Excited state spectroscopy and excited state dynamics of Rh(III) and Pd(II) chelates as studied by optically detected magnetic resonance techniques.

In this paper we review optically detected magnetic resonance (ODMR) investigations of a series of Rh3+ (4d6) and Pd2+ (4d8) complexes in the lowest excited electron spin triplet state. Starting with a brief survey of the technique of optical detection of magnetic resonance, zero-field and low-magnetic field ODMR results are reviewed for the tris-diimine chelates [Rh(phen)n(bpy)3−n]3+, where phen=1,10-phenanthroline, bpy=2,2′-bipyridine, and n=0, 2, or 3, and for the mixed cyclometalated chelates [Rh(thpy)x(phpy)2−x(bpy)]+, with thpy=2,2′- thienylpyridinate, phpy−=2-phenylpyridinate, and x=1, or 2. The ODMR data reveal fine structure splittings in the phosphorescent excited state of the complexes comparable in magnitude to those known for the nonchelated ligand molecules in the excited triplet state. Anisotropy studies of the ODMR spectra for the single crystals in low magnetic fields show that the lowest electronic excitation in the complexes is a triplet state indeed and that this state is localized on a single ligand molecule per metal ion site. From microwave recovery experiments, performed under conditions that the spin-lattice relaxation can be neglected (T< 2 K), detailed information concerning the triplet sublevel lifetimes is obtained. The lifetimes are on the millisecond timescale, i.e., three orders of magnitude shorter than for the nonchelated ligand molecules. The lifetime shortening as well as the observed spin-selective radiative decay of the triplet sublevels of the ligand molecule are discussed in detail on the basis of enhanced spin-orbit couplings caused by the central (heavy) metal ion. Optically detected spin coherence experiments (transient nutation and spin echo decay) are also discussed. The results show that the homogeneous line broadening of the ODMR transitions of the metal complexes in the emissive triplet state is approximately 100 kHz. The homogeneous broadening is attributed to the effects of flip-flop motions of ligand proton spins that randomly modulate the triplet electron spin levels on account of dipolar electron spin - nuclear spin couplings. Finally, recent ODMR and PMDR (phosphorescence microwave double resonance) experiments performed for the Pd2+-chelates, Pd(thpy)2 and Pd(qol)2 (with qol−=8-hydroxyquinolinate) in Shpol’skii matrices are discussed. The lowest excited electronic state in these molecules is also emissive and ODMR spectra at zero- and low magnetic fields have been observed. For Pd(thpy)2 only one zero-field ODMR transition could be measured, but it is argued that this transition originates in an excited triplet state. The results of the microwave recovery experiments could be related to time-resolved emission experiments in high magnetic fields. Spin selectivity in the vibronic line emission is demonstrated by means of PMDR.