TIME-RESOLVED VIBRATIONAL-STRUCTURES OF THE TRIPLET SUBLEVEL EMISSION OF PD(2-THPY)(2)

Time-resolved phosphorescence spectra from the lowest electronic triplet of Pd(2-thpy)(2) (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 +/- 1 cm(-1) (electronic origin). Its zero-field splitting is less than 1 cm(-1) 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 mu s), 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(-1) 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.