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Conformational Relaxation of p ‐Phenylenevinylene Trimers in Solution Studied by Picosecond Time‐Resolved Fluorescence
Author(s) -
Di Paolo Roberto E.,
Seixas de Melo João,
Pina João,
Burrows Hugh D.,
Morgado Jorge,
Maçanita António L.
Publication year - 2007
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.200700548
Subject(s) - picosecond , fluorescence , time resolved spectroscopy , relaxation (psychology) , photochemistry , chemistry , materials science , chemical physics , crystallography , optics , physics , psychology , social psychology , laser
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 = aη o −α ⋅ exp(− αE η / RT ), where aη o −α is the frequency factor, η o is the pre‐exponential coefficient of viscosity, E η is the activation energy of viscous flow. The empirical parameter α , relating the solvent microscopic friction involved in the conformational change to the macroscopic solvent friction ( α =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.