Electronic absorption spectroscopy of [Ru(phen)2(bpy)]2+, [Ru(phen)2(dmbp)]2+, [Ru(tpy)(phen)(CH3CN)]2+ and [Ru(tpy)(dmp)(CH3CN)]2+: A theoretical study

The electronic absorption spectroscopy of Ru(II) polyimine complexes, potentially photolabile under visible light irradiation, is investigated by means of density functional theory. The structures of [Ru(phen)2(bpy)]2+, [Ru(phen)2(dmbp)]2+, [Ru(tpy)(phen)(CH3CN)]2+ and [Ru(tpy)(dmp)(CH3CN)]2+ (phen = 1,10-phenanthroline; bpy = 2,2′-bipyridine, tpy = 2,2′; 6′,6″-terpyridine, dmbp = 6,6′-dimethyl-2,2′-bipyridine, dmp = 2,9-dimethyl-1,10-phenanthroline) have been optimized at the DFT(B3LYP) level. The main features of the theoretical absorption spectra of the four molecules have been determined by means of time-dependent DFT (TD-DFT) calculations. The electronic spectra are characterized by a high density of states between 550 nm and 350 nm assigned mainly to metal-to-ligand-charge-transfer (MLCT) states corresponding to electronic excitations to the low-lying π* orbitals, either localised on the phen, dmp and tpy ligands, or delocalised on the phen/bpy, phen/dmbp, phen/tpy and dmp/tpy ligand. The theoretical spectra of [Ru(phen)2(dmbp)]2+ and [Ru(tpy)(phen)(CH3CN)]2+ reproduce rather well the large bands observed experimentally at about 449 nm and 455 nm with a blue shift of 0.2 eV in the latter case. Four 1MLCT states (dRu→πphen∗,πphen/dmbp∗) calculated at 445 nm, 436 nm, 423 nm and 418 nm with significant oscillator strengths contribute to the band centred at 449 nm in the spectrum of [Ru(phen)2(dmbp)]2+, whereas three 1MLCT states (dRu→πphen∗,πphen/tpy∗) calculated at 419 nm, 378 nm and 374 nm contribute to the band observed in the spectrum of [Ru(tpy)(phen)(CH3CN)]2+. The theoretical spectrum of [Ru(phen)2(bpy)]2+ does not differ drastically from the spectrum of the analogous dmbp species with four intense, slightly blue-shifted 1MLCT (dRu→πphen∗,πphen/bpy∗) states, calculated at 432 nm, 429 nm, 408 nm and 395 nm. The theoretical spectrum of [Ru(tpy)(dmp)(CH3CN)]2+ is more compact with close-lying 1MLCT states (dRu→πtpy/dmp∗,πdmp∗,πtpy∗) calculated at 428 nm, 424 nm and 416 nm with rather small oscillator strengths. The four complexes are characterized by the presence of potentially dissociative metal centred (3MC) excited states between 400 nm and 350 nm.