On the nature of the fluorenone-based emission in oxidized poly(dialkyl-fluorene)s

Abstract

This study examines the underlying nature of the green emission band observed as a resultof oxidation in dialkyl-fluorene polymers. Specifically, we set out to further determinewhether an inter- (excimeric) or intra-molecular fluorenone-based excited state isinvolved. The emission properties of poly(9,9-dihexylfluorene) dispersed at lowconcentration in a solid polystyrene matrix are carefully explored. In situ, time-resolvedphotoluminescence measurements are made during photo-oxidation of the blend andduring subsequent exposure to an atmosphere saturated with the vapour of agood solvent. The polystyrene matrix suppresses the appearance of the greenemission band during oxidation but the subsequent solvent vapour exposure thenactivates it. The same effect (activation of the green emission) can be achieved bythermally annealing the matrix above its glass transition temperature. Moreover,the activation of the green emission can be reversed by dissolving the film andre-casting. This behaviour is attributed to controlling the phase structure of thepolyfluorene/polystyrene blend and is considered strong evidence for an origin of thegreen band emission in the formation of excimer-like states between co-faciallyarranged fluorenone moieties. The photoluminescence behaviour of 9-fluorenone andfluorene molecular mixtures in solution is also studied. This model system allowsanalysis of the green emission band independent of relative intra- and inter-molecularenergy transfer effects since this system is affected only by inter-molecular energytransfer. These results provide further evidence for an excimeric origin of the greenemission.