H-Bonding-Assisted Room-Temperature Phosphorescence in a Purely Organic Molecule in the Solid State via Through-Space Charge Transfer

Room-temperature phosphorescence (RTP) from metal-free organic molecules is a rapidly growing area of research due to its broad applications in OLEDs, anticounterfeiting technologies, sensors, and more. Highly phosphorescent aggregates derived from simple organic chromophores hold significant promise for practical use, owing to their economic and structural simplicity. However, inducing RTP in existing fluorescent frameworks remains a challenge. Here, we studied a conjugated fluorophore, (E)-2-((3,5-dichloro-2-hydroxybenzylidene)amino)-3′,6′-dihydroxyspiro[isoindoline-1,9′-xanthen]-3-one (AB-65), which combines a xanthene-based fluorescein moiety and salicylaldehyde, designed to promote efficient intersystem crossing via through-space charge transfer. The photophysical properties of AB-65 were examined in both solution and solid states to identify conditions that minimize nonradiative decay. While AB-65 exhibits phosphorescence in solution at cryogenic temperatures, our primary goal was to achieve RTP under ambient conditions. Although weakly emissive in aqueous solution due to poor solubility, AB-65 displayed phosphorescence at low temperature in mixed aqueous solvent. Further enhancement of RTP was observed in the purely organic solid state. These findings demonstrate that enabling RTP in typically nonphosphorescent molecules through TSCT, hydrogen bonding, and rigidification provides valuable insights for the rational design of phosphorescent materials.