Formation of the charge-transfer exciton in 1,3,5-triphenyl-2-pyrazoline nanocrystals

A series of 1,3,5-triphenyl-2-pyrazoline (TPP) nanocrystals were prepared by the reprecipitation method. The electronic transitions of TPP nanocrystals have been studied using both absorption and fluorescence spectra. An absorption feature at about 400 nm gradually appeared with increasing nanocrystal size. This feature resulted from the formation of the charge-transfer (CT) exciton in nanocrystals. It was also supported by the longer decay time of the nanocrystal emission from CT compared with that of the solution emission from S-1 at 298 K. On the other hand, the different behavior of the molecular pi-pi* and n-pi* transitions originates from the different overlapping modes of the pyrazoline pi orbital and n-electron orbital, according to the molecular model calculations. The nanocrystal emission from both the S-1 and CT states was found at 77 K; moreover, the emission intensity redistributed from S-1 to CT with increasing nanocrystal size. In contrast, at 298 K, only the emission from the CT states was observed, since the thermal fluctuation easily relaxes the excited electrons into CT states through vibration.