Utilizing ancillary ligands to optimize the photophysical properties of 4H-imidazole ruthenium dyes

Correlations between structural features and photophysical properties offer the possibility to design dyes with tailor-made properties. In this respect, the photophysical properties of a series of 4H-imidazole (4H-im) ruthenium dyes with varying chemical and electronic structures of the complex fragments [(tpy)Ru(4H-im)X] {tpy=4,4',4''-tri-tert-butyl-2,2':6',2''-terpyridine; X=Cl(-), NCS(-)} and [(bpy)2Ru(4H-im)](+) {bpy=4,4'-di-tert-butyl-2,2'-bipyridine} were investigated. Variation of the π-donor/acceptor properties of the ancillary ligands offers the possibility to tune the relative energies of the d donor and π* acceptor orbitals of 4H-im, which results in a shift in the Ru→4H-im (1)MLCT (MLCT=metal-to-ligand charge transfer) absorption band in the visible range. Further, the energies of the excited states and also the interactions and mixing of the metal d orbital with the π* 4H-im orbital are sensitive to the chemical and electronic structures of the complex fragment. This causes subtle changes in the photoinduced processes; for example, the rate of the interim population of a planarized state increases with increasing/decreasing π-acceptor/donor character of the ancillary ligand. Although ground-state repopulation in the tpy species follows the energy-gap law, local symmetry-related effects have to be considered to account for the very short lifetime of the excitation in the bpy complex. Additionally, the ultrafast intramolecular relaxation processes leading from the initially excited states to the relaxed triplet states localized on the 4H-im ligand (internal conversion, internal vibrational energy redistribution, intersystem crossing) depend on the nature of the complex fragment. The high excitation-wavelength-dependent rate for the population of the relaxed triplet states in the bpy complex points to additional interligand electron-transfer contributions upon excitation into the higher lying Ru→bpy (1)MLCT states.