Visible Organic Thermoluminescence above Room Temperature in Small Molecule Crystals

Thermoluminescence (TL) is commonly observed in inorganic luminescent materials at elevated temperatures, with applications in radiation dosimetry, archaeology, geological dating, and defect studies. However, the understanding of TL in organic small molecules above room temperature is still limited. Here, we experimentally discovered and theoretically analyzed the unusual TL exhibited by organic scintillation crystals in air. It exhibits an onset temperature of ~300 K, reaching its maximum intensity at 348 K (heating rate of 2 K s-1). Upon 365 nm ultraviolet light excitation at room temperature, the material gradually released stored energy through phosphorescence, achieving a record-breaking organic afterglow lasting over 10 hours. The exceptional luminescence originates from excited states generated by thermally driven back electron-transfer (BET). Crucially, this work reveals that certain instances of previously reported ultralong organic afterglow are, in reality, specific cases of organic TL occurring at room temperature. These findings prompt a reevaluation of the similarities and differences between organic and inorganic phosphors. Specifically, in the absence of a comprehensive understanding of above-room-temperature thermoluminescence in organics, the information gained from this work holds significant practical implications for organic emitters and fields involving back electron transfer.