Regiocontrolled Synthesis of Poly[(p-phenylene ethynylene)-alt-(2,5-thienylene ethynylene)]s:  Regioregularity Effect on Photoluminescence and Solution Properties.

Regioregular poly[( p -phenylene ethynylene)- alt -(2,5-thienylene ethynylene)]s (PPETEs) have been synthesized to evaluate the pure regioregularity effect on the physical properties of π-conjugated polymers. Comparison of their UV−vis and 13 C NMR spectroscopy confirms the absence of steric interaction between the adjacent substituents on thiophene rings. Although both polymers emit at the same wavelength, PPETE 3 with a pure head-to-tail chain sequence exhibits a slightly stronger fluorescence from its solution than PPETE 4 with a head-to-head chain sequence (φ fl = 0.59 for 3 vs 0.53 for 4 ). The Mark−Houwink constant α and radius of gyration R g values have been determined for PPETEs 3 and 4, both indicating that 3 has a more rigid conformation than 4 . The higher molecular rigidity for 3 might be partially responsible for its higher PL quantum efficiency. A significant enhancement in PL efficiency has been observed from PPETE films. Under identical experimental conditions, a film of 4 emits about 2.5 times stronger than 3 and 6 times stronger than the regiorandom PPETE, respectively.