CALCULATION OF THE HYPERPOLARIZABILITIES OF SILICON-BASED DONOR-ACCEPTOR COMPOUNDS FOR NONLINEAR OPTICS

A substituted diphenyldisilane having a dimethylamino donor and a perf luoroalkylsulfonyl acceptor shows a moderate second-order nonlinear op tical efficiency while being transparent in the visible spectrum. Semi empirical quantum chemical approaches (AM1, ZINDO) as well as an ab in itio sum-over-orbitals method (HYPERPOL) were employed to calculate th e properties of this compound, of its fragments, and of model molecule s, such as monosilyl- and disilanylphenyls substituted with either a d onor or an acceptor group and donor-acceptor-substituted benzene. The calculated values of the first hyperpolarizability, beta, show the pro per trends when compared with experimental values obtained by hyper-Ra yleigh scattering. A simple two-level approach overestimates beta but predicts the trends correctly. The HYPERPOL method is found to produce too low beta values for compounds containing second-row elements; sti ll it yields a useful prediction of experimental trends. The compariso n of excited states obtained from ZINDO calculations with experimental absorption spectra suggests that d-orbitals on sulfur should be inclu ded in these calculations; the results are less conclusive with respec t to d-orbitals on silicon. Molecular geometry was considered, in part icular the orientation of donor and acceptor groups with respect to th e phenyl ring, and that of the phenyl rings with respect to the Si-Si bridge. Attention is paid to a possible pi-sigma conjugation in disila nes. ZINDO apparently underestimates such interactions. For the diphen yldisilane donor-acceptor compound, this method predicts a partial cha rge transfer, from the donor to the disilanylene bridge, associated wi th the excitation that contributes most to beta.