Color-Tunable Room-Temperature Phosphorescence from Non-Aromatic-Polymer-Involved Charge Transfer

Abstract

Polymeric room-temperature phosphorescence (RTP) materials especially multicolor RTP systems hold great promise in concrete applications. A key feature in these applications is a triplet charge transfer transition. Aromatic electron donors and electron acceptors are often essential to ensure persistent RTP. There is much interest in fabricating non-aromatic charge-transfer-mediated RTP materials and it still remains a formidable challenge to achieve color-tunable RTP via charge transfer. Herein, a charge-transfer-mediated RTP material by embedding quinoline derivatives within a non-aromatic polymer matrix such as polyacrylamide (PAM) or polyvinyl alcohol (PVA) is developed. Through-space charge transfer (TSCT) is achieved upon alkali- or heat treatment to realize a long phosphorescence lifetime of up to 629.90 ms, high phosphorescence quantum yield of up to 20.51%, and a green-to-blue afterglow for more than 20 s at room temperature. This color-tunable RTP emerges from a nonaromatic polymer to single phosphor charge transfer that has rarely been reported before. This finding suggests that an effective and simple approach can deliver new color-tunable RTP materials for applications including multicolor display, information encryption, and gas detection.

Keywords: charge transfer; color‐tunable; non‐aromatic polymer; quinoline zwitterion; room‐temperature phosphorescence.

Figure 1

a) Design sketch of the charge‐transfer‐mediated RTP systems in previous works and this work. b) Excitation spectra (green solid line), prompt (brown solid line), and delayed (dash line) phosphorescence spectra of PAM‐Cl, PAM‐Cl‐NaOH, and PAM‐Cl‐Heat. Delayed time: 1.0 ms. c) Excitation spectra, prompt and delayed phosphorescence spectra of 4‐Cl (1✗10^‐6 M) in methanol at 77 K. d) The time‐resolved emission spectra (excited at 340 nm) of PAM‐Cl‐NaOH treated with 0, 0.5, and 1 equivalent of NaOH and their phosphorescence lifetime. e) Normalized phosphorescence emission spectra of PAM‐Cl‐NaOH with 0, 0.25, 0.50, 0.75, and 1 equivalent of NaOH. f) The changes in phosphorescence quantum yield of PAM‐Cl and PAM‐Br treated with NaOH or heat.

Figure 2

a) Afterglow images of PAM‐Cl‐NaOH with different equivalents of NaOH (λ _ex = 365 nm) after ceasing irradiation in air environment. b) Excitation spectrum, prompt and delayed phosphorescence spectra of 4‐Cl‐NaOH (1 ⨯ 10^‐6 M) in methanol at 77 K. c) ¹H NMR spectra (400 MHz, D₂O) of 4‐Cl‐NaOH with addition of 0, 0.25, 0.5, 0.75, and 1 equivalent of NaOH. d) Chromaticity coordinate (CIE) of PAM‐Cl‐NaOH with 0, 0.25, 0.5, 0.75, and 1 equivalent of NaOH.

Figure 3

a) Normalized phosphorescence emission spectra of PAM‐Cl‐Heat treated at different temperatures for 10 min. b) CIE of PAM‐Cl‐Heat. c) Temperature‐dependent phosphorescence spectra of PAM‐Cl‐Heat treated at 110 °C for 10 min (excited at 340 nm, delayed time: 1.0 ms). d) Time‐resolved delayed spectra (excited at 340 nm, monitored at 434 nm) of PAM‐Cl‐Heat and its phosphorescence lifetime. e) ¹H NMR spectra (400 MHz, D₂O) of PAM‐Cl solution (down) and the solution after heating (up).

Figure 4

a) Normalized phosphorescence emission spectra of PVA‐Cl‐Heat treated at different temperatures for 10 min. b) Phosphorescence lifetime decay curves of PVA‐Cl (excited at 350 nm, monitored at 520 nm) and PVA‐Cl‐Heat (excited at 340 nm, monitored at 434 nm). c) 2D NOESY NMR spectrum (400 MHz, d ₆‐DMSO) of PVA and 4‐Cl after heating. The visualization of non‐covalent interactions of PVA‐Cl‐Heat d), PVA‐Br‐Heat e), PAM‐Cl‐Heat f) and PAM‐Br‐Heat g). h) The common interpretation of the coloring method of mapped function sign(λ₂)ρ in the independent gradient model (IGM) and independent gradient model based on Hirshfeld partition (IGMH) maps.

Figure 5

a) Electron/hole map of the excited state 2 of 4‐Cl (pink: electron; blue: hole) and the corresponding atomic number. b) Thermal maps corresponding to the atom‐atom charge transfer matrix of S₀ → S₁ of 4‐Cl. c) The heat map of atoms’ contribution to hole and electron of 4‐Cl. d) Diagrams of the TD‐DFT calculated energy levels and SOC constants of 4‐Cl. e) Electron/hole map of the excited state 2 of PAM‐Cl‐Heat (pink: electron; blue: hole) and the corresponding atomic number. f) Thermal maps corresponding to the atom‐atom charge transfer matrix of S₀ → S₁ of PAM‐Cl‐Heat. g) The heat map of atoms’ contribution to hole and electron of PAM‐Cl‐Heat. h) Diagrams of the TD‐DFT calculated energy levels and SOC constants of PAM‐Cl‐Heat.

Figure 6

a) Photographs of cotton thread infiltrated with the solution of 4‐Cl as the dye solution under daylight and after removing 365 nm irradiation, and photographs of TS‐FRET between Rh‐B and 4‐Cl on cotton. b) Schematic diagram of the TS‐FRET process. c) Photographs of different letters on A4 paper by using the solution of 4‐Cl as the ink before and after heating. d) The afterglow images of PAM‐Cl films treated with ammonia gas at different concentrations (0.02, 0.04, 0.06, and 0.08 mol L⁻¹) for 2 min or for different times (2, 4, 6, and 8 min) at 0.02 mol L⁻¹. Inset is the normalized phosphorescence emission spectra of PAM‐Cl (Original), PAM‐Cl treated with 0.08 mol L⁻¹ of ammonia gas for 2 min and 0.02 mol L⁻¹ of ammonia gas for 8 min. e) Temperature‐induced anticounterfeiting realized by PAM‐Cl and PAM‐H.