  In recent years, pi-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
photoswitchable pathways, we devised a computational framework to enhance the
sampling of the torsional 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 an
intermediate unphysical state between the ground and the first two excited
states, which are characterized by TD-DFT simulations at the B3LYP/6-31G* level
of theory. We applied the method to a 5-mer of 9,9-dioctylfluorene and found
that upon irradiation this system can undergo a dihedral inversion from 155 to
-155 degrees crossing a barrier that decreases from 0.1 eV in the ground state
(S0) to 0.05 eV and 0.04 eV in the first (S1) and second (S2) excited states.
Furthermore, S1 and even more S2 were predicted to stabilize coplanar
dihedrals, with a local free-energy minimum located at +-44 degrees. The
presence of a free-energy barrier of 0.08 eV for the S1 and 0.12 eV for the S2
state can trap this conformation in a basin far from the global free-energy
minimum located at 155 degrees. The simulation results were compared with the
experimental emission spectrum, showing a quantitative agreement with the
predictions provided by our framework.
