Syntheses and third-order NLO properties of η6 complexes of N-ethylcarbazole with Cr(CO)3 and Cr(CO)2PPh3 moieties

Abstract

Carbazole mono- and bimetallic complexes [(N-ethylcarbazole){Cr(CO)₃}₂] (3) and [(N-ethylcarbazole)Cr(CO)₂L] (L = CO (2) and PPh₃ (4)) have been synthesized, and X-ray crystal structure analyses performed for 2 and 4. Each complex possesses spectroscopically intense intraligand (IL) and metal-to-ligand charge-transfer (MLCT) excitations that are well separated in energy. The MLCT bands display a small to moderate negative solvatochromism. The third-order non-linear optical (NLO) properties, measured by the time-resolved forward degenerate four-wave mixing (DFWM) technique in solution at 532 nm, have been studied for these complexes, together with N-ethylcarbazole for comparison. It is found that the third-order NLO response can be tuned by chromium complexation. The molecular second-order hyperpolarizabilities, γ, of the synthesized complexes are determined to be in the order of 10⁻³⁰ esu and clearly depend on both the nature of the ligand and the extent of chromium tricarbonyl coordination by the carbazole ring. The carbazole-mediated metal–metal electronic interaction existing in 3 is believed to be a possible factor responsible for a large improvement in the non-linearity.