International Journal of Organic Chemistry, 2013, 3, 49-64
Published Online November 2013 (http://www.scirp.org/journal/ijoc)
Open Access IJOC
Electrochemical, Photophysical, and Magnetic Properties
of Green Emitting
Conducting Oligomer Addended Fullerene-Diol Dyad
Rachana Singh1, Rimpa Jaiswal2, Thakohari Goswami2*
1School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, USA
2Electronics and Smart Materials Division, Defence Materials and Stores Research and Development Establishment,
Received September 18, 2013; revised October 25, 2013; accepted November 5, 2013
Copyright © 2013 Rachana Singh et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Towards the development of potential new organic photovoltaic and optoelectronic materials, a simple route to synthe-
size flexibly ether linked fullerene-bis[oligo-(phenylene-alt-thiophene)] and evaluation of electrochemical, photophysi-
cal and magnetic properties is presented. Flexible ether linking of oligo-phenylene-thiophene chain to 1, 2 C60(OH)2 is
achieved employing Williamson’s ether synthesis. 7-chain phenylene-thiophene chain fluorescent conducting oligomer
is synthesized using Grignard coupling reaction with preservation of bromo end groups. Oligomer is highly ordered and
soluble in all organic solvents while on linking to fullerene-diol, solubility of adduct restricts only to dimethyl sulfoxide
(DMSO). All the synthesized materials are characterized through spectroscopic techniques and molecular weight is de-
termined by mass spectrometry and GPC. Properties of the material indicate the substantial effect of fullerene. High
quenching in fluorescence intensity and strong paramagnetic property are observed in this material.
Keywords: Fullerene-Diol; Fluorescent Conducting Oligomer; Electrochemical Properties; Photophysical Properties;
Several fullerene-based donor-acceptor dyads [1-4] syn-
thesized so far through the attachment of polymeric or
oligomeric chain to fullerene have resulted in improved
photovoltaic efficiency [5,6]. The search for flexibly at-
tached donor moiety to fullerene core is, however, almost
negligible. Ether linkages provide better flexibility and
help in film formation. Away from conventional methods
resulting in rigid cyclo-additions [7,8], exohedral addi-
tion of hydroxyl groups on fullerene provides several
sites for flexible attachment for other molecules [9-18].
Fullerenol behaves as excellent nucleophile because of
the electrophilic nature of fullerene which makes hy-
droxyl hydrogen highly acidic. Interestingly, fullerenol is
appeared to be very notorious owing to non-specified
attachment of hydroxyl groups onto its surface but sup-
posed to be the most applicable molecule due to its high
reactivity, stability and solubility. Controlled synthesis of
fullerenol with fewer number of hydroxyl groups has
opened up the way to use this magical derivative of full-
erene and in the present study 1, 2 C60(OH)2 used as the
fullerene source . Due to attachment of only two exo-
hedral OH groups, fullerene’s symmetry is not perturbed
to a great extent and it preserves its high electrophilic
character making the efficient nucleophile.
One of the important issues to improve performance of
photovoltaic material is to design donor moiety having
effective donating capability. Extensive research is on to
prepare different classes of donor molecules like poly-
mers, macrocycles, etc., but polymers find most exten-
sive application. Among the polymers, highly regioregu-
lar poly 3-hexylthiophene (rrP3HT) is being extensively
used with the best results [20-22]. Substitution on the main
chain and control over regio-regularity can tune the band
R. SINGH ET AL.
gap of polythiophene from 1.0 - 2.5 eV . The lowest
band gap allows the creation of polymer light-emitting
diodes in the near infra red range and matches well with
ultimate requirement for solar spectrum. The main draw-
back for commercial use of polymeric materials is, how-
ever, associated with their poor stability to air/oxygen
resulting in larger off-currents & lower on/off ratio and
also positive shift in the threshold voltage in case of or-
ganic thin film transistor (OTFTs). The stability towards
oxidative doping is related to ionization potential, i.e.,
the highest occupied molecular orbital (HOMO) levels in
vacuum and lowering of HOMO-level improves its sta-
bility. This can be achieved by a number of methods.
Introduction of electron donating groups, such as alkoxy
or thio-alkyl groups, at β-position of thiophene unit in-
creases the band gap by lowering HOMO level [23,24].
Insertion of suitably substituted phenylene rings into the
thiophene backbone is another efficient structural modi-
fication approach for improving photoluminescence (PL)
efficiency of thiophene-based conjugated polymers .
Substitutions on both phenylene and thiophene rings re-
markably affect the PL quantum efficiency of the result-
ing polymers and also provide better air stability.
Present article describes a simple and convenient me-
thod for stepwise synthesis of regioregular bromo end
capped 7-chain phenylene-alt-thiophene oligomer (5) and
its further attachment to 1,2-fullerene-diol to achieve
novel photovoltaic materials (6) . One of the strong ad-
vantages of this synthesis route is that the use of both
cryogenic temperatures and highly reactive metals is not
required and consequently this method offers quick and
easy preparation of oligomer in large scale. These reac-
tions are carried out either at room temperature or in re-
flux condition. Covalent attachment of 5 onto fullerene
core by nucleophilic substitution of the terminal bromo
group with the hydroxyl groups of fullerenol produces
fullerenol-adduct 6. Evaluation of thermal, photophysical
and magnetic properties of the material indicates substan-
tial effect of fullerene on the thermal behavior as well as
high quenching in fluorescence intensity and strong para-
2. Experimental Section
 fullerene was obtained from MER Co. (purity >
99.5%). The sample quality was checked by UV/vis ab-
sorption, 13C NMR, and was used without further purifi-
cation. Thiophene and hydroquinone were purchased
from E Merck and used without further purification. NBS
was recrystallized and dried freshly. All solvents were
purified and dried before use. All glass wares were flame
dried under argon before reaction.
2.2. Synthesis of Monomer Units
2.2.1. Synthesis of Diiodothiophene (1
In a 200 mL three necked flask, 4.2 g (0.05 M of thio-
phene is added into 30 mL DCM. 64.5 g of iodine (0.25
M is added in portions with high speed stirring (me-
chanical. A solution of 68 mL nitric acid: water (1:1 is
transferred to a dropping funnel fitted to the flask and
~10 mL of this nitric acid: water mixture is added slowly
with vigorous stirring. Once initiated, the reaction pro-
ceeds vigorously with the evolution of brown oxides of
nitrogen. After the evolution of gases is subsided, the
remaining nitric acid is added drop wise and reaction
proceeds smoothly at room temperature. After all nitric
acid is added, the solution is heated under refluxing for
30 min. The reaction mixture is allowed to stand and the
red organic layer is collected and washed several times
with water and dried over anhy. Na2SO4 and concen-
trated to yellow oil. On cooling the yellow oil freezes and
collected as yellow crystals of diiodothiophene. The
product is characterized by 1H & 13C NMR (CDCl3,
singlet at 6.96 ppm for the two thiophene ring protons
and two carbon peaks at 78 & 138 ppm for substituted
and unsubstituted thiophene carbons respectively clearly
supporting the formation of 1,5-diiodothiophene. UV-vis
(methanol, nm 242, 245, 252, 256 & 266; Yield = 50%,
2.2.2. Synthesis of 1,4 Dihexyloxy Phenylene (2
To the degassed alkaline (12.5 g NaOH methanolic
solution of hydroquinone (11 g, 100 mM in 150 mL
methanol, 1-bromo hexane (35.09 mL, 250 mM is
added slowly at room temperature. The reaction mixture
is then refluxed for 24 h and finally extracted with di-
ethyl ether (4 times, 50 mL each. Combined organic
fractions were washed with 10% NaOH solution (2 - 3
times; 50 mL each and further with water to remove the
impurity of hydroquinone and alkali respectively. Or-
ganic layer is dried over anhy. Na2SO 4 & concentrated to
obtain white crystals. Washed several times with chilled
methanol and further re-crystallized with dicholomethane.
1H NMR (CDCl3,
) 0.8 (t, 6H, CH3, 1.2 (m, 8H,
CH2CH2, 1.3 (q, 4H, CH2, 1.6 (q, 4H, CH2, 3.8 (t, 4H,
-O-CH2, 6.7 (s, 4H, Ph; UV-vis (methanol, nm 227,
248, 254, 260 & 290; Yield = 55%, mp: 36˚C.
2.2.3. Synthesis of 1,4 Dibromo-2,5
Dihexyloxy Phenylene (3
To a mixture of CH2Cl2/CH3COOH (100 mL, 1:1 solu-
tion, 2 (5 g, 0.018 M is added under inert atmosphere
and stirred for 15 min. N-bromo succinimide (NBS) is
added in portions (5.3 g, 0.045 M with stirring. Reaction
temperature is raised to 60˚C in stirring condition on
complete addition of NBS. After 24 h of stirring the re-
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R. SINGH ET AL. 51
action mixture is poured onto 200 mL water. Organic
layer is separated and washed 3 - 4 times with 100 mL
saturated sodium sulphate solution & further washed
with water. Organic layer is dried over anhy. Na2SO4 &
concentrated. Pinkish-white colored crystals are obtained
and recrystallized with dichloromethane. 1H NMR
: 0.8 (t, 6H, CH3, 1.2 (m, 18H, CH2CH2, 1.3
(q, 4H, CH2, 1.6 (q, 4H, CH2, 3.8 (t, 4H, -O-CH2, 7.06
(s, 2H, Ph; UV-vis (methanol, nm 227, 248, 254, 260 &
29; Yield = 67%, mp: 50˚C.
2.2.4. Synthesis of 4
1) Preparation of Grignard Reagent of Diiodothio-
phene: Few drops of mixture of diiodothiophene (1,
1.0078 g, 3 mM and dry dichloro-ethane (1.437 mL, 18
mM in 30 mL dry THF is added to the dry Mg (0.18023,
7.5 mM also in dry THF (20 mL) under inert atmos-
phere at refluxing temperature (65˚C - 70˚C. 1 mL of
dry dichloro-ethane is further added to initiate the reac-
tion. After initiation, the rest of the diiodothiophene/di-
chloro-ethane solution is added drop wise over 1 h. Re-
action mixture is refluxed for 5 h & cooled to room tem-
2) Grignard coupling reaction: The Grignard of 1
thus prepared, is slowly transferred to the ice-cooled dry
THF solution of 3 (3.48 g, 7.5 mM) and Ni (dppe Cl2
(30 mg was added under inert atmosphere with stirring
at room temperature. After 24 h, 50 mL of saturated
ammonium chloride solution is added and stirred for 30
min. Organic layer is washed several times with water
and dried over anhy. Na2SO4 and concentrated to 2 mL.
The concentrate is kept in refrigerator for crystallization.
The obtained crystals are further recrystallized with di-
chloromethane. The FTIR (KBr, cm−1 3050 (
2919, 2848 (
, C-H, 1676 (
, C=C Ph, 1495 (
Th, 1462, 1393, 1358 (
, CH3, 1269, 1210 (
, C-Br, 1062 (
, =C-O, 1020 (
, C-O ether, 805
& 726 (
, out of plane =C-H. NMR (1H & 13C, CDCl3,
0.8 - 0.9, 1.34 - 1.36, 1.45 - 1.52, 1.75 - 1.85, 3.93 -
3.97, 6.8, 6.9, 7.09; 13.9, 22.5, 25.5, 28.9, 31.4, 70.19,
111.0, 114.2, 118.3, 119.3, 149.9; UV-vis (methanol,
nm 249, 255 & 300; Yield = 2 g (83%; mp = 50˚C.
2.2.5. Synthesis of 7-Chain Oligomer (5
Grignard of 1 (0.3359 g, 1 mM, prepared as described
above, is added to the ice-cooled dry THF solution of 4
(2 g, 2.5 mM and Ni (ddpe Cl2 (30 mg) under inert at-
mosphere in stirring condition. Temperature is raised to r.
t (25˚C) and stirring continued for 24 h. Similar work-up
steps to that of product 4ii are employed and is recrystal-
lized with dichloromethane. FTIR (KBr, cm−1 3000 (
=C-H, 2932, 2848 (
, C-H, 1677 (
, C=C Ph, 1495 (
C=C Th, 1462, 1394, 1359 (
, CH3, 1269, 1210 (
CH2, 1124 (
, C-Br, 1062 (
, =C-O, 1020 (
ether, 805 & 726 (
, out of plane =C-H. 1H & 13C NMR
0.8 - 0.9, 1.22, 1.6, 1.7, 3.8, 6.7 - 6.8, 6.8,
7.01; 13.9, 22.3, 25.3, 29.1, 31.2, 70.2, 111, 112, 114,
118, 119, 138, 150, 153. UV-vis (methanol, nm 249,
255 & 300; Yield = 2 g (66%; mp = 55˚C.
2.2.6. Synthesis of Fullerene-Oligomer Adduct (6)
To the benzene solution of fullerenol (0.020 g, 0.027
mM), K2CO 3 (anhy, 0.0398 g, 2.88 mM) and 18-crown-
6-ether (catalytic amount) are added at room temperature
(25˚C). The benzene solution of oligomer 5 (0.2177 g,
1.44 mM in 10 mL) is added drop wise with stirring at
room temperature. The temperature of the reaction mix-
ture is now raised to reflux and after 24 h of refluxing;
the solid mass is collected by centrifugation and washed
several times with methanol: water (80:20) mixture to
remove the impurity of K2CO3 and crown ether. Solid
dried and collected.
Yield = 10 mg (50%; FTIR (KBr, cm−1 3187 (
=C-H, 2932, 2849 (
, C-H, 1656 (
, C=C Ph, 1495 (
C=C Th, 1451, 1376 (
, CH3, 1104 (
, C-Br, 1020 (
C-O ether, 805 & 726 (
, out of plane =C-H; 1H & 13C
0.8 - 1.0, 1.15 - 1.23, 1.6, 2.3, 3.4 - 3.5,
7.16 - 7.18, 7.23 - 7.25; 21, 29, 30, 33, 34, 69.8, 125.6,
128.5, 129.2, 130.08, 148.3, 150.9, 156.2.
3. Results and Discussion
3.1. Synthesis and Identity of Products
A simple approach towards the synthesis of highly or-
dered oligomeric chain has been adopted starting with
2,5 diiodo thiophene (1) prepared by reported method
(Scheme 1). Grignard coupling of (1’) and 1,4 dibromo
2,5-bis hexyloxy benzene (3) results in 3-chained 2,5-bis
(4-bromo-2,5(hexyloxy) phenyl) thiophene (4). Repeti-
tion of the above reaction using (4) and Grignard of (1)
converts (4) into oligomer (5) (Scheme 2) retaining ter-
minal bromo groups. The conversion efficiency of the
Grignard coupling is excellent (83%). All materials have
been purified by flash chromatography. Selective at-
tachment of the regioregular oligomer to fullerene core in
heterogeneous medium is carried out using benzene solu-
tion of fullerene-diol and oligomer (5) with K2CO3 in
presence of 18-crown-6-ether. Refluxing for 24 h yields
dyad (6) in good yield (Scheme 3). 1,2 C60(OH)2 has
been synthesized and purified according to reported
Oligomer (5) is a pale yellow crystalline solid soluble
in most of the organic solvents, while the dyad (6) is
black powder with solubility only in DMSO.
Product structures are established by spectroscopic
characterization. The critical 13C peaks assignment of the
oligomer (5 ) suggest three non equivalent carbons (C1,
C2, C3) for inner phenylene ring and six for terminal
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R. SINGH ET AL.
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HO OH C6H13OOC6H13 C6H13OOC6H13
1,4- di br omo-2, 5- bi s
(hexyloxy)b e nze ne (3)
Scheme 1. Synthesis of precursor materials 1 & 3. (i) Iodine (0.25 M), Nitric acid : water (1:1, 68 mL), refluxed for 30 min; (ii)
NaOH (12.5 g), 1-bromo hexane (0.25 M), refluxed for 24 h; (iii) CH2Cl2/CH3COOH (100 mL, 1:1), 2 (0.018 M), NBS (0.045
M), 24 h stirring at 60˚C.
Scheme 2. Synthesis of precursor materials 4 & oligomer 5. (i) Dichloro ethane (18 mL), Mg (7.5 mM), refluxed at 65˚C -
70˚C for 5 h; (ii) THF solution of 3 (7.5 mM), Ni (dppe) Cl2 (30 mg), stirred for 24 h, (iii) 1’; (iv) Ni (dppe Cl2/24 h stirring/r. t.
phenyl ring, of which C1 and C2 is common for both.
Thiophene has two non-equivalent carbons (C2, C3).
Overall one should expect eight 13C peaks for (5). On the
other hand, 1H NMR assigns four types of non-equivalent
protons; inner phenyl ring proton at C3, two different
protons for the terminal ring at C3 and C6 and single pro-
ton for thiophene at C3. The FTIR spectrum of (5) also
nicely accords with the proposed structure. The inter-ring
C-C & C-Br streching peak is appearing in product (5).
The molecular weight of the oligomer estimated by GPC
using polystyrene as standard confirms the 7-ringed oli-
gomeric phenylene-thiophene structure with poly-dis-
persity ~1.1. The molecular ion peak in ESI-MS (metha-
nol, M+-1 at m/z 1521) also adheres with the GPC result.
Attachment of oligomer to fullerene core is evident
from the disappearance of all the typical peaks of
fullerenol and appearance of aromatic & alkyl stretching
and bending peaks in FTIR spectrum of (6). The spec-
trum also shows peak for C-Br (
at 1104 cm−1 suggest-
ing single terminal bromo group involvement in the reac-
tion. Fol-O-C ether linkage is ascertained from the ap-
Scheme 3. Outline of reaction scheme for the synthesis of
phenylene-alt-thiophene addended fullerenediol dyad (6). (i
K2CO3 (2.88 mM, 18-Crown-6-Ether; (ii Benzene solution
of 5 (1.44 mM, 24 h refluxing.
R. SINGH ET AL. 53
pearance of broad (
-band at 1020 cm−1. All eight
non-equivalent carbon peaks, similar to that of (5), still
exist in 13C NMR of the dyad (6). The broad peaks in 1H
NMR are shifted down-field due to the attachment of
electronegative fullerene into oligomer. Sp3 hybridized
fullerene (at the point of attachment) as well as hexyloxy
carbon peaks are present. High symmetry of the dyad is
as well be determined by NMR spectroscopy. Broad
triplet for methyl group of hexyloxy chain, followed by
methylene proton peaks between 2.2 - 2.6 ppm are ob-
served in 1H NMR of (6). Appearance of single triplet for
methylenoxy protons envisaged the equivalent nature of
the addend oligomeric chain. Aromatic proton peaks ap-
pear between 7.1 - 7.3 ppm. Due to paramagnetic nature
of the dyad material, NMR signals are not very sharp.
3.2. Molecular Modeling
Effect of exohedral addition of OH-groups in fullerenol
and oligomers in dyad is compared with that of fullerene
by performing molecular modeling at semiemperical
AM1 level using DS visualizer (Figure 1) and with
SPARTAN for the lowest energy molecule (1597.3744
Kcal/mol) (Figure 2). C1-C2 bond length of fullerene
moiety in derivatives has increased due to addition. Mo-
lecular modeling calculation measures C1-C2 bond length
of 1.55 - 1.609 Å in diol to compared to 1.38 Å in pris-
tine fullerene suggesting conversion of sp2 carbon into
sp3. A further nominal increment by ca. 0.03 Å. of C1-C2
bond length has been observed on addition of oligomeric
chain. Slight change in bond angle is also observed on
3.3. Thermal Properties
Oligomer 5 is a crystalline solid material possessing good
Figure 1. Semiemperical AM1 molecular modeling using DS
visualizer for comparison of bond length ((a) & (b), Å) and
bond angles ((c) & (d)) between fullerene-diol and dyad (6).
Figure 2. Molecular modeling with SPARTAN for the low-
est energy molecule (1597.3744 Kcal/mol) for comparison of
bond length (Å) and bond angles (degree) between fullerene-
diol and dyad (6).
thermal stability. Melting point recorded in DSC is re-
ported to be 64.2˚C and the enthalpy of melting (m
is 121.6 J/g (Figure 3). The oligomer (heating rate of
10˚C/min in nitrogen atmosphere) shows thermal stabil-
ity up to 150˚C followed by a sharp degradation. First
derivative TGA trace indicate single degradation step
(crest temp 318.4˚C) owing to high symmetric structure.
It is attributed to the formation of more rigid planar inter-
digited π-stack 3D structure and conformational symme-
try due to thermal ordering of hexyloxy groups attached
in regular fashion at a particular distance. Flexible ether
linkages of hexyloxy groups might have assisted in at-
tending such geometry.
Symmetric structure of 5 is perturbed on chemical at-
tachment to fullerene and is clearly evident from the
TGA and first derivative TGA thermogram of 6. Adduct
6 still maintain initial thermal stability up to 150˚C and
the small weight loss (6.7%) within this temperature
range is due to removal of low boiling solvents physi-
cally absorbed on the surface. Stepwise removal of ad-
dended oligomers is also recorded in first derivative be-
fore the structural degradation of fullerene. First step is
associated with long temperature zone 150˚C - 410˚C
(crest temp 330.78˚C) followed by two sharp steps in the
temperature range 425˚C - 520˚C (crest temperature
480.16˚C) and 520˚C - 560˚C (crest temperature 550˚C)
respectively (Figure 4). Weight loss associated with each
step corresponds to sequential release of terminal bromo
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R. SINGH ET AL.
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Figure 3. (a TGA and (b D heating rate of 10˚C/min.
SC of oligomer 5 recorded under inert atmosphere and at
Figure 4. (a TGA and (b2 atm.
ontaining phenylene and attached thiophene ring fol-
3.4. Electrochemical Properties
with a computer
DSC of adduct 6 recorded at a heating rate of 10˚C /min under N
lowed by middle phenylene and thiophene rings respec-
tively. The required amount of absorbed energy associ-
ated during sequential release of addended aromatic units
is calculated to be 224.5 J/g (DSC of 6 in N2 atm., heat-
ing rate 10˚C/min).
Cyclic voltammograms are recorded
controlled Autolab model 302 Potentiostat at a constant
scan rate of 25 mV/s using 0.1 M tetrabutyl ammonium
perchlorate (n-Bu4NH4ClO4 in acetonitrile as supporting
electrolyte. A three-electrode configuration undivided
cell is used: platinum disc working electrode, platinum
wire counter electrode and Ag/AgCl (3 M KCl and satu-
rated Ag/Cl separated with a diaphragm as reference
electrode. Typical cyclic voltammogram of 5 (scans
window between –1.2 to 2.0 V) is displayed in Figure 5
and the electrochemical data are presented in Table 1.
Formal reduction potential
–0.77 V and the onset reducti re-
corded to be –0.66 V. Values of formal oxida poten-
tial (Eox and onset oxidation potential (0
E are simi-
larly measured to be 1.42 V and 1.47 V restively. The
measured redox behavior is transposed to estimate the
an empirical relationship proposed by Bredas et al. 
on potential ( is
ionization potential (I and electron affinity (E using
ox 4.4 eV
tion and reduction
and are the onset potentials for oxida-
relative to Ag/AgCl reference elec-
and oligo-phenylene analogues
trode fr vacuumvel.  Electron affinity (LUMO
level and ionization potential (HOMO level is calcu-
lated as 3.74 eV and 5.87 eV and the difference in energy
between Ip and Ea yields the band gap of the material. For
5, the value is 2.13 eV.
The measured low band gap value in 5 compared to
ggest a more regular structure in solution and incorpo-
ration of alkoxy substituted phenylene ring alternate to
each thiophene can be a better design for obtaining these
-conjugated systems. Reports suggest that for thio-
phene-phenylene oligomers, the standard formal oxida-
tion potential (Eox increases with the introduction of
p-phenylene rings into the oligomer . This increase in
oxidation potential is ca 0.15 V for one p-phenylene ring
and subsequent introduction of p-phenylene rings shift its
oxidation potential by ca. 0.10 V. It means that the thio-
R. SINGH ET AL. 55
-1.0-0.50.0 0.5 1.0 1.5 2.0
Figure 5. Cyclic voltammogram of oligomer 5 (3 mM in
acetonitrile) recorded at 25˚C using sweep rate of 25 mV·s−1
ility than the corresponding oligo-thiophenes. Electro-
and high (i.e., phenylene band gap materials. Experi-
orm at room tem-
perature (25˚C) shows maxima at 302 nm which is ap-
nergy compared to their
phene-phenylene oligomer (5 should have better air sta-
chemical properties of fol-adduct (6) could not be meas-
ured due to solubility problem of the material. Operating
window of DMSO falls in the range –1 to +1 V and the
adduct (6) does not show any characteristic redox peak in
Electrical and optical properties of
copolymers mainly depend on mean
length and -electron delocalization [29,30]. The nature
of the substituted group generally affects the electronic
and optical properties of the conjugated polymers in two
ways ; the electronic feature of the substituted group
and steric hindrance arising from the substituted group.
The side chains do not take part directly in the -bonds
delocalization, but their steric hindrance could induce a
considerable inter-ring twisting, giving rise to a substan-
tial reduction of conjugation length. It has been demon-
strated both experimentally and theoretically that alkyl
substitution in polythiophene gives rise to stronger steric
hindrance and increases the torsional angles between
aromatic rings (effectively reduces conjugation length)
resulting spectral blue shift in absorption . On the
other hand, introduction of alkoxy chains at 2- and 5-
positions of the phenylene rings induces spectral red shift
of the same polymers [33-35]. Strong electron-donating
property and less steric hindrance of alkoxy groups
compared to alkyl substituents are considered responsible
for the spectral red shift and by far, hexyloxy substituents
at 2,5-position of phenylene rings have the highest effect
on spectral red shift . Theoretical works  on the
copolymers suggest that the resultant band gap is the
weight average band gap value of the individual unit in
polymer composed of alternative low (i.e., thiophene
mental observation records 20 - 40 nm blue shifts of the
absorption maxima (higher energy) in dilute chloroform
solution on inserting a single phenylene unit in 5-ring
polymer of thiophene analogue .
3.5.1. Absorption Properties
Absorption spectrum of (4 in chlorof
preciably red-shifted (lower e
individual building blocks (1) and (3) because of easy
-* transition on increasing the conjugated chain length.
A blue-shift (higher energy) in absorption maxima by
about 50 nm compared to 3-ring thiophene homologue
(Table 2) is indeed observed. The optical-gap calculated
from absorption edge (absorption onset;
onset 326 nm3.83 eV
is substantially higher than its
thiophene homologue. The result suggests that the pres-
ence of p-phenylene groups does not break
OMO-LUMO optical band gap.
The absorption maxima of 7-chain oligomer (5 remains
the same (
max = 302 nm although optical-gap is slightly
but do influence their H
onset 340 nm3.66 eV
the chain length from 3 to 7 (Figure 6). This result indi-
cates that the absorption maxima remains uneffected on
inserting 1,4-heylene group be-
tween two thiophene units, although optical gap is
slightly decreased by ca. 0.17 eV. The optical gap is
however significantly higher compared to 7-ring thio-
phene homo-polymer (~2.8 eV and the difference is
about 0.8 eV and the optical band gap is more compara-
ble to 7-ring oligo p-phenylene homo-polymer.
The inductive effect due to electron-donating character
of an alkoxy group present in 2,5-positions of phenylene
ring would expect to contribute red shifting of abs
xyloxy substituted phen
d emission maxima. Indeed, this effect is substantial in
fluorescence but in absorption it contributes for blue-
shifting. It is attributed that although these alkoxy groups
are positioned head-to-tail in adjacent rings but still
yields a slightly greater preference for non-coplanar ge-
ometry in ground state. Thus, no shifting of absorption
maxima on increasing the conjugation chain length can
be rationalized by combining the total effect of alkoxy
substituted phenylene with thiophene and chain exten-
sion. The absorption spectrum in methanol also shows
similar spectral features. This indicates that polarity of
the solvent does not influence the absorption property.
Insolubility of the adduct (6) in common solvent (soluble
only in DMSO) prohibits comparison of absorption
property. UV-vis spectrum of (6) recorded in DMSO
shows high absorption in UV-region and extended tailing
in the entire visible region, typically matched with the
characteristic of the fullerene derivatives (Figure 6).
3.5.2. Emission Properties
Comparative emission spectra of oligomer 5, fulleren
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Table 1. Electrochemica
p-Doping (V n-y Levels (eV
l data of oligomer 5.
Doping (V Energ
Eonset Epc Eonset Epc HOMOO Band Gap(eV
Epa Epa LUM
1.47 1.55 1.42 −0.66 − 0.77 - −5.87 −3.74 2.13
Table 2. Optical data of 4, 5 and 6.
max (nm) Onset abs.
onset(nm)/band gap (eV)Emission em (nm) Stokes’ shift(nm)
3T 342/3.62 433/2.86 91
4 300/4.13 326/3.83 502/2.48
7T 409/3.03 490/2.53 81
5 302/4.10 /3512/2.43 212
6 entire UV 461/2.7 /3.00 436/2.
Figure 6. UV-vis spectra of (A) 5 and (B) 6 recoded in
nd oligomer-fol adduct (6 measured in solid state using
both the cases (Figure 7). Small perturbations of -
76 nm compared to 5. Con-
oped state is EPR inactive,
radical(s)/ion(s) and elec-
chloroform and DMSO respectively at room temperature.
excitation wavelength 350 nm (
ex = 350 nm are pre-
sented in Figure 7 and photophysical data are complied
in Table 2. Fluorescence spectra of 5 show strong single
emission peak in green region (512 nm/2.43 eV). A small
red-shifting (10 nm) of emission peak in oligomer 5
compared to 4 is observed. It is attributed that this small
red-shifting is net balance of two opposing factors oper-
ating simultaneously; the presence of phenyl ring in the
backbone and contribution of thiophene ring and chain
extension. However, both oligomer 5 and monomer 4
show a large Stokes’ shift between absorption and emis-
sion peaks. The Stokes’ shift is more than 200 nm and
can be ascribed to the formation of more rigid planar
inter-digited π-stack 3D structure and conformational
symmetry occurring after photo-excitation [37-39] al-
lowing a possible exciton migration to long conjugation
segments. The large Stokes’ shift of these materials thus
promises a possible use as active medium in laser diodes.
Fullerene-diol and adduct 6 shows similar spectral pat-
rn and the emission peaks appear at 416 and 436 nm in
symmetry of fullerene in both fullerenol and adduct (6)
result similar spectral patterns. Fullerene core governs
the emission peaks positions and are independent of the
nature of addends. Fluorescence intensity of the adduct
(6) is however highly quenched compared to both
fullerenol and oligomer (5) and is also blue shifted com-
pared to oligomer 5. This indicates an efficient intra- and
inter-molecular electron transfer from oligo-phenylene-
alt-thiophene (donor) to the fullerene (acceptor) produc-
ing a stable charge-transfer exciplex in photo-excited
(Figure 8). Electron acceptor property of fullerene is
well known and the functionalization with oligophenylene-
alt-thiophene molecules undergoes fast charge-separation
(CS) and slow charge-recombination.  The stronger
quenching in fluorescence intensity in C60-oligothio-
phene-C60 triads compared to oligo-thiophene-C60 dyads
is observed earlier [41-45].
In addition, adduct (6) also show high blue-shift in
photoluminescence spectra (
gation defect and conformational disorder arises due to
steric repulsion and distortion of torsional angle between
phenylene-thiophene rings on chemical attachment of C60
results blue-shifting .
3.6. Magnetic Properti
Oligomer 5 in its pristine un-d
indicating an absence of free
tronically defect free symmetric structure in solid state.
The oligomer behaves like a p-donor in presence of
strong acceptor and creates an electronic imbalance
within the adduct on chemical attachment to fullerene
(acceptor) making it EPR active (Figure 9) . Higher
g-value (g = 2.005) compared to both free electron (g =
2.0023) and fullerenol (g = 1.99) owing to strong elec-
tronic interaction between oligomer donor chain and
R. SINGH ET AL. 57
fullerene acceptor producing resultant dipole moment
within the dyad molecule. Deviation of g-factor from the
free-spin value is a useful indication of the extent of
spin-orbit coupling in the paramagnetic species and pro-
vides information about the molecular environment of
the unpaired electrons. Absorption at higher magnetic
field (3515 G) compared to free electron (3300 G) indi-
cate electron-electron interaction within the molecule in
ground state needing higher energy for electronic transi-
tion. Peak to peak width is inversely proportional to the
spin-spin relaxation time (8
which depends on the electron – electron interactions.
For adduct 6, T2 values are ofs
(0.48 × 10−9 s) and peak to peak width (
the order of nanosecond
) is slightly
H ). The higher peak width
compared to pure fullerenol is due to the shorter lifetime
of excited stalecule readily relax back to
tes and the mo
d bis-(2,5-hexyloxy) substitutes
ene oligomer and its attachment to
fullerene-diol are explored for their thermal, electro-
chemical, photophysical and magnetic properties. Syn-
thesis of oligomer and formation of its adduct with
fullerenol are successfully carried out. Thermal disorder
of oligomer on chemical attachment to fullerenol is evi-
dent from the multi-step degradation of the adduct (6)
compared to single step degradation in oligomer (5).
Also, the chemical attachment induced paramagnetic
character to otherwise EPR inactive material. Low band-
gap of the oligomer in solution indicates symmetric pla-
nar structure and good electron donating contribution of
alkoxy substituents towards easy migration of -elec-
trons. Attachment to fullerene moiety results partial dis-
tortion of co-planarity and is easily reflected from the
Figure 7. Comparative PL spectra of (A oligomer 5, (B
C60(OH2 and (C adduct 6 using 350 nm excitation wave-
length in solid state at room temperature.
Figure 8. Schematic diagram of proposed intra- and intermolecular CS processes in 6.
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R. SINGH ET AL.
3000 3100 3200 3300 34003500 3600 3700 3800 3900 4000
Figure 9. EPR spectra of adduct 6 recorded in solid at room
blue shifting of emission peak. There is considerable
charge-transfer interaction between p-type oligomeric
chain and n-type fullerene core resulting in the high
quenching of fluorescence intensity. Optical properties of
the polymer can be tuned by suitable manipulation of the
back-bone chain and also by changing the substituents on
both phenylene and thiophene rings. Long chain alkoxy
pendant groups substantially improve the solubility yet
ontribute to retai
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Figure 1. 1H NMR spectra of 5 recorded in CDCl3.
Figure 2. 13C NMR spectra of 5 recorded in CDCl3.
Figure 3. 1H NMR spectra of 6 recorded in DMSO.
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R. SINGH ET AL.
Figure 4. 13C NMR spectra of 6 recorded in DMSO.
Figure 5. FTIR spectra of 5.
Figure 6. FTIR spectra of 6.
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R. SINGH ET AL. 63
Figure 7. ESIMS spectra of 5 recorded in methanol. Inset showing the GPC chromatogram with molecular weight about
Table 1. Details of 1H & 13C NMR peak assignment.
Sample 1H peak Assigned 13C Peak Assigned
Code position proton position carbon
4 6.9 3H (Ph) 119.4 1C (Ph)
6.8 6H (Ph) 148 2/5C (Ph)
7.09 3H (Th) 118 3C (Ph)
111 4C (Ph)
114 6C (Ph)
138 2/5C (Th
128 3/4C (Th
5 6.7 - 6.8 (m) Ph,Th 111 4C-Br
7.01 112 3C (inner Ph)
114 6C (terminal Ph)
118 3C (Ph)
119 1C (Ph)
150 2C (Ph)
153 5C (Ph)
138 2/3C (Th)
6 0.8 - 1.0 CH3 29 - 69 C6H13
1.15 - 1.23 CH3CH2 76.7 sp3 fullerene
1.6 CH2 1 25.6 1C & 4C of inner Ph
2.3 CH2 28.5 3/4C (Th
3.4 - 3.5 O-CH2 129.2 4C- Ph attached to C60
7.16 - 7.18 Th 130.08 2C & 5C Th
7.23 - 7.25 Ph 148 2C & 5C Ph
150.4 2C terminal Ph
150.9 5C-Ph attached to C60
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B r 1
The article highlights the dyad characteristics of phenyl-alt-
thiophene addended fullerene diol through inter and intra
molecular CS leading to quenching and red shifting of emission
spectra . Significant effect of fullerene on magnetic and electro
chemical properties are also observed.