Low-lying electronic states and photophysical properties of organometallic Pd(II) and Pt(II) compounds. Modern research trends presented in detailed case studies

Two homologous organometallic compounds, Pd(2-thpy)₂ and Pt(2-thpy)₂ (with 2-thpy⁻ = 2- (2-thienyl)-pyridinate, structure formulae in Fig. 1), are chosen for case studies of photophysical properties of the lowest excited states. The triplets of these two representative compounds are marked by differences of nearly two orders of magnitude in metal/MLCT (metal-to-ligand charge transfer) character. Determination of detailed photophysical properties of both compounds is possible, since highly resolved spectra are obtained when the compounds are dissolved in an n-octane matrix (Shpol’skii matrix), when the measurements are carried out at low temperature (typically at T = 1.3 K), and when modern techniques of laser spectroscopy are applied. In addition, methods of time-resolution and of microwave double-resonance, such as optically detected magnetic resonance (ODMR), microwave recovery, and phosphorescence microwave-double resonance (PMDR) are used. In particular, it is shown that with increasing metal character of the triplet, that is when Pd(2-thpy)₂ is compared to Pt(2-thpy)₂, many properties change characteristically and in part by orders of magnitude. For example, the following properties will be addressed: Transition probabilities, emission decay times, zero-field splittings (zfs), processes of spin-lattice relaxation (slr), intersystem crossing rates, intrastate relaxation rates, excited state binding properties as compared to those of the electronic ground state, anharmonicity effects, metal mediated ligand-ligand coupling or spatial extensions of the excited state wavefunctions.Moreover, we focus on spin-selectivity in the vibrational satellite structures of the emission spectra as identified by the complementary methods of time-resolved emission and PMDR spectroscopy.We also discuss radiative deactivation processes, such as spin-vibronic Herzberg-Teller and Frank-Condon activities. Further, we specify sub-picosecond relaxation paths on the basis of micro second time resolution by applying for the first time the method of time-resolved excitation spectroscopy to transition metal complexes. It is further demonstrated that the size of zero-field splitting of the triplet state can be used as an ordering parameter, that reflects the metal participation in the lowest triplet. Thus, one can relate Pd(2-thpy)₂ and Pt(2-thpy)₂ to a larger number of other compounds, such as [Rh(bpy)₃]³⁺, [Pt(bpy)₂]²⁺, Pt(qol)₂, [Pt(mnt)₂]²⁻, [Ru(bpy)₃]²⁺, [Os(phen)₃]²⁺, [Os(bpy)₃]²⁺, etc. (compare Fig. 1 and Table 11) and one obtains a series that demonstrates chemical tunability of photophysical properties. – For several specific subjects, we present the basic background information in order to make the paper more easily readable, also for non-specialists