An organic-inorganic hybrid material was prepared from a pentacyclo[9.5.1.1(3.9).1(5,15).1(7,13)]octasiloxane bearing eight coumarin groups. The coumarin groups were photodimerized by UV-light irradiation to connect the monomer units of the octasiloxane. Using a concentrated solution, the intermolecular photodimerization of these coumarin groups leaded to the cross-linking of the octasiloxane, fabricating an organic-inorganic hybrid material with a three-dimensional nanostructure. Whereas in dilute solution the intramolecular photodimerization was preferred to the intermolecular reaction, resulting in no formation of polymeric material.