Sensitization of Nanocrystalline TiO₂ Initiated by Reductive Quenching of Molecular Excited States

An alternative mechanism for dye sensitization of wide bandgap semiconductors has been realized: reductive quenching of the dye excited state(s) followed by thermal interfacial electron transfer. The processes were identified using nanosecond-time-resolved absorption and photoluminescence spectroscopies after selective excitation of Ru(deeb)(bpy) 2 (PF 6 ) 2, where bpy is 2,2‘-bipyridine and deeb is 4,4‘-(COOC 2 H 5 ) 2 -2,2‘-bipyridine, sensitizers anchored to a nanocrystalline (anatase) TiO 2 film immersed in a 0.1 M tetrabutylammonium perchlorate acetonitrile solution with phenothiazine electron donors. With the electrolyte changed to 0.1 M lithium perchlorate, this same assembly undergoes the generally accepted mechanism for dye sensitization: the sensitizer excited state(s) transfer an electron to the semiconductor and is subsequently reduced by the phenothiazine donor.