On the Degradation Process Involving Polyfluorenes and the Factors Governing Their Spectral Stability

This study deals with an investigation of the spectral stability of differently structured polyfluorenes (PFs), deprived of 9-H defects, embodying 9,9-dialkylfluorene (P1), 9,9-diarylfluorene (P2), or 9,9-diarylfluorene/9,9- dibenzylfluorene units in a 1:1 alternating fashion (P3). Thermal annealing or UV irradiation carried out on films of P1-P3 in air revealed that their typical blue photoluminescence is invariably stained, independently of their 9-substitution, by the appearance of the low-energy band (g-band) pointing out a remarkable effect of light on the degradation process. A more comprehensive picture of the degradation pathway is proposed, including as key step a light-promoted formation of a PF radical cation generated by aerobic oxidation (photoluminescence test) or p-doping (cyclic voltammetry test). The blue emission of P1-P3 could successfully be preserved by dispersing them into a higher band gap matrix, such as polyvinylcarbazole (PVK), indicating a fundamental role of the intermolecular interactions between PF chains in the appearance of the low-energy emission band. Comparison between the optical behavior of suitably prepared PFs containing either fluorenone moieties (PFK) or 9-(bis-methylsulfanyl-methylene)fluorene moieties (PFS) holds regions of planarity within the PF backbone (inducing local intermolecular interactions) and not the fluorenone charge-transfer emission as responsible of the g-band of degraded PFs.