Ultraviolet irradiation-responsive dynamic ultralong organic phosphorescence in polymeric systems

Abstract

Room temperature phosphorescence (RTP) has drawn extensive attention in recent years. Efficient stimulus-responsive phosphorescent organic materials are attractive, but are extremely rare because of unclear design principles and intrinsically spin-forbidden intersystem crossing. Herein, we present a feasible and facile strategy to achieve ultraviolet irradiation-responsive ultralong RTP (IRRTP) of some simple organic phosphors by doping into amorphous poly(vinyl alcohol) matrix. In addition to the observed green and yellow afterglow emission with distinct irradiation-enhanced phosphorescence, the phosphorescence lifetime can be tuned by varying the irradiation period of 254 nm light. Significantly, the dynamic phosphorescence lifetime could be increased 14.3 folds from 58.03 ms to 828.81 ms in one of the obtained hybrid films after irradiation for 45 min under ambient conditions. As such, the application in polychromatic screen printing and multilevel information encryption is demonstrated. The extraordinary IRRTP in the amorphous state endows these systems with a highly promising potential for smart flexible luminescent materials and sensors with dynamically controlled phosphorescence.

Fig. 1. Schematic representation for the manipulation of an UV irradiation-responsive organic molecule-doped PVA system.

a Schematic illustration the competitive decay channels of luminescent materials. b Proposed mechanism and c molecular structures for irradiation-responsive ultralong RTP systems. Singlet excitons (S_n) produce triplet excitons (T_n) through the ISC process, and suppressing nonradiative transitions of the molecules leads to efficient phosphorescence. The formation of a rigid surrounding matrix can stabilize the triplet excitons and generate radiation-dependent phosphorescence. Abs. absorbance, Fluo. fluorescence, Phos. phosphorescence, LPL long persistent luminescence, IC internal conversion, Non-rad. nonradiation.

Fig. 2. Irradiation-responsive room-temperature phosphorescence of the eight hybrid films.

a Photographs of the eight polymeric systems before and after turning off UV 254 nm light source. b IRRTP lifetime of the SDP-based polymeric system at different doping concentrations under 254 nm light irradiation for 45 min. c Comparison of room temperature phosphorescence lifetime of the eight polymeric systems at different states. Red bar: phosphor powder without the PVA matrix. Green bar: phosphor doped PVA films without the light irradiation. Blue bar: phosphor-doped PVA films with UV irradiation for 45 min.

Fig. 3. Photophysical properties of SDP-doped film.

a Time-resolved photoluminescence spectra and b phosphorescence decay curves under irradiation with 254 nm UV light for different irradiation times. c Excitation-phosphorescence emission mapping under 0, 5, 45, and 120 min continuous irradiation with 5 ms delay at room temperature. Em.Wave. emission wavelength, Ex.Wave. exitation wavelength. d Phosphorescence emission observed at different time intervals before and after switching off the light excitation at ambient conditions. e Phosphorescence spectra of SDP-doped film after 45 min irradiation measured at different temperatures from 77 to 250 K. f Nonradiative decay rate constant with different irradiation times.

Fig. 4. Mechanism of IRRTP at room temperature.

a Hydrogen bonding interactions between SDP and SDP, SDP and PVA, and PVA and PVA in unirradiated SDP-based film. b Covalent bond (C–O–C) formation between PVA chains after 254 nm UV light irradiation for <45 min. c Rearrangement of hydrogen bonding interactions, and further formation of covalent bond (C–O–C) after irradiation for more than 45 min. d ¹H NMR spectra of SDP-based film (1 mg/mL) in DMSO-d₆ recorded after different irradiation times. e Electron paramagnetic resonance spectra and f one-dimentional scattering profiles in the q_z direction of GiWAXS pattern for SDP-based film (0.3 mg/mL) recorded after different irradiation times.

Fig. 5. UV irradiation-responsive IRRTP for green screen printing and multilevel information encryption.

a Screen printing process for patterning. Flat IRRTP films were fabricated by drop-coasting the premixed solution on 75 mm × 25 mm glass substrate, followed by drying at 65 °C for 3 h. Then, the designed images were superimposed and fixed on the film surface. The obtained layers were continuously irradiated by a portable 254 nm UV lamp for 45 min to finish the printing progress. b IRRTP photographs of panda and lotus patterns. Green and yellow panda patterns were printed with SDP-based and 2,2-DB-based films, respectively. In row 1, from left to right, printed lotus by SDP, ODP, TDP, and ABP-based films under 254 nm UV lamp. In row 3, from left to right, printed lotus by DP, 4,4-DB, 2,2-DP, and BFPE-based films under 254 nm UV lamp. In rows 2 and 4, from left to right, corresponding printed lotus after turning off 254 nm UV lamp for 0.5 s. c Irradiation time-dependent anticounterfeiting photographs (after turning off UV light for 0.5 s) of eight doped films as the inks under ambient conditions. Numbers 1–8 were encrypted with SDP, ODP, TDP, ABP, DP, 4,4-DB, 2,2-DP, and BFPE-based films, respectively. Changeable encrypted information was shown by irradiation for 0, 15, and 45 min.