A New Copper(I) Iodide Based Organic-Inorganic Hybrid Structure with Red Emission

: A new organic-inorganic hybrid structure based on copper (I) iodide staircase chain 1D-Cu 2 I 2 (5-chloropyrimidine) 2 ( 1 ) has been synthesized by a slow-diffusion method. It emits red emission peaking at 620 nm. The internal quantum yield (IQY) measured for this compound is 6.5% under 360 nm excitation. This compound exhibits potential as a non-rare-earth light-emitting phosphor


Introduction
Replacement of traditional energy-costly incandescent lamps by light-emitting diode (LED) lamps could be an important step on reducing the amount of electricity usage worldwide [1][2][3]. Since modern LEDs are typically monochromatic emitters, phosphors are usually used in combination with a monochromatic LED to generate target light with high quality for general lighting usage [4]. Examples, such as white light LED (WLED), whose emission spectrum covers the whole visible light region (400 nm-700 nm), are majorly produced by a green-yellow phosphor, Ce 3+ intermingled yttrium aluminum garnet (YAG), coating onto a blue emitting InGaN/GaN diode [5]. The combination of emission from yellow phosphor and light from blue LED through the phosphor would generate white light. This type of LEDs is called phosphor-converted (pc-) LEDs. Except yellow ones, phosphors with other colors are also in significant demand for specific usages. Examples include blue phosphors, such as BaCa 2 MgSi 2 O 8 : Eu 2+ [6] and red phosphors, such as (Ba,Sr) 2 Si 5 N 8 :Eu 2+ [7] and so on [8]. Rare-earth (RE) metal doped with inorganic ligand, like sulfides, nitrides, oxides, oxysulfides, and oxynitrides, is the major formation of commercial phosphors today. They possess the advantages of high efficiency and stability. However, the RE metal is essential either for building the inorganic structures or to control the light quality, and RE metals, such as europium and yttrium, are at supply risk and have environmental issues [8]. What is more, their synthesis usually involves extremely high temperatures (>1000 • C) and synthetic processes are always complicated. Because of these drawbacks, developing non-rare-earth new types of phosphors that could overcome these issues is of significant importance [4,9].
There have been several types of non-rare-earth phosphors studied recently, such as inorganic nanocrystals and quantum dots [10,11]. Complicated synthetic procedure and particle size dependence are the limitation of their practical application. Organic-inorganic hybrid materials are promising phosphor candidates for their novel optical properties that arise from their hybrid nature and several structure systems have been published as rare-earth free phosphors for lighting usages [12][13][14][15][16][17][18][19][20]. Among them, structure diversity, optical tunability and facile synthesis are the particular interesting characteristics of the copper(I) iodide (I-VII semiconductors)-based hybrid materials [21][22][23][24]. Copper iodidebased organic-inorganic hybrid structures have a variety of inorganic modules [25,26]. Among them, structures that are based on copper iodide rhomboid dimer are particular interesting due to the tunable band gaps and facile synthesis [27][28][29][30]. Research reveals that the valence band is majorly made of I(5p) and Cu(3d) orbitals while the lowest unoccupied molecular orbital (LUMO) of the organic ligands contribute to the conduction band. Their band gaps could be synthetically modulated through incorporating ligands with various LUMO energies into the structures, which could lead to band gap emission at various wavelengths. This is also in agreement with the two luminescence mechanisms, i.e., halide-to-ligand charge transfer (XLCT), and metal-to-ligand charge transfer (MLCT), observed for staircase chain structures [25,[31][32][33]. Among all the reported copper iodide phosphors, highly efficient low-energy emitters are particularly rare. In this paper, we report a new organic-inorganic hybrid structure on the basis of copper halide staircase chain 1D-Cu 2 I 2 (5-chloropyrimidine) 2 (1) as a promising candidate for light emitting phosphors. This compound emits red emission peaking at 620 nm. The internal quantum yield (IQY) measured for 1 is 6.5% under 360 nm excitation.

Synthesis Procedure of Compound 1
1D-Cu 2 I 2 (5-chloropyrimidine) 2 single crystals were cultured by slow diffusion that has been reported previously [23]. The reactions were operated in glass bottles with lid at room temperature. There were three layers in this reaction. The bottom layer was a CuI/KI saturated aqueous solution. The middle was acetonitrile. The top layer was ligand in ethanol. The single crystals were typically generated at the middle layer in three to five days under room temperature condition. To obtain the samples of pure phase powder, the CuI (0.019 g, 0.1 mmol) in saturated KI solution was directly mixed with ligand (0.1 mmol) in ethanol. After stirring, the powder samples would normally be generated straight away. Mild heating (60 • C for 12 h) may be helpful for obtaining samples of higher crystallinity.

Single Crystal X-ray Diffraction (SC-XRD)
To collect the SXRD data, a Bruker APEX-II CCD diffractometer was used at low temperature (193 K) with graphite-monochromatic MoKα radiation (λ = 0.71073 Å). Direct methods were applied to solve the structures, and the Bruker SHELXTL package was employed to refine the data by full-matrix least-squares on F 2 . These data can be acquired from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/ cif, accessed on 24 May 2021 without any charge. The structure was loaded into the Cambridge Structural Database (CSD) with the number 1999510 (see in Supplementary Materials). Crystal data of compound 1 are shown in Table 1.

Powder X-ray Diffraction (PXRD)
The PXRD spectrum of compound 1 was detected by a Bruker D8 Advance automated diffraction system, in which Cu Kα radiation was used with a λ value of 1.5406 Å. A 3-50 • 2θ range and a 1 • /min scan speed was adopted to gather the data. The system was operated with a power of 40 kV/40 mA at room temperature.

UV-vis Diffuse Reflectance Spectra
A Shimadzu UV-3600 UV/VIS/NIR spectrometer was used to obtain the UV-vis diffuse reflectance spectra at room temperature. The reflectance data were then converted to the Kubelka-Munk function of α/S = (1 − R) 2 /2R (where α is the absorption coefficient, S represents the scattering coefficient and R represents the reflectance), from which the band gap was estimated. To prepare the samples for reflectance measurements, the ground powder sample was evenly distributed between two quartz slides.

Thermogravimetric (TG) Analysis
A computer-controlled TG 550 (TA Instrument) was used to conduct the TG analysis. Pure powder samples were placed into platinum pans. Then, they were heated to 400 • C from room temperature with a 10 • C/min heating rate.

Photoluminescence Measurements
A FLS1000 spectrofluorometer was used to obtain steady-state photoluminescence spectra under room temperature condition.

Internal Quantum Yield (IQY) Measurements
To measure the IQY of crystals, a Hamamatsu Photonics C9920-03 absolute quantum yield measurement system was used, for which a 3.3 inch integrating sphere and 150 W xenon monochromatic light source were applied.

Results and Discussion
Various approaches have been developed to grow single crystals of copper iodidebased hybrid structures. Among them, the slow-diffusion method is an efficient method for obtaining high-purity crystals of Cu 2 I 2 rhomboid dimer type structures. Because of the fast nucleation rate, these compounds generally precipitate out immediately after direct mixing of the copper iodide with the ligands at room temperature. To slow down the reaction and to promote the crystallization process, a third solvent, namely acetonitrile, was used in our synthesis as a buffer layer and was placed between two solutions containing the copper iodide (CuI/KI aqueous solution) and the ligand (in ethanol solution). With this approach, compound 1 was obtained with fine crystal samples. The analysis from single crystal X-ray diffraction demonstrated a crystallization in the orthorhombic space group Cmca for compound 1. The inorganic part of compound 1 is discrete Cu 2 I 2 dimer. As shown in Figure 1a, the copper ions link to two iodide ions and two ligand molecules, generating a tetrahedral geometry. The Cu 2 I 2 inorganic modules have been connected by the bidentate organic ligand molecules. The phase purity was revealed by PXRD analyses. As shown in Figure 1b, the peak positions of PXRD patterns are consistent with those simulated from single crystal X-ray data, which indicates that a pure phase is gained. It is interesting to note that the two N atoms in the pyrimidine-based ligand coordinate to Cu atoms, generating one-dimensional (1D) extended hybrid structures. Excepted pyrimidine derivatives, other bidentate ligands, such as pyrazine derivatives, are commonly used for the synthesis of multidimensional copper iodide-based hybrid structures [34]. The asymmetric unit of compound 1 has been plotted in Figure 1c. The TG plot shows that 120 • C is the decomposition temperature of compound 1 (Figure 2a). This is in accordance with other 1D copper iodide staircase chain-based structures with pyrimidine/pyrazine derivatives [35]. The use of other organic ligands forming stronger Cu-N bonds may improve the stability of these compounds. The UV absorption spectrum of compound 1 is shown in Figure 2b. The estimated energy gap of compound 1 is ∼2.1 eV. The commercial CASTEP code implemented in the Material Studio 5.0 package was used to conduct the first-principles calculations of compound 1, through which the band structure (BS) and density of states (DOS) were calculated. Generalized gradient approximations (GGA) with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional (xc) were adopted for the simulations. The calculation result indicates that the band gap of compound 1 is 1.193 eV (Figure 3a). Such a discrepancy of the experimental and calculated band gap values results from the fact that the GGA functional typically underestimates the band gaps [36]. Energy states for the region in the valence band maximum are mainly contributed by the inorganic ingredients (i.e., Cu 3d and I 5p atomic orbitals), in contrast, the region of the conduction band minimum is majorly originated from the organic ingredients (i.e., C and N 2p atomic orbitals) (Figure 3b). Based on the calculation results, it is suggested that the luminescence mechanism of compound 1 is a combination of XLCT and MLCT. This property is similar to CuI staircase chain-based structures, the band gap of 1 may be affected by altering the organic ligands with various LUMO energies [23].  When the organic ligand is placed under UV excitation, it emits no detectable emission. Single crystals of 1 are red under natural light and emit strong red light under 360 nm wavelength UV irradiation. Figure 4a shows the emission spectra under room temperature and UV excitation. The red emission of compound 1 reaches the peak at 620 nm under room temperature condition, with a full width at half-maximum (FWHM) of around 100 nm. The Commission Internationale de l'Eclairage (CIE) chromaticity coordinates were calculated to be (0.52, 0.32) for compound 1 (Figure 4b). The IQYs were tested with a C9920-03 absolute quantum yield measurement system. In addition, the IQYs of compound 1 is 6.5% under an excitation wavelength of 360 nm. Compared to other staircase-chain-based structures, compound 1 shows a relatively higher IQY [25]. This might be because of the substituted group on the pyrimidine ring. It has been found that a structure with organic ligands with electron donating groups would enhance the IQYs of the hybrid, such as the chloro-group in this case [23].

Conclusions
In summary, a new copper(I) iodide staircase-chain-based inorganic-organic hybrid structure has been cultured and characterized. This compound is demonstrated as a redlight emitter, and an IQY of 6.5% under 360 nm excitation was revealed. This implies that the compound 1 has the potential to be a rare-earth-free lighting phosphor alternative.