Triplets in metal-organic compounds. Chemical tunability of relaxation dynamics

Triplets of metal-organic or related compounds of the platinum metal group split into substates. The amount of splitting at zero magnetic field (zfs) is mainly determined by the effective spin-orbit coupling, which is, for ex ample, induced by metal-d and/or MLCT participations in these triplets. The total zfs can be tuned chemically over a very wide range from about 0.1 cm (-1) to mon than 200 cm(-1) (sce Fig. 1). After excitation, the relaxation time between the substates can be as long as hundreds of nano-seconds to ma ny micro-seconds at low temperature. This relaxation, the spin-lattice rela xation (slr), depends on the splitting pattern of the triplet substates, fu rther on temperature, and on the matrix surrounding the chromophore. Four c ompounds [Pt(bpy)(2)](2+), Pt(2-thpy)(CO)(Cl), Pt(Z-thpy)(2), and [Ru(bpy)( 3)](2+) with strongly different zero-field splittings are selected as case studies, to investigate the dynamics of sir according to the direct, the Or bach, and the Raman process. Temperature dependent studies and investigatio ns at low temperature (T less than or equal to 2) under application of high magnetic fields up to B = 10 T and high pressure up to p = 20 kbar, respec tively, allow us to develop a deeper insight into the relaxation mechanisms . Moreover, several effects are pointed out that result from slow spin-latt ice relaxation and that can be important at low temperature, like the non-v alidity of a Boltzmann distribution for closely lying states, the occurrenc e of super-imposed emission spectra from different excited states, the depe ndence of emission decay properties on excitation and detection wavelengths , effects of spectral shifts with time, and a specific behavior of radiatio nless energy transfer. In an outlook, a number of further transition metal complexes is presented to underline the general importance of the effects o f relatively slow spin-lattice relaxation. (C) 2000 Elsevier Science S.A. A ll rights reserved.