Photoinduced electron transfer in perylene-TiO₂ nanoassemblies

The photosensitization effect of three perylene dye derivatives on titanium dioxide nanoparticles (TiO₂ NPs) has been investigated. The dyes used, 1,7-dibromoperylene-3,4,9,10-tetracarboxy dianhydride (1), 1,7-dipyrrolidinylperylene-3,4,9,10-tetracarboxy dianhydride (2) and 1,7-bis(4-tert-butylphenyloxy)perylene-3,4,9,10-tetracarboxy dianhydride (3) have in common bisanhydride groups that convert into TiO₂ binding groups upon hydrolysis. The different substituents on the bay position of the dyes enable tuning of their redox properties to yield significantly different driving forces for photoinduced electron transfer (P_eT). Recently developed TiO₂ NPs having a small average size and a narrow distribution (4 ± 1 nm) are used in this work to prepare the dye-TiO ₂ systems under study. Whereas successful sensitization was obtained with 1 and 2 as evidenced by steady-state spectral shifts and transient absorption results, no evidence for the attachment of 3 to TiO₂ was observed. The comparison of the rates of P_eT (k_PeT) for 1- and 2-TiO₂ systems studied in this work with those obtained for previously reported analogous systems, having TiO₂ NPs covered by a surfactant layer (Hernandez et al. [2012] J. Phys. Chem. B., 117, 4568-4581), indicates that k_PeT for the former systems is slower than that for the later. These results are interpreted in terms of the different energy values of the conduction band edge in each system.