Crystalline matrix-activated spin-forbidden transitions of engineered organic crystals.

Spin-forbidden excitation is an efficient way to obtain triplet excitons directly from the ground state of organic semiconductors. According to perturbation theory under the framework of Fermi's golden rule, this process requires spin-orbit coupling (SOC) and the transition dipole moment (TDM) to be combined through an intermediate state that mixes the initial and final states. While previous research has focused mostly on enhancing SOC, little attention has been paid to engineering the coupling between SOC and the TDM in organic materials. In this study, a series of engineered crystals were designed by doping guest molecules into host organic crystals. The confinement of the guest molecule within a crystalline matrix of the host provides a strong intermolecular interaction to couple both SOC and the TDM. This in turn activates the spin-forbidden excitation directly from the ground state to a "dark" triplet state. Based on a comparison of different engineered crystals, strong intermolecular interaction is identified to induce a distortion of the ligands and further enhancing the spin-forbidden excitation. This work outlines a strategy for designing spin-forbidden excitation.