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 example, 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 many micro-seconds at low temperature. This relaxation, the spin-lattice relaxation (slr), depends on the splitting pattern of the triplet substates, further on temperature, and on the matrix surrounding the chromophore. Four compounds [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 Orbach, and the Raman process. Temperature dependent studies and investigations 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, respectively, allow us to develop a deeper insight into the relaxation mechanisms. Moreover, several effects are pointed out that result from slow spin-lattice relaxation and that can be important at low temperature, like the non-validity of a Boltzmann distribution for closely lying states, the occurrence of super-imposed emission spectra from different excited states, the dependence of emission decay properties on excitation and detection wavelengths, effects of spectral shifts with time, and a specific behavior of radiationless energy transfer. In an outlook, a number of further transition metal complexes is presented to underline the general importance of the effects of relatively slow spin-lattice relaxation. (C) 2000 Elsevier Science S.A. All rights reserved.