Multi-replica biased sampling for photoswitchable π-conjugated polymers.

Area of Science:

• Materials Science
• Computational Chemistry
• Polymer Physics

Background:

• π-conjugated polymers exhibit unique light-emitting properties due to light-induced torsional reorganization.
• Designing polymers with tunable photoswitchable pathways is of significant interest.
• Understanding the dynamics of torsional conformational space is crucial for controlling these properties.

Purpose of the Study:

• To devise a computational framework for enhanced sampling of torsional conformational space.
• To estimate ground- to excited-state free-energy differences in π-conjugated polymers.
• To investigate the photoswitchable pathways and conformational changes upon irradiation.

Main Methods:

• Combination of Hamiltonian Replica Exchange Method (REM), parallel bias metadynamics, and free-energy perturbation theory.
• REM simulations sample intermediate unphysical states between ground and excited states.
• Time-dependent density functional theory (TD-DFT) simulations at B3LYP/6-31G* level for state characterization.

Main Results:

• A computational framework was developed and applied to a 5-mer of 9,9-dioctylfluorene.
• Upon irradiation, a dihedral inversion from -155° to 155° was observed, with a decreasing energy barrier in excited states.
• Excited states (S1 and S2) stabilize coplanar dihedrals, with potential trapping in local free-energy minima.

Conclusions:

• The developed computational framework accurately predicts photoswitchable pathways and conformational changes in π-conjugated polymers.
• The study reveals how excited states influence torsional dynamics and light-emitting properties.
• Simulation results show quantitative agreement with experimental emission spectra, validating the framework.