A quantitative substituent effect on the second-order hyperpolarizabilities of polymers based on the quantum chemical studies

To obtain the in-depth identification of the electronic nature of the subst ituent effects, which have been recognized as a facilitated means to elevat e the second-order hyperpolarizabilities (gamma) of polymers, the systemati c quantum chemical studies have been carried out on the tried systems (poly phenyls) of longer chain length (17 phenyl units) and more versatile substi tution types (seven kinds) than ever. All polymers are fully optimized and the nonlinear optical properties are based on the finite-field model with P M3 Hamiltonian. The computational results have been validated by available experimental and theoretical data. As found in the experiments, the compute d energy levels converge in the range of long chain length, indicating a gr adually decreased coupling between components. Both the heat of formations and gamma values of the studied polymers with substituents are found to par allel to those without substituents, which can be explained by the first-or der approximation of the additivity law of properties for weakly coupled mu lti-component systems. This quantitative component effect or the substituen t effect may build a theoretical ground to extrapolate long chain polarized polymers on the basis of oligomers. As for the molecular design guidelines , the preferential site for better conjugation, the larger number of substi tuents, and the more powerful polar groups are favored to elevate gamma. (C ) 2001 Elsevier Science B.V. All rights reserved.