ELECTRONIC-STRUCTURE AND QUADRATIC HYPERPOLARIZABILITIES IN ORGANOTRANSITION-METAL CHROMOPHORES HAVING WEAKLY COUPLED PI-NETWORKS - UNUSUALMECHANISMS FOR 2ND-ORDER RESPONSE

This contribution describes the use of the computationally efficient, chemically-oriented INDO-SOS electronic structure model (ZINDO) to elu cidate the electronic origins of the second-order nonlinear optical (N LO) response in molecules with extended pi-architectures weakly couple d to transition-metal fragments. ZINDO-derived quadratic hyperpolariza bilities are found to be in excellent agreement with experiment for a variety of group 6 pyridine pentacarbonyl complexes in which coordinat ion to the low-valent metal fragments enhances the NLO response of the free ligands. The metal-pyridine chromophores are found to obey the c lassical two-level model. However, the beta-dictating MLCT transitions possess significantly larger Delta mu(ge) values and markedly lower o scillator strengths relative to the traditional organic chromophore pi -donor-acceptor architectures by virtue of weak coupling between the m etal and the ligand pi-network. The computed quadratic hyperpolarizabi lities of group 6 stilbazole pentacarbonyl derivatives are in good agr eement with experiment. In contrast to conventional organic chromophor es, an increase in pi-conjugation length of the stilbazole ligands doe s not result in a dramatic increase in the second-order response or a decrease in the HOMO --> LUMO transition energy. The molecular orbital analysis indicates that the metal pentacarbonyl fragment acts as sigm a-acceptor, forcing the adjacent pyridine ring to become the molecular LUMO. As a consequence, the seemingly innocent pyridine ring becomes a primary charge acceptor in these structures, regardless of the deriv atization or conjugation length. The synthesis and characterization of the donor-functionalized chromophore (4-(dimethylamino)4'-stilbazole) W(CO)(5) is also reported. The large observed beta(vec) value supports the proposed NLO response model.