Polymorph- and molecular alignment-dependent lasing behaviors of a cyano-substituted thiophene/phenylene co-oligomer

Abstract

Four types of crystals with different molecular orientations and crystal morphologies (green platelet, yellow platelet, yellow fiber, and orange disk) consisting of 5,5′-bis(4′-cyanobiphenyl-4-yl)-2,2′-bithiophene (BP2T-CN), a type of thiophene/phenylene co-oligomer (TPCO), were obtained by changing the crystal preparation method. Each crystal exhibited high photoluminescence quantum efficiency values of 61–85%. For green- and yellow-emitting (platelet and fiber) crystals, lasing was observed from the crystal end facets acting as Fabry–Pérot (F–P) resonators. Strong exciton–photon coupling was demonstrated by energy-wavevector dispersion plots obtained from the energies of each interference peak in the mode structure of the F–P resonator. Moreover, a low lasing threshold of 17 μJ cm⁻² was achieved in the case of yellow-emitting fiber-like crystals, owing to the high absorption efficiency resulting from the lying-down molecular orientation in the crystal. For the orange-emitting crystal, whispering-gallery mode lasing was observed above 156 μJ cm⁻². This higher threshold resulted from the low absorption efficiency that is attributable to the 50° angle (slip angle) formed between the transition dipole moment and crystal basal plane ((100) plane). The crystal structure- and molecular alignment-dependent optical properties can be obtained by controlling the molecular packing and magnitude of intermolecular interactions.