CALCULATION AND ELECTRONIC DESCRIPTION OF QUADRATIC HYPERPOLARIZABILITIES - TOWARD A MOLECULAR UNDERSTANDING OF NLO RESPONSES IN ORGANOTRANSITION METAL CHROMOPHORES

This contribution explores the use of the computationally efficient, chemically-oriented INDO electronic structure model (ZINDO) in concert with perturbation theory to relate molecular quadratic hyperpolarizabilities to molecular architecture and electronic structure in transition metal chromophores, The ZINDO-derived second-order nonlinear optical responses are found to be in excellent agreement with the experiment for a variety of ferrocenyl and (arene)chromium tricarbonyl derivatives. The assumptions needed to describe nonlinear optical response in simple molecular orbital terms are presented, and their reliability is analyzed in a quantitative fashion. All of the ferrocenyl chromophores examined are found to closely resemble traditional organic pi-electron chromophores in that intense MLCT transitions dominate the second-order response. A detailed examination of the modest second-order nonlinearities of the chromium arenes identifies two shortcomings that may be characteristic of many organometallic architectures: the intrinsic hyperpolarizability may be far greater than the experimentally accessible vectorial component of beta (that directed along the dipole moment direction), and the electronic distribution about the metal centers in many organometallic structures is pseudo-centrosymmetric. This explains the relatively low nonlinearities of a number of recently reported organometallic chromophores. The design utility of the present computational formalism is illustrated by the calculation of the second-order response of a hypothetical organometallic chromophore having a very acentric electron distribution and, correspondingly, a larger calculated second-order response than any measured to date for an organometallic chromophore.