Conformational change, intrachain aggregation and photophysical properties of regioregular poly(3-octylthiophene) in alkanes

Abstact This study explores the role of segmental solubility of regioregular poly(3-octylthiophene) (rr-P3OT) on chain organization and its photophysical properties. In good solvent chlorobenzene (CRB), rr-P3OT chain adopts an extended conformation, allowing long conjugation length of π-electrons. Cyclohexane is a good solvent for octyl side chain but a poor solvent for the thiophene backbone. The selective segmental interactions of rr-P3OT with this solvent induce conformational change of the polymer. Addition of cyclohexane into the CRB solution leads to chain coiling, which in turn causes significant decrease of the conjugation length. Absorption and photoluminescence spectra of the rr-P3OT in cyclohexane exhibit a blueshift of about 16 nm compared to those of the CRB solution. The change of chain conformation is also detectable by monitoring the variation of quantum yield upon increasing cyclohexane ratio. The quantum yield drops from 0.17 ± 0.01 to 0.11 ± 0.01 when the extended rr-P3OT chain transforms into coiled conformation. Hexane is a nonsolvent for rr-P3OT due to its relatively low solubility parameter. The addition of hexane into rr-P3OT solution in cyclohexane forces dense packing of thiophene rings within the coiled chain. An intrachain aggregation occurs in this system, leading to the appearance of three distinct redshift peaks in absorption spectra and the drastic drop of quantum yield. Correlation between the growth of redshift peaks and the decrease of quantum yield is clearly observed.