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

Qing Jiang Pan, Yuan Ru Guo, Li Li, Samuel O. Odoh, Hong Gang Fu, Hong Xing Zhang

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17 Scopus citations

Abstract

Scalar relativistic density functional theory (DFT) has been used to explore the spectroscopic and redox properties of Ruthenium-type photovoltaic sensitizers, trans-[Ru( RL)(NCS) 2] ( RL = 4,4′′′-di-R-4′,4′′-bis(carboxylic acid)-2,2′:6′,2′′:6′′,2′′′ -quaterpyridine, R = H (1), Me (2), tBu (3) and COOH (4); RL = 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 CN-Ru angle representing the SCN → Ru coordination approaches 180° 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 π* 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), [NH 4][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).

Original languageEnglish (US)
Pages (from-to)14481-14489
Number of pages9
JournalPhysical Chemistry Chemical Physics
Volume13
Issue number32
DOIs
StatePublished - Aug 28 2011
Externally publishedYes

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