Molecular Packing versus Strength and Effective Mass of the Emitting Exciton of β-1,1,4,4-Tetraphenyl-1,3-butadiene

Abstract

Molecular packing in molecular crystalline solids and aggregates influences both the redistribution of oscillator strength between the excitons involved in optical transitions and also their effective masses. 1,1,4,4-Tetraphenyl-1,3-butadiene is taken into consideration because it is often used in light-emitting solid-state devices and it is also a promising material for solid-state laser applications, owing to the observed amplified spontaneous emission at room temperature of its β polymorph. This paper concerns a combined experimental and computational study of the photoluminescence emission of the β phase. The emission is demonstrated to originate from the lowest-energy exciton, which takes almost the whole available oscillator strength from the four molecules in the unit cell, and it is also the heaviest one in terms of effective mass. On the basis of the balance between nuclear relaxation energy, free-exciton Davydov splitting, and also exciton effective mass, as discussed in the framework of the recent reclassification reported in J. Chem. Phys. 2007, 127, 184703, the exciton–phonon coupling turns out to be effective in limiting the delocalization of this heavy exciton and in influencing the phonon progression in the emission spectra.