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 complex, potentially photolabile under visible light irradiation, is investigated by means of density functional theory. The structures of [Ru(phen)(2)](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 (MCLT) states corresponding to electronic excitations to the low-laying pi* orbitals, either localised on the phen, dmp and tpy ligands, or delocalized on the phen/bpy, peh/dmpb, phen/tpy and dmp/tpy ligand. The theoretical spectra of [Ru(phen)(2)(dmpb)](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 (MLCT)-M-1 states (d(Ru) -> pi(phen*), pi(phen/dmbp*)) calculated at 445 nm, 436 nm, 423 nm and 418 with significant oscillator strengths contribute to the band centered at 449 nm the spectrum of [Ru(phen)(2)(dmbp)](2+), whereas three (MLCT)-M-1 (d(Ru) -> pi(phen)*, pi(phen/tpy)*) calculated at 419 nm and 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)(pby)](2+) does not differ drastically from the spectrum of the analogous dmbp species with four intense, slightly blue-shifted (MLCT)-M-1 (dRu -> pi(phen)*, pi(phen/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 (d(Ru) -> pi(tpy/dmp)*, pi(dmp)*, pi(tpy)*) 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 center ((MC)-M-3) excited states between 400 nm and 350 nm. (C) 2008 Elsevier B.V. All rights reserved.