Thermodriven Conformational Transition of Optically Active Poly[2,7-{9,9-bis[(<i>S</i>)-2-methyloctyl]}fluorene] in Solution

Abstract

We report the first solution state circular dichroism (CD) spectroscopic properties of a newly designed polyfluorene (PF) featuring highly enantiopure β-branched chiral alkyl side chains, poly[2,7-{9,9-bis[(S)-2-methyloctyl]}fluorene] (PFMO), in dilute THF solution. Upon cooling the solution temperature, the single positive-sign CD band at 380 nm at +50 °C gradually changed into an apparent bisignate CD signal with two extrema of 403 and 355 nm at −80 °C, while the broad UV−vis band centered at 380 nm changed into a major intense band at 399 nm with a shoulder around 380 nm. On the basis of these data combined with photoluminescence (PL) studies and viscometric measurement, we concluded that PFMO in THF solution underwent a certain thermodriven, order−disorder conformational transition around 0 °C. It was considered that, upon cooling, the randomly twisted local PF backbone motifs in the wormlike PF backbone at higher temperature were transformed into a highly ordered PF backbone motif, possibly a P-52 helix (the P-handedness is assumed here corresponding to the negative-sign CD band, and vice versa), and other unclear local backbone motifs with the opposite screw sense at lower temperature.