Titanium dioxide visible light photocatalysis: surface association enables photocatalysis with visible light irradiation

Titanium dioxide (TiO₂) is a widely employed and inexpensive photocatalyst, but its use in organic synthesis has been limited by the short-wavelength ultraviolet irradiation typically used. We have discovered that TiO₂ particles efficiently mediate photocatalytic radical cation Diels–Alder cycloadditions using a simple visible light source, enabled by the formation of a visible light absorbing complex of the substrate on the semiconductor surface.