Evolution of Linear Absorption and Nonlinear Optical Properties in V-Shaped Ruthenium(II)-Based Chromophores

Abstract

In this article, we describe a series of complexes with electron-rich cis-{RuII(NH3)4}2+ centers coordinated to two pyridyl ligands bearing N-methyl/arylpyridinium electron-acceptor groups. These V-shaped dipolar species are new, extended members of a class of chromophores first reported by us (Coe, B. J. et al. J. Am. Chem. Soc. 2005, 127, 4845−4859). They have been isolated as their PF6− salts and characterized by using various techniques including 1H NMR and electronic absorption spectroscopies and cyclic voltammetry. Reversible RuIII/II waves show that the new complexes are potentially redox-switchable chromophores. Single crystal X-ray structures have been obtained for four complex salts; three of these crystallize noncentrosymmetrically, but with the individual molecular dipoles aligned largely antiparallel. Very large molecular first hyperpolarizabilities β have been determined by using hyper-Rayleigh scattering (HRS) with an 800 nm laser and also via Stark (electroabsorption) spectroscopic studies on the intense, visible d → π* metal-to-ligand charge-transfer (MLCT) and π → π* intraligand charge-transfer (ILCT) bands. The latter measurements afford total nonresonant β0 responses as high as ca. 600 × 10−30 esu. These pseudo-C2v chromophores show two substantial components of the β tensor, βzzz and βzyy, although the relative significance of these varies with the physical method applied. According to HRS, βzzz dominates in all cases, whereas the Stark analyses indicate that βzyy is dominant in the shorter chromophores, but βzzz and βzyy are similar for the extended species. In contrast, finite field calculations predict that βzyy is always the major component. Time-dependent density functional theory calculations predict increasing ILCT character for the nominally MLCT transitions and accompanying blue-shifts of the visible absorptions, as the ligand π-systems are extended. Such unusual behavior has also been observed with related 1D complexes (Coe, B. J. et al. J. Am. Chem. Soc. 2004, 126, 3880−3891).