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Chemical Sciences Section IV: Chemical Sciences PROCEEDINGS OF THE NINETY NINETH SESSION OF THE INDIAN SCIENCE CONGRESS BHUBANESWAR, 2012 PART II SECTION OF CHEMICAL SCIENCES President: Prof. A. K. Bakhshi CONTENTS I. Presidential Address 01 II. Abstract of Platinum Jubilee Lecture 25 III. Abstract of Young Scientist Award Programme 29 IV. Abstracts of Symposium/Invited Lecture 35 V. Abstracts of Oral/Poster Presentation 45 Organic Chemistry 45 Inorganic Chemistry 157 Physical Chemistry 223 VI. List of Past Sectional President 257 Section IV: Chemical Sciences 99th Indian Science Congress 3-7 January 2012, Bhubaneswar I PRESIDENTIAL ADDRESS PRESIDENT:-Prof. A. K. Bakhshi Sir Shankar Lal Professor of Chemistry, Department of Chemistry Universtity of Delhi, Delhi & Vice-Chancellor UR Rajarshi Tandon Open University (UPRTOU), Allahabd Section IV: Chemical Sciences Theoretical Designing of Novel Low Band Gap Conducting Polymers A.K.Bakhshi Email: [email protected] 1. Introduction In the fascinating world of polymers, conducting polymers have always been the focus of attraction. The low electrical conductivity of polymers has been immensely used in the manufacture of insulators and dielectric substances. According to a report entitled “Conductive Polymers-Global Strategic Business Report”, published by Electronics.ca on March 23, 2011, the US market for conductive polymers is forecast to reach 240.5 thousand tons by the year 2015. Conductive polymers market is expected to grow at a healthy rate due to various advantages offered by these electricity-conducting plastics. Lightweight, easy fabrication, low cost and high resistance to heat is driving the growth of conductive polymers market in the United States. Rising demand for high performance and low cost materials for electrical and electronics product components and devices are likely to augment the conductive polymers utilization by the electronics industry. Conductive polymers are organic polymers that conduct electricity. Such compounds may have metallic conductivity or can be semiconductors. The conductivity of such polymers is the result of several processes. Conducting polymers have backbones of continuous sp2 hybridized carbon centers. One valence electron on each center resides in a pz orbital, which is orthogonal to the other three sigma-bonds. The electrons in these delocalized orbitals have high mobility when the material is “doped” by oxidation, which removes some of these delocalized electrons. Thus, the conjugated p-orbitals form a one-dimensional electronic band, and the electrons within this band become mobile when it is partially emptied. An increasing number of academic, governmental and industrial laboratories throughout the world are involved in basic research and assessment of possible applications of conducting polymers1-4. Figure 1 shows the chemical structures of most commonly studied conjugated polymers. These polymers have a wide range of conductivities despite having many structural similarities. 1 Proc. 99th Indian Science Congress, Part-II: Presidential Address Fig. 1:- Chemical structures of monomers of most commonly studied conjugated polymers. Being semiconductors, CPs do not conduct unless free charge carriers are produced by excitation of electrons from the valence into the conduction band5 or by chemical or electrochemical oxidation or reduction6 referred to as p- and n-doping, respectively. The coupling between electronic and structural changes is known as electron-phonon coupling. The entity consisting of charge and associated geometry distortion is known as a “polaron”. Upon removal of a second electron either a second separate polaron may form or, a “bipolaron”, if the second electron is removed form the same site as the first7, 8. The aim of theoretical investigation of conducting polymers (CPs) is, firstly, to understand electrical and optical properties and secondly, to design new systems with specifically tailored properties depending on molecular structure. Theoretical design of CP implies that their key properties i.e. structures in the neutral, doped, and excited states, IPs, EAs, band gaps, and band widths, can be predicted from theoretical calculations depending on their chemical structure. Band-gap of a polymer is a measure of 2 Section IV: Chemical Sciences its ability to show intrinsic conductivity, while IP and EA values of a polymer determine its ability to form conducting polymers through oxidative and reductive doping respectively. The problems related to the industrial bulk applications of CPs namely low solubility and processibility explain the sudden emergence for the need of bandgap control. A key requirement for a polymer to become intrinsically electrically conducting is that there should be an overlap of molecular orbitals to allow the formation of delocalized molecular wave function. Reduction of the bandgap (Eg) will enhance the thermal population of the conduction band and thus increase the number of intrinsic charge carriers. Besides this, molecular orbitals must be partially filled so that there is a free movement of electrons throughout the lattice. The decrease of Eg can also lead to true “organic metals” showing intrinsic electrical conductivity without resorting to oxidative or reductive doping. The lower oxidation potential associated with narrow gaps will result in a stabilization of the corresponding doped state. From the moment polymers were found to be able to conduct electricity, upon appropriate doping, a number of technological applications have been implemented and a large number of other applications are envisioned. Applications of conducting polymers are driven by the unique combination of electronic, mechanical, as well as fabrication properties of this class of materials. The delocalized electronic structure enables high electrical conductivity which in conjunction with traditional properties of polymers (e.g., good mechanical properties, ease of fabrication, low cost, and low weight) enable conducting polymers to be commercially important. Li et al9 have recently reported the synthesis and electrochemical applications of the composites of conducting polymers. These conducting organic molecular electronic materials have attracted much attention largely because of their many projected applications in solar cells, light weight batteries10, electrochromic devices, sensors and molecular electronic devices. Polymeric heterojunctions, solar cells have been fabricated by electrochemical deposition of PPY on n-silicon. Many conducting polymers such as polyacetylene, polythiophene, polyindole, polypyrrole, polyaniline etc. have been reported as electrode materials for rechargeable batteries11. It has been reported that the conducting polyheterocycles are good candidates for electrochromic displays and thermal smart windows12. The potential for conducting polymers in the area of electronics and photonics (nonlinear optics)13 is enormous and has been used to fabricate diodes, capacitors, field-effect transistors (FET) and printed circuit boards. 3 Proc. 99th Indian Science Congress, Part-II: Presidential Address 2. Band gap engineering Intensive research has been dedicated to synthesizing novel conjugated polymers with high conductivity upon doping. The process of doping of electrically conducting polymers is often the source of chemical instability in them. Another problem often associated with doped polymers is their poor processibility which is due to the insolubility and infusibility of these polymers. The possible elimination of doping in preparing conducting polymers while still achieving high conductivity is one of the original motivations for band gap engineering of polymers. Band gap engineering of conjugated polymers refers to the tuning of band gap to obtain desired properties or functionality. It includes band gap minimization and processibility improvement. Many researchers have focused on synthesizing conjugated polymers with small or even zero band gaps (Eg) (figure 2) because of their expected good intrinsic electrical conductivity13 and nonlinear optical properties14. Name Structure Band gap (eV) 1. Trans-PA 2. Polythiophene 2.1 3. Polyisothian- 1.0 aphthene 4. Polyisonapht- 1.4 hothiophene Fig. 2:- Some low band gap conjugated polymers. Still at the present moment, the development of stable, processable polymeric materials with a low band-gap is an important issue for further advancements in these fields. Thus, substantial efforts have been devoted to the design of conjugated organic 4 Section IV: Chemical Sciences polymers with a small band-gap. Several parameters control the band gap of conducting polymers, including bond length alternation, inter-ring torsion angle, resonance energy, inter-chain effects and substituent effects15. Based on these parameters, several strategies for obtaining narrow-band gap systems have been suggested. These include: 2.1. Substitution/Fusion Various kinds of substituents are in use for improving solubility, decreasing band gaps, increasing polarizabilities, and optimizing luminescence efficiencies. Alkyl substituents on the backbone of polythiophene are often employed for increasing the solubility of polymer 16, for example, polar substituents on polymer backbone increase hydrophilicity. Electropolymerization of 1 (figure 3) resulted in a polymer that is water-soluble and fully electroactive in aqueous solution 17,18. The effect of substituents on band gap reduction, however, is not known very properly. In particular, dicyano 19,20 groups and dithia groups such
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