Femtosecond two-photon photoemission probing electron injection from the excited singlet state of perylene attached to a long rigid tripod anchor-cum-spacer on rutile TiO2(110).

Electron injection from the excited singlet state of the molecular chromophore perylene into the rutile single crystal of the wide band gap semiconductor TiO₂ was investigated with two photon photoemission (2PPE) spectroscopy in ultra-high-vacuum. The chromophore was attached to the (110) surface via acidic anchors of a tripod spacer-cum-anchor group. The corresponding wet-chemistry preparation step was performed in a special ultra-high-vacuum chamber that can accommodate solvents. Two electron injection times, i.e., 700 fs and 7 ps, were obtained with about equal weight factors from the fit to the measured time-dependent decay of the excited singlet state of the chromophore. UPS and 2PPE data showed the energetic position of the donor state at 0.5 eV above the bottom of the conduction band of rutile (110). The latter position confirmed the validity of the so-called wide band limiting case, where electron transfer is controlled only by the strength of the electronic coupling and not in addition by Franck–Condon factors. The two different injection times were attributed to two different adsorption configurations with two different distances for the chromophore from the crystal surface. A plausible configuration for the largest reaction distance suggests 1 nm distance between the rim of the chromophore and the surface atoms. The latter distance was identified with the 7 ps injection time, whereas the 10-times shorter injection time was attributed to a non-identified second adsorption configuration with an about 0.25 nm shorter reaction distance. [ABSTRACT FROM AUTHOR]