Theoretical studies of the structures and spectroscopic properties of the photoelectrochemical cell ruthenium sensitizers, C101 and J13

A variety of heteroleptic ruthenium sensitizers have been engineered and synthesized because of their higher light-harvesting efficiency and lower charge-recombination possibility than the well known homoleptic N3 dye. As such, a great deal of attention has been focused on sensitizers with the general formula Ru(ancillary-ligand)(anchoring-ligand)(NCS)2, among which important examples are Ru(4,4′-bis(5-hexylthiophen-2-yl)-2,2′-bipyridine)(4,4′-carboxylic acid-4′-2,2′-bipyridine)(NCS)2 (C101) and Ru(N-(4-butoxyphenyl)-N-2-pyridinyl-2-pyridinamine)(4,4′-carboxylic acid-4′-2,2′-bipyridine)(NCS)2 (J13). In order to simulate experimental conditions with different pH values, the photosensitizing processes of these sensitizers possessing different degrees of deprotonation (2H, 1H to 0H) have been explored theoretically in this work. Their ground/excited state geometries, electronic structures and spectroscopic properties are first calculated using density functional theory (DFT) and time-dependent DFT (TDDFT). The absorption and emission spectra of all the complexes in acetonitrile solution are also predicted at the TDDFT (B3LYP) level. The calculated results show that the ancillary ligand contributes to the molecular orbital (MO) energy levels and absorption transitions. It is intriguing to observe that the introduction of a thiophene group into the ancillary ligand leads directly to the increased energy of the absorption transitions in the 380–450 nm region. The calculations reveal that although deprotonation destabilizes the overall frontier MOs of the chromophores, it tends to exert a greater influence on the unoccupied orbitals than on the occupied orbitals. Consequently, an obvious blue shift was observed for the absorptions and emissions in going from 2H, 1H to 0H. Finally, the optimal degree of deprotonation for C101 and J13 has also been evaluated, which is expected to lead to further improvements in the performance of dye-sensitized solar cells (DSSCs) coated with such sensitizers.