Influence of Triplet Surface Properties on Excited-State Deactivation of Expanded Cage Bis(tridentate)Rutheniunn(II) Complexes

Calculations of excited-state potential energy surfaces (PESs) are useful to predict key properties relating to the deactivation cascade of transition-metal complexes. Here, we first perform full free optimizations of the relevant excited state minima, followed by extensive two-dimensional PES calculations based on the minima of interest. Maps of the lowest triplet excited-state surfaces of two bistridentate Ru-II-complexes, [Ru(DQP)(2)](2+) and [Ru(DQzP)(2)](2+), are used to explain recent experimental findings including an unexpected order of magnitude difference in an excited-state lifetime. The calculations reveal significant differences in the fundamental shapes and spin transitions of the lowest triplet excited-state energy surfaces of the two complexes and, in particular, show that the metal-to-ligand charge transfer (MLCT) surface region of [Ru(DQzP)(2)](2+) with a shorter excited-state lifetime significantly smaller than that of [Ru(DQP)(2)](2+). This leads to a minimum energy crossing between the triplet and singlet surfaces near the MLCT for [Ru(DQzP)(2)](2+) or near the MC for [Ru(DQP)(2)](2+). These results indicate that the experimentally observed difference in the excited-state lifetime is closely related to the set of energetically accessible (MLCT)-M-3 conformations.