Chromophore Bending Controls Fluorescence Lifetime in Single Conjugated Polymer Chains.

Single-molecule spectroscopy of conjugated polymers offers unique insight into the interplay between the spatial arrangement of monomer units-twisting and bending-and the characteristics of the primary excitonic photoexcitation, provided that a single conjugated segment can be isolated. β-phase polyfluorene constitutes an ideal model to study variations in intermonomeric coupling, determined by nanoscale molecular shape, on the fundamental optical transition. If structural relaxation in the excited state is weak, exciton self-trapping occurs stochastically along the conjugated segment. Bending of the π-system is then revealed by a reduction in single-photon polarization anisotropy, correlating directly with increased fluorescence lifetime. Strong relaxation raises emission anisotropy because of deterministic exciton localization, decelerating fluorescence decay due to a decrease in exciton coherence length.