Exciton trapping with a twist.

In electron donor-acceptor (D-A) molecules, the relative orientation of constituents has a dramatic influence over their performance. However, the D and A subunits are generally composed of planar aromatic backbones, and the effect of curvature is rarely explored. Here, we investigate how the twist of the aromatic core of a symmetric double-branched D-π-A molecule affects the nature and dynamics of its lower singlet excited state. We show that the twisting of the central donor not only affects the chiroptical properties, and increases the triplet yield, but also facilitates excited-state symmetry breaking (ESSB) and the trapping of the exciton on one D-π-A branch of the molecule. This enhancement is attributed to the decrease in the interbranch coupling upon distortion. Because of this, the loss of the coupling upon ESSB requires a smaller gain in solvation energy to be compensated for and, thus, exciton trapping occurs in a less polar solvent. Consequently, distortion can be viewed as an additional tuning knob for controlling the localisation of electronic excitation in large conjugated systems.