Scintillation is the process that ionizing radiation converts its energy into photons through the interaction of a material. It has widespread applications, including X-ray detection in radiological imaging and nondestructive inspection[1, 2], γ-ray detection in radioisotope identification and positron emission tomography (PET)[3, 4], beta ray detection in electron microscopes[5], and electromagnetic calorimeters in high-energy physics experiments[6, 7]. Development of scintillators with higher efficiency and faster timing properties is always the chasing goal of the field as high efficiency could generate more photons for a given radiation and produce better images without dosage increase, and fast scintillation enables quicker imaging and less lag, both of which are crucial for computed tomography (CT) and PET as well as high-energy physics application.
The study of inorganic scintillators has been ongoing for more than seventy years since the discovery of Sodium Iodide by Hofstadter in 1949.[7] Generally, inorganic scintillators have high efficiency (> 50000 photons MeV− 1) but inferior lifetime properties (tens of ns to thousands of ns) and thus limits their application with the urgent demand of fast scintillators (sub ns to ns) for high-energy physics and medical imaging.[8–11] Hence, organic scitllators have been studied due to their intristic fast lifetime (ns) as a result of the large electron-hole wave function overlap in molecular crystals. In addition, organic scintillators have advantages in low-temperature processing, abundant resourece supplies, high mechanical flexibility, and cost-effective fabrication in large volumes.[12] However, their performance is fundamentally limited by the inefficient exciton utilization and thus low radioluminescence efficiency (typically < 20000 photons MeV− 1), as well as small radiation stopping power inherent to their low atomic number. The low efficiency is due to that the weak spin-orbit coupling in conventional organic scintillators only generates fluorescence from singlet excitons upon high-energy X/γ-ray excitation, and approximately 75% triplet excitons relax primarily via non-radiative pathways. To circumvent these limitations, thermally activated delayed fluorescence (TADF) organic molecules and organic phosphors based on high-Z organometallics and halogenated organic materials have been extensively studied as organic scintillators to exploit the triplet excitons for emission recently.[13–15] Despite these increase the radioluminescence yield, the scintillation lifetime is unwantedly severely prolonged to microsecond or even second time scale via the delayed fluorescence or phosphorescence process, which lose the intrinstic advantage of organic scintillators.
We propose a different approach to achieve undelayed fluorescence by manipulating the hot excitons and thus circumvent this dilemma of organic scintillators. Figure 1a schematically shows the interaction process between high-energy X/γ-rays and organic materials. X/γ-ray photons firstly transfer all or partial of their energy to the materials, producing primary high-energy electrons. These primary photoelectrons further collide with organic scintillators, releasing secondary high-energy electrons[16]. Then the interaction between the primary/secondary electrons and the organic molecules could lead to the removal of an electron completely from an atom (ionization) or to the promotion of an electron to a higher-lying shell (excitation)[17]. The excited electrons maintain the same spin state as the ground state electrons, forming only singlet excitons. The ionized electrons, which dominate the scintillation process, populate the excited states and consist of singlet and triplet excitons in the ratio of 1:3 following the rule of spin conservation.[14] In conventional organic scintillators and recently reported phosphorescent and TADF scintillators, the high-lying singlets and triplets would rapidly, within ps and driven by thermodynamics, relax to the lowest singlet (S1) and triplet (T1) states through internal conversion, respectively.[13–15] The occupation of spin forbidden T1 states either creates non-radiative recombination channel in the case of conventional scintillators, as in anthracene, or prolongs decay time in phosphorescent and TADF scintillators. Thus, the key to realize efficient and fast organic scintillators is to exclude the occupation of T1 states without sacrificing hot excitons. However, this dreaming process is fundamentally challenging and so far, to our best knowledge, there is no report on such hot exciton scintillators.
We analyze the possibility of manipulating all hot excitons into S1 states without involving T1 states. Fundamentally, in order to achieve the above effect, the hot excitons should relax from the higher triplet states (Tn) to the singlet states (Sm) and subsequent S1 states via high-lying reverse intersystem crossing (hRISC), avoiding the population of T1 states (Fig. 1a). To realize efficient hRISC, it is obligatory to suppress the internal conversion (IC) of Tn→T1 and accelerate the hRISC of Tn→Sm. If the hRISC rate is sufficiently high, the IC could be completely suppressed, and all the high-energy triplet excitons could be converted to singlet excitons.[18] According to the photochemical theory, the rates of hRISC and IC are inversely proportional to the energy gap between the initial and final states. Therefore, for material screening, the S1 and T1 states of the targeted molecules should be localized exciton with large energy differences, and Tn and Sm should be delocalized excitons with small energy splitting.[19]
Following these rules here we have successfully developed a series of hot exciton scintillators (HES), which also contain heavy halogen atoms to enhance the X/γ-ray attenuation efficiency. Figure 1c presents three typical molecules, 1,1,2,2-tetrakis(4-bromophenyl)ethane (TPE-4Br), 1,1,2,2-tetrakis(3’-bromo-[1,1’-biphenyl]-4-yl)ethane (m-BrTBE) and benchmark organic scintillator anthracene. We compared the scintillator performance of TPE-4Br and m-BrTBE with representative organic and inorganic scintillators, as shown in Fig. 1b and Table S1. TPE-4Br (72600 photons MeV− 1, 1.79 ns) and m-BrTBE (64500 photons MeV− 1, 1.33 ns) exhibits an unprecedented combination of high light yield and short decay time, showcasing the strength of hot exciton scintillators.
We now investigate the scintillation performance of the HES in detail. Figure 2a shows the X-ray absorption spectrum of the organic molecules in a broad range of X-ray photon energies from 1 to 1000 keV. It should be noted that the linear attenuation coefficient of photoelectric effect µ is determined by Z4/E3 (Z is the atomic number and E is the energy of X-ray photons or gamma ray photons). Due to the presence of heavy atom Br (Z = 35), the attenuation efficiency of TPE-4Br and m-BrTBE is much higher than that of anthracene across the whole energy region. The X-ray attenuation efficiency versus thickness of these materials for RQA3 spectrum is reported in Fig. S1. Obviously, a thickness of 1.34 mm is enough for TPE-4Br to attenuate 80% of the incident X-ray photons, while anthracene needs 54.46 mm. The X-ray excited radiation luminescence (RL) spectra of these organic molecules at room temperature are shown in Fig. 2b. In sharp contrast to anthracene with multiple emission peaks, TPE-4Br and m-BrTBE exhibit a single emission peak at 448 nm and 456 nm, respectively. Moreover, the peak shape and position of these organic molecules are almost identical to their photoluminescence (PL) spectrum (Fig. S2), indicating the singlet fluorescence nature of the radioluminescence. The transient photo luminescence (TRPL) of these organic molecules is shown in Fig. 2c. These curves are well-fitted by a single-exponential function. The time constant of TPE-4Br and m-BrTBE are 1.79 ns and 1.33 ns, respectively. Moreover, the photoluminescence quantum yields of TPE-4Br and m-BrTBE are as high as 93.7% and 61.7% (Fig. S3), respectively, demonstrating that the nonradiative recombination is negligible for these two molecules. In comparison, TADF material DMAC-TRZ and TADF-I exhibits long decay tail of 2150 ns and 1420 ns respectively, and the decay time of phosphorescent scintillator 9,9'-(6-iodophenoxy-1,3,5-triazine-2,4-diyl)bis(9H-carbazole) even reaches tens of ms.[14, 15]
Given the efficient absorption efficiency of TPE-4Br, we then selected it as a model to evaluate the steady-state X-ray to light conversion efficiency, lowest detectable dose rate and radiation hardness. To measure the emission intensity and obtain the light yield, TPE-4Br and anthracene powder are fabricated with different thicknesses (Table S2). Here we employ two separate methods for strictly evaluating the light yield, one is the X-ray excited steady radioluminescence intensity[20] and the other is the γ-ray excited pulse radioluminescence intensity (also known as pulse height spectrum)[21]. For the X-ray excited steady RL emission, one side of the scintillator is coupled onto a silicon photomultiplier (SiPM) and the other side is exposed to X-ray (Fig. S4). The light yield is calculated by correcting the wavelength-dependent detection efficiency (Fig. S5) of SiPM for scintillators. The corrected response amplitude of TPE-4Br and anthracene versus thickness is shown in Fig. 2d. Due to the large absorption efficiency, the steady RL intensity of TPE-4Br reaches saturation at a smaller thickness than anthracene. It is worth noting that the resolution and contrast of the image in practical X-ray imaging is directly determined by the steady RL intensity and thus we calculate the relative light yield by comparing the saturated RL intensity of TPE-4Br with that of anthracene. The saturated RL intensity of TPE-4Br is about 4.54 times larger than that of anthracene (16000 photons MeV− 1), which gives the equivalent light yield of TPE-4Br under X-ray excitation as 72600 photons MeV− 1. The light yield of m-BrTBE is also charactered as 64500 photons MeV− 1 with the same method (Fig. S6). To derive the light yield under γ-ray, we recorded the pulse height spectrum of the organic scintillators toward 241Am source (γ-ray: 59.5 keV) (Fig. S7). Fig. S8 shows the γ-ray pulse height spectrum. After correcting the wavelength-dependent detection efficiency of PMT for TPE-4Br and anthracene, the pulse height of TPE-4Br is estimated as 2.94 times larger than that of anthracene. The relatively small light yield of TPE-4Br under γ-ray may be due to the limited absorption capability of the pellets.
Additionally, the lowest detectable dose rate is a crucial parameter for the scintillator. Figure 2e shows the signal-to-noise ratio (SNR) versus irradiation dose rate. We record the lowest detectable dose rate of TPE-4Br as 109 nGyair s− 1 at SNR = 3, which is four times lower than that of anthracene (447 nGyair−1 s− 1) and 50 times smaller than the regular dose rate required in medical diagnostics (5.5 µGyair s− 1)[22], demonstrating its great potential in low dose radiation detection. As for the radiation stability, the TPE-4Br is continuously irradiated by the X-ray without any encapsulation. As shown in Fig. 2f, the light output of the TPE-4Br single crystal exhibits negligible degradation during continuous X-ray irradiation with a total dose of about 250 Gyair at a high dose rate of 5.5 mGyair s− 1 in ambient atmosphere (25 oC, 30% humidity).
To validate the above claimed hot-exciton harvesting mechanism, the geometrical and electronic structures of TPE-4Br is calculated. The results are shown in Fig. 3a (more information is in Fig. S9 and Table S3). The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) almost spread over the whole molecule skeleton. The singlet and the triplet energies are calculated based on TD-DFT method with Tamm-Dancoff approximation with singlet energy levels as S1 (2.70 eV), S2 (3.60 eV), and triplet energy levels as T1 (1.49 eV), T2 (3.06 eV), T3 (3.14 eV), T4 (3.42 eV) and T5 (3.55 eV). Moreover, the calculated energy of S1 (2.70 eV) well matched the PL spectrum of TPE-4Br powder (peak at 448 nm), demonstrating the accuracy of theoretical calculation. Obviously, the energy gap between T2 and T1 is as large as 1.57 eV, while that between S2 and T5-T2 is relatively small (0.05 eV, 0.18 eV, 0.46 eV, 0.54 eV, respectively). Thus, the TPE-4Br indeed has a large Tn-T1 energy gap and a small Tn-Sm energy splitting, which is necessary to suppress the internal conversion of Tn to T1 and accelerate the hRISC of Tn to Sm. Furthermore, according to the perturbation theory, the rate of ISC and hRISC, kISC and kRISC, is determined by the energy gap (△EST) and the spin-orbit coupling (SOC) constant (ξST) between the involved triplet and singlet states, and can be expressed by the formula[23]:
$${k}_{ISC/RISC}\propto \frac{{{{\xi }}_{ST}}^{2}}{{e}^{{{△E}_{ST}}^{2}}}$$
The SOC strengths between S1 and T1-T5 are calculated as 0.54 cm− 1, 6.22 cm− 1, 9.38 cm− 1, 2.11 cm− 1, 5.21 cm− 1 (Table S4), respectively. Obviously, the SOC strength between S1 and T2-T5 is much larger than that between S1 and T1. The stronger SOC strength as well as smaller energy gap between S1 and T2-T5 of TPE-4Br together guarantee the considerable hRISC rate from Tn to Sm.
Furthermore, from the experimental view, there are two processes need to be identified to validate the hot-exciton harvesting mechanism: (1) fast hRISC process and (2) negligible ISC efficiency from S1 to T1. To verify the fast hRISC process, we directly use a sensitizer to populate only the high triplet states (Tn) of TPE-4Br. If the high triplet states (Tn) of TPE-4Br relax to T1 state, we then could observe phosphorescence or delayed fluorescence with long lifetime. Otherwise, the high triplet states of TPE-4Br take the relaxation channel to S1 state through the fast hRISC process. Here benzophenone (BP) is chosen as the sensitizer and has a well-known T1 state level of 3.01 eV,[24] which is very close to the T2 state (3.06 eV) of TPE-4Br. The matched energy level and spin state guarantees the successful population of T2 states. The inset of Fig. 3b shows the PL spectrum of the pure TPE-4Br (T) solution and the TPE-4Br solution sensitized by BP (TB). There is negligible difference of PL spectrum shape and the peak position (473 nm) for T solution and TB solution, indicating that the luminescence of the two solutions originates from the same S1 radiative channel of TPE-4Br. The PL decay profiles of the T solution and TB solution at room temperature with an excitation wavelength of 340 nm are shown in Fig. 3b. Both solutions demonstrate a fast decay time of 0.96 ns for T solution and 1.44 ns for TB solution, which is identified as the the S1 state emission of TPE-4Br. In addition to the fast decay time, the TB solution exhibits another slightly slow decay time of 4.56 ns, which is originated from the sensitizing process from BP to TPE-4Br. Anyway, there is no long lifetime component for phosphorescence or delayed fluorescence, confirming the fast hRISC process in TPE-4Br. The illustration of the photo-physical processes existing in the TPE-4Br solutions with BP are shown in Fig. S10.
Subsequently, we adopt the previously established transient absorption method to evaluate negligible ISC from S1 to T1 of TPE-4Br.[25, 26] We prepared TPE-4Br and benzil (reference) solutions. β-carotene was added to both TPE-4Br and benzil solutions. TPE-4Br and benzil could serve as the triplet exciton donors to sensitize β-carotene. β-carotene is selected because the negligible excitation by the selected pump wavelength (343 nm) and its well-known insignificant ISC (\({\text{Փ}}_{isc}\approx 0\)). Thereby, the observation of triplet emission of β-carotene could directly reflect the occupation of T1 states of the sensitizer. The illustration of the photo-physical processes in the solutions with and without β-carotene is shown in Fig. S11. For the solutions without β-carotene, after the excitation of donors (TPE-4Br or benzil), the excitons are populated to S1 state, and then some of the S1 state excitons could transfer to the T1 state by ISC process. In pump-probe spectra, it is possible to observe the S1-Sn and T1-Tn transient absorption of the donors. As shown in Fig. S12, we indeed observed two photo absorption (PA) features at 529 nm and 487 nm for the pure benzil solutions without β-carotene, where the PA signal at 529 nm (life time: 2.39 ns) is originated from the S1-Sn process[27] and that at 487 nm[28] (life time: 2.62 µs) is ascribed to the T1-Tn process of benzil. The same process also occurs in TPE-4Br solutions, but we only observed one PA features at 458 nm (life time: 1.39 ns), which is originated from the S1-Sn process (Fig. S13). For the solutions with β-carotene addition, the T1 excitons of the donors (TPE-4Br or benzil) could principllly sensitize T1 state of the acceptor (β-carotene). Therefore, there will be the additional PA signal caused by T1-Tn transient absorption of the acceptors. We can see an additional PA features at 519 nm of benzil solutions with β-carotene, which is ascribed to the T1-Tn process of β-carotene (Fig. 3c and Figure S14).[29] However, there is still only one PA signal features at 458 nm for TPE-4Br solutions with β-carotene. The absence of T1 state PA signal in both TPE-4Br solutions with and without β-carotene strongly testifies the negligible ISC process form S1 to T1 in TPE-4Br.
In addition to the effective hRISC and negligible S1-T1 ISC processes, the fluorescence efficiency (S1-S0) is also very important to guarantee the scintillation yield. Commonly, the halogen atoms of organic materials can lead to the enhancement of phosphorescence and the quenching of the fluorescence by promoting the ISC process from S1 to T1 (also known as heavy-atom effect).[30] We then measured the fluorescence efficiency of TPE-4Br and its parent molecule without bromide TPE (the molecular structure is shown in Fig. S16a) at room temperature by an absolute photoluminescence measurement system with an integrating sphere. The fluorescence efficiency of TPE-4Br and TPE are 93.7% and 60.5% via calculation of emission divided by absorbance (Fig. S16b). To our surprise, the introduction of bromine atoms in TPE-4Br exhibits improved emission efficiency compared to TPE. The anomalous anti-heavy-atom effect of TPE-4Br is quite different from the traditional heavy-atom effect. As TPE-4Br has the properties of aggregation-induced emission (AIE), we also calculate the density of states (DOS) for TPE-4Br as shown in Fig. 3e, the valence band maximum (VBM) and conduction band minimum (CBM) are both derived from the carbon atoms with little contribution from the hydrogen and bromine atoms, consistent with the calculation results of HOMO and LUMO. As a result, the heavy bromine atoms do not contribute to the S1-T1 ISC process. More importanly, the presence of Br in TPE-4Br molecule also results in some additional strong intermolecular forces. The Hirshfeld surface mapped over density of normal distribution (dnorm) and decomposed fingerprint plots of TPE-4Br and TPE are shown in Fig. 3f and Fig. S17. The intermolecular interactions of Br···H and Br···Br bonds can be easily found in TPE-4Br molecules with ordered packing pattern in the crystal structure. The proportion of Br···H and Br···Br interaction to total intermolecular interaction are 37.2% and 9.9% respectively, playing important roles in the intermolecular interactions among TPE-4Br molecules. The presense of the hydrogen bonding and halogen bonding is responsible for the restriction of intermolecular motions and thus suppression of the phonon-mediated emission quenching.
Efficient and ultrafast detection of X/γ-rays plays an important role in medical imaging, flash X-ray radiography and high-energy physics (Fig. 4a).[31–33] For example, as a result of relying on the time of flight (TOF) technique, fast scintillators are critical for PET and CT to get better temporal resolution.[34.35] Moreover, scintillators with ultrafast decay time have the capability to distinguish almost simultaneous events and mitigate the pile-up effect (Fig. 4b), which is vital in flash X-ray radiography and high-energy physics. The luminescence decay curve (X-ray pulse excited) of TPE-4Br is shown in Fig. 4c and the decay time is fitted as 1.32 ns, consistent with the photoluminescence decay time (1.79 ns). We demonstrate the high resolution and flash X-ray imaging capability by TPE-4Br scintillators. The X-ray imaging system is constructed with a digital camera, a reflector and the Amptek Mini X-ray tube (Fig. S18). To fabricate a large scintillator film, we mixed the TPE-4Br molecules into sucrose octaacetate (SO) to obtain a plastic scintillator screen. We optimized the solubility as 1.7 wt% of TPE-4Br in SO matrix (Fig. 4d: top left) for later imaging. The X-ray image of an opaque capsule with a built-in metal spring clearly shows the internal structure (Fig. 4d: top right). The bottom of Fig. 4d shows the image of a standard X-ray resolution pattern and indicates that the line pair notches at ~ 20.0 lp mm− 1 could be well resolved. Furthermore, the modulation transfer function (MTF) curve (Fig. 4e) is obtained by adopting the slanted-edge method (Fig. S19). The spatial resolution at MTF = 0.2 of TPE-4Br scintillator, commercial CsI:Tl film and Gd2O2S2:Tb (GOS) film are 18.69 lp mm− 1, 4.82 lp mm− 1 and 2.78 lp mm− 1, respectively. The high spatial resolution exceeds most scintillator screens (Table S5) and is ascribed to the uniformly distributed refractive index and weak optical scattering of the transparent scintillator screen. In addition, we also characterized the normalized noise power spectra (NNPS) of TPE-4Br scintillator (Fig. 4f), which is almost flatten and outperforming commercial scintillators (TPE-4Br: 10− 4 mm2, CsI:Tl: 10− 3 mm2),[36] indicating the small cross-talk between neighbouring pixels. Figure 4g shows the flash X-ray imaging of the scintillator screen. The fusion process of bubbles in the saturated solution of lead based compounds (0.6 g MAPbI3 in 1 mL γ-butyrolactone) could be clearly recorded by our high-speed imaging facility. The frame rates reach 10000 frame per second (fps). The right of Fig. 4g exhibits the specific merging process of two bobbles. The video of the mobile bubbles process is in Supplementary Information Movie 1. We note that considering the decay time constant of TPE-4Br, the imaging speed could be even increased to 108 fps, and the frame rates of the current system is restricted by the image camera. The ultrafast imaging could provide significant information for interior, intermediate, exterior and terminal ballistics and detonation research applications, as well as validating computer models and determining material parameters in transient interactions.
We also investigate the potential utility of TPE-4Br for PET imaging. The ultra-short pulse of the TPE-4Br crystals is of great benefit to time-of-flight PET (TOF-PET) which is a definite trend toward reducing image noise and improving the identification of cancerous lesions. To evaluate the coincidence time resolution (CTR) of the TPE-4Br crystals, we recorded the event data with two TPE-4Br based detectors (Fig. 4h). Figure 4i shows typical waveforms of coincidence pulse pair from the two detectors. We calculate the time difference (Δt) for 10,000 coincidence events and plot the histogram. Through the same way, we acquire the waveforms and the histogram of LYSO, which is commercially used for present PET system. The CTRs for TPE-4Br and LYSO, which are determined from Gaussian fitting to the data (Fig. 4j), are calculated to be 180 ps and 270 ps (full width at half maximum), respectively. The CTR obtained with TPE-4Br crystals is much better than the commercial PET scintillator LYSO, implying the ability to diagnose smaller lesions or tumors according to the formula in the inset of Fig. 4i.
In conclusion, we have developed a new family of hot-exciton scintillators to achieve efficient and ultrafast radioluminescence. The large energy gap between high-lying and lowest triplet state (Tn-T1, n ≥ 2), small energy splitting between high-lying triplet and singlet states (Tn-Sm, n ≥ 2, m ≥ 1), and strong spin-orbit coupling between S1 and Tn (n ≥ 2) facilitates the fully harvesting of radiation-induced hot excitons into the fast radiative S1 state without population of T1 state. Our strategy provides a solution to overcome the intrinsic dilemma between high scintillation efficiency and fast timing properties of scintillators, and our hot-exciton scintillator TPE-4Br shows a balanced performance outperforming nearly all existing scintillators. Our finding not only provides a paradigm-shifting approach for organic scintillator design, but also broaden the utility of organic scintillators in various ultrafast radiation detection applications.