Excited‐state Intramolecular Proton–transfer‐induced Charge Transfer of Polyquinoline

Abstract

The excited-state intramolecular proton-transfer-induced charge transfer of semirigid polyquinoline (PQH) is explored in 1,1,2,2-tetrachloroethane (TCE) and N-methyl-2-pyrrolidinone (MP) using picosecond time-resolved fluorescence spectroscopy. Reaction mechanisms are found to depend on the rotational conformations of PQH at the moment of excitation; whereas the trans-enolic form does not undergo intramolecular proton transfer within its excited-state lifetime, the cis-enolic form does within 15 ps to form a tautomeric zwitterion species. While the subsequent intramolecular charge transfer of the zwitterionic species to yield a tautomeric keto species takes place on time scales of 25 ps in TCE (ε = 8.50) and 62 ps in MP (ε = 32.55), its reverse reaction is also followed on time scales of 28 ps in TCE and 20 ps in MP. The lack of a kinetic isotope effect in both forward and reverse charge-transfer reactions support our proposed mechanisms.