CHARACTERIZATION OF TRIPLET SUBLEVELS BY HIGHLY RESOLVED VIBRATIONAL SATELLITE STRUCTURES - APPLICATION TO PT(2-THPY)(2)

The cyclometalated Pt(2-thpy)(2) complex with thpy(-) as the deprotonated form of 2-(2-thienyl)pyridine shows highly resolved phosphorescence and triplet excitation spectra at low temperatures when the complex is isolated in Shpol'skii matrices, as is shown for the first time. Sharp-line Shpol'skii spectra were obtained by dissolving Pt(2-thpy)(2) in n-hexane, n-heptane, n-octane, n-nonane, and n-decane matrices. The highest resolution was reached using n-octane. In this matrix only one dominant site governs the spectra. The lowest electronic origins lie at 17 156 (I), 17 163 (II), and 17 172 cm(-1) (III) (+/-1 cm(-1)). They represent triplet sublevels that are split by the relatively large zero-field splitting of 16 cm(-1). These sublevels are assigned as pi-pi* ligand-centered (LC) with an appreciable metal-to-ligand charge transfer (MLCT) admixture. The emission from the lowest triplet sublevel \I] to the ground state \0] (origin line I) is strongly forbidden (emission lifetime at T = 1.3 K: 110 mu s), but due to vibronic (Herzberg-Teller) coupling, additional radiative deactivation paths are opened and thus a large number of ''false origins'' occur. The emission and excitation spectra corresponding to the sublevels \II] and \III] show relatively strong origin lines due to direct spin-orbit coupling. Thus, one observes a large number of vibrational satellites of the Franck-Condon type and combinations. A comparison of the highly resolved vibrational satellite structures allows one to conclude that the emitting tripIet state (all three sublevels) and the singlet ground state exhibit very similar force constants and nuclear equilibrium positions. Interestingly, a comparison to the properties of the homologous Pd(2-thpy)(2) (with triplets exhibiting only a very small MLCT or d-d* contribution) indicates that with increasing MLCT admixture the discussed distortions become less pronounced. Thus, an increase of MLCT character leads to a more pronounced covalency in the involved states.