Intraligand charge transfer in the Pd(II) oxinate complex Pd(qol)(2). Site-selective emission, excitation, and optically detected magnetic resonance

The first spectroscopic investigation of Pd(qol)(2) (qol(-) = 8-quinolinola to-N,O = oxinate) dissolved in an n-octane matrix (Shpol'skii matrix) is re ported. Application of several spectroscopic methods at liquid helium tempe ratures (typically, T = 1.2 K), such as site-selective and highly resolved luminescence and excitation spectroscopy, time-resolved emission spectrosco py, optically detected magnetic resonance, microwave recovery, phosphoresce nce microwave double-resonance, and magnetic fields, allows us to character ize the lowest excited electronic states in detail. In accord with previous assignments for the related Pt(qol)(2) it is shown that these lowest state s represent intraligand charge-transfer states, namely, (ILCT)-I-1 and (ILC T)-I-3. The electronic origin of the (ILCT)-I-1 state lies at 20 617 cm(-1) (site A). It exhibits a nearly homogeneous line width with a half-width of about 80 cm(-1) (fwhm), which corresponds to a lifetime of tau((ILCT)-I-1) approximate to 2 x 10(-13) s. This value is even shorter than that found f or Pt(qol)(2), presumably due to intersystem crossings and relaxations to d d* states. The electronic origin of the (ILCT)-I-3 state lies at 16 090 cm( -1) (site A), and its zero-field splittings (zfs) into three sublevels are 2E = 2356 MHz (0.0785 cm(-1)) and D - E = 5241 MHz (0.175 cm(-1)). The emis sion decay times of the three sublevels are determined as tau(I) = 90 +/- 3 0 ms, tau(II) = 180 +/- 10 mu s, and tau(III) = 80 +/- 10 mu s. (Slightly d ifferent values are found for a second site B at 16 167 cm(-1).) From the s mall values of zfs and the long emission decay times it is concluded that m etal-d or MLCT admixtures to (ILCT)-I-3 are very small. This result clearly reflects the ligand-centered character of the transition. The assignment a s an ILCT transition is supported by the occurrence of relatively strong vi brational satellites of Pd-N and Pd-O character in highly resolved emission spectra. Although the transition is ascribed to a charge-transfer process, the geometry changes between the ground state and (ILCT)-I-3 are very smal l. The results found for Pd(qol)(2) are compared to those of companion stud ies of Pt(qot)(2) and Pt(qtl)(2) (qtl- = 8-quinolinethiolato-N,S).