Exciton self‐trapping in MEH‐PPV films studied by ultrafast emission depolarization

Abstract

Depolarization of the transient absorption with a time constant of <120 fs is observed in poly[2-(2'-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] (MEH-PPV) film. It is too fast to be explained by incoherent hopping. We attribute it to dynamic localization (self-trapping) of the initially delocalized excitons driven by structural relaxation of excited segments. Implications for charge carrier photogeneration are briefly discussed.1 Introduction Conjugated polymers with a strong electronic coupling along their backbone represent interesting model quasi-1D systems and are promising materials for light emitting and photovoltaic applications. Formation and dynamics of emitting electronic states (excitons) is therefore of particular interest. The extent of exciton delocalization along the conjugated chain, which is determined by electronic coupling, conformational defects and electron-phonon interaction, is still unresolved issue in these materials. Dynamic localization of the exciton (self-trapping) on six to ten repeat units has been theoretically predicted for poly(phenylene-vinylene) (PPV) as an outcome of structural relaxation [1][2][3]. Such a relaxation is likely to be the cause of the rapid decay of the three-pulse stimulated echo peak shift within 50 fs observed in a PPV derivative poly[2-(2'-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] (MEH-PPV) in solution [4], but no information on exciton localization length can be extracted from these data. The relaxation energy in the polymer was in fact very similar to that in a PPV pentamer, where delocalization of the primarily exciton is limited by pentamer length. Ultrafast vibrational coherence gives an indication of structural relaxation within 100 fs in another conjugated polymer polydiacetylene [5]. Some information on exciton size (maximal distance between electron and hole) is obtained from polarizabilities, which can be measured by electroabsorption in the Franck-Condon region (before structural relaxation) [6,7] and by GHz and THz conductivity for the relaxed excitons [8,9]. Polarizability increases with the length of conjugated oligomers [7,8], and thus, with the size of exciton. Electroabsorption experiments on MEH-PPV films showed that polarizability along the conjugated backbone of the non-relaxed excitons decreases with an increase of the photon energy, which generates the exciton, and ranges between 3000 and 12000 Å 3 for the photon energies of 2.2 and 2.1 eV [6]. The THz conductivity of relaxed excitons measured after excitation with 3.1 eV photons gives polarizability of ~2300 Å

Excited-State Self-Trapping and Ground-State Relaxation Dynamics in Poly(3-hexylthiophene) Resolved with Broadband Pump–Dump–Probe Spectroscopy

“…Theoretical and transient absorption/emission studies have presented arguments both for and against exciton localization by conformational disorder in amorphous, solution-phase thiophene systems. ,− The observed large dynamic Stokes shifts are generally attributed to either (1) EET between disordered conjugated segments and/or (2) self-trapping dynamics resulting in planarization and conjugation extension. ,, Self-trapping, as discussed here, is defined as a monomolecular structural and/or vibrational relaxation resulting in the energetic stabilization of an exciton. Exciton localization was previously considered to result from stochastic excited-state self-trapping, as has been observed in other polymers like MEH-PPV. , However, two-color three-photon echo peak shift (2C-3PEPS) experiments, performed by Blank and co-workers, found that the initial (<200 fs) exciton relaxation in low-concentration P3HT in chloroform is highly correlated and not the result of random dynamics. Instead, the initial Stokes shift is driven by coherent low-frequency torsional excited-state self-trapping, which reorganizes the polymer to lower the free energy of the exciton and creates a local minimum, or trap, on the excited-state potential energy surface.…”

Dynamics of Energy Transfer in a Conjugated Dendrimer Driven by Ultrafast Localization of Excitations

“…Dynamical localization of the wave function of a transient vibrationally hot excited state has been previously studied on π conjugated polymers. − Adiabatic vibrational relaxation of the lowest electronic excited state has been linked to the spatial localization (self-trapping) of an exciton strongly coupled to torsional and C–C nuclear motions. ,, The time-scale associated with such adiabatic process is relatively slow (ps) being defined by molecular vibrations and the flow of excess vibrational energy to the bath. Our previous measurements of the vibrational relaxation in several phenylene-ethynylene dendrimers yield a time constant of between 2 and 6 ps, while vibrational relaxation of the lowest excited state in polyfluorenes due to slow torsional motions can take up to tens of picoseconds resulting in a formation of self-trapped states …”

Vibrational fine structures revealed by the real-time vibrational phase and amplitude in MEH-PPV using few cycle pulses

“…[17][18][19] Pulse lasers with pulse durations of several tens to hundreds of femtoseconds were used to study MEH-PPV. [20][21][22] However, no study has been reported on the coherent molecular vibration induced by an impulsive excitation using a short enough pulse to excite coherent vibration.…”