Electronic Structures and Nonlinear Optical Properties for Cross-Conjugated Polyene
- PDF / 291,561 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 53 Downloads / 226 Views
ELECTRONIC STRUCTURES AND NONLINEAR OPTICAL PROPERTIES FOR CROSS-CONJUGATED POLYENE YUHEI MORI AND YOSHIMICHI OKANO ATR Optical and Radio Communications Research Laboratories, Sanpeidani, Inuidani, Seika-cho, Soraku-gun, Kyoto, 619-02, Japan
ABSTRACT For several cross-conjugated molecules, molecular excited states and thirdorder nonlinear susceptibilities -y are calculated by semi-empirical molecular orbital method. The calculated -y are compared with each other and the reason for the differences are discussed in terms of the orbital shapes which contribute mainly to -y. The reason that long polyene chains have large Y, is ascribed to the large transition moment which is explained by the signs of the LCAO coefficients. On the other hand, it is found that the cross-conjugated molecules in which two similar-length chain cross at the center is not good for large thirdorder susceptibility. INTRODUCTION Over the past 10 years or so, nonlinear optical properties for conjugated molecules have attracted much interest, in particular the third-order susceptibility -y of polyenes. In the early stages, Rustagi et.al. 11] and Agrawal et.al. [21 lead the analytic investigation of the dependence of 7yon the chain length. In recent years, some works were done using the molecular orbital method to calculate the third-order susceptibilities for finite polyenes [3-61. Garito et.al. 151 and Yamaguchi et.al. 161 calculated -y by CNDO/S-CI and indicated the significance of upper excited A. state for third-order susceptibility in addition to the ground and B, excited state which is observed in an absorption spectrum. Pierce 171 also did calculations for short polyenes and pointed out that Cl calculation including doubly excited states is necessary, especially to obtain a correct sign of -f for ethylene and butadiene. These works concern unsubstituted polyenes. Flom et.al.[8] measured third-order susceptibilities for three derivatives of polyacene quinone and suggested that the third-order coefficients are not simply correlated with the conjugation length. But there are few works which treat substituted polyene systems. In this work, we calculate the molecular orbitals and third-order susceptibilities for some cross-conjugated polyenes. For the calculation of various molecules, it is important to know which part of the molecule contributes mainly to the -y value. This will be clarified if we investigate the shapes and distributions of the molecular orbitals. Mat. Res. Soc. Symp. Proc. Vol. 173. c1990 Materials Research Society
666
CALCULATION Molecular structures, orbitals and third-order susceptibilities are calculated for molecules listed in Fig. 1. The structures i.e. bond lengths and angles for these molecules are calculated using a MNDO program, assuming planar configuration. With these optimized structures, excitation energies and transition moments are obtained by the CNDO/S-CI method. 60 configurations are taken into account for Cl only for singly excited states. Third-order susceptibilities are calculated by the following equ
Data Loading...
