Chain morphologies in semi-crystalline polyfluorene: evidence from Raman scattering

Organic semiconductors, such as short-chain oligomers and long- chain polymers, are now a core constituent in numerous organic and organic-inorganic hybrid technologies. Polyfluorenes (PF) have emerged as attractive alternatives to other polymers due to their strong blue emission and high charge carrier mobilities. Almost all PF derivatives utilize side-chain substitution that improves solution processing and confers new functionality. There are many ambiguities regarding structure- property relationships in various side-chain substituted PFs. Di-octyl substituted PF (PF8) is known for its mesomorphic behavior, multitude of crystalline phases, and several conformational isomers that depend on the torsional angle between monomer units. In particular, the $\beta$ phase that represents a more planar backbone conformation dominates the optical emission although it appears as a minority constituent. We present Raman scattering studies of PF8 as a function of thermal cycling, which establishes a connection between the conformational isomers and the side and main chain morphology. Density-functional calculations of the vibrational spectra of single chain oligomers in conjunction with the experimental results demonstrate the incompatibility of the $\beta$ phase with the overall $\alpha$ crystalline phase in PF8.