Picosecond dynamics of an IR sensitive squaraine dye. Role of singlet and triplet excited states in the photosensitization of TiO2 nanoclusters

The photosensitization properties of a novel squaraine dye is probed using picosecond laser flash photolysis. The dye which exists in a protonation equilibrium with a pKa of 4.9 has a strong absorption in the IR region. The protonated form of the dye is highly fluorescent and is readily quenched in colloidal TiO2 suspensions. The association constant as determined from the quenching studies is 1.25×103 M−1. The singlet excited state of the protonated form of the dye participates in the charge injection process with a heterogeneous electron transfer rate constant of 8.3×109 s−1. The unprotonated form of the dye injects electrons from both the singlet and triplet excited state with electron transfer rate constants of 1×1010 s−1 and 1.25×109 s−1, respectively.