Optical switching, photophysical, and electrochemical behaviors of pendant triazole-linked indolylfulgimide polymer

Article abstract of DOI:10.1002/pola.24528

Triazole-linked 2-indolylfulgimide polymer has been synthesized and its photochromic switching behavior has been characterized by NMR, IR, GPC, TGA, DSC, and UV-Vis spectroscopy. The synthesized photochromic polymer showed absorption peak maxima at 386 and 510 nm wherein the absorption at 510 nm was attributed to charge transfer from triazole ring nitrogen to carbonyl carbon of fulgimide unit. Fluorescence lifetime studies on exciting at 550 nm reveals triexponential behavior with fluorescence decay around 0.1, 1 and 4.2 ns, which correspond to open (E), closed (C) form of fulgimide and triazole ring, respectively. Whereas exciting at 470 nm evidences biexponential fit with fluorescence decay around 0.1 and 2.2 ns, which corresponds to the closed (C) form and triazole ring, respectively. Fluorescence decay of triazole ring was found to be influenced by the excitation wavelength. The cyclic voltammogram of open form of polymer depicts irreversible reductive wave at -1.4 V. On illumination with 360-nm light, the reduction wave of polymer was shifted toward less cathodic wave at -0.9 V; this leads to formation of the closed form of fulgimide unit.

Full text of DOI:10.1002/pola.24528

Optical Switching, Photophysical, and Electrochemical Behaviors
of Pendant Triazole-Linked Indolylfulgimide Polymer
SIVASANKARAN NITHYANANDAN,1 PALANINATHAN KANNAN,1 KARUPPANNAN SENTHIL KUMAR,2 PERUMAL RAMAMURTHY2
1Department of Chemistry, Anna University, Chennai 600025, India
2National Center for Ultra Fast Processes, University of Madras, Taramani, Chennai 600113, India
Received 25 November 2010; accepted 29 November 2010
DOI: 10.1002/pola.24528
Published online 3 January 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Triazole-linked 2-indolylfulgimide polymer has been
synthesized and its photochromic switching behavior has been
characterized by NMR, IR, GPC, TGA, DSC, and UV–Vis spec-
troscopy. The synthesized photochromic polymer showed
absorption peak maxima at 386 and 510 nm wherein the
absorption at 510 nm was attributed to charge transfer from
triazole ring nitrogen to carbonyl carbon of fulgimide unit.
Fluorescence lifetime studies on exciting at 550 nm reveals
triexponential behavior with fluorescence decay around 0.1, 1
and 4.2 ns, which correspond to open (E), closed (C) form of
fulgimide and triazole ring, respectively. Whereas exciting at
470 nm evidences biexponential fit with fluorescence decay
around 0.1 and 2.2 ns, which corresponds to the closed (C)
form and triazole ring, respectively. Fluorescence decay of tria-
zole ring was found to be influenced by the excitation wave-
length. The cyclic voltammogram of open form of polymer
depicts irreversible reductive wave at ꢀ1.4 V. On illumination
with 360-nm light, the reduction wave of polymer was shifted
toward less cathodic wave at ꢀ0.9 V; this leads to formation of
C
V
the closed form of fulgimide unit.
2011 Wiley Periodicals,
Inc. J Polym Sci Part A: Polym Chem 49: 1138–1146, 2011
KEYWORDS: biexponential; charge transfer (CT); click chemistry;
electrochemistry; fluorescence; fluorescence lifetime; function-
alization of polymers; 2-indolyl fulgimide; optical switching;
photochromism; photoreactive effects; photochemistry; triex-
ponential fit
chemical configurations on exciting with different wave-
lengths of light. In any case, a photochromic transformation
is always accompanied by profound absorbance changes in
the visible region.2 The two configurations may display
distinct physicochemical properties such as refractive indi-
ces, dielectric constants, oxidation–reduction potential, and
geometrical structures. The instant property changes by pho-
toirradiation without processing lead to their use in various
optoelectronic devices, such as optical memory, optical
switching, displays, and nonlinear optics.3,4 The photostation-
ary state emits fluorescence, and it can be exploited to mo-
lecular electronics and information storage. Thermally irre-
versible fulgides/fulgimides exhibit good thermal stability of
their photoisomers and show high photostability during
repeated illumination cycles.5,6 The photoresponsive charac-
ter associated with this fascinating class of organic molecules
has suggested a wealth of potential applications in a wide di-
versity of areas, ranging from biomedical research to infor-
mation technology.7 It has been viewed in rewritable optical
memories and photofunctional switching applications.8
INTRODUCTION Recent years, optical data storage is becom-
ing significant and everyone seek to store their data like
multimedia technology, scientific information in the form of
compact disk, memory stick, hard disk, etc. The write–read–
erase mechanism of data storage is a key factor in storing
the information. Photochromic materials are promising as re-
cording media for optical memory because they erasably or
rewritably store data in photon mode. As the data-recording
mechanism is based on the photochemical reaction of each
molecule in the matrix, extremely high spatial resolution
should be achievable. The organic photochromic materials
have been responding with light and repeatedly change from
chromatic state to achromatic state or vice versa, such as
isomerization reaction with light by fulgide, ring opening/
closing reaction, and redox reaction used by spiropyran and
diarylethenes.1 Light can induce circulation of p electrons for
the electrocyclic reaction to occur. As per Woodward-Hoff-
man electocyclic reaction fulgide/fulgimide molecule can
undergo conrotatory mode of E-configuration to C-configura-
tion, which occurs via thermally irreversible or photochemi-
cally reversible. Light is responsible for coloration and decol-
oration of these molecules. Photochromic compounds can
undergo reversible transformation between two distinct
In particular, photochromic polymers meet the requirements
in many practical applications ascribed to their excellent
photoreponsive behavior in the solid state (thin film).9–11
Additional Supporting Information may be found in the online version of this article. Correspondence to: P. Kannan (E-mail: pakannan@annauniv.
edu)
C
V
Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 1138–1146 (2011) 2011 Wiley Periodicals, Inc.
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ARTICLE
Belfield et al.12 investigated the laser-induced two-photon
photochromism of indolylfulgide doped with a copolymer of
phosphorylated poly(vinylbenzyl chloride (VBC)-co-methyl
methacrylate (MMA)); a thin film was prepared by spin coat-
ing onto a glass substrate and it can be used in two-photon
holographic and multilayer three-dimensional (3D) informa-
tion storage. Photochromic materials were used in 3D optical
memory devices, based on two-photon absorption. Concen-
trations of 10ꢀ1 M and higher are needed for the device to
perform at the required writing and reading efficiencies.13
To achieve the high concentrations of photochromism in the
polymer matrix, Rentzepis and coworkers14 synthesized pho-
tochromic crosslinked copolymers by attaching the photo-
chromic fulgimide molecules to the polymer chain and
copolymerizing it with methyl methacrylate and characteriz-
ing their switching properties. Cu (I)-catalyzed ligation of or-
ganic azides and terminal alkynes belong to a group of reac-
tions referred as ‘‘Click Chemistry,’’ a term coined by Huisgen
et al.15 These compounds are well known for their biological
properties such as antiallergic,16 antiviral applications and
also in materials chemistry.17 However, no work has been
reported regarding the click chemistry with indolylfulgimide
polymer. This work deals with synthesis of triazole-substi-
tuted indolylfulgimide polymer, and their optical switching,
photophysical, thermal, and electrochemical properties were
studied.
1H NMR (400 MHz, CDCl3, d, ppm): 1.8 (s, 3H), 2.5 (s, 1H),
4.2 (s, 2H), 5.6 (s, 1H), 6.1 (s, 1H) (Supporting Information
Fig. S1). 13C NMR (400 MHz, CDCl3, d, ppm): 20.2, 51.0, 62.8,
78.4, 125.5, 138.9, and 168.1(C¼¼O) (Supporting Information
Fig. S2). Anal. calcd for C7H8O2: C, 67.73, H, 7.70. Found: C,
67.74, H, 8.0.
Synthesis of 1-Azido-3-aminopropane
1-Chloro-3-aminopropane hydrochloride (6.5 g, 49.9 mmol)
and sodium azide (9.75 g, 149 mmol) in water (40 mL) were
heated at 80 ꢁC for 15 h. After removing most of the water
under vacuum distillation, the reaction mixture was cooled
in an ice bath; diethyl ether (50 mL) and then KOHꢁpellets
(4 g) were added keeping the temperature below 10 C. The
organic phase was separated, and the aqueous layer was fur-
ther extracted with diethyl ether (2ꢂ 20 mL). The combined
organic layers were died over K2CO3 and concentrated to
give oil which was purified by bulb-to-bulb distillation to
obtain a colorless liquid. Yield: 3.43 g (70%). FTIR (KBr,
cmꢀ1): 2097 (AN3), 3390 (ANH2).
1H NMR (400 MHz, CDCl3, d, ppm): 1.7 (m, 2H), 2.5 (t,
ACH2A J ¼ 7.1), 2.8 (t, J ¼ 7.1, 2H), 3.4 (s, 2H) (Supporting
Information Fig. S3). 13C NMR (400 MHz, CDCl3, d, ppm):
32.0 (CH2), 40.4 (CH2), and 50.1(CH2) (Supporting Informa-
tion Fig. S4). Anal. calcd for C3 H8 N4: C, 35.99, H, 8.05, N,
54.96. Found: C, 34.53, H, 7.83, N, 53.96.
Synthesis of 2-[1,3-Dimethyl-2-indolylmethene]-3-
isopropylidene-N-(3-azidopropyl) Succinimide
EXPERIMENTAL
Materials
A flask is charged with a solution of 2-indolylfulgide (1 g,
3.5 mmol) in benzene (15 mL). Through a dropping funnel,
3-azidoamine (0.35 g, 3.5 mmol) in benzene (4 mL) was
added dropwise, and the reaction mixture was stirred for 1
h at room temperature. To this reaction mixture, zinc chlo-
ride powder (0.47 g, 3.5 mmol) was added in one portion.
Subsequently, the reaction mixture was raised to reflux tem-
perature, followed by the addition of HMDS (0.36 mL, 4.9
mmol) in benzene (5 mL), through dropping funnel, over a
period of 10 min. This reaction mixture was then refluxed
for 20 h. The benzene was removed by rotary evaporation.
The crude product was purified by flash silica gel chroma-
tography column (9:1 hexane–ethylacetae as eluent) to afford
dark red syrup (0.635 g, 50%). FTIR (KBr, cmꢀ1): 1745
(C¼¼OO), 1727, 1690 (AC¼¼O), 1630 (ACH¼¼CAC¼¼O), 1605
(AC¼¼CA(CH3)2), 1712 (NAC¼¼O), 3023 (ACH3), 2094
(AN3), 2982(ACH2A).
Diethylsuccinate, acetone, t-butyl alcohol, potassium-tertiary-
butoxide, sodium hydride, sodium hydroxide, methacrylic acid,
hydroquinone, and 2,20-azobis(isobutyronitrile) (AIBN) (Merck,
Germany) were used as received. Methanol, ethanol, phenol,
THF, diethyl ether, chloroform, triethylamine, dimethylforma-
mide (DMF) (SRL, India) and other solvents were purified by
reported procedures.18 2-[1,3-Dimethyl-2-indolylmethylene]-3-
isopropylidenesuccinicanhydride was synthesized based on a
similar reported procedure.19 Methacryloylchloride was pre-
pared according to procedure reported elsewhere.20 Hexame-
thyldisilazane (HMDS) and 3-chloropropylamine hydrochloride
(Fluka, Switzerland) were used as supplied.
Synthesis of Propargyl Methacrylate
Propargyl alcohol (4.75 g, 84 mmol) was dissolved in dry
THF (0.25 mL); triethyl amine (10.1 g, 100 mmol) was
added to a reaction vessel and stirred under nitrogen for 30
min keeping the temperature below 10 ꢁC. Methacryloyl-
chloride (9.3 g, 89 mmol) was then added at the same tem-
perature and stirred overnight at room temperature. The
reaction mixture was monitored by thin layer chromatogra-
phy. After completion of the reaction, mixture was poured
into ice cold water and extracted with ethyl acetate, washed
several times with water, brine and dried over anhydrous so-
dium sulfate. The solvent was evaporated by rotary evapora-
tor and purified by column chromatography using hexane as
eluent to obtain a colorless liquid product, yield: 8.3 g
(80%). FTIR (KBr, cmꢀ1): 3297 (ACBCAH), 2980, 2877
(ACH2A), 2147 (AC¼¼CA).
1H NMR (400 MHz, CDCl3, d, ppm): 1.6 (m, 5H), 1.82 (s, 6H),
2.29 (s, 3H), 3.21 (t, J ¼ 4.8 Hz, 2H), 3.99 (s, 3H), 4.40 (t, J
¼ 5.8 Hz, 2H), 7.02 (s, 1H), 7.37 (m, 1H),7.52 (m, 1H), 7.55
(s, 1H), 7.57 (s, 1H) (Supporting Information Fig. S5). Anal.
calcd for C22H26N5O2: C, 66.83; H, 6.14, N,17.84. Found: C,
66.84; H, 6.10, N, 17.44.
Synthesis of 2-[1,3-Dimethyl-2-indolylmethene]-3-
isopropylidene-N-[3-(4-methacryloyloxymethyl-1H-[1,2,3]
triazole-4-yl)propyl] Succinimide
To solution of 2-[1,3-dimethyl-2-indolylmethene]-3-isopropy-
lidene-N-(3-azidopropyl) succinimide (0.7 g, 1.92 mmol) and
TRIAZOLE-LINKED INDOLYLFULGIMIDE POLYMER, NITHYANANDAN ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
propargyl methacrylate (0.239 g, 1.92 mmol) was dissolved
in t-butanol–H2O (3:1) mixture; 30 mol % of CuSO4ꢃ5H2O
(0.14 g, 0.57 mmol) with 10 mol % of sodium ascorbate
(0.07 g, 0.192 mmol) were stirred at room temperature for
18–28 h. The reaction was monitored by thin layer chroma-
tography. The mixture was poured into cold water, and the
resulting solution was extracted with chloroform (3ꢂ 50
mL). The combined organic phase was dried with sodium
sulfate, concentrated, and purified by column chromatogra-
phy over silica gel using chloroform as eluent, yield: 0.6 g
(70%). FTIR (KBr, cmꢀ1): 1748 (C¼¼OO), 1734, 1693
(AC¼¼O), 1637 (ACH¼¼CAC¼¼O), 1609 (AC¼¼CA(CH3)2),
1714 (NAC¼¼O), 3024 (ACH3).
400 MHz spectrometer in CDCl3 with TMS as an internal
standard. The thermogravimertic analysis (TGA) and DSC
measurements were performed on a Mettler Toledo STARe
system used to scan the polymer in aꢁn unsealed aluminum
pan as reference at a heating rate of 5 C/min.
Photoexcitation was carried out using a 150 W Xenon arc
lamp (Oriel). Light of the appropriate wavelength was
selected either by a monochromator or cutoff filters (Hoya).
The quantum yield of the photochromic reaction, ring clo-
sure (coloration), and ring opening (bleaching) in solution or
in thin film were determined by comparison with photochro-
mic reaction of E-form of polymer in chloroform. Fluores-
cence quantum yield of the sample was measured relative to
fulgimide as described in the literature.24
1H NMR (400 MHz, CDCl3, d, ppm) : 1.9 (m, 16H), 3.9 (t, J ¼
5.0 Hz, 2H), 4.3 (t, J ¼ 5.6 Hz, 2H), 5.2 (s, 2H), 5.5 (s, 1H),
6.1 (s, 1H), 6.8 (s, 1H), 7.2 (d, J ¼ 6.8 Hz, 2H), 7.3 (s, 1H),
7.5 (s, 1H), 7.6 (s, 1H) (Supporting Information Fig. S6). 13C
NMR (400 MHz, CDCl3, d, ppm): 25.1, 26.3, 27.2, 28.4, 30,
50, 58.1, 68.6, 103, 114.4, 122.2, 124.3, 126.7(aliphatic), 128,
130.5, 133, 135.7, 142, 143.5, 153.9(aromatic), 158.9,
166,167,168 (AC¼¼O) (Supporting Information Fig. S7). Anal.
calcd for C28H31N5O4: C, 67.05, H, 6.23, N, 13.96. Found: C,
66.54; H,6.10, N, 13.50.
Preparation of Thin Films
A pair of quartz slides were cleaned with water, acetone, and
chloroform in an uꢁltrasonic bath for 30 min and dried in a
hot air oven at 60 C for 3 h. The polymer was dissolved in
chloroform (10%, w/w, 0.1); the solution was spin coated on
the quartz slide (size: 25 ꢂ 25 ꢂ 1.5 mm3) and dried in air.
The film thickness was adjusted to get the absorbance
between 0.05 and 0.10. The photoswitching property was
carried out in a discontinuous mode: that is, the samples
were exposed to UV radiation from a 500-W high-pressure
mercury lamp, kept at a distance of 10 cm from the sample
for varying intervals of time. The irradiated film was subse-
quently subjected to spectral analysis.
Synthesis of 2-[1,3-Dimethyl-2-indolylmethene]-3-
isopropylidene-N-[3-(4-polymethacryloyloxymethyl-1H-
[1,2,3] triazole-4-yl) propyl] Succinimide
The polymer was synthesized by free-radical solution addi-
tion polymerization technique. The homopolymerization of
monomer was carried out using AIBN as an initiator. The
monomer (0.5 g, 4 mmol) and AIBN (2 wt % of the mono-
mer) were taken in a glass ampoule and dissolved in 2 mL
of dry THF. The solution was degassed by several freeze-
pump-thaw cycles under vacuum and sealed. The ampoule
Cyclic Voltammetry Measurements
Cyclic voltammogram was carried out in a CH-620B instru-
ment using conventional three-electrode electrochemical cell
with platinum foil as auxiliary electrode, Ag/Agþ electrode
as reference electrode, and glassy carbon as working elec-
trode with 0.1 M tetrabutylammonium perchlorate (TBAP)
as the electrolyte; all experiments were performed at ambi-
ꢁ
was then kept at 60 C for 48 h, subsequently it was opened
and the content was poured into 200 mL of methanol. The
precipitated solid was separated by filtration and purified by
ꢁ
ent temperature (25 6 0.1 C) in argon atmosphere.
Fluorescence Lifetime Measurements
repeated precipitation from chloroform into methanol. Mw
¼
Time-resolved fluorescence measurements were carried out
using (IBH) time-correlated single photon counting techni-
ques by exciting the sample at 375 and 550 nm. Fluores-
cence decay was measured at 470 and 550 nm. The data
analysis was carried out by the software provided by IBH
(DAS-6), which is based on deconvolution techniques using
nonlinear least square method, and quality of the fit is nor-
mally identified by the value v2 < 1.2 and weighted residual.
2,16,698, Mn ¼ 1,95,334, polydispersity index (PDI) ¼ 1.109.
FTIR (KBr, cmꢀ1): 1758 (AC¼¼OO), 1734, 1639 (AC¼¼O),
1637 (ACH¼¼CAC¼¼O), 1609 (AC¼¼CA(CH3)2), 1720
(NAC¼¼O), 3030 (ACH3).
1H NMR (400 MHz, CDCl3, d, ppm) : 1.9 (m, 16H), 3.9 (t, J ¼
5.0 Hz, 2H), 4.3 (t, J ¼5.6 Hz, 2H), 5.2 (s, 2H), 6.8 (s, 1H), 7.3
(d, J ¼ 6.4 Hz, 2H), 7.4 (s, 1H), 7.6 (s, 1H), 7.7 (s, 1H) (Sup-
porting Information Fig. S8). 13C NMR (400 MHz, CDCl3, d,
ppm): 20.2, 23.4, 25.1, 26.3, 27.2, 28.4, 30, 50, 58.1, 68.6, 103,
114.4, 122.2, 124.3, 126.7 (aliphatic), 128, 130.5, 133, 135.7,
142, 143.5, 153.9 (aromatic), 158.9, 166,167,168 (AC¼¼O).
RESULTS AND DISCUSSION
Characterization of Polymer
The triazole-linked photochromic indolylfulgimide polymer
was prepared as per the synthetic route presented in
Schemes 1 and 2. The molecular weight of the polymer was
determined by GPC (see Supporting Information Fig. S9) and
found to be Mw ¼ 2,16,698, Mn ¼ 1, 95,334, PDI¼ 1.109.
The thermal stability of polymer was evaluated by TGA
under argon atmosphere (Fig. 1), and the thermogram
revealed that the polymer was stable up to 300 ꢁC. A single
stage decomposition was observed around 410 ꢁC is due to
Measurements
Viscosity of polymer was measured with an Ubbelohde vis-
cometer in a temperature bath at 30 ꢁC 6 1 using CHCl3 as
solvent. Molecular ꢁweight determination was performed by
GPC in THF at 25 C reported versus polystyrene standards.
The infrared spectra were obtained with a Bruker IFS 66V
Fourier Transform spectrometer using KBr pellets. High-reso-
lution 1H- and 13C-NMR spectra were recorded on a Bruker
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SCHEME 1 Synthesis of precursors.
the breakdown of methacrylic unit and imꢁide group. A high
char yield of 29.69% was observed at 750 C. This high char
yield might be attributed to increase of heteroatoms like
nitrogen and oxygen. The DSC thermogram of polymer (Fig.
2) indicates a sharp melting point followed by decomposi-
tion around 368 ꢁC without glass transition which in turn
indicates that the polymer is highly crystalline in nature,
attributed to high molecular weight as evidenced by GPC
and XRD analysis.
Powdered XRD pattern of polymer was recorded using PAN
analytical instrument. The 2h values were recorded in the
range of 10ꢁ to 70ꢁ (see Supporting Information Fig. S10).
It has been observed that the maximum relative intensity
peak of XRD pattern was well matched with crystallinity
of poly(2-chloromethylpropylene oxide) reported in the
JCPDS Powder Diffraction.22 Based on the observation, the
crystallinity of polymer was ascribed to stereo-regular
arrangement of bulky side chain.23 The noticed d-spacing
TRIAZOLE-LINKED INDOLYLFULGIMIDE POLYMER, NITHYANANDAN ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 2 Synthesis of polymer.
FIGURE 2 DSC thermogram of polymer in argon atmosphere
at a heating rate of 5 ꢁC/min.
values (Table 1) and relative intensity confirms the primary
phase of the structure of polymer.
However, the triazole ring is not affected during photocycli-
zation process (photostationary state) because it is separated
by three-carbon methylene chain. To reach the photostation-
ary state attributed to lone pair of electrons in indole nitro-
gen, p–p* transitions of the closed C-isomer of an indolylful-
gimide moiety is extended. Indeed, the yellow solution turns
pink color, with a concomitant appearance of broad absorp-
tion band at 510 nm in the visible region (as given in inset
of Supporting Information Fig. S11). Earlier, it has been
reported that the photocyclization forces the system into a
more planar conformation, thereby increasing conjugation
and lowering energy of photochromic unit. On irradiation
with 520-nm light in the different time intervals, the C-form
reverts back to original position. Alternative irradiations of
UV and visible light, the photochromic molecule can behave
as a photoswitch. It has been shown that photoswitching
Photophysical Studies of Polymer
The absorption spectrum of the polymer is shown in Sup-
porting Information Fig. S11. The typical p–p* transitions
exhibited a broad absorption band in the UV domain, which
corresponds to open (E) form (kmax (CHCl3)/386 nm) of
indolylfulgimide moiety. Similar observation was reported by
Liang et al.24 and Saravanan et al.,25 herein, there is a little
difference noticed in absorption peak maxima at 395 and
396 nm for 2-indoylfulgides and 2-indolylfulgimide polymer,
respectively.24
When absorption spectrum was recorded for polymer dis-
solved in chloroform, the kmax was noticed at 510 nm that
was attributed to charge transfer (CT) from triazole ring to
carbonyl carbon of fulgimide. On the other hand, for polymer
dissolved in THF and DMF, CT band was not observed at
510 nm (Fig. 3), attributed to the fact that hydrogen bonding
between carbonyl carbon of fulgimide and nitrogen present
in the triazole ring of polymer is pronounced only in chloro-
form solution. On irradiation with UV light at 360 nm in the
discontinued mode of various time intervals, a peak maxi-
mum at 510 nm increases with increasing irradiation time.
property is
operations.
a pivotal requirement for write and read
The absorption maxima of polymer (in chloroform) in quartz
thin film illuminated with 360-nm UV light is shown in Sup-
porting Information Figure S12. It was observed that the
absorption maximum appeared at 374 nm in thin film indi-
cates that the E-form of fulgimide moiety was same when
compared with that in solution. While increasing illumination
time of the thin film polymer, the absorption maximum at
518 nm indicates the C-form of fulgimide moiety. While com-
paring the absorption maximum of polymer in chloroform
TABLE 1 d-Spacing And Relative Intensity Values of Polymer
Relative
Pos [2h]
d-Spacing
Intensity (%)
13.63
14.33
15.03
17.03
22.19
6.48
6.17
5.88
5.20
4.00
85.66
100
26.75
19.83
26.65
FIGURE 1 TGA and DTA thermogram of polymer in N2 atmos-
phere at a heating rate of 5 ꢁC/min.
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FIGURE 5 Emission spectrum of polymer kex ¼ 510 nm at dif-
FIGURE 3 Charge transfer band at 510 nm (chloroform). [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
ferent time intervals (THF).
peak indicates the increase in emission intensity of C-form of
indolylfulgimide at 587 nm during illumination with UV
light. The emission spectrum of polymer recorded in THF
solvent shows emission peak maximum at 557 nm (shown
in Fig. 5). On illumination with UV light, the emission inten-
sity increases with red shift. Based on these observations, it
is suggested that, on illumination with light at 360 nm,
absorption at 510 nm corresponding to C-form increases,
when exciting the sample at 510 nm most of the excitation
photon observed by C-form, which leads to decrease in fluo-
rescence intensity. The higher fluorescence quantum yield of
C-form in THF was noticed when compared with fluores-
cence quantum yield of C-form in chloroform. However, with
the increase of FWHM emission on illumination of UV light,
the C-form of indolylfulgimide emission was red shifted with
increasing intensity and constant emission of CT band. The
photogenerated C-form of the polymer extends considerably
the conjugation across the molecular assembly enhancing the
fluorescence intensity with red shift. The red shift around 11
nm was observed on irradiation at 360 nm with different
time intervals. This remarkable change in shift revealed that
the lone pair of electrons in indole nitrogen extended in the
closed form of indolylfulgimide unit. This process is fully re-
versible. Photochromic component within the same skeleton
can be modulated with optical stimulations. Specifically, the
electronic structure of chromophores changes with state of
the photochromic component when delocalized action is pos-
sible across their covalent connector. Under these criteria,
and thin film, the absorption maximum of polymer in thin
film blue shifted around 12 nm when compared with that of
polymer in chloroform, as the polymer in thin film experi-
ence a confined environment.
Photoluminescence Spectra of Polymer
The photoluminescence spectra of polymer was recorded in
chloroform, and the spectra show an emission maximum at
569 nm, ascribed to CT band emission (shown in Fig. 4). On
illumination with light at 360 nm, emission maximum at 569
nm decreases with red shift because of higher fluorescence
quantum yield of CT band as compared to that of photoregu-
lated C-form of indolylfulgimide polymer. On other hand, the
decrease in full width half maximum (FWHM) of emission
TABLE 2 Fluorescence Quantum Yield of Indolyl Fulgide and
Indolyl Polymer
Acetonitrile
FIGURE 4 Emission spectrum of polymer kex ¼ 510 nm at dif-
ferent time intervals (chloroform). [Color figure can be viewed
in the online issue, which is available at wileyonlinelibrary.
com.]
FL
k
max, EM (nm)
/
Indolylfulgide
618
590
0.033
0.019
Indolyl polymer
TRIAZOLE-LINKED INDOLYLFULGIMIDE POLYMER, NITHYANANDAN ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 3 Thermal, Photophysical and Electrochemical Properties of Polymer
Optical
Band
(CHCl3)a
k
(CHCl3)b
max
abs
em
k
max
Isomer
(nm)
emax
(nm)
Ufc
Gap (eV)
Td (ꢁC)
Polymer
E
C
386
510
13,400
14,400
569
590
0.010
0.19
2.73
373
a
Longest wavelength absorption maximum in THF.
Fluorescence emission maximum in CHCl3.
Fluorescence quantum yield relative to Rhodoamine B as standard (U ¼ 0.65).
b
c
the reversible transformation of photochromic switch results
in modulation of emission intensity.26,27
Fluorescence Lifetime Studies
The fluorescence lifetime of triazole-containing indolylfulgi-
mide moiety of the polymer in chloroform on excitation at 375
nm exhibited triexponential behavior with lifetime of 0.1
(25%), 1 (23%) and 4.2 ns (51%) (Supporting Information
Fig. S15). Fluorescence lifetime was recorded after irradiation
of sample at 360 nm with various time intervals, and fitted
values are given in Table 4. From the results, it is observed
that the relative amplitude of C-form of indolylfulgimide moi-
ety increased whereas the relative amplitude of E-form
decreased. Whereas, triazole ring show no remarkable change
in relative amplitude. The observed fluorescence decay
revealed that 0.1 and 1 ns corresponds to E-form and C-form
of indolylfulgimide, respectively. The fluorescence lifetime of
4.2 ns for the triazole ring is unaffected during the irradiation
at 360 nm. To conform, whether the absorption maxima at
510 nm correspond to C-form of indolylfulgimide moiety or
CT band of triazole ring and carbonyl carbon of indolylfulgi-
mide moiety, we have calculated the fluorescence lifetime for
the same polymer on excitation at 470 nm. The fluorescence
decay well fitted in biexponential function with lifetime of 1
and 2.2 ns, which corresponds to closed (C) form of indolylful-
gimide and triazole ring of the polymer, respectively.
Fluorescence Excitation Spectral Studies
The fluorescence excitation spectra of polymer are illustrated
in Supporting Information Figures S13 and S14 for polymer
in chloroform/THF before and after irradiation with light at
360 nm has been recorded and emission maximum fixed at
550 nm. Fluorescence excitation spectrum had two peak
maxima at 476 and 507 nm that corresponds to indolylfulgi-
mide moiety and triazole ring, respectively. This fluorescence
excitation was conformed by the exactly matched absorption
spectrum of C-form of indolylfulgimide and triazole ring
present in the polymer. It confirms that emission occurs
from both indolylfulgimide and triazole ring present in the
polymer.
Fluorescence Quantum Yield of Polymer
The fluorescence of C-form of polymer consists of broad emis-
sion band with its maximum intensity at 587 nm (see Table
2), which is due to the photoregulated colored forms of indo-
lylfulgimide moiety rather than triazole ring present in the
polymer. Fluorescence quantum yield of the polymer was cal-
culated and given in Table 3. On comparison with quantum
yield of indolyl polymer in E-form (0.05) and C-form (0.11)
reported by Liang et al.,24 in our studies, the quantum yield
decrease to 0.02 (E-form) and 0.01 (C-form), respectively. The
decrease in quantum yield of polymer is due to highly efficient
vibrational relaxation of three methylene groups in the poly-
mer, which leads to interconversion of the nonradiative transi-
tions. However, it is difficult to examine the emission of E-
form; we were able to detect emission peak maximum 557
nm, which could be restored by CT band28 of triazole ring and
carbonyl carbon in presence of solvent (chloroform).
Electrochemical Studies of Polymer
Cyclic voltammograms of 2-indolylfulgide and 2-indolylfulgi-
mide polymers were recorded 0.1 M TBAP in acetonitrile
TABLE 4 Fluorescence Lifetime Decay Profile of Polymer at
Different Time Intervals of Illumination Under 360 nm UV Light
Time (s)
s1 (ns)
s2 (ns)
s3 (ns)
B1
B2
B3
0
0.16
0.10
0.11
0.08
0.08
1.0
1.0
1.0
0.97
1.0
4.2
4.7
5.2
5.0
4.3
25.00
26.85
16.14
20.44
22.19
23.99
23.90
29.99
23.01
23.48
51.01
49.25
53.87
56.55
54.33
30
60
90
120
FIGURE 6 Cyclic voltammogram of open E-isomer of polymer
in acetonitrile (1 ꢂ 10ꢀ3 M) and (0.1 M TBAP) on a platinum
electrode (vs. Fc/Fcþ) with scanning rate of 50 mV sꢀ1. [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
kexc ¼ 375 nm, kem ¼ 550 nm.
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WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
of polymer interestingly shows peak maximum at 557 nm,
which attributed the CT band of triazole ring to carbonyl
carbon of indolyl fulgimide unit. On irradiation with 360 nm,
the emission maximum was red shifted from 557 to 590 nm
because of formation of photoexcited C-form. We have stud-
ied optical switching properties of this material by fluores-
cence lifetime techniques. On excitation at 375 nm, triexpo-
nential behavior was exhibited with lifetime of 0.1, 1, and
4.2 ns. On excitation at 470 nm, biexponential behavior was
exhibited with lifetime of 1 and 2.2 ns. Cyclic voltammo-
grams of E-form of polymer show irreversible reduction
peak at ꢀ1.4 V; during photolysis, the reduction peak at
ꢀ1.4 V shifted toward less cathodic peak at ꢀ0.9 V, which
suggested the closed C-form of indolylfulgimide unit. Owing
to interesting photophysical, electrochemical and thermal
properties, these materials make them potentially useful for
applications in optical storage and optical switching devices.
FIGURE 7 Cyclic voltammogram of close C-isomer of polymer
in acetonitrile (1 ꢂ 10ꢀ3 M) and (0.1 M TBAP) on a platinum
electrode (vs. Fc/Fcþ) with scanning rate of 50 mV sꢀ1. [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
The authors acknowledge the financial support received from
the Board of Research in Nuclear Science (Sanction No. 2006/
37/13/BRNS/205) Government of India.
REFERENCES AND NOTES
solution at a scan rate of 10 mV sꢀ1 as shown in Supporting
Information Figures S16, S17 and Figures 6 and 7, respectively.
The open form of indolylfulgide shows no characteristic oxida-
tive and reductive peaks. After illumination at 360 nm, the
electrochemical signal arising as irreversible cathodic peak at
ꢀ1.3 V was tentatively assigned to closed form of 2-indolylful-
gide. While, the open form of 2-indolylfulgimide polymer
shows irreversible reductive peak at ꢀ1.4 V, after illumination
at 360 nm (time interval of 90 s) two new irreversible reduc-
tive peaks appeared at less positive potentials, that is, ꢀ1.2
and ꢀ0.9 V. Increasing the illumination (time interval of 180
s) at the same wavelength abruptly shows single irreversible
reductive peak at ꢀ0.9 V and this indicates the formation of
photogenerated closed form of indolylfulgidmide polymer. Sim-
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electrochemical cyclization of diarylethenes. As a result, on
photochromic ring-closing reaction, the conversion of the 2-in-
dolyl rings to 5,9b,10,10-tetramethyl-2-propyl-9b,10-dihydro-
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chemical signal arising from the open form and the formation
of new cathodic peak due to the reductions of the closed form.
The occurrence of these new reduction waves at less cathodic
peak is in good agreement with the more conjugated nature of
the nitrogen present in the fulgimide polymer. The peculiar
interaction of conjugated nitrogen with light is essential for
making optical devices and optical data storage applications.
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