Organic phosphorescent scintillation from copolymers by X-ray irradiation

Abstract

Scintillators that exhibit X-ray-excited luminescence have great potential in radiation detection, X-ray imaging, radiotherapy, and non-destructive testing. However, most reported scintillators are limited to inorganic or organic crystal materials, which have some obstacles in repeatability and processability. Here we present a facile strategy to achieve the X-ray-excited organic phosphorescent scintillation from amorphous copolymers through the copolymerization of the bromine-substituted chromophores and acrylic acid. These polymeric scintillators exhibit efficient X-ray responsibility and decent phosphorescent quantum yield up to 51.4% under ambient conditions. The universality of the design principle was further confirmed by a series of copolymers with multi-color radioluminescence ranging from green to orange-red. Moreover, we demonstrated their potential application in X-ray radiography. This finding not only outlines a feasible principle to develop X-ray responsive phosphorescent polymers, but also expands the potential applications of polymer materials with phosphorescence features.

Fig. 1. Rational design of amorphous copolymers for organic phosphorescent scintillation and the related emissive processes under X-ray irradiation.

After X-ray irradiates the polymers and ejects the inner electrons of the halogen atoms, plenty of electrons and holes are produced. Then the recombination of holes and electrons produces singlet and triplet excitons. Among them, radiative transitions of the excitons generate radioluminescence under ambient conditions. Fluo. and Phos. refer to fluorescence and phosphorescence, respectively.

Fig. 2. Photophysical properties of the PBBr copolymers by X-ray irradiation under ambient conditions.

a Normalized steady-state photoluminescence (PL, blue line) and phosphorescence (gray line) spectra of the PBBr-50 film under UV light excitation, as well as radioluminescence (RL) spectrum (green line) at a dose rate of 278 μGy s⁻¹. Insets are the corresponding ultraviolet (UV) light- and X-ray-excited photographs. b A lifetime decay curve of emission band at 496 nm for PBBr-50 film. c PL efficiency and RL intensity variation of copolymer films from PBBr-3 to PBBr-800. d RL spectra of different PBBrs at a dose rate of 278 μGy  s⁻¹. Note: the examination was quantitative for each group. e RL measurements of the PBBr-5 film as a function of dose rate in the range of 0.688 to 278 μGy s⁻¹. f Emission photostability of the PBBr-5 film at 500 nm under repeated on-off cycles of X-ray at a dose rate of 278 μGy s⁻¹.

Fig. 3. Mechanism of phosphorescent scintillation from organic copolymers under X-ray irradiation.

a X-ray absorption spectra of the BBr, BPh, and AA monomers. The insert shows the normalized radioluminescence (RL) intensity of the PBBr, PBBh, and PAA polymers at a dose rate of 278 μGy s⁻¹. b WAXS pattern of PBBr-5 polymer film. c PXRD patterns of the copolymers from PBBr-3 to PBBr-800. d Natural transition orbitals (NTOs) for the lowest triplet state of BBr, and calculated excitation energies and spin-orbit coupling (SOC) constants (ξ) of the BBr and BPh monomers. e Proposed mechanism of radioluminescence for amorphous copolymers. After X-ray irradiation, the electron in the inner shells is excited by high-energy X-ray photons and ejected out of the Br atom. Then, the high-energy electrons generate lots of secondary electrons by interacting with other atoms in the polymer. The generated electrons and holes are rapidly thermally dissipated and gradually accumulate at the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of the organic phosphors, respectively. Eventually, the electrons and holes recombine to form excited states, generating singlet and triplet excitons in a ratio of 1:3, which produces fluorescence (Fluo.) and phosphorescence (Phos.) via the radiative decay processes, respectively.

Fig. 4. Colorful phosphorescent radioluminescence in organic copolymers under X-ray irradiation.

a Normalized radioluminescence spectra and the related photographs of PNBr, PDBr, and PIBr copolymer films under the irradiation of a dose rate of 278 μGy s⁻¹, respectively. b CIE chromaticity coordinate diagram of the radioluminescence color of the copolymers. c Lifetime decay curves of emission bands at 546, 555, and 575 nm for PNBr, PDBr, and PIBr copolymer films, respectively, under UV light excitation. d Radioluminescence (RL) intensity variation of PNBr, PDBr, and PIBr copolymer films with different molar feed ratios of two monomers by X-ray irradiation at a dose rate of 278 μGy s⁻¹. Note that the examination was quantitative for each group.

Fig. 5. Demonstration of the flexible phosphorescent copolymers for potential applications in digital radiography.

a A schematic of the radiography set-up, and the preparation process of X-ray imaging background substrate using PNBr-10 aqueous solution. b Photographs of large-scale, transparent, and flexible PNBr-10 copolymer film under daylight and UV light, respectively. c X-ray images of the shell and guitar-shaped sheet metal under X-ray irradiation.