Multi-replica biased sampling for photoisomerization processes in conjugated polymers.

In recent years, conjugated polymers are attracting considerable interest in view of their light-dependent torsional reorganization around the pi-conjugated backbone, which determines peculiar light-emitting properties. Motivated by the interest in designing conjugated polymers with tunable photoisomerization pathways, we devised a computational framework to enhance the sampling of the polymer conformational space and at the same time estimate ground to excited-state free-energy differences. This scheme is based on a combination of Hamiltonian Replica Exchange (REM), Parallel Bias metadynamics, and free-energy perturbation theory. In our scheme, each REM replica samples different intermediate states connecting the ground to the first two excited states, which are characterized by TD-DFT simulations at the B3LYP/6-31G* level of theory. We applied the method on a 5-mer of poly(9,9-dioctylfluoren-2,7-diyl) and compared the results with the emission energies measured experimentally, showing a quantitative agreement with the prediction provided by our simulation framework.