Pathway-dependent electron transfer for rod-shaped perylene-derived molecules adsorbed in nanometer-size TiO2 cavities

A broad distribution of time constants was found for photoinduced heterogeneous electron transfer (PHET) from the excited-state of a perylene chromophore when the latter was attached via long rigid bridge/anchor groups to the inner walls of nanometer-size cavities formed in a colloidal anatase TiO2 layer. In contrast, in the same environment PHET was dominated by only one short time constant when the perylene chromophore was attached via a short anchor/bridge group. The same results were obtained irrespective of the specific chemical composition of the short or long rigid anchor/bridge groups. To verify that the set of different time constants was caused by different microscopic environments in the nanometer-cavities, PHET was also measured for the same perylene compounds on the (110) surface of TiO2 rutile single crystals, employing here the more sensitive femtosecond two-photon photoemission technique in place of transient absorption. On the surface of the single crystals only one long time constant was measured for PHET also in the case of the long rigid bridge/anchor groups. Thus, the broad distribution of time constants observed in the nanometersize cavities for the long rigid bridge/anchor groups can be attributed to different microscopic environments giving rise to different distances between the chromophore and the nearest TiO2 wall. Consequences of this pathway dependent PHET are discussed for the design of dye molecules and electrodes in dye-sensitized solar cells.