Size-dependent optical properties of 9,10-bis(phenylethynyl)anthracene crystals

The optical properties of 9,10-bis(phenylethynyl)anthracene (BPEA) were studied for various states: nanoparticles (~100 nm), microcrystals (~1 μm), bulk crystals (~1 mm), and vacuum-deposited film. The absorption spectra showed a red shift from ethanol solution to bulk crystals with an increase in crystal size. The vacuum-deposited film seemed to have mixed spectral characteristics of nanoparticles and bulk crystals. The fluorescence of BPEA showed two peaks for the various crystals. The fluorescence of the shorter wavelength peak was shifted toward the blue from 556 to 527 nm from the bulk to nanoparticles. On the other hand, the position of the longest wavelength peak changed little, staying around 596 nm. The relative intensity of the former and latter increased with a decrease in the crystal size. The time-resolved fluorescence spectra clearly confirmed the existence of two emissive sites in the bulk crystals. These sites are probably located at the peripheral and inner positions of the crystals. Change in the relative proportions of the two sites could be the main reason for the size-dependent fluorescence.