Effect of solvent aromaticity on poly(9,9-dioctylfluorene) (PFO) chain solution behavior and film condensed state structure

Abstract

Solvent media is crucial to the dynamic evolution process of conjugated polymer condensed state structures from solution state to film. Especially, aromatic solvent can arouse strong π-π interaction with conjugated polymer backbone. Poly(9,9-dioctylfluorene) (PFO) is a classic hairy-rod conjugated polymer. Chlorobenzene (CB) and toluene (Tol) were used as aromatic solvents, while tetrahydrofuran (THF) and chloroform (CF) were used as non-aromatic solvents. It was firstly found that PFO chain solution behaviors were closely connected to solvent aromaticity with broad concentration range from 0.005 mg/mL to 5 mg/mL. Owing to the bigger chain exclude volume in CB and Tol solvents, in dilute solution with PFO concentration ≤ 0.7 mg/mL, PFO single chains adopted more extended and rigid chain conformation than those in THF and CF solvents, thus, needed conjugated length of β conformation could be easily achieved, and β conformation could be firstly formed; PFO solution concentration increased to ≥ 1 mg/mL, chain aggregation began formation, but aggregation sizes were larger meanwhile chain packing density decreased according to the fractal dimension parameter df; Until PFO solution concentration increased to ≥ 3 mg/mL, only β conformation characteristic peaks began appearance in the all PL spectra. The different solution state behaviors directly affect film condensed structures. A fiber and ordered structure formed in films from CB and Tol solvents, meanwhile circular sheet and short rod-like morphology formed in films from THF and CF solvents, respectively. Solvent effect is significant not only to deeply understand the physical essence both condensed state structure formation and photoelectric performance enhancement but also to well control conjugated polymer condensed state structure to achieve photoelectric devices with high efficiency and stability.