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 π-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 β (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.
ASJC Scopus subject areas
- Colloid and Surface Chemistry