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) invest igations of a series of Rh3+ (4d(6)) and Pd2+ (4d(8)) complexes in the lowe st excited electron spin triplet state. Starting with a brief survey of the technique of optical detection of magnetic resonance, zero-field and low-m agnetic field ODMR results are reviewed for the tris-diimine chelates [Rh(p hen)(n)(bpy)(3-n)](3+), where phen = 1,10-phenanthroline, bpy = 2,2'-bipyri dine, and n = 0, 2, or 3, and for the mixed cyclometalated chelates [Rh(thp y)(x)(phpy)(2-x)(bpy)](+), with thpy = 2,2'-thienylpyridinate, phpy(-) = 2- phenylpyridinate, and x = 1, or 2. The ODMR data reveal fine structure spli ttings in the phosphorescent excited state of the complexes comparable in m agnitude to those known for the nonchelated ligand molecules in the excited triplet slate. Anisotropy studies of the ODMR spectra far the single cryst als in low magnetic fields show that the lowest electronic excitation in th e complexes is a triplet state indeed and that this state is localized on a single ligand molecule per metal ion site. From microwave recovery experim ents, performed under conditions that the spin-lattice relaxation can be ne glected (T less than or equal to2 K), detailed information concerning the t riplet sublevel lifetimes is obtained. The lifetimes are on the millisecond timescale, i.e., three orders of magnitude shorter than for the nonchelate d ligand molecules. The lifetime shortening as well as the observed spin-se lective radiative decay of the triplet sublevels of the ligand molecule are discussed in detail on the basis of enhanced spin-orbit couplings caused b y the central (heavy) metal ion. Optically detected spin coherence experime nts (transient nutation and spin echo decay) are also discussed. The result s 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 o f ligand proton spins that randomly modulate the triplet electron spin leve ls on account of dipolar electron spin - nuclear spin couplings. Finally, r ecent ODMR and PMDR (phosphorescence microwave double resonance) experiment s performed for the Pd2+-chelates, Pd(thpy)(2) and Pd(qol)(2) (with qol(-) = 8-hydroxyquinolinate) in Shpol'skii matrices are discussed. The lowest ex cited electronic state in these molecules is also emissive and ODMR spectra at zero- and low magnetic fields have been observed. For Pd(thpy), only on e zero-field ODMR transition could be measured, but it is argued that this transition originates in an excited triplet state. The results of the micro wave recovery experiments could be related to time-resolved emission experi ments in high magnetic fields. Spin selectivity in the vibronic line emissi on is demonstrated by means of PMDR.