Geometries and Terahertz Motions Driving Quintet Multiexcitons and Ultimate Triplet-Triplet Dissociations via the Intramolecular Singlet-Fissions

Importance of vibronic effects has been highlighted for the singlet-fission (SF) that convert one high-energy singlet exciton into doubled triplet excitons, as strongly correlated multiexcitons. However, molecular mechanisms of spin conversion processes and ultimate de-couplings in the multiexcitons are poorly understood. We have analyzed geometries and exchange couplings of the photoinduced multiexcitons in the pentacene dimers bridged by a phenylene at ortho and meta positions [denoted as o-(Pc)₂ and m-(Pc)₂] by simulations of the time-resolved electron paramagnetic resonance spectra. We clarified that terahertz molecular conformation dynamics plays a role on the spin conversion from the singlet strongly coupled multiexcitons ¹(TT) to the quintet state ⁵(TT). The strongly coupled ⁵(TT) multiexcitons are revealed to possess entirely planar conformations stabilized by mutually delocalized spin distributions, while the intramolecular de-coupled spin-correlated triplet pairs generated at 1 microsecond are also stabilized by distorted conformations resulting in two separately localized biradical characters.