Dynamics of conformational relaxation in photoexcited oligofluorenes and polyfluorene

Hintschich, Susanne I.; Dias, Fernando B.; Monkman, A. P.

doi:10.1103/physrevb.74.045210

Cited by 47 publications

(69 citation statements)

References 42 publications

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“…[7] Alternative interpretations, also based on the biexponential nature of the decays, claim that Fçrster energy transfer dominates the dynamics of the decay process in MEH-PPV both in solution and solid state, but with interaction between different chains. [2] In the case of oligofluorenes [4,8] this interpretation was excluded on the basis of the strong dependence of the observed relaxation time on both solvent viscosity and temperature, which is compatible with conformational relaxation and incompatible with energy transfer.…”

Section: Introductionmentioning

confidence: 95%

Conformational Relaxation of p‐Phenylenevinylene Trimers in Solution Studied by Picosecond Time‐Resolved Fluorescence

Paolo¹,

et al. 2007

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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.

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Section: Introductionmentioning

confidence: 95%

Conformational Relaxation of p‐Phenylenevinylene Trimers in Solution Studied by Picosecond Time‐Resolved Fluorescence

Paolo¹,

et al. 2007

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“…Since we have to maintain hydrodynamic equilibrium a decrease by a in the shear modulus demands a compensatory increase in strain, also related to the fraction of occupied volume. (10)]. (10)].…”

Section: Viscosity Model: Pure N-alkanesmentioning

confidence: 99%

Model for Conformational Relaxation of Flexible Conjugated Polymers: Application to p‐Phenylenevinylene Trimers in Nonpolar Solvents

et al. 2013

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Photoexcitation of flexible conjugated polymers is invariably followed by a fast conformational/torsional relaxation towards a configuration favouring coplanarity of the conjugated segments. In general, the experimental relaxation rate constant (k(CR)) depends on the solvent viscosity (η) and temperature (T), and is not proportional to T/η. A theory capable of explaining the observed dependence of k(CR) on T and η over a wide range of these variables is not available. This gap is filled here by presenting a stochastic model that includes the participation of the oligomer side chain in storing and dissipating the stresses induced by photoexcitation. The model is able to account for the softening of solute-solvent interactions and its predictions are found to be in excellent agreement with the observed relaxation rate constants of a series of substituted p-phenylenevinylene trimers [ChemPhysChem 2009, 10, 448-454] on T, η and the size of the side-chains.

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“…82,83 Those have previously been investigated with oligomeric model systems in solvents of varying viscosity. For example, oligomeric phenylene(ethynylenes) undergo planarization in the excited state.…”

Section: Relaxation Of the Emission Spectrum Following Smentioning

confidence: 99%

Exciton Formation, Relaxation, and Decay in PCDTBT

et al. 2010

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Abstract:The nature and time evolution of the primary excitations in the pristine conjugated polymer, PCDTBT, are investigated by femtosecond-resolved fluorescence up-conversion spectroscopy. The extensive study includes data from PCDTBT thin film and from PCDTBT in chlorobenzene solution, compares the fluorescence dynamics for several excitation and emission wavelengths, and is complemented by polarization-sensitive measurements. The results are consistent with the photogeneration of mobile electrons and holes by interband π-π* transitions, which then self-localize within about 100 fs and evolve to a bound singlet exciton state in less than 1 ps. The excitons subsequently undergo successive migrations to lower energy localized states, which exist as a result of disorder. In parallel, there is also slow conformational relaxation of the polymer backbone. While the initial self-localization occurs faster than the time resolution of our experiment, the exciton formation, exciton migration, and conformational changes lead to a progressive relaxation of the inhomogeneously broadened emission spectrum with time constants ranging from about 500 fs to tens of picoseconds. The time scales found here for the relaxation processes in pristine PCDTBT are compared to the time scale (<0.2 ps) previously reported for photoinduced charge transfer in phaseseparated PCDTBT:fullerene blends (Phys. Rev. B 2010, 81, 125210). We point out that exciton formation and migration in PCDTBT occur at times much longer than the ultrafast photoinduced electron transfer time in PCDTBT:fullerene blends. This disparity in time scales is not consistent with the commonly proposed idea that photoinduced charge separation occurs after diffusion of the polymer exciton to a fullerene interface. We therefore discuss alternative mechanisms that are consistent with ultrafast charge separation before localization of the primary excitation to form a bound exciton.

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Dynamics of conformational relaxation in photoexcited oligofluorenes and polyfluorene

Cited by 47 publications

References 42 publications

Conformational Relaxation of p‐Phenylenevinylene Trimers in Solution Studied by Picosecond Time‐Resolved Fluorescence

Conformational Relaxation of p‐Phenylenevinylene Trimers in Solution Studied by Picosecond Time‐Resolved Fluorescence

Model for Conformational Relaxation of Flexible Conjugated Polymers: Application to p‐Phenylenevinylene Trimers in Nonpolar Solvents

Exciton Formation, Relaxation, and Decay in PCDTBT

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