Singlet exciton fission (SEF) is a key process for developing efficient optoelectronic devices. We have performed femtosecond electron diffraction experiments to directly probe the structural dynamics accompanying the SEF process in pentacene single crystals. The data reveal coherent atomic motions at 1 THz, incoherent motions, and an anisotropic lattice distortion representing the polaronic character of the triplet excitons. By combining real-time time–dependent density-functional theory, molecular dynamics simulations and experimental structure factor analysis, we have identified the coherent motions as collective motions of the pentacene molecules along their long axis. These long-range intermolecular motions heavily modify the excitonic coupling between adjacent molecules. In doing so, they efficiently neutralize the forces that keep the two triplet excitons together right after they have been generated, providing a possible explanation about the origin of the ultrafast timescales related to the fission.

Full publication: Seiler et al., Science Advances Vol. 7, no. 26, eabg0869 and link to press release.