Photoinduced Electron Transfer in Squaraine Dyes: Sensitization of Large Band Gap Semiconductors

Squaraine dyes are known to photosensitize large band gap semiconductors via a charge-injection process from a singlet excited state. We have modeled this process with the simplest squaraine chromophore and titanium dioxide (TiO 2 ). A TiO 2 −squaraine excited-state complex has been optimized with an adjacent conical intersection. This suggests that the charge injection from the excited-state squaraine into the conduction band of the semiconductor should be an ultrafast photochemical process in agreement with recent experimental results. In terms of Marcus−Hush electron transfer theory, the charge transfer for the TiO 2 −squaraine model occurs in the inverted region, which is a strong indication for the equivalence between electron transfer in the inverted region and radiationless decay through a sloped conical intersection.