Femtosecond studies of exciton dynamics in a novel main chain chiral conjugated poly(arylenevinylene)

Abstract

The formation and decay dynamics of photogenerated excitons in an optically active poly(arylenevinylene), PAV, in solution have been studied using femtosecond transient absorption spectroscopy. Photoexcitation initially creates hot excitons which quickly (<200 fs) relax geometrically towards the equilibrium position in the excited state. The exciton subsequently decays following a double exponential with time constants of 6.5 and 420 ps in toluene. The decays become faster (5 and 250 ps) in pyridine, indicating a dependence of the relaxation process on the solvent environment. The fast decay is attributed to vibrational relaxation and internal conversion (recombination) of the exciton from the excited to the ground electronic state through tunneling or thermal-activated barrier crossing before thermalization. The slow decay is assigned to conversion of the thermalized exciton to the ground state through both radiative and nonradiative pathways. Anisotropy decay shows a fast component (6 ps in toluene and 10 ps in pyridine) and an offset which persists up to 650 ps. Possible explanations for the fast decay include internal conversion, vibrational relaxation, conformational change, and exciton migration. The offset may decay on a longer time scale through local reorientation of the conjugation segments, exciton migration, or rotational diffusion of the polymer. Comparison to a well-studied system, MEH-PPV [poly(2-methoxy, 5-(2-ethylhexoxy)-p-phenylenevinylene], provides further insight into the relaxation mechanism of photoexcitations in this PAV polymer.