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 deprotona ted form of 2-(2-thienyl)pyridine shows highly resolved phosphorescenc e and triplet excitation spectra at low temperatures when the complex is isolated in Shpol'skii matrices, as is shown for the first time. Sh arp-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 on e 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)). Th ey 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 tr ansfer (MLCT) admixture. The emission from the lowest triplet sublevel \I] to the ground state \0] (origin line I) is strongly forbidden (em ission 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 e xcitation spectra corresponding to the sublevels \II] and \III] show r elatively strong origin lines due to direct spin-orbit coupling. Thus, one observes a large number of vibrational satellites of the Franck-C ondon type and combinations. A comparison of the highly resolved vibra tional satellite structures allows one to conclude that the emitting t ripIet state (all three sublevels) and the singlet ground state exhibi t very similar force constants and nuclear equilibrium positions. Inte restingly, a comparison to the properties of the homologous Pd(2-thpy) (2) (with triplets exhibiting only a very small MLCT or d-d contribut ion) indicates that with increasing MLCT admixture the discussed disto rtions become less pronounced. Thus, an increase of MLCT character lea ds to a more pronounced covalency in the involved states.