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

This contribution describes the use of the computationally efficient, chemically-oriented INDO-SOS electronic structure model (ZINDO) to elucidate the electronic origins of the second-order nonlinear optical (NLO) response in molecules with extended pi-architectures weakly coupled to transition-metal fragments. ZINDO-derived quadratic hyperpolarizabilities are found to be in excellent agreement with experiment for a variety of group 6 pyridine pentacarbonyl complexes in which coordination to the low-valent metal fragments enhances the NLO response of the free ligands. The metal-pyridine chromophores are found to obey the classical two-level model. However, the beta-dictating MLCT transitions possess significantly larger Delta mu(ge) values and markedly lower oscillator strengths relative to the traditional organic chromophore pi-donor-acceptor architectures by virtue of weak coupling between the metal and the ligand pi-network. The computed quadratic hyperpolarizabilities of group 6 stilbazole pentacarbonyl derivatives are in good agreement with experiment. In contrast to conventional organic chromophores, an increase in pi-conjugation length of the stilbazole ligands does 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 sigma-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 derivatization 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.