Abstract

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)(CH 3CN)] 2+ and [Ru(tpy)(dmp)(CH 3CN)] 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)(CH 3CN)] 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 ( d Ru → π phen ∗ , π phen / 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 ( d Ru → π phen ∗ , π phen / tpy ∗ ) calculated at 419 nm, 378 nm and 374 nm contribute to the band observed in the spectrum of [Ru(tpy)(phen)(CH 3CN)] 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 ( d Ru → π phen ∗ , π phen / bpy ∗ ) states, calculated at 432 nm, 429 nm, 408 nm and 395 nm. The theoretical spectrum of [Ru(tpy)(dmp)(CH 3CN)] 2+ is more compact with close-lying 1MLCT states ( d Ru → π tpy / dmp ∗ , π dmp ∗ , π 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 centred ( 3MC) excited states between 400 nm and 350 nm.

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