Excimer formation and site selectivity in single pyrene microcrystals

Abstract

Thin microcrystals of pyrene have been recrystallized by using a surfactant. With a size-range of several hundred nm to a few micrometers, the crystals have been characterized by using standard techniques of X-ray and Raman spectroscopy besides the atomic force and scanning electron microscopy. The fluorescence microscopy of single microcrystal shows fluorescence due to monomer as well as the red-shifted broad, structureless excimer-type emission. The observed decay profiles of the red-shifted emission reveal existence of dimer and higher aggregates in the crystalline state when excited at the red-edge of the absorption band. This is in contrast to the excimer emission observed at higher energy excitation range corresponding to the monomer absorption. Distinctly different nature of time-resolved decay profiles differentiates the two mechanisms. The observations have been used to propose existence of different sites of crystal packing under two different excitations (370 nm and 408 nm). The structural anisotropy of these crystals makes them useful as a probe in crystal structure based molecular and biological sensing.