A Müller‐Type Diradicaloid With Efficient Spin‐Allowed Emission for X‐Ray Scintillation

ABSTRACT Limited exciton utilization efficiency—primarily due to spin‐forbidden triplet transitions—remains a fundamental challenge in organic optoelectronics. Unlike conventional closed‐shell luminophores, open‐shell diradicaloids inherently overcome this limitation by enabling spin‐allowed emission from both singlet and triplet excited states. However, achieving high luminescence efficiency in diradicaloids remains challenging due to competing nonradiative decay pathways and structural instability. Herein, we report a super‐stable and highly luminescent Müller‐type diradicaloid, TPT‐CzPh, which achieves a record photoluminescence quantum yield (PLQY) of 16.9%—an order‐of‐magnitude improvement over the prototypical Müller hydrocarbon TTM‐PhTTM. Comprehensive spectroscopic and theoretical analyses attribute the enhanced emission to a rationally engineered donor–acceptor framework that enables efficient spin‐allowed transitions and suppresses nonradiative decay via minimized structural reorganization. Remarkably, TPT‐CzPh also exhibits pronounced X‐ray‐induced scintillation, representing the first luminescent diradicaloid functioning as a metal‐free organic scintillator. This dual achievement not only deepens understanding of excited‐state dynamics in diradicaloids but also bridges open‐shell photophysics with radiation detection, paving the way for next‐generation high‐efficiency optoelectronic materials.