We investigate the energetic relaxation and spatial localization of photoexcited states in conformationally disordered p-conjugated models, choosing poly(para-phenylene vinylene) as a model system. Assuming vertical excitations, the initial photoexcited eigenstates are obtained via the disordered Frenkel model. The subsequent relaxation and localization of the excited statesis determined via the disordered Frenkel-Holstein model coupled to a dissipative environment. In particular, we solve the Lindblad master equation via the time-evolving block decimation (TEBD) and quantum jump trajectory methods.
The values of the model parameters physically relevant to polymer systems naturally lead to a separation of time scales, with the ultra-fast dynamics corresponding to energy transfer from the exciton to the internal phonon modes (i.e., the C-C bond oscillations), while the longer time dynamics correspond to damping of these phonon modes by the external dissipation. Associated with these time scales, we investigate the following processes that are indicative of the system relaxing onto the emissive chromophores of the polymer: 1) Exciton-polaron formation occurs on an ultra-fast time scale, with the associated exciton-phonon correlations present within half a vibrational time period of the C-C bond oscillations. 2) Exciton decoherence is driven by the decay in the vibrational overlaps associated with exciton-polaron formation, occurring on the same time scale. 3) Exciton density localization is driven by the external dissipation, arising from ‘wavefunction collapse’ occurring as a result of the system-environment interactions. Finally, we show how fluorescence anisotropy measurements can be used to investigate the exciton decoherence process during the relaxation dynamics.