Polymorphism and microcrystal shape dependent luminescence, optical waveguiding and resonator properties of coumarin-153

Abstract

Single dye based crystalline organic materials displaying molecular packing and conformation-dependent luminescent properties are attractive for opto-electronic and photonic applications. Hence, in this work the opto-structural analysis of two polymorphs (forms I and II) of a well-known coumarin-153 (C-153) dye is presented. Crystal structural analysis based on the syn or anti position of C-β1 and C-β2 carbons adjacent to nitrogen in the julolidyl moieties showed that form I and II polymorphs possess three and two molecular conformers, respectively. Optical excitation of single crystals of forms I and II radiated green and yellow fluorescence (FL) bands, respectively. The presence of different conformers, strong intermolecular π–π stacking, hydrogen bonding interactions, and charge transfer states was attributed to the energetically different emission bands of these polymorphs. Depending on the acetonitrile : water ratio, during self-assembly, C-153 formed polymorphic micro-crystalline rods, tubes and Japanese twin tubes on glass substrates. Analysis of the single-particle Raman spectra and FL life-time values of the rods, tubes and Japanese twin tubes clearly confirmed dissimilar molecular packing within the micro-solids. Single-particle microscopy studies revealed remarkable polymorphism-dependent FL emissions, optical waveguiding, and optical cavity and resonator effects of the rods, tubes and Japanese twin tubes. The yellow emitting hexagonal and rectangular tubes possibly supported whispering-gallery-modes (WGMs) and Fabry–Pèrot (FP) modes, respectively, while the blue-green emitting rods and Japanese twin tubes displayed FP-type cavity modes. Overall this work establishes a simple and economical self-assembly route for the fabrication of C-153 photonic micro-structures displaying polymorphism-dependent emission colours suitable for band-width tunable organic-based micro-lasers, opto-electronic and other polaritonic devices.