Crystalline confinement leads to broadening of absorption spectra through activated spin-forbidden transitions in Alq3-Irppy2acac engineered crystals

Abstract

In organic materials, triplet excitons can be directly generated from the ground state through spin-forbidden excitations. These excitations can be realized by efficiently mixing “bright” singlet states with strong transition dipole moments (TDMs) and “dark” triplet states through spin–orbit coupling (SOC). The primary challenge for small-molecule organic materials is achieving this mixture within single molecules. However, most of the previously demonstrated spin-forbidden excitations were generally extremely weak. In this study, we explore a different mechanism where the TDM and SOC originate from neighboring molecules. Specifically, we selected 8-hydroxyquinoline aluminum (Alq₃) as a fluorescence molecule to provide the TDM and a phosphorescent Ir complex (Irppy₂acac) to provide the SOC. We doped Alq₃ into the crystal of Irppy₂acac to generate strong intermolecular interactions, which couple SOC and TDM. Under this material design, spin-forbidden excitation was activated. The photo-excitation range was significantly extended and new excitation peaks were observed. Correspondingly, room-temperature long-wavelength phosphorescence induced by the activation of the spin-forbidden excitation was also observed. Excited-state species of the Alq₃-Irppy₂acac supramolecular system were identified and details of the intermolecular coupling characteristics of SOC and TDM were investigated using a theoretical model. The proposed photoexcitation pathway sufficiently extends the material's absorption range at longer wavelengths due to the low-energy triplet excited states and has a potential for use in solar cell applications.