Role of Side Chains in Charge Localization and Transfer in Azothiazol-Dithiophene Conjugated Copolymers

Abstract

Charge localization across conjugated polymers is one of the most important features to control their overall photophysical processes including the intermolecular charge transfer (CT) with molecular acceptors at interfaces. Herein, we synthesize two azothiazole (ATz) conjugated polymers coupled with a dithiophene (BT) donor using either esters or non-esters as side chains. The non-ester side chain polymer (P 1 ATz-BT) shows a strong emission quenching upon successful addition of 1,4dicyanobenzene (DCB), as a molecular acceptor, via intermolecular CT. On the other hand, the ester side chain polymer (P 2 ATzE-BT) does not exhibit such changes, indicating that the side chain has a significant impact on the photophysical processes of these polymers at the interface with molecular acceptors. Density functional theory (DFT) calculations and time-resolved photoluminescence measurements show that unlike P 1 ATz-BT, the introduction of ester chains causes significant steric effects on the conformation of polymer backbones and the localization of electron density distributions in P 2 ATzE-BT, which leads to the suppression of the charge transfer to the molecular acceptor. Our study provides the experimental and theoretical clues for understanding the tremendous impact of polymer side chains on photophysical processes with molecular acceptors, at interfaces. These findings pave the way toward controlling and optimizing the interfacial charge transfers in conjugated polymers.