Conjugated Thiophene-Containing Polymer Zwitterions: Direct Synthesis and Thin Film Electronic Properties

We report a direct and facile synthesis of novel conjugated polymeric zwitterions (CPZs) as a simple route to electronically active homopolymers and copolymers containing dipole-inducing pendent zwitterions. Sulfobetaine-containing polythiophenes (PTSB-1 and PTSB-2) and alternating thiophene-benzothiadiazoles (PTBTSB-1 and PTBTSB-2) were prepared and characterized relative to alkylated polymer analogues (POT-a-T and POT-a-BT). The polar zwitterionic side chains make these polymers hydrophilic and salt-responsive, with interesting electronic properties that depend on zwitterion distance from the conjugated polymer backbone (tether length), as characterized by UV-vis absorption and ultraviolet photoelectron spectroscopy (UPS). Close proximity (CH2 spacer) of the sulfobetaine groups to the polymer backbone results in increased ionization potential and enlarged band gaps of 2.19 and 2.04 eV for PTSB-1 and PTBTSB-1, respectively. On Au and Ag surfaces, the zwitterionic pendent groups significantly alter the work function due to the presence of an interfacial dipole, with the largest interfacial dipoles measuring -1.29 eV (PTBTSB-1/Au) and -0.69 eV (PTBTSB-1/Ag).