Modulating Emission of Boric Acid into Highly Efficient and Color-Tunable Afterglow via Dehydration-Induced Through-Space Conjugation

Abstract

Inorganic boric acid (BA) is generally not considered an efficient afterglow material, and several groups have reported its extremely weak room-temperature phosphorescence (RTP) in the blue spectral region. It is discovered that heat treatment of BA results in increased afterglow intensity (27-fold increase) and prolonged emission lifetime (from 0.83 to 1.59 s), attributed to enhanced through-space conjugation (TSC) of BA. The afterglow intensity of BA can be increased further (≈415 folds) by introducing p-hydroxybenzoic acid (PHA), which contains a conjugated molecular motif, to further promote the TSC of the BA system. This combination results in the production of afterglow materials with a photoluminescence quantum yield of 83.8% and an emission lifetime of 2.01 s. In addition, a tunable multicolor afterglow in the 420-490 nm range is achieved owing to the enhancement of the RTP and thermally activated delayed fluorescence of PHA, where BA exerts a confinement effect on the guest molecules. Thus, this study demonstrates promising afterglow materials produced from extremely abundant and simple precursor materials for various applications.

Keywords: afterglow; boric acid; room-temperature phosphorescence; thermally activated delayed fluorescence; through-space conjugation.

Figure 1

Schematic illustration of the heat treatment process and the increase of TSC in the system.

Figure 2

Photophysical properties of BA and related molecules. a) Normalized delayed PL spectra (excitation: 260 nm) of BA of different purities from different manufacturers. b) EPR spectra and c) current density as a function of the irradiation time for BA (red line) and heated BA (blue line). d) TGA and DSC curves of BA. e) Powder XRD patterns of BA after heat treatment at different temperatures. f) Delayed PL spectra of BA, heated BA, and BA incorporated with PHA; the enlarged spectra of BA and BA@PHA are shown in the inset. Normalized delayed PL spectra of g) BA, (h) metaboric acid, and (i) B₂O₃, under different excitation wavelengths.

Figure 3

Characterization of BA@PHA. a) SEM image of BA@PHA. b) FTIR spectra of BA (black line), PHA (red line), and BA@PHA (cyan line). c) XRD patterns of heated BA (blue line), BA@PHA (cyan line), and simulated metaboric acid (black line). High‐resolution d) C1s, (e) O1s, and (f) B1s XPS profiles of BA@PHA.

Figure 4

Delayed PL properties of BA@PHA and mechanism investigation of the afterglow. a) Prompt and delayed PL spectra of BA@PHA. b) Emission decay curves of BA@PHA were detected at 420 and 335 nm (excited at 280 nm). c) Prompt PL emission spectrum of PHA (blue line, excited at 280 nm), delayed PL excitation spectrum of heated BA (purple line, detected at 430 nm), and absorption spectrum of BA@PHA (red line). d) Digital photographs of afterglow materials were obtained by doping different guest molecules into BA under daylight, under UV light, and after different time intervals following the switching off of the UV light. e) Delayed PL decay curves of the afterglow materials obtained by doping different guest molecules into BA (excited and recorded at 280 and 430 nm, respectively). f) Normalized delayed PL excitation spectrum of heated BA (purple line, detected at 430 nm), and prompt PL emission spectra (excited at 280 nm) of OHA, MHA, and PHA. g) Simplified Jablonski diagram showing the energy transfer from PHA to heated BA (ET: energy transfer, ISC: intersystem crossing).

Figure 5

Mechanism investigation of the multiple afterglow colors in of BA@PHA. a) Digital photographs of BA@PHA under UV light and after different time intervals following the switching off of UV light (the sample was excited at different wavelengths). b) Excitation‐delay PL mapping of BA@PHA (top), and the delayed PL spectra obtained under different excitation wavelengths (bottom). Delayed PL spectra of BA@PHA recorded at different temperatures by exciting at c) 280 nm and d) 360 nm. e) Temperature‐dependent delayed PL intensity at different detection temperatures (top) and the plot of emission lifetime against the detection temperature for BA@PHA (bottom). f) Simplified Jablonski diagram for the emission of PHA.

Figure 6

Information encryption applications of BA@PHA. a) Schematic illustration of the screen‐printing process. b) Digital photographs of the QR code of the official website of the College of Chemistry and Environmental Science of Hebei University on a piece of printing paper upon switching on (top) and switching off (bottom) 254 nm UV light. c) Patterns of a number and hidden code printed using BA@PHA samples produced under different pH upon switching on (top) and switching off (bottom) 254 nm UV light. d) Digital photographs of a flower pattern under the irradiation of 254 and 365 nm UV light, and after switching off the UV light for 2 s.