Optical investigation of spin-crossover in cobalt(II) bis-terpy complexes

The spin transition of the [Co(terpy)(2)](2+) complex (terpy = 2,2':6',2-terpyridine) is analysed based on experimental data from optical spectroscopy and magnetic susceptibility measurements. The single crystal absorption spectrum of [Co(terpy)(2)](ClO(4))(2) shows an asymmetric absorption band at 14400 cm(-1) with an intensity typical for a spin-allowed d-d transition and a temperature behaviour typical for a thermal spin transition. The single crystal absorption spectra of suggest that in this compound, the complex is essentially in the highspin state at all temperatures. However, the increase in intensity observed in the region of the low-spin MLCT transition with increasing temperature implies an unusual partial thermal population of the low-spin state of up to about 10% at room temperature. Finally, highspin -> low-spin relaxation curves following pulsed laser excitation for [Co(terpy)(2)](ClO(4))(2) dispersed in KBr discs, and as a comparison for the closely related [Co(4-terpyridonc)(2)](ClO(4))(2) spin-crossover compound are given. (c) 2007 Elsevier B.V, All rights reserved.