Theoretical Prediction of the Donor/Acceptor Site-Dependence on First Hyperpolarizabilities in Conjugated Systems Contai
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Theoretical Prediction of the Donor/Acceptor Site-Dependence on First Hyperpolarizabilities in Conjugated Systems Containing Azomethine Bonds Azuma Matsuura and Tornoaki Hayano Fujitsu Laboratories Ltd., 10-1 Morinosato-Wakamiya, Atsugi 243-01, Japan
ABSTRACT We calculated frequency-dependent first hyperpolarizabilities for electro-optic Pockels effect, f3(-w ; 0, w), in wr-conjugated molecules containing azomethine bonds using the CNDO/S-CI sum-over-states method. First, we examined the dependence of static 63 on formulas for the two-center Coulomb integral, YAB. The choice of the 7AB formula has little effect on the relative /3 value, but a considerable effect on the absolute value. Then we investigated the relationship between the magnitude of /3(-c ; 0, w) and the donor/acceptor substitution sites. We found that the molecule with no donors or acceptors can be divided into donor and acceptor regions whose existence causes the donor/acceptor site-dependence of 3(-W; 0, W). I.
INTRODUCTION
The design of nonlinear optical materials with the molecular orbital (MO) method has been applied mainly to small molecules. However, since most practical materials are polymers, this procedure must be extended to larger molecules. Nonlinear optical polymers are promising materials because of their large-area deposition capability, processability, and thermal stability. Recently, it was reported that terephthalaldehyde and p-phenylenediamine are connected by azomethine bonds (RCH=NR') through -CHO 1 and -NH 2 groups to form polymer films via chemical vapor deposition (CVD) or molecular 2 layer deposition (MLD) . These deposition techniques enable monomolecular-order control of one-dimensional polymer structures, which have great potentialities for enhancing optical nonlinearity. However, experimental study is costly and time-consuming. Therefore, polymer design based on theoretical computation has played an increasingly important role in the efficient development of nonlinear optical polymers. In this paper, we report the MO calculation of frequency-dependent first hyperpolarizabilities for electro-optic Pockels effect, /3(-w ; 0, w), in Ir-conjugated systems containing azomethine bonds, especially the dependence on the donor/acceptor site-dependence. To the best of our 3 knowledge, few attempts ', have been made at the MO calculation of the donor/acceptor sitedependence on the hyperpolarizabilities. We also present the dependence of static /3 on formulas for the two-center Coulomb integrals, _YAB-
II.
COMPUTATIONAL PROCEDURE Model molecules
Model molecule structures are shown in Fig. 1. These molecules differ in the donor/acceptor substitution sites. In Fig. 1, D is the donor (NH 2 ), A the acceptor (NO 2 ), and H the hydrogen atom. The optimized geometry of HHHH was used throughout in calculating /3 for all the model molecules to focus attention on the donor/acceptor site-dependence. The HHHH geometry was 5 optimized using the Austin model 1 (AM1) semiempirical MO method. The AM1 calculation was carried out using MOPAC V
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