title: Conformational relaxation of p-phenylenevinylene trimers in solution studied by picosecond time-resolved fluorescence.
abstract: Two p-phenylenevinylene (PV) trimers, containing 3'-methylbutyloxyl (in MBOPV3) and 2'-ethylhexyloxyl (in EHOPV3) side chains, are used as model compounds of PV-based 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 decays of MBOPV3 and EHOPV3 reveal the presence of similar fast components, which are assigned to excited-state conformational relaxation of the initial population of non-planar 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)=aeta(o) (-alpha)exp(-alphaE(eta)/RT), where aeta(o) (-alpha) is the frequency factor, eta(o) is the pre-exponential coefficient of viscosity, E(eta) is the activation energy of viscous flow. The empirical parameter alpha, relating the solvent microscopic friction involved in the conformational change to the macroscopic solvent friction (alpha=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.
authors.affiliation: Centro de Química Estrutural, Departamento de Engenharia Química e Biológica, Instituto Superior Técnico, Av. Rovisco Pais, s/n. Lisboa 1049-001, Portugal. roberto.paolo@mail.ist.utl.pt
authors.forename: Roberto E
authors.initials: RE
authors.lastname: Di Paolo
authors.forename: João
authors.initials: J
authors.lastname: Seixas de Melo
authors.forename: João
authors.initials: J
authors.lastname: Pina
authors.forename: Hugh D
authors.initials: HD
authors.lastname: Burrows
authors.forename: Jorge
authors.initials: J
authors.lastname: Morgado
authors.forename: António L
authors.initials: AL
authors.lastname: Maçanita
mesh_terms: D019281:Dimerization; D004563:Electrochemistry / Q000379:methods; D004735:Energy Transfer; D008956:Models, Chemical; D008968:Molecular Conformation; D010777:Photochemistry / Q000379:methods; D011108:Polymers / Q000737:chemistry; D011145:Polyvinyls / Q000737:chemistry*; D012997:Solvents / Q000737:chemistry; D013050:Spectrometry, Fluorescence / Q000379:methods*; D013053:Spectrophotometry / Q000379:methods; D013696:Temperature; D013995:Time; D014783:Viscosity
chemical_list: D011108:Polymers; D011145:Polyvinyls; D012997:Solvents; C470024:poly(4-phenylenevinylene)