Characterization of triplet sublevels by Highly resolved vibrational satellite structures. Application to Pt(2-thpy)2

Abstract

The cyclometalated Pt(2-thpy)~ complex with thpy-as the deprotonated form of 2-(2-thienyl)pyridine shows highly resolved phosphorescence and triplet excitation spectra at low temperatures when the complex is isolated in Shpol'skii matrices, as is shown for the first time. Sharp-line Shpol'skii spectra were obtained by dissolving Pt(2-thpy)z in n-hexane, n-heptane, n-octane, n-nonane, and n-decane matrices. The highest resolution was reached using n-octane. In this matrix only one dominant site governs the spectra. The lowest electronic origins lie at 17 156 (I), 17 163 (11), and 17 172 cm-' (111) ( f l cm-I). They represent triplet sublevels that are split by the relatively large zero-field splitting of 16 cm-I. These sublevels are assigned as n-n* ligandcentered (LC) with an appreciable metal-to-ligand charge transfer (MLCT) admixture. The emission from the lowest triplet sublevel 11) to the ground state 10) (origin line I) is strongly forbidden (emission lifetime at T = 1.3 K: 110 ,us), but due to vibronic (Herzberg-Teller) coupling, additional radiative deactivation paths are opened and thus a large number of "false origins" occur. The emission and excitation spectra corresponding to the sublevels 111) and 1111) show relatively strong origin lines due to direct spin-orbit coupling. Thus, one observes a large number of vibrational satellites of the Franck-Condon type and combinations. A comparison of the highly resolved vibrational satellite structures allows one to conclude that the emitting triplet state (all three sublevels) and the singlet ground state exhibit very similar force constants and nuclear equilibrium positions. Interestingly, a comparison to the properties of the homologous Pd(2-thpy)z (with triplets exhibiting only a very small MLCT or d-d* contribution) indicates that with increasing MLCT admixture the discussed distortions become less pronounced. Thus, an increase of MLCT character leads to a more pronounced covalency in the involved states.

Characterization of excited electronic and vibronic states of platinum metal compounds with chelate ligands by highly frequency-resolved and time-resolved spectra

“…Such a behavior is well known for organic molecules (e.g., see [91,[213][214][215]) and other transition metal complexes [38,92,93,101,216]. For example, the related [Os(bpy)3] 2 § doped into the [Ru(bpy)3](PF6) 2 matrix displays this behavior much more distinctly.…”

“…4.1 and 4.2; cf. also the detailed studies of Pt(2-thpy)2 [93]). (3) The formation of guest aggregates introduces a large number of different types of guest-guest and guest-host nearest neighbors.…”

“…(For the theoretical background see e.g. [86][87][88][89][90][91][92][93][94][95][96][97]99]). Equivalent arguments hold also for the second and the third excited state [74].…”

“…Thus, a Franck-Condon activity, according to totally symmetric modes, can be excluded as dominant process. ~6b Therefore, it is concluded that vibronic coupling according to the Herzberg-Teller mechanism induces the vibrational satellite structure (e. g., see [86][87][88][89][90][91][92][93]217] ). The forbiddenness of the purely electronic transition [ 0 ) ~ [ I ) seems to imply that direct spin-orbit coupling (soc) to state [I) is very small.…”

“…Therefore, one has to consider such vibronic coupling mechanisms that provide singlet character to state [I}. Two processes have been proposed [92], which may be responsible for a radiative deactivation of the lowest excited state of [Os(bpy)3] z § The theoretical background is discussed in [86][87][88][89][90][91][92][93].…”