Resonance Raman Studies of Bis(terpyridine)ruthenium(II) Amino Acid Esters and Diesters

Resonance Raman (rR) spectroscopy in combination with DFT calculations was used to elucidate the nature of the (1)metal-to-ligand charge-transfer states ((MLCT)-M-1) of ester derivatives of homo- and heteroleptic bis(terpyridine)ruthenium(II) complexes [Ru-II(tpy-COOC2H5)(tpy-R)](PF6)(2) with R = NH2 (1a), R = COOC2H5 (1b) and R = NHCOCH3 (1c). The rR spectra provide evidence that the (MLCT)-M-1 states of 1b and 1c are well described by the expected (1)[(t(2g))(5){pi*{(tpy-COOC2H5)}(1)] electron configuration, while the (MLCT)-M-1 state of the donor/acceptor-substituted complex la also involves the amine-substituted terpyridine ligand. The excited state of la can be described by a (1)[{d(yz)/pi(tpy-NH2)}(1){pi*{(tpy-COOC2H5)}(1)] electron configuration with ligand-to-ligand charge-transfer ((LLCT)-L-1) character. The mixed MLCT/LLCT character of the singlet excited state of la is characterized by a low-energy absorption maximum (lambda(max) = 500 nm). The pi-donor character of the tpy-NH2 ligand is also corroborated by DFT calculations on model compounds (small HOMO/LUMO gap, short C-N bond, longer Ru-N bond). The emitting triplet state features a low-energy emission maximum (lambda(max) = 744 nm) as well as a longer lifetime of the emitting state (tau = 33.7 ns) at room temperature in acetonitrile as compared to the data of 1b and 1c. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)