EFFECT OF THE PI-BONDING SEQUENCE ON 3RD-ORDER OPTICAL NONLINEARITY EVALUATED BY AB-INITIO CALCULATIONS

We have conducted ab initio coupled-perturbed Hartree-Fock calculation s on five different well-defined pi-bridges of varying chain lengths, N=CH-CH=N, CH=N-N=CH, CH=N-CH=N, CH=CH-CH=CH, and C=C-C=C end-capped w ith electron-donating amino groups, to examine the effect of the natur e of the pi-bonding sequence on polarizabilities (alpha) and second hy perpolarizabilities (gamma). Second hyperpolarizabilities of the azine (N-CH=CH-N) conjugated system were found to be the largest, and those of CH=N-N=CH, the smallest for the monomeric model compounds of the s ame chain length. On the other hand, the gamma value varied in the ord er CH=CH-CH=CH > C=C-C=C > N-CH=CH-N > CH=N-CH=N > CH=N-N=CH for the d imeric model compounds, and the same order was observed for the trimer ic model compounds. Polarizabilities and second hyperpolarizabilities were found to be associated with the nature of the pi-bonding sequence where the presence of the nitrogen atom in the conjugated backbone ha s a diminutive effect. Polarizabilities and second hyperpolarizabiliti es increase as the length of pi-electron delocalization increases and were found to be larger for polyene and polyyne systems than those of nitrogen-containing pi-conjugated backbones. Our ab initio calculation s demonstrate that the nature of the pi-bonding sequence plays an impo rtant role in determining the magnitude of the second hyperpolarizabil ity, which should be considered while designing novel organic molecule s for third-order nonlinear optics.