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.

Time-resolved phosphorescence spectra from the lowest electronic triplet of Pd(2-thpy)~ (with 2-thpy-= ortho-C-deprotonated form of 2-(2-thienyl)pyridine) (see the inset of Figure 2) are presented. The complex was isolated in a Shpol'skii matrix to obtain high resolution. The emitting triplet lies at 18 418 f 1 cm-' (electronic origin). Its zero-field splitting is less than 1 cm-I and could not be resolved optically. However, at 1.3 K, when the spin-lattice relaxation is slow compared to the emission lifetimes of the sublevels (130, 235, 1200 p), the individual sublevels emit independently. Thus, by time-resolved spectroscopy it is possible to separate a fast-decaying emission spectrum from a slow-decaying one. A highlight of this investigation is that these spectra exhibit different vibrational satellite structures. This shows that different spin-orbit coupling mechanisms (direct spin-orbit coupling and Herzberg-Teller coupling) govern the radiative deactivation of the sublevels. In particular, it is found that specific vibrational modes couple very selectively to individual sublevels. For example, the 528 cm-I mode couples only to the slow-decaying sublevel. Thus, these optically well resolvable vibrational satellites display directly properties of the individual sublevels, which are unresolvable by conventional optical spectroscopy. This effect is observed for the first time for transition metal complexes. IntroductionThe extensive studies of photophysical and photochemical properties of transition metal complexes with organic ligands during the past 20 years were stimulated by possible applications, especially in photocatalytic processes like solar energy These are related in many cases to the properties of the lowest excited electronic states. Moreover, there is substantial scientific interest to develop a better understanding of these compounds, especially since they exhibit a series of electronic and vibronic properties which are found neither in pure organic molecules nor in simple transition metal compounds with metal centered transitions. Valuable information about the nature of the excited states is obtained from highly resolved emission and excitation spectra as well as from emission decay properties. This was demonstrated for the wellknown [Ru(bpy)3I2+ and other polypyridine Cyclometalated complexes form a related class of compounds.2s6s8 Recently, one representative, the Pd(2-thpy)~ complex (with 2-thpy-= ortho-C-deprotonated form of 2-(2-

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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The emission of Pt(2-thpy) 2 (2-thpy -) 2-(2-thienylpyridinate)) doped into an n-decane Shpol'skii matrix has been studied at T ) 4.2 K (0 kbar) and at different pressures up to 14 kbar at T ≈ 5 K. This type of matrix is used for the first time for high-pressure investigations of transition metal complexes. The emitting state of Pt(2-thpy) 2 is of 3 ππ* character with a significant MLCT admixture, and the ground state is a singlet. The spectra are well resolved and thus show the electronic origin (0-0 transition) of the most intensively emitting triplet sublevel as well as a number of vibrational Franck-Condon satellites. The corresponding pressure-induced shifts are determined. In particular, the value for the electronic origin is with ∆ν j/∆p ) -( 3( 1) cm -1 /kbar -1 relatively small, due to an apparently small inherent shift of the singlet-triplet transition. In addition, the smallness of this value seems to be related to the very weak interaction between the chromophore and the environment for this Shpol'skii matrix, whereby this interaction does not strongly change with pressure. The intensity distribution of the vibrational satellite structure shows that the equilibrium positions of the potential hypersurfaces of ground and excited states are very similar at ambient pressure. Apparently this situation is not changed up to 14 kbar.