Excitation‐Wavelength‐Dependent Phosphorescence‐Fluorescence Switching Though Modulating Heavy Atom Effect

ABSTRACT Excitation‐wavelength‐dependent emission (Ex‐De) direct ultra‐long room temperature (URTP) materials have garnered significant attention due to their versatile applications. However, most organic materials are dominated by fluorescence. This work demonstrates bright direct phosphorescence emission via the combined effects of the heavy atom effect and a rational host‐guest doping strategy. The host molecule, dimethyl terephthalate (DTT), undergoes energy transfer and conformational changes between the ground and excited states, resulting in Ex‐De performance that enables reversible switching between URTP and fluorescence. Low‐temperature measurements confirm that the URTP originates from the phosphorescence emission of the guest molecules. Theoretical studies reveal that S‐Br dual‐substituted SBrCz exhibits significantly enhanced spin‐orbit coupling (SOC) and a high proportion of n‐π* transitions compared to the S‐S dual‐substituted SCz. These factors collectively contribute to a high phosphorescence quantum yield of up to 35% and an ultralong lifetime exceeding 450 ms. The Ex‐De properties endow the spray‐coated samples with excellent capabilities for anti‐counterfeiting, information encryption, and illumination applications. This work demonstrates an efficient and facile RTP molecular design and host‐guest doping strategy to achieve Ex‐De direct URTP through modulating of the heavy atom effect.