Synthesis, structure and band gap of novel thiophene based conductive polymers

Article abstract of DOI:10.1080/10587250216164

A novel poly 3-(2-benzo thiazolyl) thiophene based conductive polymer have been synthesized and characterized using various experimental techniques. The structure and band gap (difference between Highest Occupied Molecular Orbital and Lowest Unoccupied Molecular Orbital) for the above polymer have been calculated using oligomeric approach employing B3LYP/3-21G* method and semi-empirical AM1 and ZINDO methods. The calculated band gap for monomer, dimmer and trimmer has been plotted versus 1/N (where N is the number units). The value at 1/N= 0, is taken as the band gap of the polymer having long chain length. It is evident from the theoretical investigations, the band gap of poly 3-(2-benzothiazolyl) thiophene is 3.77 eV. It is possible to change the band gap by designed alteration in the thiophene framework. Copyright

Full text of DOI:10.1080/10587250216164

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Synthesis, Structure and Band
Gapof Novel Thiophene Based
Conductive Polymers
S. Radhakrishnan a , V. Subramanian a , N.
Somanathan a , K. S. V. Srinivasan a & T. Ramasami a
a Central Leather Research Institute , Adyar,
Chennai , 600020 , India
Published online: 18 Oct 2010.
To cite this article: S. Radhakrishnan , V. Subramanian , N. Somanathan , K. S.
V. Srinivasan & T. Ramasami (2003) Synthesis, Structure and Band Gapof Novel
Thiophene Based Conductive Polymers, Molecular Crystals and Liquid Crystals, 390:1,
113-120, DOI: 10.1080/10587250216164
To link to this article: http://dx.doi.org/10.1080/10587250216164
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Mol. Cryst. Liq. Cryst., Vol. 390, pp. 113–120
Copyright # 2003 Taylor & Francis
1542-1406/03 $12.00+.00
DOI: 10.1080/15421400390193495
SYNTHESIS, STRUCTURE AND BAND GAP
OF NOVEL THIOPHENE BASED CONDUCTIVE
POLYMERS
S. Radhakrishnan, V. Subramanian*, N. Somanathan*,
K. S. V. Srinivasan, and T. Ramasami
Central Leather Research Institute, Adyar,
Chennai 600020, India
A novel poly 3-(2-benzo thiazolyl) thiophene based conductive polymer have
been synthesized and characterized using various experimental techniques. The
structure and band gap (difference between Highest Occupied Molecular
Orbital and Lowest Unoccupied Molecular Orbital) for the above polymer have
been calculated using oligomeric approach employing B3LYP/3-21G* method
and semi-empirical AM1 and ZINDO methods. The calculated band gap for
monomer, dimmer and trimmer has been plotted versus 1/N (where N is the
number units). The value at 1/N ¼ 0, is taken as the band gap of the polymer
having long chain length. It is evident from the theoretical investigations, the
band gap of poly 3-(2-benzothiazolyl) thiophene is 3.77 eV. It is possible to
change the band gap by designed alteration in the thiophene framework.
Keywords: polythiophene; band gap; LEDs; molecular orbital calculations
INTRODUCTION
Molecular electronic materials based on conjugated polymers have poten-
tial applications in various fields in recent years [1–3], wide spread efforts
have been made to develop novel light emitting devices (LED). Poly
thiophenes (PT), Poly phenelene vinylene (PPV) and Poly para phenelene
(PPP) etc. are such conjugated polymers whose electroluminescence (EL)
emission falls in the visible range. Among the above, PT are good candi-
dates since they can be easily processible when compared to other systems.
Other advantages are the environmental stability and versatility because of
their tailorable properties. One of the disadvantages is its poor solubility
that can be overcome by functionalisation at the 3- position of the
thiophene ring.
*Corresponding author
113

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S. Radhakrishnan et al.
The principal requirements of polymeric light emitting devices are high
efficiency and reliability [4]. PT based LEDs show low quantum yield due to
the presence of sulfur. Hence in the case of PT based LEDs, the goal is to
achieve maximum efficiency and brightness at minimum drive voltage. The
present alternative method to improve efficiency in the case of PLEDs is to
use multi layer technology using Electron conducting hole blocking
(ECHB) low molecular materials [5]. The disadvantages of using these
materials are the phase separation and crystallization at higher tempera-
tures. The compatibility of these materials with the light emissive polymer
layers is also affected. The Present study aims to incorporate the properties
and functions of the above mentioned ECHB, by suitably substituting
groups containing p-electron deficient hetero cyclics such as oxadiazole,
triazoles, triazines, pyridines and extended benzo derivatives of the above.
The proper structure and groups suitable for the above function viz. ECHB,
was initially studied using molecular orbital calculations and subsequently
synthesis of the modeled compounds have been made.
Molecular orbital calculation is a versatile tool in designing and predicting
properties of new materials. Recent investigations [6,7] using molecular
orbital calculations, exploits with the chain length of the geometric and
electronic structure and optical properties of the various systems. The oli-
gomeric approach of the polymers is used to establish relations between the
chain length and physical properties and assumed to allow a more precise
structure-activity relationship to be determined and also to allow extra-
polation of these relationships towards those expected for polymers. Com-
puting Highest Occupied Molecular Orbital- Lowest Unoccupied Molecular
Orbital (HOMO-LUMO) energies with different substituents may be helpful
in predicting the substituent effect on polymer band gap.
Based on the theoretical predictions, Benzo thiazole is used as a sub-
stituent attached to the 3- position of the thiophene (BTT) ring. In order to
understand the influence of the substituent at the 4- position of the BTT,
methyl (MBTT) and cyano (CBTT) groups substituted at the 4- position of
the thiophene ring of BTT have also been studied by Density Functional
and Semi- empirical calculations.
EXPERIMENTAL
The synthetic method used for the synthesis of BTT and its polymer is
outlined in Scheme 1. In a three necked flask fitted with a condenser, a
dropping funnel and nitrogen inlet, magnesium turnings and 3-bromo
thiophene were introduced together with dry diethyl ether. The entrainer
1, 2-dibromo ethane in anhydrous diethyl ether is then added at ice-cold
temperature for a period of 8 hrs. After the reaction begins, the solution

Synthesis, Structure and Band Gap of Polymers
115
SCHEME 1
was brought to ambient temperature. The resulting Grignard compound
was transferred to a second dropping funnel fitted to a second three necked
flask containing 2-chloro benzothiazole and 1,3- bis (diphenyl phosphino
propane) Nickel (II) chloride (Nidppp) in anhydrous diethyl ether. After
cooling with an ice bath, the Grignard compound was added drop wise and
the resulting adduct allowed to warm to ambient temperature before being
stirred for 24 h. The mixture obtained is poured onto very dilute aqueous
hydrochloric acid. The organic layer is washed with water, dried and con-
centrated. The crude product is purified on a silica gel column (eluting with
Petroleum ether containing 10% ethyl acetate). NMR has confirmed the
1
structure of the monomer. H NMR (300 MHz, CDCl3, ppm): 8.02(t, 2H);
7.85(d,1H); 7.69(d,1H); 7.39(m,3H). 13C NMR (75 MHz, CDCl3, ppm):
162.6; 153.8; 135.9; 134.6; 126.8; 126.5; 126.2; 126.0; 125.0; 123.0; 121.5.
Chemical Polymerisation
Polymer of (3-(2-benzothiazolyl)) thiophene was prepared by chemical
oxidation of the monomer using Ferric chloride adopting a procedure
similar to that of Sugimoto et al. [8]. In a three-nacked flask, 0.012 M Ferric
chloride was dissolved in chloroform under nitrogen to which a solution of
0.003 M of monomer in chloroform was added dropwise. The mixture was
stirred for 24 h at room temperature under inert atmosphere and the
solution was precipitated using methanol.
UV/visible spectra were recorded in chloroform solution using a
Shimatzu-160 A spectrometer. The Photoluminescence of the polymers was
studied in chloroform using Hitachi 650-40 fluorescence spectro-
photometer.

116
S. Radhakrishnan et al.
DFT (B3LYP/3-21G*), AM1 and ZINDO calculation have been performed
using the G 98 w suite of packages [9]. The starting geometries have been
generated using model-building tools available in the Cerius2 package from
Molecular Simulation Inc (SanDiego, CA). The initial geometries have been
optimized using AM1 Hamiltonian. All calculations have been made for AM1
optimized geometries. For comparison, the parent thiophene oligomers (T)
also have been optimized. Excitation energies (HOMO-LUMO gap) calcu-
lated using B3LYP/3-21G*, AM1 and ZINDO methods and extrapolated to
infinite chain length. The first excited state with significant oscillator
strength has been used for calculating the excitation energies at the
ZINDO.
RESULTS AND DISCUSSION
The calculated excitation energies for thiophene oligomers at the various
levels of theory are shown in Table 1. The calculated excitation energies are
slightly higher than experimental values obtained for polythiophene. This
may be due to the fact that the theoretical values have been obtained in the
gaseous phase, while the obtained experimental value is in solution phase,
TABLE 1 Calculated Eexc(eV) of the Different Thiophene Systems at the Various
Computational Levels
System
AM1
B3LYP/3-21Gq (AM1 opt)
ZINDO (AM1 opt)
T
Monomer
Dimer
Trimer
9.79
8.02
7.29
6.12
6.52
4.50
3.71
2.38
2.27
4.86
3.74
3.13
2.38
Polymer (T)
Polymer (Exptl)
BTT
Monomer
Dimer
Trimer
8.10
8.02
7.95
7.87
4.68
4.41
4.18
4.00
3.99
3.77
3.94
3.92
3.74
3.72
Polymer (T)
Exptl-monomer
-polymer
MBTT
CBTT
Monomer
Dimer
Trimer
8.21
8.22
8.20
8.19
8.08
8.02
7.92
7.88
4.64
4.77
4.74
4.82
4.61
4.62
4.55
4.56
4.02
4.03
4.05
4.05
3.99
3.99
4.03
4.03
Polymer (T)
Monomer
Dimer
Trimer
Polymer (T)

Synthesis, Structure and Band Gap of Polymers
117
which may add some non-planarity to the system [7]. In addition to the
environmental effects, the electron correlation would also influence to
excitation energies obtained from MO calculations. The values obtained for
poly thiophene using B3LYP/3-21G* and ZINDO method are in close
agreement with each other.
Calculated excitation energies for BTT, MBTT and CBTT are plotted
against the inverse number of monomer units [7] and extrapolated to an
infinite number of units. The obtained band gap values are presented in
Table 1. The band gap calculated using ZINDO and B3LYP/3-21G* is pre-
sented respectively in Figure 1a and 1b. The extrapolated band gap from
ZINDO excitation energy using AM1 optimized geometries for BTT polymer
is in agreement with the obtained experimental values, while B3LYP/3-21G*
overestimates about 0.2 eV. In the case of BTT monomers, the theoretical
values agree with the experimental values obtained in solution phase. The
solution phase absorption spectrum of monomer and polymer of BTT is
shown in Figure 2a–2b respectively. For BTT, as the conjugation length
increases, the red shift is observed as expected. The extrapolated value is
3.72 eV, which agrees well with experimental value of 3.77 eV corresponds
to an absorption maximum at 329 nm (in solution phase). In the experi-
mental data, in addition to the peak at 329 nm (3.77 eV), a peak at 345 nm
(3.59 eV) is also obtained. The overall studies suggest that the obtained
329 nm peak may be due to regio regular coupling while 345 nm may be due
to mis coupling [10]. In contrast to BTT, in the case of MBTT and CBTT, as
the chain length increases, blue shifting is observed in theoretical level.
Due to the electron releasing effect of methyl group, blue shift is expected
in the case of MBTT, when compared to BTT, whereas in the case of CBTT,
due to electron withdrawing effect, red shift is expected when compared to
BTT. Table 1 shows that, in both CBTT and MBTT, the obtained value does
not show any characteristic change at ZINDO level, while a red shift (cal-
culated using B3LYP/3-21G*) is obtained for CBTT when compared to
MBTT. The blue shifting of CBTT and MBTT (compared to BTT) can be
attributed to the overcoming steric effect produced by the methyl and
cyano substituents at the 4 position of the thiophene ring, dominating over
the electronic effect produced by the electron withdrawing and releasing
natures of the substituents. In BTT monomer for an excitation of 299 nm,
the emission is falls at 370–444 nm, while in the case of Poly BTT for an
excitation at 345 nm the emission at 420 nm is observed which is shown in
Figure 3.
Since the geometries are optimized at AM1, further optimizations at
reasonable level of theory would also influence the results. But optimiza-
tion at HF and/or DFT level would require more computational time for
dimmer and trimmer. Hence calculation have been made at AM1 level to
derive reasonable trend of excitation energies.

118
S. Radhakrishnan et al.
FIGURE 1 Band gaps extrapolated from plots of Eexc versus the inverse number of
monomer units for different thiophenes: (a) ZINDO and (b) B3LYP/3-21G*.

Synthesis, Structure and Band Gap of Polymers
119
FIGURE 2 Absorption spectrum of (a) BTT and (b) Poly BTT.
FIGURE 3 Emission spectrum of (a) BTT and (b) PolyBTT.

120
S. Radhakrishnan et al.
The over all studies show that the computational methods help to pre-
dict the properties and the band gap obtained from experimental results of
BTT polymers, coincides well with the theoretical one. The photoemission
of the BTT polymer falls in the blue light range with high intensity, which
will be useful for the development of white light.
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