The cyclometalated Pt(2-thpy)~ 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)z 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 (11), and 17 172 cm-' (111) ( f l cm-I). They represent triplet sublevels that are split by the relatively large zero-field splitting of 16 cm-I. These sublevels are assigned as n-n* ligandcentered (LC) with an appreciable metal-to-ligand charge transfer (MLCT) admixture. The emission from the lowest triplet sublevel 11) to the ground state 10) (origin line I) is strongly forbidden (emission lifetime at T = 1.3 K: 110 ,us), 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 111) and 1111) 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 triplet 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)z (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.
Pd(2-thpy)2 is a representative of the interesting new class of ortho-metalated compounds. For the first time, we present highly resolved emission and excitation spectra. This could be achieved by using the Shpol'skii matrix isolation technique. From the intensity distribution of the highly resolved vibrational satellite structures and the corresponding vibrational energies it is concluded that the excited triplet lying at 18 418 f 1 cm-l and the singlet ground state exhibit nearly the same force constants and equilibrium positions of the potential hypersurfaces. The type of the electronic transition is assigned as being ligand centered with a relatively small MLCT admixture. The zero-field splitting of the triplet could not be resolved (experimental resolution 1 cm-l), but at T = 1.3 K the three sublevels emit independently with 71 = 155 f 20 ps, 7 2 = 200 ps, and 7 3 = 1200 f 100 ps, respectively. With increasing temperature and thus increasing spin-lattice relaxation the emission lifetime becomes monoexponential with ~( 4 . 2 K) = 235 f 10 ps.
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