Article pubs.acs.org/IC
Tetranuclear Copper(I) Iodide Complexes: A New Class of X‑ray Phosphors Kaplan Kirakci,*,† Karla Fejfarová,‡ Jiří Martinčík,‡,§ Martin Nikl,‡ and Kamil Lang*,† Institute of Inorganic Chemistry of the Czech Academy of Sciences, v.v.i., Husinec-Ř ež 1001, 250 68 Ř ež, Czech Republic Institute of Physics of the Czech Academy of Sciences, v.v.i., Cukrovarnická 10/112, 162 00 Praha 6, Czech Republic § Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Břehová 7, 115 19 Praha 1, Czech Republic † ‡
S Supporting Information *
ABSTRACT: We report intensive visible light radioluminescence upon X-ray irradiation of archetypal tetranuclear copper(I) iodide complexes containing triphenylphosphine or pyridine ligands in the solid state. These properties, attractive for the design of X-ray responsive materials, can be attributed to the heavy {Cu4I4} cubane-like core, the absence of oxygen quenching of the emissive triplet states, and the high photoluminescence quantum yields. Radioluminescence originates from the same emissive triplet states as those produced by ultraviolet excitation as confirmed by the observed radioluminescence thermochromism. The radioluminescence properties are also preserved after incorporation of these complexes into polystyrene films, making them appealing for the development of plastic scintillators.
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and pressure-sensing purposes.16−20 As these complexes have affordable synthetic protocols and high photoluminescence yields and are built from high-Z elements, we suggest that complexes with the {Cu4I4} cluster core are suitable for the design of scintillating materials. We now report uncharted radioluminescence properties of archetypal copper(I) iodide complexes [Cu4I4(PPh3)4] (1) and [Cu4I4py4] (2a) with cubane-like structure (Figure 1). In addition, we describe the structure and properties of a new polymorph, [Cu4I4py4]·0.5C4H8O (2b). The radioluminescence properties of powders of these complexes upon X-ray irradiation were studied at varying temperatures and compared with the corresponding photoluminescence behavior under
INTRODUCTION Scintillating materials, emitting visible light radioluminescence, are widely used for the detection of ionizing radiation in medicine, astrophysics, homeland security, and industry. The emergence of new medical modalities taking advantage of the deeper penetration of living tissue by X-rays, such as X-ray luminescence-computed tomography or X-ray-induced photodynamic therapy, makes the study of new scintillating materials relevant.1,2 In this context, transition metal cluster complexes appear to be highly attractive for the design of X-ray responsive materials because of the combination of a high atomic number (Z), good luminescence efficiency, and the possibility of being incorporated into inorganic or organic matrices, or nanocarriers.3 We have recently reported radioluminescence properties of an octahedral Mo(II) cluster complex and suggested that other transition metal cluster complexes could exhibit similar features.4 Tetranuclear copper(I) iodide complexes [Cu4I4L4] (L = cyclic amine or phosphine) with cubane-like structure have been investigated extensively because of their peculiar photoluminescence properties.5−7 These complexes have two distinct emission bands in the solid state: a high-energy band with a maximum in the range of 400−450 nm, which is predominant at low temperatures and is assigned to the halide (X)-to-ligand charge (3XLCT) transition state, and a low-energy band around 520−600 nm that prevails at room temperature and arises from a cluster-centered excited state (3CC).6,8−11 The position of the low-energy band can be correlated with Cu−Cu distances in the {Cu4I4} cluster core, a parameter that is affected by various factors.12−15 The photoluminescence properties together with the successful incorporation of these complexes into suitable matrices have allowed their utilization for gas-, temperature-, © XXXX American Chemical Society
Figure 1. Schematic structures of 1 (left) and 2a and 2b (right). Color coding: dark yellow for Cu, magenta for I, yellow for P, blue for N, and black for C. Hydrogen atoms have been omitted for the sake of clarity. Received: January 30, 2017
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DOI: 10.1021/acs.inorgchem.7b00240 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
on a single grating monochromator with a grating blazing wavelength of 500 nm and a model TBX-04 photon counting detector. For excitation was used a tungsten DX-W 10×1-S 2400 W X-ray tube (short anode, Be window, Seifert GmbH) powered by an ISODEBYEFLEX 3003 high-voltage supply set to 40 kV and 15 mA. Measured spectra were corrected for the spectral dependence of detection sensitivity. The emission spectrum of BGO powder was used as a reference of absolute emission intensity.
ultraviolet (UV) excitation. Complexes 1 and 2a were also incorporated into polystyrene matrices to fabricate plastic scintillators.
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EXPERIMENTAL SECTION
General. Compounds were prepared by using methods derived from previously published procedures.5,24 Copper iodide, pyridine, triphenylphosphine, and polystyrene (average Mw ∼ 192000) were purchased from Sigma-Aldrich and used as received. Solvents for syntheses were purchased from Penta and were dried over molecular sieves. The reactions were performed under an argon atmosphere using a standard technique. Synthesis of [Cu4I4(PPh3)4] (1). CuI (500 mg, 2.63 mmol) and triphenylphosphine (690 mg, 2.63 mmol) were introduced into a Schlenk flask, and 100 mL of chloroform was added. The mixture was left to stir for 18 h at room temperature, heated at 60 °C for 30 min, and hot filtered. The solvent from the resulting solution was removed on a rotary evaporator, and the remaining solid was washed three times with 20 mL of diethyl ether, yielding 920 mg of a white powder (77% yield): 1H NMR (400 MHz, CDCl3) δ 7.28 (d, 24H, ArH), 7.37 (t, 12H, ArH), 7.54 (t, 24H, ArH); 31P NMR (400 MHz, CDCl3) δ −20.60 (br s). Synthesis of [Cu4I4py4] (2a). CuI (500 mg, 2.63 mmol) and pyridine (0.21 mL, 2.63 mmol) were introduced into a Schlenk flask, and 100 mL of chloroform was added. The mixture was left to stir for 18 h at room temperature and filtered. The solvent was removed on a rotary evaporator, and the resulting solid was washed three times with 20 mL of diethyl ether, yielding 480 mg of a white powder (68% yield): 1H NMR (400 MHz, CDCl3) δ 7.62 (br s, 8H, ArH), 7.80 (br s, 4H, ArH), 9.59 (br s, 8H, ArH). Synthesis of [Cu4I4py4]·0.5C4H8O (2b). 2a (200 mg, 185 μmol) was dissolved in 20 mL of tetrahydrofuran (THF). The resulting solution was filtered and allowed to slowly evaporate in a glass Petri dish. After 3 days, orange needles were collected in a virtually quantitative yield. Preparation of the 1/PS Film with 2 wt % Loading. Polystyrene beads (980 mg) and 1 (20 mg) were added to 30 mL of chloroform. The mixture was stirred for 1 h at room temperature; the resulting orange dispersion was transferred to a glass Petri dish (35 mm × 15 mm), and the solvent was allowed to slowly evaporate, yielding a film with a thickness of approximately 0.2 mm. Preparation of the 2a/PS Film with 10 wt % Loading. Polystyrene beads (900 mg) and 2a (100 mg) were added to 30 mL of chloroform. The mixture was stirred for 1 h at room temperature; the resulting orange dispersion was transferred to a glass Petri dish (35 mm × 15 mm), and the solvent was allowed to slowly evaporate, yielding a film with a thickness of approximately 0.2 mm. Crystallographic Study of 2b. Diffraction data were collected at 293 K on a four-cycle Gemini diffractometer (Rigaku Oxford Diffraction) equipped with an Atlas CCD detector, using monochromated Mo Kα radiation (λ = 0.71073 Å) from a sealed X-ray tube, monochromatized with a graphite monochromator, and collimated with a fiber-optic Mo-Enhance collimator. Integration of the CCD images was performed using CrysAlisPro software (Agilent Technologies). The same program was used to index the crystal shape and absorption correction. The crystal structure was determined by direct methods with Sir2002.21 The obtained solution was used for the subsequent refinement based on F2 utilizing Jana2006.22 Luminescence Measurements. Luminescence spectra were recorded on a Fluorolog 3 spectrometer (Horiba Jobin Yvon) with a cooled TBX-05-C photon detection module. The same instrument was used for luminescence lifetime experiments using excitation at 390 nm (SpectraLED-390, Horiba Scientific). The decay curves were fitted to exponential functions by the iterative reconvolution procedure of the DAS6 software (version 6.4, Horiba Jobin Yvon). Absolute photoluminescence quantum yields were measured using a Quantaurus QY C11347-1 spectrometer (Hamamatsu). Corrected radioluminescence spectra were recorded with a Horiba Jobin Yvon 5000M spectrofluorometer. The detection part was based
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RESULTS AND DISCUSSION Synthesis and Crystallography. Complexes 1 and 2a were prepared by reacting copper(I) iodide with triphenylphosphine and pyridine, respectively, in deaerated chloroform for 18 h at room temperature, followed by filtration, evaporation of the solvent, and subsequent washing with diethyl ether. The identity and purity were checked by 1H NMR and powder X-ray diffractions (Figures S1−S8). As previously reported, 1 and 2a crystallize in the P21/n and P21 space groups, respectively.23,24 An unreported polymorph, [Cu4I4py4]·0.5THF (2b), was obtained by recrystallization of 2a in THF (Table S1). This phase (CCDC 1522700) crystallizes in the P42bc tetragonal space group and contains THF molecules in the voids between the copper(I) cubanes (Figure 2). Analysis of the intramolecular interatomic distances
Figure 2. Crystallographic representation of the determined molecular structure of 2b with 50% thermal ellipsoids (light brown for copper, magenta for iodine, blue for nitrogen, red for oxygen, and black for carbon; hydrogen atoms omitted for the sake of clarity). Projection of the crystal structure of 2b along the b direction.
in 2b revealed average Cu−Cu, Cu−I, and Cu−N bond lengths of 2.644(1), 2.695(1), and 2.027(7) Å, respectively, indicating that the Cu−Cu distances in 2b are shorter than those in 2a. Polymorph 2b is metastable and partially converts to 2a after several weeks at room temperature (Figure S4). The conversion can be achieved quickly and quantitatively by washing 2b with diethyl ether, leading to the removal of cocrystallized THF (Figure S5). General Aspects of Photoluminescence Properties. Even though the photoluminescence properties of 1 and 2a in the solid state have already been extensively studied, we reevaluated them to allow for their accurate comparison with radioluminescence properties. Figure 3 shows the corresponding normalized photoluminescence spectra at room temperature. Large Stokes shifts indicate significant structural reorganization in the excited states. Complex 1 emits intense green photoluminescence with a maximum at 547 nm, and 2a produces yellow emission peaking at 580−585 nm, in accordance with previous reports (Figure 2).5,9,12,25 For comparison, the luminescence maxima of two other B
DOI: 10.1021/acs.inorgchem.7b00240 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
and 2a, respectively, were obtained upon 350 nm excitation in air and are slightly higher than the previously reported yields of 0.64 and 0.51, respectively.7,9 For comparison, polymorph 2b is less emissive than 1 and 2a under the same conditions, with a photoluminescence quantum yield of 0.28. Radioluminescence Properties. Irradiation of the powdered compounds with X-rays generated by a tungsten tube leads to visible light emissions. In the case of 1 and 2a, the radioluminescence spectra are identical to the photoluminescence spectra obtained by UV excitation of the corresponding powders (Figures 3 and 4). The radioluminescence band of 2b is slightly red-shifted to approximately 620 nm when compared with its photoluminescence band, and this discrepancy is reminiscent of the wavelength dependence of the photoluminescence spectra discussed above (Figure 3). The applied X-ray excitation does not change the emission characteristics of the samples as their repeated measurements provide the same intensities of the radioluminescence spectra. Evidently, the complexes do not show radiation damage within the estimated excitation doses up to 0.1 Gy. The scintillating efficiencies of 1 and 2a are approximately 4 and 2.5 times greater, respectively, than that of Bi4Ge3O12 (BGO) powder, a typical scintillating standard (Figure 5). The
Figure 3. Normalized photoluminescence spectra (PL) of 1, 2a, and 2b powders excited at 350 nm compared with corresponding radioluminescence spectra (RL).
[Cu4I4(PPh3)4] polymorphs were reported at 535 and 520 nm. These blue shifts with respect to 1 were correlated with larger Cu−Cu distances within the {Cu4I4} core.9,12 Compound 2b shows orange photoluminescence with a maximum at 607 nm (Figures 3 and 4). The red shift of the emission band with
Figure 4. (A) Photoluminescence of 1, 2a, and 2b powders under 365 nm excitation compared with (B) radioluminescence of 1 and 2a under X-rays (Cu Kα, 40 kV, 30 mA).
respect to 2a can be attributed to more intensive cuprophilic interactions evidenced by shorter Cu−Cu distances in 2b than in 2a. Interestingly, the luminescence spectrum of 2b shows an excitation wavelength dependence probably due to variations in the local environment of the complexes, slightly affecting the triplet state energy. Presumably, the distribution of THF molecules is not homogeneous (evidently, fewer THF molecules are at the crystallite surface than in the volume), in accordance with the liberation of this cocrystallization solvent over time. This hypothesis is corroborated by the fact that fresh unground crystals of 2b, which should have a larger amount of THF than a fine powder sample, have a red-shifted emission with a maximum at approximately 635 nm (excited at 350 nm). The luminescence lifetimes of the low-energy bands of 1 and 2a are 5.2 and 10.8 μs, respectively, and are in accordance with the literature. Complex 2b displays a lifetime of 9.6 μs. It is noteworthy that the removal of oxygen from powders did not trigger changes in the emission intensities and lifetimes, pointing out that the emissive triplet states are not quenched by oxygen in the solid state. This observation contrasts with previous studies evidencing quenching of the emission by oxygen when 2a is dissolved in toluene or loaded into mesoporous silica.5,26 The absence of quenching probably originates from limited diffusion of oxygen within the structures and is beneficial in terms of the luminescence efficiency in air. The photoluminescence quantum yields of 0.86 and 0.75 for 1
Figure 5. Absolute emission intensities of radioluminescence spectra of 1 (black), 2a (red), and 2b (blue) powders compared with that of Bi4Ge3O12 (BGO) powder (magenta). The spectra were recorded in an air atmosphere upon excitation with an X-ray tungsten tube (40 kV, 15 mA) at room temperature.
experiments were performed in an air atmosphere or in vacuum with no effects on the radioluminescence intensities, indicating no radioluminescence quenching by oxygen. This feature contrasts with the recently reported hexanuclear molybdenum cluster complex and substantiates the high radioluminescence efficiencies of 1 and 2a.4 Complex 2b displays a lower scintillating efficiency, comparable to that of BGO, and this feature can probably be correlated with the lower photoluminescence quantum yield. The results indicate that radioluminescence originates from the same excited triplet states as those produced by UV excitation with the preservation of high emission efficiencies. To confirm this assumption, we studied the temperature dependence of the radioluminescence spectra that are key signatures of the nature of the excited emissive states. Because of the metastable nature of 2b, we did not select this compound for further investigation. The temperature dependence of radioluminescence from 295 to 77 K revealed a progressive red shift of the low-energy band C
DOI: 10.1021/acs.inorgchem.7b00240 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
separation of phases was clearly observed at higher loadings. The photoluminescence spectra of the films have maxima at 555 and 620 nm for 1/PS and 2a/PS, respectively (Figure 7).
to approximately 575 and 605 nm for 1 and 2a, respectively (Figure 6). A blue-emitting band appears around 90 K at
Figure 7. Normalized photoluminescence (a and c, black) and radioluminescence (b and d, red) spectra of 1/PS (a and b, 2 wt %, excited at 300 nm) and 2a/PS (c and d, 10 wt %, excited at 316 nm). The right panels show the corresponding foils under UV light (365 nm).
The embedding of complexes in the polystyrene matrix causes a decrease in absolute photoluminescence quantum yields to 0.22 for 1/PS and 0.02 for 2a/PS (excitation at 350 nm) when compared with the yields of respective powders. The low quantum yield of 2a/PS can be attributed to the presence of nonemissive absorption bands in the visible region as indicated by the transmittance spectrum (Figure S11). The effect of the polystyrene environment is also reflected by shortening of photoluminescence lifetimes, i.e., by the appearance of a shortlived luminescence component in contrast to corresponding powders. Thus, photoluminescence decays are best analyzed by a biexponential function giving 2.3 μs (52%) and 5.3 μs (48%) components for 1/PS and 5.5 μs (39%) and 10.6 μs (61%) components for 2a/PS. Irradiation of the films with X-rays led to the same emission spectra that were observed under UV excitation (Figure 7 and Figure S12). No radioluminescence of the polystyrene scintillating matrix was observed, indicating that all energy emitted by the polystyrene matrix is transferred to the luminescent copper complexes. This feature is not surprising as polystyrene fluorescence in the UV region favorably overlaps with the absorption bands of both complexes. This energy transfer probably increases the efficiency of the scintillating films. Typical X-ray plastic scintillators are based on polystyrene doped with wavelength shifters and luminophores such as anthracene or p-terphenyl. Eventually, an X-ray absorber containing heavy metals such as lead or bismuth may also be added. Thus, it is reasonable to compare the performance of 1/ PS and 2a/PS with a commercial organic scintillator based on polystyrene under the same experimental conditions. Preliminary comparison of the commercial plastic scintillator (NUVIA Inc.) with the studied films shows that 1/PS and 2a/PS reach approximately 16 and 47% of the scintillating efficiency of the commercial scintillator, respectively. These results are encouraging considering the moderate loading of the active complexes within the polystyrene matrix and document that the use of the studied complexes combining more functions can simplify the architecture of plastic scintillators.
Figure 6. Normalized radioluminescence spectra of (A) 1 and (B) 2a at various temperatures.
approximately 420 and 435 nm for 1 and 2a, respectively, which is concomitant with the decrease in the intensity of the low-energy emission band. The observed radioluminescence thermochromism following X-ray absorption is equivalent to the temperature dependence of the photoluminescence, reported by previous authors5,9,12,13,25 and measured by us (Figures S9 and S10). These results confirm that excitation with UV light and X-rays leads to the same emissive triplet states. Overall, the pathway from X-ray irradiation to visible light emission seems to be the same for these complexes and for a recently reported hexanuclear molybdenum cluster complex.4 In general, X-rays with
