We have designed tripyridine-thiolato (4,4',4"-tricarboxy-2,2':6',2"-terpyridine)ruthenium(II) [complex 1], a novel efficient sensitizing dye for dye sensitized TiO2 solar cells, based on the DFT MO calculations with PBE0 functional. Complex I is a modified BD (black dye: trithiocyanato (4,4',4"-tricarboxy-2,2' : 6',2"-terpyridine)ruthenium(II) complex) molecule where NCS ligands of BD are replaced by C5H4NS ligands. Molecular and electronic structures of complex I have been theoretically characterized. Complex I is expected to have the following two advantages over BD, in addition to the advantage of high electron transfer rate from the photoexcited dye to TiO2 realized in BD: (1) higher electron transfer rate from redox systems to oxidized dyes; (2) higher absorption efficiency to solar spectrum. We propose complex I as a novel efficient sensitizing dye which provides the higher efficiency than does BD for dye sensitized solar cells. (C) 2004 Elsevier B.V. All rights reserved.