Ultralong room temperature phosphorescence from amorphous organic materials toward confidential information encryption and decryption

Abstract

Ultralong room temperature phosphorescence (URTP) emitted from pure amorphous organic molecules is very rare. Although a few crystalline organic molecules could realize URTP with long lifetimes (>100 ms), practical applications of these crystalline organic phosphors are still challenging because the formation and maintenance of high-quality crystals are very difficult and complicated. Herein, we present a rational design for minimizing the vibrational dissipation of pure amorphous organic molecules to achieve URTP. By using this strategy, a series of URTP films with long lifetimes and high phosphorescent quantum yields (up to 0.75 s and 11.23%, respectively) were obtained from amorphous organic phosphors without visible fluorescence and phosphorescence under ambient conditions. On the basis of the unique features of URTP films, a new green screen printing technology without using any ink was developed toward confidential information encryption and decryption. This work presents a breakthrough strategy in applying amorphous organic materials for URTP.

Fig. 1. Rational design strategy of URTP from amorphous organic materials.

(A) Chemical structures of G and PVA and the fabrication of G-doped PVA films for URTP. (B) Schematic illustration of URTP processes in G-doped PVA films. DIW, deionized water.

Fig. 2. Photophysical properties of the PVA film doped with different concentrations of G.

Phosphorescence spectra of the PVA film doped with different concentrations of G (λ_ex = 280 nm) at room temperature. (A) Unirradiated. (B) 254-nm UV light irradiation for 65 min. (C) Phosphorescent lifetime of these G-doped PVA films at room temperature, monitored at 480 nm and λ_ex = 280 nm. (D) Snapshots of these G-doped PVA films upon irradiation by a 254-nm UV lamp for 65 min, followed by recording URTP at different time intervals in the dark under ambient conditions.

Fig. 3. Irradiation time-dependent measurements of the photophysical properties of the PVA-100-3mg film.

(A) Phosphorescence spectra of the PVA-100-3mg film upon light irradiation at different times (λ_ex = 280 nm). (B) Phosphorescent decay profiles of the PVA-100-3mg film at different irradiation times at room temperature (monitored at 480 nm, λ_ex = 280 nm). (C) Time-dependent luminescent spectra of the PVA-100-3mg film at different irradiation times followed by turning off the UV light (delayed time: approximately 50 ms) at room temperature. (D) Comparison of the emission observed at different time intervals before and after turning off the light excitation at room temperature.

Fig. 4. Schematic diagram for achieving URTP in G-doped PVA films and irradiation time-dependent ¹H NMR spectra of the PVA-100-3mg film.

(A) Unirradiated G-doped PVA films with substantial H-bond formation from G-G, G-PVA, and PVA-PVA. (B) Irradiated G-doped PVA films by a 254-nm UV light (irradiation time ≤65 min) with cross-linked bond (C–O–C) formation between PVA chains. (C) Irradiated G-doped PVA films (irradiation time >65 min) with the rearrangement of H-bonds from G-G, G-PVA, and PVA-PVA. (D) ¹H NMR spectra of G, PVA-100, and PVA-100-3mg films at different irradiation times in DMSO-d₆. The inset shows the chemical structures of G (up) and PVA-100 (down).

Fig. 5. Photophysical properties of different G-doped PVA matrices upon irradiation by a 254-nm UV light for 65 min.

(A) Phosphorescence spectra of PVA-100-3mg, PVA-87-3mg, and PVA-80-3mg films upon irradiation by a 254-nm UV 1ight (λ_ex = 280 nm) for 65 min. (B) Phosphorescent decay profiles of PVA-100-3mg, PVA-87-3mg, and PVA-80-3mg films (monitored at 480 nm, λ_ex = 280 nm). The inset shows the chemical structures of PVA-100, PVA-87, PVA-80, and PVAc. (C) Plots of phosphorescence emission intensity at 480 nm versus temperature (293 to 363 K) for PVA-100-3mg, PVA-87-3mg, and PVA-80-3mg films (λ_ex = 280 nm). (D) Plots of phosphorescent lifetime versus temperature (293 to 363 K) for PVA-100-3mg, PVA-87-3mg, and PVA-80-3mg films (monitored at 480 nm, λ_ex = 280 nm).

Fig. 6. Luminescent G-doped PVA film patterns fabricated via green screen printing.

(A) Green screen printing without any inks using the lotus flower screen plate. First, G-doped PVA films were fabricated through drop-coating the G-PVA solution on a clean glass substrate, followed by drying at 65°C for 3 hours. Second, the screen plate of the lotus flower was covered on G-doped PVA films. Third, the covered PVA films were irradiated by a 254-nm UV lamp for 65 min to finish the screen printing progress. No patterns on the films were observed under sunlight. Clear patterns were observed upon exposure to the 254-nm UV lamp, and these patterns were still visible for several seconds after turning off the UV lamp. (B) Several complicated patterns by green screen printing technology after removing the excitation source. (C) More advanced anti-counterfeiting technology through doping AlQ₃ into G-doped PVA films. (D) Reversible patterns of the lotus flower after removing the excitation source under different conditions. (E) Schematic illustration of the URTP process in the G-doped PVA films.