Gain-narrowed emissions of thiophene/phenylene co-oligomer single crystals

In order to elucidate the origin of gain-narrowed photoluminescence spectra of 5,5 ''-bis(4-biphenylyl)-2,2':5',2 ''-terthiophene (BP3T) single crystals, the dependence of excitation energy threshold on excitation lengths was studied at 12 K. The emission peak of the 0-1 vibrational band was narrowed at a constant excitation density irrespective of the excitation length, indicating that the gain-narrowing followed a stimulated emission process. On the other hand, the excitation density threshold for the 0-2 peak decreased with increasing excitation length. The latter behaviour is suggestive of a cooperative interaction among uniaxially oriented transition dipoles in the BP3T crystal. [GRAPHICS] Fluorescence micrograph of a BP3T single crystal (left) and its photoluminescence spectral change taken at lambda(ex) = 457 nm, 12 K as a function of excitation density (right). (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim