LARGE MOLECULAR HYPERPOLARIZABILITIES - QUANTITATIVE-ANALYSIS OF AROMATICITY AND AUXILIARY DONOR-ACCEPTOR EFFECTS

The linear and nonlinear optical properties of donor-acceptor ct-elect ron chromophores having conjugated bridges varying in aromaticity and electron excessivity are investigated using the INDO/1 semiempirical H amiltonian. NLO response properties of such chromophores are more sens itive to the electron excessivity of the bridges than to bridge aromat icity. Electron-excessive/deficient heterocyclic bridges have lesser/g reater tendency to deplete the electron density from the donor/accepto r groups and thus increase their donor/acceptor ability. Thus, electro n-excessive heterocyclic bridges act as auxiliary donors while electro n-deficient heterocyclic bridges act as auxiliary acceptors, significa ntly enhancing NLO response. Interposing charged five-membered auxilia ry donor and acceptor fragments between strong donor or acceptor group s results in large computed second-order NLO response.