A photoreactive composition comprises (a) at least one reactive species that is capable of undergoing an acid- or radical-initiated chemical reaction; and (b) a photoinitiator system comprising photochemically-effective amounts of (1) at least one type of semiconductor nanoparticle quantum dot that has at least one electronic excited state that is accessible by absorption of two or more photons, and (2) a composition, different from said reactive species, that is capable of interacting with the excited state of the semiconductor nanoparticle quantum dot to form at least one reaction-initiating species.