Electronic Structure and Quadratic Hyperpolarizabilities in Organotransition-Metal Chromophores Having Weakly Coupled π-Networks. Unusual Mechanisms for Second-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 π-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 β-dictating MLCT transitions possess significantly larger Δµ_gevalues and markedly lower oscillator strengths relative to the traditional organic chromophore π-donor-acceptor architectures by virtue of weak coupling between the metal and the ligand π-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 π-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 (r-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) -4ʹ-stilbazole)W(CO)₅is also reported. The large observed β_vecvalue supports the proposed NLO response model.