Organic molecules with dual triplet‐harvesting channels enable efficient X‐ray scintillation and imaging

Organic scintillators have recently gained considerable attentions in X‐ray detection for their potential applications in biomedical radiograph and security inspection. However, the weak X‐ray absorption and/or inefficient exciton utilization have limited the development and commercialization of organic scintillators. Currently, high‐performance X‐ray organic scintillators are scarce and organic scintillators with dual triplet‐harvesting channels have not been explored before. Here, we develop several proof‐of‐concept sulfone‐based organic molecules, C1–C7, using different alkoxy chains to manipulate molecular packing mode. These materials exhibit dual triplet‐harvesting channels of thermally activated delayed fluorescence (TADF) and room‐temperature phosphorescence (RTP) in aggregated state. Inspiringly, these molecules display distinct radioluminescence under the X‐ray stimulation. Among them, C6 behaves the highest light yield of 16,558 photons MeV−1. Moreover, clear X‐ray images are demonstrated in both aggregated state and single‐molecule level. High spatial resolutions of 15.0 and 10.6 line pairs per millimeter (lp mm−1) are achieved for rigid and flexible scintillator screens, exceeding most reported organic and conventional inorganic scintillators. These results highlight the great potential of organic molecules with TADF and RTP nature for efficient X‐ray scintillation and imaging.


INTRODUCTION
X-ray detection is of great importance in the fields of radiation detection, medical imaging, and scientific research. [1,2]ne of the stable and cheap way to realize X-ray detection is to use scintillators with the ability of converting high-energy X-rays into low-energy visible photons. [3][6] Albeit the impressive X-ray scintillating performance, there are many issues to these scintillators, such as ultrahigh-temperature (∼1700 • C) treat-ment or poor moisture and light stability together with high toxicity. [7]0] However, organic scintillators have still slowly evolved, although the typical fluorescent scintillator of anthracene has been reported since 1979.One primary issue is the weak X-ray absorption of organic molecules that mainly consist light atoms such as carbon, hydrogen, and nitrogen. [11]The other limitation is inefficient exciton utilization of organic molecules. [12]Under X-ray irradiation, X-ray photos generally interact with the heavy atoms of organic molecules by the Compton scattering and photoelectric effect, inducing the ejection of hot electrons.Following by the interaction with other atoms in organic molecules, lots of secondary electrons are produced, generating electron-hole pairs.Similar to electroluminescence process in some degree, singlet and triplet excitons are then generated in a ratio of 1:3 according to spin statistics.In this sense, the widely used fluorescent organic scintillatos can only use a small portion (approximately 25%) of singlet excitons under the X-ray excitation, whereas approximately 75% triplet excitons could be wasted due to the dark state characteristic in fluorescent organic scintillators.
][15] To access TADF, one has to minimize single-triplet energy gaps of organic molecules to boost reverse intersystem crossing (RISC) process from the lowest triplet state to the lowest singlet state. [16,17]In 2021, Yang and co-workers firstly used several classical TADF molecules as organic scintillators to achieve strong radioluminescence (RL) and clear X-ray imaging. [18]The basic strategy to realize RTP is to use heavy atoms or carbonyl groups to boost intersystem crossing (ISC) and meantime to construct a rigid environment to suppress nonradiative decay. [19,20]To illustrate, Wang et al. developed several organic RTP crystals as X-ray scintillators to enhance RL and realized a very low detection limit of 33 nGy s −1 . [21]Albeit these impressive reports, high-performance organic scintillators are still scarce.Therefore, the search for new classes of organic emitters with satisfactory X-ray absorption and efficient exciton utilization is still highly desired. [22,23][26][27] In our previous report, a sulfone-based simple molecule mono-DMACDPS [28] was developed to exhibit both TADF and RTP properties in the crystalline state.In this context, we conceive to use organic scintillators having TADF and RTP nature simultaneously for X-ray imaging.To validate our concept, we herein developed a series of sulfone-based organic molecules, C1, C3, C5, C6, and C7, via alkoxy chain engineering on the prototype molecule of mono-DMACDPS (Figure 1B).The relatively high Z of the sulfur atom could favor X-ray absorption during the X-ray excitation.The oxygen atom in the alkoxy chain could further enhance n-π* transitions, facilitating the ISC and RISC process.Meanwhile, the different length alkoxy chains may induce different molecular packing, providing a feasible platform to understand the relationship between molecular structures and RL of organic scintillators.All resultant molecules showed nearly the same photoluminescence (PL) properties together with obvious TADF and free reabsorption feature in the dilute solutions and doped films.However, in the crystalline state, C1-C7 exhibited distinct TADF and RTP nature simultaneously.Remarkably, C1-C7 not only displayed different RTP characteristics (emission peaks, lifetimes, and PL quantum yields) but also supported different RL under the X-ray irradiation, mainly depending on molecular packing, as revealed by single-crystal analysis and theoretical calculations.Moreover, C6 depicted the strongest RL with the highest light yield of 16,558 photons MeV −1 under the X-ray stimulation.We demonstrated the potential applications of C6 on X-ray radiography both in single molecule and aggregated states.Inspiringly, the rigid and flexible C6-based scintillator screens displayed high X-ray imaging resolutions of 15.0 and 10.6 line pairs per millimeter (lp mm −1 ), respectively.These values outperformed most reported organic scintillators and conventional inorganic scintillators.

Molecular design and theoretical simulation
As a proof-of-concept, we designed a series of sulfone-based organic molecules, C1, C3, C5, C6, and C7, by introducing alkoxy chains into the prototype TADF molecule of mono-DMACDPS.Before synthesis, theoretical calculations were performed on these molecules (Figure S1).As expected, C1-C7 showed nearly the same optimized configuration with perpendicular torsion between donor and acceptor units.The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) mainly distributed on the acridine and diphenyl sulfone moieties, respectively.The good separation of frontier molecular orbitals are favorable to obtain small singlet-triplet energy gap (ΔE ST ) values of 0.02 eV, indicative of the possible TADF nature of C1-C7.Furthermore, no frontier molecular orbitals were dispersed on the alkyl chains, establishing that the alkyl chain would not affect the electronic structures of these molecules. [29]ith a simple Pd-catalyzed C-N coupling reaction and a nucleophilic substitution reaction (Scheme S1), the target compounds were obtained and characterized by 1 H/ 13 C nuclear magnetic resonance, high-resolution mass spectra, and single-crystal analysis (Figures S17-S33 and Table S1).

Photophysical properties
As shown in Figure 2A,B, all compounds showed the same absorption spectra in toluene.The strong absorption bands around 280 nm was assigned to locally excited transitions.The weak absorption bands peaking at 370 nm belonged to intramolecular charge transfer transitions.Similarly, these five molecules displayed nearly the same steady-state PL profiles peaking at ∼458 nm in toluene (Table 1).Due to large Stokes shifts, all compounds exhibited free reabsorption feature, which is beneficial to construct high-performance X-ray scintillating layers. [30,31]Moreover, these compounds displayed similar PL and phosphorescence spectra in polymethyl methacrylate (PMMA) films (with a low doping concentration of 1 wt%) (Figure S2).S3).Notably, the delayed fluorescence lifetimes of C1-C7 are much shorter than that of mono-DMACDPS, rooting from the enhanced RISC induced by the introduction of the alkoxy chains.Such fast light decay of C1-C7 clearly outperform most inorganic scintillators, beneficial to the fast-response-time X-ray scintillation for potential applications.The above results reveal that the alkoxy chain engineering has a negligible influence on electronic structures and associated optical properties of these emitters at the single-molecule level.
To explore the influence of the alkoxy chain on optical properties at aggregated state, we further studied photophysical properties of these emitters in the crystalline state.As shown in Figure 2E, the steady-state fluorescence of these compounds varied from deep blue at 425 nm for C7 to pure blue at 466 nm for C5.After a 50 ms delay, all emitters displayed red-shifted emissions (Figure 2H) with visible afterglows at room temperature, which could be attributed to the RTP emission.Take C6 as an example, the temperaturedependent PL spectra exhibited double-exponential decay curves in the microsecond scale.The enhanced delayed fluorescence ratio with increasing temperature clearly proved the TADF feature of C6 (Figure S4a).In contrast, the millisecond scale decay curves displayed negative temperature-dependent behavior (Figure S4b), which demonstrated the phosphorescence nature of C6 in millisecond scale.Besides, the time-resolved PL spectra of C6 (Figure S5) revealed the emissive process in the crystalline state.After a 5.1 μs delay, the PL spectrum was nearly the same as the initial PL profile peaking at 450 nm, which is attributed to the TADF nature of C6.When the time was extended to the millisecond scale, the PL spectra red shifted to 480 nm, which originated from the RTP feature.These results clearly proved the dual tripletharvesting channels of these compounds in the crystalline state.
It is worth noting that, with the different alkoxy chains, C1-C7 exhibited different afterglow colors together with variable decay lifetimes (Figures 2G,F,I and S6).For example, C6 displayed a distinct blue afterglow peaking at 450 nm, accompanied by a RTP lifetime of 483 ms, while C5 exhibited a green afterglow peaking at 513 nm, coupled with the longer RTP lifetime of 848 ms.Such large RTP discrepancies suggested that the alkoxy chain can significantly affect the triplet excited states and associated optical properties.Similar to mono-DMACDPS, the observed microsecondscale lifetimes of these emitters in the crystalline state could belong to the intrinsic TADF nature.Owing to the different TADF and RTP emission shifts in the aggregated states, the ΔE ST s of C1-C7 were different from each other and also vary from those at the single-molecule level.These discrepancies leaded to various delayed fluorescence lifetimes for C1-C7, as depicted in Figure 2F.Moreover, C6 depicted the highest PLQY of 55% (Table 1) among these emitters, comparative to the highest values reported for sulfone-based organic RTP materials (75% PLQY reported by Xu et al. [32] ).

Single-crystal analysis and emissive origin
To understand the influence of the alkoxy chain on the optical properties in the crystalline state, single-crystal Xray diffraction was performed.C3 and C6 were chosen as examples because of their large differences in PLQY values and RTP lifetimes.As shown in Figure 3A, there was only one pair of head-to-tail π⋅⋅⋅π interaction (3.531 Å) between neighbor molecules in the C3 single crystal (we define the acridine and alkoxy-substituted phenylene units as the head and tail, respectively), resulting in the loose packing model with a 1D linear arrangement.While for the C6 single crystal, molecular dimers were formed by two types of reversely head-to-head π⋅⋅⋅π interaction (3.432 Å) between adjacent molecules, together with multiple C-H⋅⋅⋅π (2.796 and 2.84 Å) and O⋅⋅⋅H (2.513 Å) hydrogen bonds (Figure S7).This stronger intermolecular interaction induced a tight molecular packing model of C6 with a 2D dimer connection.This can significantly restrain the non-radiative decay and lead to the higher PLQY (55%) together with longer RTP lifetime (483 ms) and DF lifetime (5.6 μs). [33]The similar tendency was also observed for the single-crystal structures of C1 and C5 (Figure S8).These results strongly established that the alkoxy chain can effectively regulate the molecular packing model and thus affect RTP of these compounds.
To understand the emissive origin of the dual tripletharvesting channels of TADF and RTP for C1-C7, the time-dependent density functional theory calculations were performed on both isolated monomers and dimers on the basis of the single-crystal structures.To illustrate, the C6 monomer possessed high singlet (3.10 eV) and triplet (3.08 eV) excited states together with a small ΔE ST of 0.02 eV (Figure 3B).Accordingly, the C6 monomer is in charge of TADF emission.However, the T 1 of the C6 dimer significantly decreased to 2.91 eV, leading to the 10-fold enlarged ΔE ST of 0.20 eV.Moreover, the SOC value from T 1 to S 0 increased threefold from 0.759 cm −1 for the monomer to 2.273 cm −1 for the dimer.In addition, the crystalline form provided a rigid environment to suppress the non-radiative decay of triplet excitons.All the above factors facilitate the RTP channel of the C6 dimer.Similar experimental and theoretical results were also obtained for the other three compounds of C1, C3, and C5 (Figures S9-S11).To sum up, the monomer and dimer in the crystal structures account for TADF and RTP channels of these compounds, respectively (Figure 3C).

Radioluminescence properties
Inspired by the excellent optical properties of these compounds, we further explored RL of C1-C7 under the X-ray excitation.As depicted in Figure 4A, C1-C7 exhibited distinct but very different RL spectra in terms of both RL intensity and emission peaks.C6 demonstrated the strongest RL intensity together with the highest light yield of 16,558 MeV −1 (Figure S12) among these compounds.Taken in mind that these molecules had nearly the same X-ray absorption coefficient (Figure S13), the highest light conversion efficiency of C6 could be attributed to its highest PLQY under the photo-excitation. [14]As depicted in Figure 4C, the tendency of the RL light yields of C1-C7 matches well with their PLQY order.In this sense, the RL behavior of organic scintillators can be manipulated by the alkoxy chain engineering, for the first time demonstrated for organic scintillators.Furthermore, the RL spectra of these compounds are very close to their phosphorescence spectra (Figures 4B  and S14).These results indicated that the triplet excitons play a critical role in the RL process. [15,34,35]Because all these compounds had dual triplet-harvesting channels of both TADF and RTP under the UV excitation, triplet excitons could be effectively harnessed via these two pathways for phosphorescence-dominated RL emissions.
In view of the strongest RL under X-ray excitation, we applied C6 to X-ray radiography.Firstly, we used the pressed C6 crystalline film that had both TADF and RTP properties as the organic scintillator screen (Figure 4D).As shown in Figure 4E, a clear chip image was observed under the Xray excitation.A feasible resolution of 5.3 lp mm −1 (at a modulation transfer function [MTF] = 0.2) was obtained, according to the MTF calculation of standard X-ray edge images (Figure 4F,G).
To further improve X-ray scintillation performance, we fabricated a doped scintillator screen by mixing C6 into sucrose octaacetate (SO) with a low concentration of 1 wt%.This C6-SO scintillator was very transparent (Figure S15a) and the transmittance was as high as 95% (Figure S16), which may favor the imaging resolution by reducing optical scattering.The much clearer X-ray image of an electronic chip was observed (Figure 5A).Furthermore, a vivid example of a copper sheet with letters of "WHU WUHAN" was clearly identified in Figure 5B, where the margin and inner details of the copper mask were distinctly imaged.As expected, the C6-SO scintillator achieved a high spatial resolution of 15.0 lp mm −1 (Figure 5C), much higher than the medical imaging standard (10 lp mm −1 ) and exceeding most reported organic scintillators (Table S2).
To explore the potential use of C6 in a flexible X-ray detector, we further fabricated a circular large-size screen with a diameter of 6 cm by doping C6 into a polydimethylsiloxane (PDMS) matrix (at a concentration of 1 wt%) (Figure S15b).The flexible C6-PDMS scintillator can be folded and stretched easily.Moreover, the film was transparent and displayed distinct blue emission under UV light and X-ray excitation (Figure 5D).To demonstrated the ability of X-ray imaging, an electronic chip was placed between the X-ray source and the C6-PDMS film.As shown in Figure 5E, the inner structure of the chip can be clearly visualized under the X-ray radiation.Moreover, the flexible C6-PDMS film delivered a high resolution of 10.6 lp mm −1 (Figure 5F), much higher than that of conventional flat-panel X-ray detectors (typically < 5.0 lp mm −1 ).These results suggest that C6 is a promising candidate as the flexible large-scale X-ray scintillator for multiple application scenarios.

CONCLUSION
In summary, this work proposed a new strategy to design organic scintillators by integrating both TADF and RTP features into organic compounds.Different length alkoxy chain was introduced to afford five molecules of C1-C7, which exhibited almost the same TADF characteristic in the single-molecule state, but displayed molecular-packingdependent RTP nature in the crystalline state.C6 behaved the highest PLQY of 55% among these five compounds.More excitingly, all five molecules demonstrated distinct RL signal under the X-ray excitation.Single-crystal analysis and theoretical simulation revealed that the alkoxy chain was the key point to manipulate the molecular packing and then significantly regulate both the photophysical and RL properties.Meanwhile, the C6-based scintillator screens exhibited clear X-ray imaging graph, together with high resolutions of 15.0 and 10.6 lp mm −1 in rigid and flexible films, respectively.This work not only propose a simple and feasible strategy to develop new organic scintillators with triplet-harvesting ability, but also open a new avenue to understand the relationship between molecular structure, molecular packing, and RL for organic scintillators.

A C K N O W L E D G M E N T S
We acknowledge financial support from the National Natural Science Foundation of China (52022071) and the Shenzhen Science and Technology Program (ZDSYS20210623091813040).The numerical calculations in this paper have been done on the supercomputing system in the Supercomputing Center of Wuhan University.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare they have no conflicts of interest.

D ATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are available from the corresponding author upon reasonable request.

F
I G U R E 1 (A) Emissive mechanism of organic scintillators with thermally activated delayed fluorescence (TADF) and room-temperature phosphorescence (RTP) properties.(B) Design principle of sulfone-based organic scintillators.

F I G U R E 2
Fluorescent photos of C1-C7 in different forms: (A) dilute toluene solution, (D) crystalline powder with 365 nm ultraviolet (UV) on, and (G) afterglow with UV off.Photoluminescence (PL) spectra of C1-C7 in different forms: (B) UV-vis absorption and normalized fluorescence spectra of C1-C7 in toluene (10 −5 M) at room temperature.(E) Normalized fluorescence and (H) phosphoresce spectra of C1-C7 in crystalline state at room temperature (excited at 340 nm).Transient PL spectra of C1-C7 in (C) toluene and (F) crystalline state at microsecond scale.(I) Transient PL spectra of C5 and C6 in the crystalline state at second scale.

F
I G U R E 3 (A) Single-crystal analysis of C3 and C6.From left to right, single-crystal structure, π⋅⋅⋅π intermolecular interactions, molecular packing mode with 2 × 2-unit cell, and corresponding simplified model.To make clearer displaying, the hydrogen atoms were all hidden.(B) Theoretical simulation and energy level diagrams of the monomer and dimer structures of C6 obtained from single-crystal structure.(C) Emissive mechanism of these emitters at the form of monomer and dimer under the excitation of ultraviolet (UV) light.F I G U R E 4 (A) Radioluminescence of C1-C7 (pressed crystalline films).(B) Normalized photoluminescence (PL) and radioluminescence (RL) spectra of C6 at room temperature.(C) Line chart of PLQYs and RL light yields of C1-C7.(D) Photographs of C6 with ultraviolet (UV) on, UV off, under the daylight, and X-ray excitation.(E) Bright-field (left) and X-ray (right) images of a chip.(F) X-ray image of a standard X-ray test pattern plate (from 2 to 16 lp mm −1 ).(G) Modulation transfer functions (MTFs) of the X-ray images.

F I G U R E 5
Potential applications of 1 wt% C6 in rigid (upper, sucrose octaacetate [SO] film) and flexible (down, polydimethylsiloxane [PDMS] film) scintillator screen for X-ray radiography.Bright-field (left) and X-ray (right) images of (A) a chip and (B) copper sheet taken by the C6-SO film.(D) Brightfield (left), ultraviolet UV-excited (middle), and X-ray excited images of the C6-PDMS film.(E) Bright-field (left) and X-ray (right) images of a chip taken by the C6-PDMS film.Modulation transfer functions (MTFs) of the X-ray images taken by the (C) C6-SO and (F) C6-PDMS films (inset: X-ray image of a standard X-ray test pattern plate).

TA B L E 1 Photophysical properties of C1-C7 in different forms. Compound λ Fl a,b,c (nm)
a Measured in toluene (10 −5 M, 300 K). b Measured in polymethyl methacrylate (PMMA) film (with a doping concentration of 1 wt%).c Measured in the crystalline state (300 K).