Syntheses and Spectroscopic and Quadratic Nonlinear Optical Properties of Extended Dipolar Complexes with Ruthenium(II) Ammine Electron Donor and N-Methylpyridinium Acceptor Groups

In this paper, we describe the extremely unusual optical properties of RuII-based electron donor-acceptor (D-A) polyene and some closely related chromophores. For three different polyene series, the intense, visible d→π* metal-to-ligand charge-transfer bands unexpectedly blue-shift as the number of E-ethylene units (n) increases from 1 to 3, and the static first hyperpolarizabilities β0 determined via hyper-Rayleigh scattering and Stark spectroscopy maximize at n = 2, in marked contrast to other known D-A polyenes in which β0 increases steadily with n. Time-dependent density-functional theory and finite field calculations verify these empirical trends, which arise from the orbital structures of the complexes. This study illustrates that transition metal-based nonlinear optical chromophores can show very different behavior when compared with their more thoroughly studied purely organic counterparts.