Photophysical Studies of Beta Phase Formation in
Poly(9,9-di-n-alkylfluorenes)

Abstract

The photophysical changes that take place in Poly(9,9-di-n-alkylfluorenes) upon formation of the beta phase in methylcyclohexane solution are observed by optical spectroscopy. The equilibrium absorption spectra as a function of temperature show that conformational changes occur for all five polymers studied, from the hexyl (PF6) to decyl (PF10) side chains. The spectroscopic indicators of beta phase formation are not observed in PF6, and the trend of beta phase formation efficacy shows an optimal side chain length of 8 carbons. The beta phase formation in PF8, PF9 and PF10 is modelled using a previously reported aggregation model, with limited success.
A mechanism for the beta phase formation is proposed, where the interactions between the alkyl side chains provide the chemical energy to overcome the activation energy barrier to planarise the polymer backbone, leading to the extended conjugation length that characterises the beta phase. Excitation spectra show that the beta phase can occur reversibly in dilute solution, most likely by chain folding leading to side chain interactions. The presence of side chain interactions is confirmed by evidence of a PF7-PF9 alternating structure formed in a mixed solution.
The same trend of beta phase formation is observed in thin films of these polymers after thermal cycling and warm toluene vapour exposure, showing that side chain interactions are also required for beta phase formation in the solid state. Spectra of PF8 films with controlled keto content show that the energy transfer to the keto sites is mediated by migration, indicating that the beta phase is formed in domains rather than isolated chains, a result which is consistent with the side chain interaction model and other published results. The fraction of beta phase formed is shown to decrease linearly with a greater content of dibenzothiophene (DBT) co-monomer units, up to a cut-off limit of 20%. A statistical model of the distribution of DBT units in the chain is used to find a conjugation length of 9 monomer units, in contrast to a previous estimate but in agreement with the persistence length of PF8 in toluene.
These results characterise the beta phase formation mechanism and its effects on the photophysical properties of Poly(9,9-di-n-alkylfluorenes), which is under widespread investigation for more use in efficient blue and white organic LED applications.