TD-DFT for transitions in bimetallic sandwich complexes

TD-DFT calculations were performed to clarify how the electronic properties of the catecholate - bipyridine complex of Pt(II) (Pt(II)-cat-bpy) are modified by the formation of stable sandwich complexes formed by a series of pentamethylcyclopentadienyl-transition metals (Cp-TM, TM = Ir(III), Rh(III), and Ru(II)) . The molecular modelling takes advantage of a previous investigation on analogous systems1. The theoretical investigation assigns all the observed transitions and elucidates the origin of the large blue-shift observed in the first band of the absorption spectra of the sandwich complexes with respect to that of Pt(II)-cat-bpy. Transitions to the first excited state are largely characterized by ligand-to-ligand charge transfer (LL'CT) excitations, analogously to what found for the lowest absorption band of Pt(II)-cat-bpy1. Nevertheless, the energy of such transitions, which involve charge transfer from the Pt/catecholate to diimine2, are modulated in a dierent manner by the Cp- TM substituents, which stabilize the energy of the HOMO. Besides, excitations occurring within the sandwich moiety also contribute to the characterization of these transitions. The phosphorescent emissions detected at low temperature were investigated to rationalize the dierent behaviour of the examined sandwich complexes in triplet states. 1. A. Damas, B. Ventura, J. Moussa, A. Degli Esposti, L.-M. Chamoreau, A. Barbieri, and H. Amouri, Inorg. Chem. 51 1739-1750 (2012). 2. J. Best, I. V. Sazanovich, H. Adams, R. D. Bennett, D. E. Stephen and A. J. H. M. Meijer, M. Towrie, S. A. Tikhomirov, O. V. Bouganov, M. D. Ward, J. A. Weinstein, Inorg. Chem. 49, 10041-10056 (2010).