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

Abstract

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.

Photophysical Study of Twop-Phenylenevinylene Oligomers End-Capped with Triphenylamine or Cyanoacetate Groups.

“…The α values, being lower than 1, indicate that the microscopic frictional force that hinders the conformational relaxation of the compounds is less effective than that predicted by the Stokes‐Einstein model, that is, a force proportional to the macroscopic solvent viscosity. A similar situation ( α < 1) has been observed before with different systems, and it was related with the fact that the viscosity is a macroscopic parameter. The solvent viscosity used to describe solute‐solvent friction processes does not take into account, for example, specific solute‐solvent interactions or structural differences of the evolved molecules, which affect the hydrodynamic boundary condition (stick‐slip) of a microscopic description of these systems.…”

Restricted Aggregate Formation on Tetraphenylethene-Substituted Polythiophenes

“…As listed in Table , the pre-exponential factors are positive when the decays are collected in the onset of the emission band, although when the emission decays are collected at lower energies (on the tail of the emission band), a negative amplitude (rise-time), associated with the shortest decay time (and in the case of P3HT also for the intermediate decay), is present. As described elsewhere, for different types of families of organic conjugated polymers and oligomers, the presence of a fast component, which appears as a decay time in the high energy part of the emission spectra and as a rise-time for low energies, is in general assigned to fast relaxation processes (excitation energy migration or conformational relaxation) in the excited state. ,,− For all the polymers, the pre-exponential factors associated with the slower decay time, τ 1 (see Table ), increases when going from the onset to the tail of the emission band and is assigned to the relaxed and more stable (lower energy) conformation of the polymers. An opposite behavior was found for the fastest decay component (and intermediary decay component in the P3HT case) where, respectively, a decrease and appearance of negative pre-exponential values were found when going to lower energies in the emission band.…”

“…23 Nevertheless, for homoPT and coPT, the sterically bulky TPE pendant groups hinders fast torsional molecular movements of the polymer backbone, thus explaining the absence of conformational relaxation for the homoPT and coPT polythiophenes. 35 To further understand the nature of the different dominant decay mechanism observed for P3HT in comparison with homoPT and coPT, the time-resolved behavior of the regioregular poly(3-dodecylthiophene-2,5-diyl), P3DDT, in toluene solution as a function of temperature was investigated. Again, three decay components (with decay times of 30, 190, and 520 ps at 293 K) were found when collecting the decay profiles along the emission band of the polymer, (see Table S4; and for the P3DDT detailed study and discussion, see the Supporting Information).…”

“…The higher activation energy found for P3DDT when compared to P3HT, together with the increase in the lifetime values associated with this process when going from P3HT to P3DDT, shows that the size of the lateral side chains (from -hexyl to -dodecyl with higher solute solvent interactions) slows down the conformational relaxation process. 35 Singlet Oxygen Measurements. Singlet oxygen sensitization quantum yields were measured by direct measurement of the NIR singlet oxygen phosphorescence emission at 1275 nm of aerated toluene solutions of the polythiophenes after laser irradiation at 355 nm.…”

“…Nevertheless, for homoPT and coPT, the sterically bulky TPE pendant groups hinders fast torsional molecular movements of the polymer backbone, thus explaining the absence of conformational relaxation for the homoPT and coPT polythiophenes …”