Distance dependence of heterogeneous electron transfer probed in ultra-high vacuum with femtosecond transient absorption

Electron injection from the excited singlet state of the molecular chromophore perylene into the wide-band-gap semiconductor TiO₂ was investigated in ultra-high vacuum employing femtosecond transient absorption (fs-TA) spectroscopy. The chromophores were bound to saturated spacer-cum-anchor groups. The latter formed bonds on the surface of nm-structured anatase TiO₂ layers. Both the latter preparation step and the fs-TA measurements were performed in a special ultra-high-vacuum chamber that can accommodate solvents. The rate constants were measured in the so-called wide band limiting case where they are controlled only by the electronic coupling strength and not by Franck–Condon factors. Measurements with different saturated anchor-cum-spacer groups revealed an exponential dependence of the rate constants on the reaction distance. The distance parameter was 0.1 nm in ultra-high-vacuum. The shortest injection time of 13 fs was measured for the carboxyl anchor group and the longest of 4 ps for the long rigid so-called tripod anchor-cum-spacer group.