly linked to azoben-
zene unit in T-ME, which does not alter photoisomerization
reaction of azobenzene unit in chloroform solution. It was
also found that the chloroform solution of T-ME represents
similar absorption spectral pattern and absorption maximum
as that of ME, indicating triazole ring has an electron donat-
ing nature.24 Representative absorption spectral pattern
noticed for NI, CY, FL, ME and T-ME in thin film upon irradi-
ation with 360 nm UV light are shown in Supporting Infor-
mation sFigures S6B, S7B, S8B, S9B and S10B, respectively.
Thin films of NI, CY, FL, ME, and T-ME show an absorption
at 348, 354, 331, 340, and 334 nm, which were hypso-
chromically shifted from the solution at 379, 366, 350, 361,
and 360 nm, respectively. This behavior could be revealed to
stacking of azobenzene chromophore (H-aggregation) in the
solid phase.26 The film NI, CY, FL, ME, and T-ME was irradi-
ated with 360 nm UV light at room temperature causes to
decrease intensity at 348, 354, 331, 340, and 334 nm with
slight increase at 481, 453, 442, 447, and 455 nm, respec-
tively, attributed to trans–cis isomerization reaction of azo-
benzene units. Except FL, absorption spectral pattern
observed for thin film of NI, CY, ME and T-ME, become
asymmetric as compared to corresponding spectral pattern
obtained from solution phase. Similarly, excluding FL,
remaining azobenzenes containing homopolymers displayed
an isobestic point in solution phase was completely disap-
peared in solid phase revealed to aggregation of azobenzene
unit in the film. On the other hand, thin film of FL was irra-
diated with 360-nm UV light causes to decrease in intensity
POM ( ꢀC)
Polymer
Tm
Ti
DT
41
Types of Mesophases
ME
FL
104
101
106
100
109
145
32
Nematic and Smectic
Nematic
31
45
CY
151
63
Nematic and Smectic
Nematic
NI
63
T-ME
36
127
Nematic and Smectic
homopolymers NI, CY, FL, ME, and T-ME alone evidenced
mesophases between their Tm and Ti. The type of mesophase
and their corresponding phase transition temperatures
observed from POM are summarized in Table 4.
On cooling from isotropic melt, NI and FL displayed nematic
phases at 126 ꢀC and 104 ꢀC, respectively. On cooling, CY
exhibited nematic phase at 144 ꢀC and focal conic fan tex-
ꢀ
ture at 118 C confirmed smectic-A phase. POM investigation
of ME and T-ME affirm nematic and smectic mesophases. On
cooling, ME and T-ME displayed schlieren texture at 129 ꢀC
and 228 ꢀC confirmed nematic phases, respectively. On fur-
ther cooling to room temperature, appearance of focal conic
fan texture at 112 ꢀC and 202 ꢀC, respectively, confirmed
smectic-A phases.24
Optical Property of Homopolymers
Herein, optical property of fulgimide (F) and six azobenzenes
(NI, CY, FL, CL, HY, ME, and T-ME) containing homopolymers
with respect to substituent present on terminal position
were investigated. Before irradiation, chloroform solution of
F displayed an absorption maximum at 396 nm. Upon irradi-
ation with 360 nm UV light, absorption at 396 nm was
slowly decreased with appearance of new band at 528 nm.
The same solution was irradiated with 560 nm visible light,
band at 528 nm decreased and band at 396 nm slowly
regain to original position. This observation suggested that
fulgimide unit in F can be toggled between E-(nonpolar) and
C-form (polar) by alternate irradiation with UV and visible
light. Fulgimide unit in thin film F is likewise switching
between E- and C-form by alternate irradiation with UV/Vis
light. The optical properties of F in solution and thin film
have been discussed in detail in our previous reports.21,24
Before irradiation, chloroform solution of azobenzene con-
taining homopolymers displayed two absorptions, one in the
range of 350–379 nm corresponding to strong symmetry
allowed p–p* electronic transition and another one in the
range of 440–473 nm corresponding to weak symmetry for-
bidden n–p* transition of trans azobenzene. It was irradiated
with 360-nm UV light causes to decrease the intensity of ab-
sorbance in the UV region; consequently, intensity of band in
visible region was showing fairly increased trend with main-
taining isobestic point. Such spectral changes testify trans–
cis isomerization of azobenzene unit in polymer under UV
light actions.21,23 Representative spectral pattern observed
FLUORINE-SUBSTITUTED AZOBENZENE IN COPOLYMER SYSTEM, CHINNUSAMY AND PALANINATHAN
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
zene unit in film as well except in T-ME. The same film was
irradiated with 560 nm visible light also causes backward
cis–trans isomerization rapidly. Time taken to attain photo-
stationary state in thin film was longer as compared to solu-
tion phase ascribed to steric effect.27 Azobenzene unit in
thin film of T-ME has trans–cis isomerization reaction upon
irradiation with 360 nm UV light like normal azobenzene
unit but cis–trans back isomerization is forbidden either
upon irradiation with visible light or subjected to heat.24
Optical Property of Copolymers
Chloroform solution of F-co-NI, F-co-CY, F-co-FL, F-co-CL,
F-co-HY, F-co-ME, F-co-T-ME demonstrated an absorption at
380, 368, 353, 358, 352, 361, 364 nm, respectively. Absorp-
tion maximum corresponding to fulgimide and azobenzene
units were merged together in the copolymers (Table 3). As
azobenzene has lower energy band gap as compared with
fulgimide unit results, high intensity of absorption maximum
of azobenzene was noticed in a given molar solution. Hence,
the lesser intensity absorption of E-form fulgimide unit at
396 nm in solution phase can not differentiate from single
band in the range 352–380 nm for copolymers. Upon irradia-
tion with 360 nm UV light, pale yellow color solution of co-
polymer was switched to dark red due to the formation of C-
form fulgimide unit. Intensity of absorption in UV region
decreased attributed to the formation of C-form; accordingly,
a new absorption emerged out in visible region around 523
nm for all copolymers except F-co-FL, which displayed at
482 nm. Absorption maximum appeared in the visible region
attributed to delocalization of p-electron from indole nitro-
gen atom to anhydride carbonyl oxygen atom in C-form fulgi-
mide unit of copolymers. Absorption spectral pattern
observed for F-co-NI and F-co-CY, F-co-ME and F-co-T-ME, F-
co-CL, and F-co-HY in chloroform solution upon irradiation
with 360 nm UV light as a function of time is shown in
FIGURE 3 Absorption spectra of polar form of copolymers in
chloroform solution at photostationary state. [Color figure can
be viewed in the online issue, which is available at www.
interscience.wiley.com.]
at 331 nm with increase at 442 nm through isobestic point
at 403 nm. Absorption spectral pattern of thin film of FL is
not asymmetric as that of remaining polymers. The trans–cis
isomerization reaction occur in thin film of FL through iso-
bestic point at 403 nm with symmetric manner indicates the
fluorine atom present on the terminal position of azoben-
zene unit is smaller in size than the other substituents and
revealed the size of substituent present on terminal position
of azobenzene unit decisive on photoisomerization reaction
in solid phase. When irradiation was ceased after reached
the photostationary state, the absorption band in the range
of 331–348 nm was gradually recovered to original state
due to backward cis–trans thermal isomerization of azoben-
SCHEME 4 Schematic representa-
tion of interaction between fluo-
rine substituted azobenzene and
polar form of fulgimide unit in
F-co-FL. [Color figure can be
viewed in the online issue, which
is available at www.interscience.
wiley.com.]
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ARTICLE
FIGURE 5 Change in fluorescence intensity of F (A) and F-co-NI
FIGURE 4 Absorption spectral patterns of F-co-FL in (A) chloro-
(B) in chloroform solution upon irradiation with 360-nm UV light.
form solution, (B) thin film upon irradiation with 360 nm UV light.
attaining photostationary state, the same solution was irradi-
ated with 560 nm visible light, red colored solution was
bleached back to pale yellow color with regain of absorbance
in UV region due to reappearance of C-form to E-form. In the
Supporting Information Figures S11, S12, S13, respectively.
F-co-FL solution was irradiated with 360 nm UV light; C-
form appeared at 482 nm, which is around 40 nm lesser
than C-form of other copolymers (Fig. 3).
Blue-shift in absorption maximum of C-form fulgimide unit
in F-co-FL relative to other copolymers indicate that fluorine
atom present on the terminal position of azobenzene unit
attract lone pair of electrons of indole nitrogen atom in
C-form fulgimide unit and causes to reduce delocalization as
depicted in Scheme 4. This kind of perturbation in delocali-
zation leads to reduce absorption maximum of C-form to
482 nm. To confirm the interaction between fluorine atom in
azobenzene and polar form fulgimide unit, thin film of F-co-
FL was prepared and irradiated with 360 nm UV light; as
anticipated C-form appeared at 522 nm as that of other
copolymers (Fig. 4).
C-form fulgimide in film of F-co-FL appeared at 522 nm
which is around 40 nm red shifted from solution phase
appeared at 482 nm. Interaction represented in Scheme 4 is
not favorable in solid phase ascribable to lack of mobility
between two photochromic units upon irradiation with UV
light. It is interesting to note that fluorine substituted azo-
benzene alone contributed significant change in the maxi-
mum of C-form fulgimide unit in copolymer system. After
FIGURE 6 Fluorescent switch cycles of FI-co-ME in chloroform
solution upon alternate irradiation with UV/visible light.
FLUORINE-SUBSTITUTED AZOBENZENE IN COPOLYMER SYSTEM, CHINNUSAMY AND PALANINATHAN
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 7 Change in fluores-
cence intensity of F-co-FL in
chloroform solution upon irra-
diation with 360-nm UV light as
a function of time. [Color figure
can be viewed in the online
issue, which is available at
www.interscience.wiley.com.]
case of F-co-NI, upon irradiation with 360 nm UV light, nitro
group present on terminal position of azobenzene stabilizes
polar form of fulgimide unit.21
absorption maxima of C-form fulgimide unit in F-co-FL. The
solution F-co-FL was irradiated with UV light; one new emis-
sion band emerged at 644 nm to a certain level then
decreased accordingly two new emission bands at 475 nm
and 524 nm raised for subsequent irradiation as shown in
Figure 7.
Fluorescence Property of Copolymers
Fluorescence property of copolymers was investigated to
understand the fluorescence efficiency of C-form fulgimide
unit with adjacent substituted azobenzenes. E-form fulgimide
converted to more polar C-form upon irradiation with 360
nm UV light, which is a good fluorescent chromophore with
high thermal, acid and base stability.28 Chloroform solution
of F, F-co-NI, F-co-CY, F-co-FL, F-co-CL, F-co-HY, F-co-ME,
and F-co-T-ME was irradiated with UV light with k excitation
at their corresponding absorption maximum as a function of
time, one broad emission band originating in the range
of 635–647 nm. Except F-co-FL, fluorescence intensity of
copolymers were increased with increase in time of UV irra-
diation ascribable to more and more amounts of ring clo-
sure, and then reached photostationary state after 180 s.
This kind of fluorescence quenching could be explained that
the fluorine atom of azobenzene attracts lone pair of elec-
trons of nitrogen atom in C-form fulgimide unit as depicted
in Figure 7. Due to this sort of attraction, lone pair of elec-
trons utilized in the delocalization of C-form is perturbed
completely; then the fluorescence quenched to 524 nm. After
Representative emission patterns of
F and F-co-NI in
chloroform solution upon irradiation with UV light is shown
Figure 5 and the same solution was irradiated with 560 nm
visible light, emission intensity was slowly decreased to orig-
inal level attributed to conversion of C-form to E-form.
Fluorescence intensity of copolymers was reversibly
switched by alternate irradiation with UV and visible light,
and this change in intensity was recorded several cycles and
plotted against change in intensity.21 Representative fluores-
cence switching cycles of F-co-ME in chloroform solution
upon irradiation with UV/Vis lights is depicted in Figure 6.
It displayed the fatigue resistance of photochemical fluores-
cence switch of all the copolymers except F-co-FL.
FIGURE 8 Change in fluorescence intensity of F-co-FL in chloro-
form solution upon irradiation with visible light after reaching
photostationary state. [Color figure can be viewed in the online
issue, which is available at www.interscience.wiley.com.]
When comparing the emission pattern of copolymers in solu-
tion upon irradiation with UV light, F-co-FL shows entirely
different pattern attributed to hypsochromically shift of
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ARTICLE
attaining photostationary state, F-co-FL was irradiated with
560 nm visible light for 600 s, the fluorescence at 524 nm
grew again and decrease at 644 nm. It indicates the occur-
rence of ring opening reaction of C-form by visible light irra-
diation solely altering the fluorescence at 644 nm and not at
524 nm as shown in Figure 8. It disclosed that the presence
of fluorine atom on the azobenzene unit attracts lone pair of
electrons completely on nitrogen atom of C-form fulgimide
unit.
Finally, it is concluded that among the substituents, fluorine
substituted azobenzene solely afford the significant switching
to the C-form fulgimide unit in the copolymer.
The authors gratefully acknowledge Board of Research in
Nuclear Sciences (BRNS), Department of Atomic Energy
(DAE), Government of India, Mumbai (Sanction No 2006/37/
13/BRNS/205) for providing financial support for this
research work. S.C. sincerely acknowledges Council of Scien-
tific and Industrial Research (CSIR), New Delhi, India, for the
award of Senior Research Fellowship.
In the case of F-co-NI, F-co-CY, and F-co-T-ME, the fluores-
cence maximum was red shifted by 10, 11, and 12 nm and
appeared at 645, 646, and 647 nm, respectively. This may be
referable to substituents on the azobenzene unit interact
with C-form fulgimide unit in the copolymer system. Photo-
chromic cyclization reaction of fulgimide unit upon irradia-
tion with UV light took place much faster than the ring open-
ing reaction of fulgimide unit upon irradiation with visible
light for all the copolymers.
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