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.