The Role of Charge Transfer and Linker States in the Magnetoluminescence of a Diradical with Two Spatially Proximate Triarylmethyl Units

Luminescent diradicals with magnetic field-sensitive emissive properties, aka magnetoluminescence ML, have gained great attention due to their potential for spin-optical manipulation technologies. Recent studies provide evidence of ML effects in covalently-linked triarylmethyl-based diradical emitters, ascribed to the modulation of the ground state singlet and triplet populations at sufficiently low temperatures. The ML response in these systems originates in the different emissive properties of the singlet and triplet excited states, which may display distinct emission wavelengths, intensities, and/or bandwidths, depending on the constituent radical and linker units. Understanding the mechanism behind a particular ML effect is crucial to control the read-out outcome and advance in the development of novel molecular diradicals with target ML properties. In this work, quantum chemical calculations are employed to elucidate the nature of the emissive states underlying the ML properties of the first reported ML-active diradical. This system comprises two m-PyBTM ((2,4-dichloro-3-pyridyl)bis(2,4,6-trichlorophenyl)methyl) units covalently linked through a THDBA (5,6,8,9-tetrahydro-7-phenyldibenz[c,h]acridine) bridge, specifically designed to bring the radicals into close proximity while suppressing through-bond spin exchange interactions. Our results provide evidence of a through-space radical-to-radical charge transfer interaction in the low-lying singlet exciton of this system and unravel the non-innocent role of the bridge in tuning its ML response. These findings contribute to the collective effort to understand and control ML in molecular systems.