Twist Angle Plays an Important Role in Photophysical Properties of a Donor-Acceptor-Type Conjugated Polymer: A Combined Ensemble and Single-Molecule Study

We investigated photophysical properties of a donor acceptor-type conjugated polymer by means of ensemble and single-molecule spectroscopy as well as density functional theory (DFT) calculation. The polymer is based on an alkyne-linked 1,8-carbazole (Cz) and possesses a benzothiadiazole (BT) as an electron acceptor moiety. A comparison with model compounds demonstrated that the dimer structure is the spectroscopic unit of the polymer. Single-molecule two-color excitation fluorescence and fluorescence lifetime experiments showed that the polymer molecules displayed broad distributions of fluorescence intensity ratio and fluorescence lifetime. Together with the DFT calculation, we demonstrated that the twist angle between the Cz and BT moieties played a central role in deciding those spectroscopic properties of the polymer. This result was further supported by single molecule fluorescence spectral measurement The spectral measurement also suggested intersegment interactions within the single chains. Furthermore, single molecule defocused and polarization fluorescence imaging suggested efficient exciton migration and trapping occurring within the single polymer chain. These experiments also revealed changes of the lowest energy site within the single polymer molecules.