Formation of singlet oxygen in solution by using Si nanocrystals as photosensitizers has been demonstrated. It has been shown that the absorption band of 1,3-diphenylisobenzofuran (DPBF) in benzene centered at 416 nm decreases by irradiating green (514.5 nm) or red (632.8 nm) light if fresh porous Si powder is dispersed in the solution. The decomposition of DPBF was observed only when fresh porous Si was irradiated by light, i.e., without light irradiation no effects were observed. Furthermore, the effect was drastically suppressed if porous Si powder was annealed and a monolayer of oxide was formed on the surface of nanocrystals. The rate of the decomposition of DPBF was accelerated when the solution was bubbled by oxygen gas. These results indicate that electronic excitation of Si nanocrystals is transferred to molecular oxygen dissolved in solution, resulting in the formation of singlet oxygen. Generated singlet oxygen reacts with DPBF (1,4-cycloaddition reaction), forming endoperoxides, which in turn decompose to yield irreversible products. In addition to the singlet-oxygen-mediated decomposition of DPBF, the possibility of direct reaction between triplet excited states of Si nanocrystals and DPBF is discussed. (C) 2004 American Institute of Physics.