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

摘要

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-beta 1 and C-beta 2 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 p-p 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 microsolids. 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-Perot (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.