Singlet Fission‐Based High‐Resolution X‐Ray Imaging Scintillation Screens

Abstract X‐ray imaging technology is critical to numerous different walks of daily life, ranging from medical radiography and security screening all the way to high‐energy physics. Although several organic chromophores are fabricated and tested as X‐ray imaging scintillators, they generally show poor scintillation performance due to their weak X‐ray absorption cross‐section and inefficient exciton utilization efficiency. Here, a singlet fission‐based high‐performance organic X‐ray imaging scintillator with near unity exciton utilization efficiency is presented. Interestingly, it is found that the X‐ray sensitivity and imaging resolution of the singlet fission‐based scintillator are dramatically improved by an efficient energy transfer from a thermally activated delayed fluorescence (TADF) sensitizer, in which both singlet and triplet excitons can be efficiently harnessed. The fabricated singlet fission‐based scintillator exhibits a high X‐ray imaging resolution of 27.5 line pairs per millimeter (lp mm−1), which exceeds that of most commercial scintillators, demonstrating its high potential use in medical radiography and security inspection.


Materials and methods
All chemicals were purchased from commercial suppliers and used without further purification. 1 H NMR spectra were recorded on Bruker Avance III 400 MHz or 500 MHz instruments. UV-vis absorbance studies were carried out with PerkinElmer Lambda 950 UV/VIS Spectrometer.
PerkinElmer LS45 Photoluminescence spectrometer having 450 W Xenon lamp was used for steady-state photoluminescence studies.
Preparation of the scintillation screens. 5 mg of TADF-Br chromophores were dissolved in 0.6 mL chloroform; n mg of rubrene was then added. After sonicating for 10 minutes in the dark, different amounts of polysulfone (PSF) were added. The mixture was sonicated for another 5 minutes and then shaken on a shaker for 5 hours to ensure that all materials were well mixed. The viscous solution was then carefully coated on the quartz plates to get the films for UV and X-ray correlated measurements. It is noteworthy that the films were covered with a beaker (covered with Aluminum foil) during the evaporation of the solvent to make sure the uniformity of the films. , and the corresponding excitation wavelengths were selected using a parametric optical amplifier (Newport, Spectra-Physics) that was pumped with an Astrella femtosecond pulsed laser (800 nm, 150 fs, 1 kHz, Coherent). Photoluminescence at different wavelengths was collected and recollimated by a pair of parabolic mirrors passed through a long-pass filter (422 nm, Newport) and finally focused on an optical fiber coupled to a monochromator and a PMT detector. The energy at each excitation wavelength was set constant with the help of a pair of variable neutral density filters (Thorlabs) to ensure that less than 1 % of excitation events resulted in a detected photon. TCSPC histograms were fitted using the Lavenberg-Marquart algorithm implemented in Ultrafast System software. The overall time resolution for the system was better than 120 ps. [1,2] Computational Methods. The ground-state geometries of TADF-Br and Rubrene molecules were optimized using B3LYP functional together with 6-31G(d) basis set. The interfacial dimer structure for TADF-Br/Rubrene was built by placing two molecules together with an initial distance of ~3.0 Å and further optimizations were carried out. All the calculations were performed with Gaussian09 program (Rev D. 01).

Time-Correlated
The key intermediate in the singlet fission is the multiexciton ( 1 TT) state, a correlated triplet-triplet pair that facilitates the conversion of singlet exciton into two spin-triplet excitons. The singlet spin 1 TT state is of double excitation nature, and this state cannot be described by typical TDDFT calculations and requires multireference methods to describe 1 TT state. The excitation energy is calculated at the complete active space self-consistent field (CASSCF) level. Active space orbitals consist of two highest-occupied and two lowest-unoccupied molecular orbitals of the dimers, CASSCF (4,4). The calculations are carried out with a def2-TZVP basis set using the ORCA program. [3] The dimer geometry is taken from the experimental structure that displays a sizeable electronic coupling between rubrene dimers. Radioluminescence (RL) measurement. Steady-state radioluminescence spectra were collected in a spectrometer (Fluoromax-4, Horiba) coupled with an X-ray tube (Tungsten target, Moxtex).
The X-ray dose was controlled by tube current and tube voltage. By measuring the dose-dependent radioluminescence (RL) spectra, a linear relationship between the RL intensity against dose rate was obtained. A commercial scintillator LYSO:Ce (size: 1.5 × 1.5 cm, thickness: 500 µm, light yield ~ 33000 photons/MeV) was used as a reference to estimate the light yield of the film samples.
The films were fabricated to keep the same size as the LYSO:Ce wafer, and RL curves were obtained using the spectrometer under an identical configuration.

Material synthesis
General information. All reactions were carried out with oven-dried glassware using standard Schlenk techniques under an inert atmosphere of dry argon or in an argon-filled glovebox.
Analytical TLC was performed on Select Scientific 200 µm silica gel plates. Column chromatography was performed on Silicycle ultrapure silica gel with particle size 40-63 µm (230-400 mesh). All the solvents used were dried and distilled according to literature methods and degassed prior to use unless otherwise noted.