Hereditary Character of Alkyl-Chain Length Effect on β-Phase Conformation from Polydialkylfluorenes to Bulky Polydiarylfluorenes

Although alkyl chain engineering provides an effective tool for regulation of polymorphic behaviors of conjugated polymers, there is no report on extracting universal rule from complicated structure-property relationships among various molecule systems. Here, a series of bulky polydiarylfluorenes with incremental length of n-alkyl chains (PnDPF, n = 6, 7, 8, 9) was investigated, examining conformation, gelation and crystallinity behavior. By analyzing the UV-vis absorbance and photoluminescence (PL) spectra of PnDPF from single chains to the aggregate state, β-phase was found in P7DPF and P8DPF. P8DPF adopted the largest torsional angle inferring from the degree of red-shift in PL and Raman spectra. By thermal annealing at 220 °C, crystallization process was observed in these four polymers, but β-phase only appeared in P7DPF and P8DPF. Finally, both thermodynamic and kinetic paths were used to illustrate the mechanism of β-phase formation for PnDPF. Our results suggested that the alkyl-chain length effect on PnDPF exhibited the characters inheriting from the corresponding polydialkylfluorenes (PFn, n = 6, 7, 8, 9). By comparative study between PnDPF and PFn, an explicit relationship between the side-chain length and polymorphic behaviors is established for the first time in bulky polymer semiconductors.