Photon-echo studies of collective absorption and dynamic localization of excitation in conjugated polymers and oligomers

Investigations of the origins of line broadening and excited state dynamics for the conjugated polymer poly[2-methoxy,5-(2-ethyl-hexoxy)-1,4- phenylenevinylene] (MEH-PPV) and a model pentamer, p-bis{[o,m-di(2-ethylhexy) oxy-p-methylstyryl]styryl}benzene are reported. The time-integrated three-pulse stimulated echo peak shift (3PEPS) experiment is employed to elucidate dephasing, spectral inhomogeneity arising from conformational disorder, and dynamical processes, otherwise obscured by ensemble averaging. We progressively discuss three dynamical models to describe the experimental data. The multiphonon model describes coupling of the electronic transitions to high frequency vibrational modes, and is able to fit the absorption spectra well, highlighting the importance of a distribution of conjugation length. However, it fails to model the 3PEPS data. A two-level system approach is found to reproduce the absorption line shapes as well as 3PEPS data, however, it cannot simultaneously describe the fluorescence data since the homogeneous linewidth is grossly overestimated. In light of these analyses, we propose the three-stage relaxation model, that (1) describes absorption into delocalized states that arise from electronically coupled conformational subunits; (2) explains the fast decay of the 3PEPS data as a rapid dynamic localization of excitation; and (3) provides a homogeneous line broadening that is consistent for both the absorption and fluorescence processes. Simultaneous modeling of the 3PEPS, absorption, and fluorescence data, establishes a consistent picture to understand the line broadening, dephasing mechanisms, and excited state dynamics for conjugated polymers and oligomers.