Substituent group regulated delayed emissions for thermally activated delayed fluorescence and room temperature phosphorescence observed in an indole@metaboric acid system

Organic afterglow materials with tunable emission colors and persistent long-lived lifetimes have exhibited promising applications in organic light-emitting diodes, bioimaging, encryption and sensing. However, precise regulation on the emission pathway remains a great challenging because of the uncontrollable energy gap between the lowest singlet and triplet excited states (ΔE_ST) and slow intersystem coupling rate. Herein, three metaboric acid (HBO₂)-confined composites, ICA@HBO₂, Cl-ICA@HBO₂ and COOH-ICA@HBO₂, were prepared by powerful matrix-assisted strategy with 1H-indole-4-carboxylic acid (ICA), 7-chloro-1H-indole-4-carboxylic acid (Cl-ICA) and 1H-indole-2,5-dicarboxylic acid (COOH-ICA) as chromophore and boric acid as matrix precursor. The orthorhombic metaboric acid matrix generated from dehydration of boric acid has uniformly anchored the indole chromophore by strong intermolecular hydrogen-bonding interactions, resulting in three white loose powdery composites. Under the excitation of low-energy visible light, both ICA@HBO₂ and Cl-ICA@HBO₂ emit cyan and pale-green thermally activated delayed fluorescence at ambient conditions with quite distinct lifetimes (1559 vs. 429 ms) and photoluminescence quantum yields (PLQYs, 13.71 % vs. 4.64 %), due to small ΔE_ST and favorable reverse intersystem crossing. By contrast, COOH-ICA@HBO₂ modified by two carboxylic acid groups exhibits bright green room temperature phosphorescence with medium-length lifetime of 1066 s and the highest PLQY up to 22.42 %. Thus, substituted groups on the indole-parent with well-tunable conjugation and heavy atom effect, together with the rigid micro-environment and slight light-induced charge separation effect by metaboric acid matrix, have significantly manipulated the channel of the delayed emission through optimizing the ΔE_ST values. Furthermore, the opposite temperature-dependent emission intensity and naked-eye distinguishable lifetime have endowed these composites promising applications in thermal-responsive security display and multilevel information encryption.