Tripler sublevels of metal organic complexes temperature dependence of spin-lattice relaxation

Triplets of transition metal complexes with organic chelate ligands can act as important pathways in photo-redox processes. Detailed information on these states is available from highly resolved optical spectra and time-resolved investigations. The lowest triplets are often zero-field split into sublevels by several cm(-1) (zero-field splitting, ZFS) due to spin-orbit interactions. Interestingly, the relaxation between these sublevels can be very slow (nanoseconds up to thousands of nanoseconds) at low temperatures. This is reflected in the emission decay times and even in the emission spectra. The population dynamics and the relaxation times are governed by the interaction between the triplet sublevels and the surrounding matrix (spin-lattice relaxation, SLR). Due to a low phonon density of states at energies of the size of the ZFS, the relaxation between the triplet sublevels is slow. It is possible to understand the relevant relaxation processes (direct, Orbach, Raman) in detail by investigating the temperature dependence of the emission decay behavior and thus of the SLR. In order to take various ZFS patterns correctly into account, an extended description for the Orbach process is derived. In the present investigation, three compounds, Pt(2-thpy)(2), Pt(2-thpy)(CO)(C1), and Pt(phpy)(2),are analyzed as case studies. Important data that describe the emission properties of the triplet substates are derived. In particular, it is possible to determine the relative importance of the three different relaxation processes for these systems. The SLR data are in accordance with qualitative models for the chromophore-cage interactions. (C) 2000 Elsevier Science B.V. All rights reserved.