Sensitization of nanocrystalline TiO₂ films with carboxy-functionalized bis(indolyl)maleimide

The immobilization on a semiconductor surface of a bis(indolyl)maleimide functionalized with two carboxylic acid groups by alkylation of the indole nitrogen atoms is presented and its synthesis is described. The compound, 3,4-bis[1-(carboxymethyl)-3-indolyl]-1H-pyrrole-2,5-dione is strongly colored and emissive and the imide part can coordinate an oxidizable substrate. Its absorption and emission spectra on TiO₂ are substantially changed as compared to the spectra obtained in neat acetonitrile, indicating surface bonding through the carboxy groups. The quenching of the fluorescence of the sensitizer by the TiO₂ surface is almost complete, reflecting the high degree of association between the TiO₂ and the dye, fast charge injection and good electronic coupling between the sensitizer and the semiconductor. Nanosecond transient absorption spectra of the free sensitizer and of TiO₂ surface bound sensitizer are recorded and compared. While the free chromophore in neat acetonitrile shows a transient absorption spectrum that decays on the nanosecond timescale (like the emission), the transient absorption spectra of the sensitized TiO₂ film show a band at 360 nm, and a decay on the microsecond time scale. This is assigned to a slow recombination reaction of the charge-separated state. The properties discussed indicate that our system can be considered as a model compound for the development of photocatalysts immobilized on surfaces.