ELSEVIER 11 October 1996 Chemical Physics Letters 261 (1996) 28-34 CHEMICAL PHYSICS LETTERS Ab initio polarizabilities of polyenic chains with conformational defects Celso P. de Melo, Tertius L. Fonseca Departamento de Ffsica, Universidade Federal de Pernambuco, 50.670-901 Recife PE, Brazil Received 17 May 1996; in final form 5 August 1996 Abstract We present an ab initio Hartree-Fock study of the electronic polarizabilities of the C2n+~H+n+ 3 (2 < n < 11) and C H ++ (2 < n < 15) oligomers of polyacetylene. After a complete geometry optimization implemented through the 2n 2n+2 GAUSSIAN 92 program, the longitudinal components of the linear polarizabilities were analytically determined and the second hyperpolarizabilities calculated through a finite field procedure. While the first hyperpolarizabilities of the bipolaron- like structures vanish (since inversion symmetry is preserved), the dominant component fl~xy of the soliton chains was obtained analytically. We confirm that the polarizabilities of these polyenic oligomers are quite sensitive 1o the nature of the eonformational defect present. 1. Introduction Even though controversy still remains as to the spe- cific details of how doping leads to the extraordinary increase in the electrical conductivity of conjugated polymers, it is now well understood [ 1 ] that at least at some basic level conformational defects play a role in the corresponding global charge transport mecha- nisms [2,3]. Less evident, however, has been the ef- fect of conformational defects upon the nonlinear op- tical (NLO) properties of these compounds. In the last decade there has been a burst of inter- est in the NLO properties of zr conjugated polymers, since it has been recognized that the corresponding figures of merit make these systems promising materi- als for the development of optoelectronic devices [4- 6]. The exceptionally large NLO response identified in several classes of conjugated polymers has been at- tributed both to the high degree of delocalization of the ¢r electrons and to the inherent spatial anisotropy of the polymer backbone. The characteristic wealth of possibilities of organic chemistry for changing the specific electronic environment of a given compound can then be used to control the amount of bond alter- nation along the main conjugated chain [7], and this, in turn, opens up the interesting possibility of tuning the polarization response of newly synthesized sys- tems to a desired range of values. There is a need for theoretical calculations of increasing degree of sophis- tication that could assess the relative importance of the various factors determining the optical properties of these compounds and, as a consequence, several ab initio calculations of the nonlinear polarizabilities of different conjugated systems have appeared [ 8-16 ]. It should be noted that in spite of the fact that the relevance of conformational defects for the electronic properties of organic chains is well recognized, the number of theoretical investigations dedicated to the analysis of their role in the NLO properties of con- jugated polymers remains surprisingly low. At the 0009-2614/96/$12.00 Copyright(~) 1996 ElsevierScience B.V. All rights reserved. PII S0009-2614 ( 96 ) 00937-2