Conformational relaxation of p-phenylenevinylene trimers in solution studied by picosecond time-resolved fluorescence

Two p-phenylenevinylene (PV) trimers, containing 3'-methylbutyloxyl (in MBOPV3) and 2'-ethylhexyloxyl (in EHOPV3) side chains, are used as model compounds of PV-bosed conjugated polymers (PPV) with the purpose of clarifying the origin of fast (picosecond time) components observed in the fluorescence decays of poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV). The fluorescence decoys of MBOPV3 and EHOPV3 reveal the presence of similar fast components, which are assigned to excited-state conformational relaxation of the initial population of nonplanar trimer conformers to lower-energy, more planar conformers. The rate constant of conformational relaxation k(CR) is dependent on solvent viscosity and temperature, according to the empirical relationship k(CR)=a eta(-a)(o).exp(-aE(eta)/RT), where a eta(-a)(o) is the frequency factor, eta(o) is the pre-exponential coefficient of viscosity, E 17 is the activation energy of viscous flow. The empirical parameter a, relating the solvent microscopic friction involved in the conformational change to the macroscopic solvent friction (alpha = 1 1), depends on the side chain. The fast component in the fluorescence decays of MEH-PPV polymers (PPVs), is assigned to resonance energy transfer from short to longer polymer segments. The present results call for revising this assignment/interpretation to account for the occurrence of conformational relaxation, concurrently with energy transfer, in PPVs.