Coupled electronic states in trans-MCl2(H2O)4n+ complexes (M: Ni2+, Co2+, V3+, Cr3+) probed by absorption and luminescence spectroscopy

The electronic spectra of trans-MCl2(X2O)(4)(n+) complexes (M: Ni2+, Co2+, V3+, Cr3+ and X: H, D) are analyzed in order to understand interactions between electronic states. We present detailed low-temperature polarized absorption spectra of single crystals. Spectra were measured over a large range, from near-infrared to UV, and they show several spectroscopic effects arising from interactions between electronic states. The lowest-energy electronic transition in VCl2(H2O)(4)(+) consists of sharp weak bands between 950 and 1030 nm, well separated from the more intense spin allowed bands. Observed energy differences and the temperature dependence of the transition intensities lead to a quantitative characterization of the ground state splitting. The analogous CrCl2(H2O)(4)(+) chromophore has a much smaller energy separation between its lowest energy spin forbidden transition and the first spin allowed band. We examine the effect of this difference on the resolved spectra and compare it to related transition metal complexes. The strongest interaction between electronic states occurs in NiCl2(H2O)(4), where the lowest energy singlet state is very close in energy to a spin allowed crystal field band, giving rise to intense vibronic patterns. Experimental spectra are analyzed quantitatively using the time dependent theory of spectroscopy and established crystal field and Jahn-Teller approaches. (C) 2001 Elsevier Science B.V. All rights reserved.