Structures, spectroscopic properties and redox potentials of quaterpyridyl Ru(II) photosensitizer and its derivatives for solar energy cell: a density functional study

Scalar relativistic density functional theory (DFT) has been used to explore the spectroscopic and redox properties of Ruthenium-type photovoltaic sensitizers, trans-[Ru(L-R)(NCS)(2)] (L-R = 4,4'''-di-R-4',4 ''-bis(carboxylic acid)-2,2' : 6',2 '' : 6 '',2'''-quaterpyridine, R = H(1), Me (2), Bu-t (3) and COOH (4); L-R = 4,4'''-di-R-4',4 ''-bis(carboxylic acid)-cycloquaterpyridine, R = COOH (5)). The geometries of the molecular ground, univalent cationic and triplet excited states of 1-5 were optimized. In complexes 1-4, the quaterpyridine ligand retains its planarity in the molecular, cationic and excited states, although the C=N-Ru angle representing the SCN -> Ru coordination approaches 180 degrees in the univalent cationic and triplet excited states. The theoretically designed complex 5 displays a curved cycloquaterpyridine ligand with significantly distorted SCN -> Ru coordination. The electron spin density distributions reveal that one electron is removed from the Ru/NCS moieties upon oxidation and the triplet excited state is due to the Ru/NCS -> polypyridine charge transfer (MLCT/L'LCT). The experimental absorption spectra were well reproduced by the time-dependent DFT calculations. In the visible region, two MLCT/L'LCT absorption bands were calculated to be at 652 and 506 nm for 3, agreeing with experimental values of 637 and 515 nm, respectively. The replacement of the R- group with -COOH stabilizes the lower-energy unoccupied orbitals of pi* character in the quaterpyridine ligand in 4. This results in a large red shift for these two MLCT/L'LCT bands. In contrast, the lower-energy MLCT/L'LCT peak of 5 nearly disappears due to the introduction of cycloquaterpyridine ligand. The higher energy bands in 5 however become broader and more intense. As far as absorption in the visible region is concerned, the theoretically designed 5 may be a very promising sensitizer for DSSC. In addition, the redox potentials of 1-5 were calculated and discussed, in conjunction with photosensitizers such as cis-[Ru(L-1)(2)(X)(2)] (L-1 = 4,4'-bis(carboxylic acid)-2,2'-bipyridine; X = NCS-(6), Cl-(7) and CN-(8)), cis-[Ru(L-1')(2)(NCS)(2)] (L-1' = 4,7-bis(carboxylic acid)-1,10-phenanthroline, 9), [NH4][Ru(L-2)(NCS)(3)] (L-2 = 4,4',4 ''-tris(carboxylic acid)-2,2' : 6',2 ''-terpyridine, 10) and [Ru(L-2)(NCS)(3)]-(11).