Photoinduced fluorescence modulation through controllable intramolecular [2+2] photocycloaddition in single molecules and molecular aggregates.

We report a general molecular design strategy of spatial proximity, which allows intramolecular [2+2] photocycloaddition reaction to take place in both single molecules and molecular aggregates. Sharply contrasting photoinduced fluorescence changes in solution and in the solid state were found and attributed to the aggregation-induced quenching property of the monomers and the aggregation-induced emission nature of the photodimers.