Determination of Relaxation Paths in the Manifold of Excited States of Pt(2-thpy)2and [Ru(bpy)3]2+by Time-Resolved Excitation and Emission

Abstract

Pt(2-thpy)₂ and [Ru(bpy)₃]²⁺, studied as representatives of transition metal complexes with zero-field splittings (zfs) of the lowest triplets of several cm⁻¹, exhibit a series of generally not-well known time dependencies of emission decay properties. These are strongly determined by relatively slow spin-lattice relaxation (slr) processes. Thus, one finds emission decays for [Ru(bpy)₃]²⁺ and Pt(2-thpy)₂ of 220 and 600 ns at T = 1.3 K, respectively, which are in both compounds controlled by relaxation processes from the second to the lowest excited state, while the lowest state itself emits with a long decay of 230 and 110 μs, respectively. According to these distinctly different emission decay times observed for the two lowest excited states (of the same compound), it is possible to gain a more detailed insight into the properties of the different states by applying the techniques of spectrally highly resolved and time-resolved emission spectroscopy. In particular, this deeper insight results from the possibility to register high-quality low-temperature emission spectra also of the second excited state, hitherto not known. Moreover, from the temperature dependencies of the slr rates in Pt(2-thpy)₂, it is concluded that at low temperature the direct process of slr dominates, while for T > 2.3 K the Orbach process becomes increasingly important. For [Ru(bpy)₃]²⁺ the situation is similar, but the Orbach process grows in for T > 6 K. It is the highlight of the present investigation that the specific properties of slr can be used to study details of relaxation paths in the manifold of the electronically and vibrationally excited states by introducing - for the first time - the method of time-resolved excitation spectroscopy. In particular, it can be shown - without applying a sub-picosecond time resolution - that after a pulsed excitation the relaxations occur within the vibrational potential hypersurfaces of each triplet sublevel. A crossing between the triplet sublevels does not occur via excited vibrational states, but it takes place after the zero-point vibrational levels are reached. However, this selectivity of the relaxation paths is lost when a higher lying singlet is excited. Moreover, this new method provides access to a series of further excited state properties.