Triplet Bandgap Engineering Using an Ion-Trapped Single Dye for Color-Tunable Phosphorescence at Cryogenic Temperatures

This study interprets the changes in the molecular structure resulting from the ion trap of a single dye, thioflavin T (ThT), where the proton and hydroxide directly combine. The triplet emission characteristics with altered bandgap at cryogenic temperatures were analyzed using chemical analysis and quantum calculation. In ThT solutions with added hydrochloric acid, H⁺ binds to N₁₇ in the aniline group resulting in protonation, while upon the addition of sodium hydroxide, OH^- binds to C₈ in the benzothiazole leading to hydroxylation. Ion binding not only affects intramolecular charge distribution but also alters the dihedral angle of the rotor for energy minimization, calculated as 50° for protonated ThT and 90° for hydroxylated ThT compared to the pristine state (33°). At a cryogenic temperature of 77 K, where the nonradiative energy relaxation process is suppressed, triplet emission lasting for ∼1 s was observed, exhibiting different luminescent colors depending on the kind of trapped ions. In 1 mM ThT solution, the ion trapping resulted in the bandgap changes, resulting in red, green, and blue emissions for pristine, protonated, and hydroxylated states, respectively, which can also be reversed. However, at a relatively high concentration of 10 mM, strong interactions between ThT molecules prevented ion trapping.