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

Mariagrazia Fortino1, Concetta Cozza1, Massimiliano Bonomi2

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Researchers developed a computational method to study how light changes the shape of π-conjugated polymers. This helps design polymers with controllable light-emitting properties by analyzing torsional reorganization.

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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.