(740f) Coarse-Grained Model of Exciton Dynamics on Long-Chain Conjugated Polymer System

Conjugated polymers are important components in optoelectronic devices such as light-emitting diodes, field-effect transistors, and photovoltaic cells. A comprehensive understanding of exciton dynamics in conjugated polymers is challenging, given the effects of electron-electron interactions , electron-nuclear coupling, and disorder on a wide range length scales—from molecular level up to the device scale—has on electronic and optical properties of polymers.

Here we present a new phenomenological model for simulating the dynamics of excitons in long-chain organic conjugated molecules. In our model, the polymer is described as a time-dependent array of ring-ring torsion angles. Torisonal angles define the conjugation environment that determines the position, size, and energy of the excitons. Exciton dynamics arise in direct response to the evolution of this torsional landscape along its excited state potential energy surface, which includes exciton-induced forces (such as those that lead to self-trapping). The framework for generating an accurate description of these heterogeneous excited state forces was developed based on the analysis of mixed QM/MM simulations known as QCFF/PI with a semi-empirical Pariser-Parr-Pople (PPP) Hamiltonian to describe the pi-electron system to accurately reproduce the local structure and dynamics of an atomistic long-chain polymer (e.g. polythiophene)

We show that this model can reproduce transient pump-probe experiments; we remark on the importance on the excited state force field when describing these systems. Then we go on to present molecular-level physical insights into exciton dynamics in these polymer materials, which have been previously speculative, to help better engineer organic solar cell devices.