On October 20, the top-tier chemistry journal Journal of the American Chemical Society (JACS) published online a research paper by Professor Dong Shaoqiang's team from Tianjin University, a user of the National Pulsed High Magnetic Field Science Center. The paper is titled "Deciphering Spin-Governed Dual Emissions in TTM Diradicaloids: Unraveling the Interplay of Singlet and Triplet Fluorescence Pathways". This research, leveraging the Center's self-developed pulsed high magnetic field magneto-spectroscopy experimental platform, achieved a significant breakthrough in understanding the luminescence physical mechanisms of organic diradical molecules. 

In recent years, organic luminescent radicals have rapidly developed as a new class of open-shell luminescent materials. Triarylmethyl radicals represented by TTM and PTM have been successfully applied in organic light-emitting diodes (OLEDs), achieving efficient doublet emission. However, most diradical materials still suffer from low luminescence efficiency and unclear luminescence mechanisms. Recently, several research groups domestically and internationally have developed diradical systems with near-infrared luminescence capabilities and proposed new mechanisms such as ionic excited state luminescence, symmetry-breaking charge-transfer luminescence, and excimer luminescence. Nevertheless, the intrinsic photophysical mechanisms of luminescent diradicals remain incompletely elucidated. Currently, the singlet state of most diradicals either does not luminesce or has low efficiency, and their non-radiative decay pathways are still unclear. Simultaneously, understanding of the evolution pathways of spin transition processes between the singlet and triplet states in diradicals is still insufficient, lacking a comprehensive theoretical framework analogous to the traditional Jablonski diagram, which greatly hinders in-depth understanding of the photophysical processes in these materials.

In the figure, (a) and (b) show the fluorescence spectra of the singlet diradical molecules DTA and DTA(*t*-Bu)₂ under different magnetic fields at 100 K, respectively. (c) is a schematic diagram of the energy levels of the S₀ and T₀ states for DTA-type diradical molecules as a function of temperature and magnetic field.

Addressing this issue, Professor Dong Shaoqiang's team designed and synthesized the monoradical MTA and the diradicals DTA and DTA(t-Bu)₂ based on the TTM skeleton. Using techniques such as single-crystal X-ray diffraction and variable-temperature electron paramagnetic resonance, they revealed for the first time the coexistence of singlet and triplet ground states in the diradical system at room temperature. Based on the energy gap between them (-0.34 kcal/mol), the population distribution in the singlet (approx. 64%) and triplet (approx. 36%) ground states was calculated via the Boltzmann distribution. This unique ground state distribution endows the material with rare dual-channel fluorescence emission: high-efficiency triplet fluorescence dominates in the high-temperature region (maximum PLQY up to 50%), while singlet fluorescence in the near-infrared region appears in the low-temperature region. Through magneto-spectroscopy measurements under 0-44 T pulsed high magnetic fields, the team observed significant magnetic field modulation of the luminescence around 100 K, confirming the synergistic regulatory effect of temperature and magnetic field on the S₀→T₀ spin conversion. Further heavy-atom solvent effect experiments and theoretical calculations revealed the unique intersystem crossing process and excited-state structural evolution in the diradicals. This study establishes, for the first time, a photophysical picture distinct from the traditional Jablonski diagram for luminescent diradical systems, providing key experimental evidence and theoretical support for addressing the scientific challenge of the "unclear luminescence mechanism in diradicals," laying a solid foundation for developing new types of efficient high-spin luminescent materials.

This research utilized the extreme magnetic fields and variable temperature spectroscopic measurement capabilities of the Pulsed High Magnetic Field Facility, successfully observing the magneto-optical effects in the diradical material under high magnetic fields. This key experimental data directly revealed the synergistic regulatory mechanism of temperature and magnetic field on the material's spin states, providing decisive evidence for elucidating the dual-emission mechanism of the diradical.

Paper link: https://pubs.acs.org/doi/10.1021/jacs.5c10608