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 th ese states is available from highly resolved optical spectra and time-resol ved investigations. The lowest triplets are often zero-field split into sub levels by several cm(-1) (zero-field splitting, ZFS) due to spin-orbit inte ractions. Interestingly, the relaxation between these sublevels can be very slow (nanoseconds up to thousands of nanoseconds) at low temperatures. Thi s is reflected in the emission decay times and even in the emission spectra . The population dynamics and the relaxation times are governed by the inte raction between the triplet sublevels and the surrounding matrix (spin-latt ice 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, Orb ach, Raman) in detail by investigating the temperature dependence of the em ission decay behavior and thus of the SLR. In order to take various ZFS pat terns correctly into account, an extended description for the Orbach proces s 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. Importan t data that describe the emission properties of the triplet substates are d erived. 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 interac tions. (C) 2000 Elsevier Science B.V. All rights reserved.