Interfacial charge-transfer transitions between TiO2 and indole

Aromatic amines were reported to show visible light absorption by the adsorption on TiO2 nanoparticles, enabling visible light responsive photocatalytic reactions. However, the origin of the visible light absorption and the chemisorption structure have not been elucidated in detail yet. Here, we demonstrate experimentally and computationally that the visible light absorption in the TiO2-indole surface complex is attributed to interfacial charge-transfer transitions (ICTT) from the adsorbed indole to TiO2, supporting the chemisorption structure of indole via a Ti-N bond formed by the dehydration reaction with a surface hydroxy group. The ICTT originates from pi(indole)-d(pi)(Ti) electronic couplings through the Ti-N bond.