Synthesis, Isolation, and Properties of All Head-to-Tail Cyclic Poly(3-hexylthiophene): Fully Delocalized Exciton over the Defect-Free Ring Polymer

The synthesis of defect-free cyclic poly(3-hexylthiophene) (P3HT) was achieved for the first time by the GRIM polymerization using an appropriate initiator, cyclization by a Pd-mediated end-to-end coupling reaction, and two isolation processes using functionalized resins by selective capture of the impurities by taking advantage of the discriminable end groups. Various properties were investigated for defect-free linear and cyclic P3HT with all head-to-tail couplings as well as those with inverted 3-hexylthiophene units formed through the conventional GRIM polymerization. M-n,M-SEC, M-w,M-SEC, R-h, and [eta] were significantly smaller for cyclic P3HT, and the topologies were visualized by STM. Remarkably, fluorescence spectroscopy revealed the essentially forbidden 00 transition with an extended lifetime for the defect-free cyclic P3HT arising from the circular symmetry, while cyclic P3HT with inverted units showed a strong 00 transition, which was similar to linear P3HT, due to the defects causing the symmetry breaking in the main chain. Thus, defect-free cyclic P3HT in the excited state was in a precise circular form with all coplanar thiophene rings and fully conjugated to follow the selection rules on polycyclic aromatic hydrocarbons. Moreover, defect-free cyclic P3HT showed effective stacking in the aggregate and the considerably prolonged half-lifetime of charge carriers.