Theoretical study on the electronic excitations of a porphyrin-polypyridyl ruthenium(II) photosensitizer

In this work, we investigated the UV-vis spectra of the Ru(bipy)(2)(MPyTPP)Cl](+) (MPyTPP = 5-pyridyl-15,20,25-triphenylporphyrin) complex and its related species Ru(bipy)(2)(py)Cl](+) and MPyTPP, by using time-dependent density functional theory and a set of functionals (B3LYP, M05, MPWB1K, and PBE0) in chloroform with the basis set 6-31++G(d,p) for nonmetal atoms and the pseudopotential LANL2DZ for Ru. Practically no geometrical changes are observed in the Ru environment when py ligand is replaced by MPyTPP. This replacement favors the electronic redistribution from bipy ligands to Ru, and from the metal to MPyTPP ligand, as indicated by NBO analysis. We found that M05 functional predicts very well the UV-vis spectra, as it shows a low deviation with respect to the experimental data, with a maximum error of 0.19 eV (11 nm). M05 theoretical electronic spectrum of Ru(bipy)(2)(MPyTPP)Cl](+) complex indicates that the presence of the Ru complex does not alter Q porphyrin bands, while charge transfer bands from Ru to bipy and porphyrin ligands mixes up in the region close to the porphyrin Soret band. Theoretical analysis allows the decomposition of this broad experimental band into specific ones identifying the Soret band and new metal to ligand charge transfers toward porphyrin at 425 and 478 nm, which were not possible in none of the moieties MPyTPP and Ru(bipy)(2)(Py)Cl](+) complex. In the UV region, the most intense intraligand band of bipy ligands becomes slightly blue-shifted both in the experimental and in the theoretical spectrum of Ru(bipy)(2)(MPyTPP)Cl](+) complex compared to that in Ru(bipy)(2)(py)Cl](+) complex. Some of the bands of Ru(bipy)(2)(MPyTPP)Cl](+) showed in this theoretical study may have practical applications. That is the case for the band at 478 nm, with potential use in PDT, and those more energetic at 348 and 329 nm, which could help in the cleavage mechanism of DNA performed by this ruthenium complex.