The organosilsesquioxane of claim 2, in which Y represents a radical of the general formula --Pv --(Bf -Er)c --R4 (VI) in which P is a radical selected from the group consisting of an oxygen atom, a sulfur atom, --Cd He --, where d is an integer of from 0 to 12 and e is an integer of from 0 to 24, and an N-alkylamino radical, v is an integer of from 0 to 12, E is a radical selected fro