Size-dependent color and luminescences of organic microcrystals

Perylene microcrystals with different sizes from about 50 nm to about 200 nm were prepared using the reprecipitation method. Their excitonic absorption peaks were found to shift to the high energy side with decreasing crystal size, that is to say, the shifts from that of the bulk crystal were about 500 cm^-1 and 1420 cm^-1 for 200 nm and 50 nm microcrystals, respectively. Moreover, strong fluorescence from the free-exciton energy level in the microcrystals was observed even at room temperature, which is not usually observed in bulk crystals.