Three Iodoargentate-Based Hybrids Decorated by Metal Complexes: Structures, Optical/Photoelectric Properties and Theoretical Studies

So far, the development of new iodoargentate-based hybrids, especially those compounds with metal complex cations, and the understanding of their structure–activity relationships have been of vital importance but full of challenges. Herein, using the in-situ-generated metal complex cations as structural directing agents, three new iodoargentate-based hybrids, namely, [Co(phen)3]Ag2PbI6 (phen = 1,10-phenanthroline; 1), [Ni(5,5-dmpy)3]Ag7I9·CH3CN (5,5-dmpy = 5,5-dimethyl-2,2-bipyridine; 2) and [Co(5,5-dmpy)3]Ag5I8 (3), have been solvothermally prepared and then structurally characterized. Compound 1 represents one new heterometallic Ag–Pb–I compound characteristic of the chain-like [Ag2PbI6]n2n− anions. Compound 2 features the straight one-dimensional (1D) [Ag7I9]n2n− anionic moieties, while compound 3 contains infrequent two types of curved [Ag5I8]n3n− anions. Optical properties reveal that the title compounds exhibit interesting semiconductor behaviors with the band gaps of 1.59–2.78 eV, which endow them with good photoelectric switching performances under the alternate light irradiations. We also present their Hirshfeld surface analyses, and the theoretical studies (band structures, density of states (DOS) and partial density of states (PDOS)).


Introduction
Organic-inorganic hybrid metal halides continue to fascinate researchers, which not only profit from their diversified structures, but also from their unusual photophysical performances inherited from the synergistic combinations of both organic and inorganic components [1][2][3]. Among them, iodoargentate-based hybrids are of special interest by virtue of the wide applications in many fields, such as white light emission, thermo-/photochromism, photocurrent response, dye sorption/separation and photocatalysis, gaining more and more attention in recent years [4][5][6][7][8][9].

Structural Description of [Ni(5,5-dmpy)3]Ag7I9·CH3CN
As presented in Figure 1b, two [AgI 4 ] tetrahedra are condensed by edge-sharing to form the [Ag 2 I 6 ] dimer. Noteworthily, the distance of Ag· · · Ag in the [Ag 2 I 6 ] dimer reaches 3.234(4)Å, which is less than the sum of the van der Waals radii of Ag (3.44Å), suggesting the significant metal-metal interactions [24]. This phenomenon is also observed in the case of [Fe(bipy) 3  ] n 2n− anionic chain extended along the a-axis. The in-situ-produced [Co(phen) 3 ] 2+ complexes serve as the structural directors and charge balancers, separating the anionic components and forming the 3D supramolecular framework through the extensive hydrogen bond contacts ( Figure 1c). The C-H· · · I hydrogen bond lengths and C-H· · · I hydrogen bond angles are between 3.78(2)-3.968 (16) A and 129.9-155.0 • . In addition to the C-H· · · I hydrogen bonds, compound 1 also contains the π· · · π and C-H· · · π interactions (Figure 1d,e).
Compound 3 exhibits a captivating structural characteristic, wherein it encompasses two distinct types of anionic chains. As depicted in Figure S4a Compound 3 exhibits a captivating structural characteristic, wherein it encompasses two distinct types of anionic chains. As depicted in Figure S4a

Hirshfeld Surface Analyses
In order to visually and quantitatively determine the percentage of the area occupied by different types of intermolecular interactions, we performed the Hirshfeld surface analyses for title compounds. The full interactions were depicted in Figure 4a-c, with a scattering range of 2.6 ≤ de + di ≤ 5.8 Å for 1, 2.4 ≤ de + di ≤ 5.8 Å for 2, 2.0 ≤ de + di ≤ 5.8 Å for 3, respectively. The decomposed 2D fingerprint plots showed that the H···I/I···H hydrogen bonds emerged like two wings of a bird, which accounted for the highest proportion of the total Hirshfeld surface area (35.1% for 1, 32.0% for 2 and 40.8% for 3; Figure 4d-f). For compound 1, the ratio of H···I contacts is 13.9% (top left), which is smaller than the ratio of I···H contacts (21.2%, bottom right). Similar case has also been observed in compounds 2 and 3. Further investigations revealed that the second contributors are due to the H···H contacts. As shown in Figure 4g-i, they mainly occurred in the intermediate region, making up 20.7% (1), 31.0% (2) and 26.2% (3) of the total surface. Furthermore, the crystal packing of compound 1 is significantly influenced by the C···H/H···C and C···C contacts. In general, these contacts are frequently employed to highlight the C-H···π (17.2%) and π···π (3.2%) interactions, thus underscoring their vital significance in

Hirshfeld Surface Analyses
In order to visually and quantitatively determine the percentage of the area occupied by different types of intermolecular interactions, we performed the Hirshfeld surface analyses for title compounds. The full interactions were depicted in Figure 4a-c, with a scattering range of 2.6 ≤ d e + d i ≤ 5.8Å for 1, 2.4 ≤ d e + d i ≤ 5.8Å for 2, 2.0 ≤ d e + d i ≤ 5.8Å for 3, respectively. The decomposed 2D fingerprint plots showed that the H· · · I/I· · · H hydrogen bonds emerged like two wings of a bird, which accounted for the highest proportion of the total Hirshfeld surface area (35.1% for 1, 32.0% for 2 and 40.8% for 3; Figure 4d-f). For compound 1, the ratio of H· · · I contacts is 13.9% (top left), which is smaller than the ratio of I· · · H contacts (21.2%, bottom right). Similar case has also been observed in compounds 2 and 3. Further investigations revealed that the second contributors are due to the H· · · H contacts. As shown in Figure 4g-i, they mainly occurred in the intermediate region, making up 20.7% (1), 31.0% (2) and 26.2% (3) of the total surface. Furthermore, the crystal packing of compound 1 is significantly influenced by the C· · · H/H· · · C and C· · · C contacts. In general, these contacts are frequently employed to highlight the C-H· · · π (17.2%) and π· · · π (3.2%) interactions, thus underscoring their vital significance in the overall structure ( Figure S5a,c). Although compound [Ni(phen) 3 ]Ag 2 PbI 6 is isomorphic with 1, the C-H· · · π and π· · · π interactions have different contribution proportions, accounting for 10.0% and 16.0% of total Hirshfeld surface, respectively [19]. It is noteworthy that the π· · · π interactions are not obvious in compounds 2 and 3. These findings are consistent with the results of crystal structure analyses. The comparative contributions from other interactions are illustrated in Figure 4j-l. Clearly, the interactions associated with hydrogen atoms contribute greatly to their structural stabilities. The occurrence of this phenomenon is unsurprising and frequently observed in numerous metal halides, like [NH 4 ][Fe(bipy) 3 ] 2 [Ag 6 Br 11 ], [Zn(bipy) 3 ] 2 Ag 2 BiI 6 (I) 1 worthy that the π···π interactions are not obvious in compounds 2 and 3. These findings are consistent with the results of crystal structure analyses. The comparative contributions from other interactions are illustrated in Figure 4j-l. Clearly, the interactions associated with hydrogen atoms contribute greatly to their structural stabilities. The occurrence of this phenomenon is unsurprising and frequently observed in numerous metal halides,  [19,[28][29][30][31]. More Hirshfeld surface comparisons of compounds 1-3 with some related analogues are listed in Table S10.

Optical Properties
The UV-Vis diffuse reflectance spectra, obtained from powder samples at room temperature, show that the optical absorption edges of title compounds are estimated to be 2.24 eV for 1, 2.78 eV for 2, and 1.59 eV for 3, respectively (Figure 5a-c). This indicates that the title compounds are potential visible light responsive semiconductors. In addition, these band values are very consistent with their respective crystal colors and match well with those of some reported Ag-based metal halide analogues, e.g.,

Optical Properties
The UV-Vis diffuse reflectance spectra, obtained from powder samples at room temperature, show that the optical absorption edges of title compounds are estimated to be 2.24 eV for 1, 2.78 eV for 2, and 1.59 eV for 3, respectively (Figure 5a-c). This indicates that the title compounds are potential visible light responsive semiconductors. In addition, these band values are very consistent with their respective crystal colors and match well with those of some reported Ag-based metal halide analogues, e.g., K[Fe(bipy) 3

Photocurrent Responses
Inspired by the semiconductor nature of title compounds, we have further tested their photoelectric performances in a KCl solution using a classic three-electrode configuration, which were commonly used to evaluate the potential applications in the photovoltaic field. The photocurrent-time curves with switching interval of 20 s were recorded in Figure 6. It can be seen that they have the obvious photocurrent response behaviors under the alternating light irradiation. The average visible light photocurrent densities of compounds 1-3 are 0.16, 0.14 and 0.14 μA cm −2 (Figure 6a-c), respectively, which are

Photocurrent Responses
Inspired by the semiconductor nature of title compounds, we have further tested their photoelectric performances in a KCl solution using a classic three-electrode configuration, which were commonly used to evaluate the potential applications in the photovoltaic field. The photocurrent-time curves with switching interval of 20 s were recorded in Figure 6. It can be seen that they have the obvious photocurrent response behaviors under the alternating light irradiation. The average visible light photocurrent densities of compounds 1-3 are 0.16, 0.14 and 0.14 µA cm −2 (Figure 6a-c), respectively, which are well comparable with some high-performance metal halides, such as [Ag 2 I 2 (phen)] n , {[Nd 2 (dpdo)(DMF) 14 [20,29,[35][36][37][38]. High photocurrent densities indicate that they possess the satisfactory transfer capacity of charge carriers. In addition, there are no substantial declines of the photocurrent switching ratios after multiple cycles, which prove the excellent stability and repeatability. This is significantly superior to [CH 3 NH 3 ]PbI 3 , a classic perovskite photovoltaic material, which usually became unstable due to the hydrolysis instability when exposed to the light irradiation. More importantly, their photoelectric switching abilities could be further enhanced with the conversion of visible light to the full spectrum condition (Figure 6d- [21,25,28,39].  (1 (a), 2 (b) and 3 (c)).

Photocurrent Responses
Inspired by the semiconductor nature of title compounds, we have further tested their photoelectric performances in a KCl solution using a classic three-electrode configuration, which were commonly used to evaluate the potential applications in the photovoltaic field. The photocurrent-time curves with switching interval of 20 s were recorded in Figure 6. It can be seen that they have the obvious photocurrent response behaviors under the alternating light irradiation. The average visible light photocurrent densities of compounds 1-3 are 0.16, 0.14 and 0.14 μA cm −2 (Figure 6a-c) [20,29,[35][36][37][38]. High photocurrent densities indicate that they possess the satisfactory transfer capacity of charge carriers. In addition, there are no substantial declines of the photocurrent switching ratios after multiple cycles, which prove the excellent stability and repeatability. This is significantly superior to [CH3NH3]PbI3, a classic perovskite photovoltaic material, which usually became unstable due to the hydrolysis instability when exposed to the light irradiation. More importantly, their photoelectric switching abilities could be further enhanced with the conversion of visible light to the full spectrum condition (Figure 6d-

Theoretical Studies
In order to gain a deeper understanding of the optical properties and photoelectric behaviors of title compounds, this study utilizes density functional theory (DFT) and the first-principles approach to analyze their electronic structures (Figure 7). According to the Figure 7a, the valence band (VB) maximum and the conduction band (CB) minimum of compound 1 are located at the same symmetric point (Γ), indicating the direct band gap semiconductor character. Compounds 2 and 3, in contrast to 1, are indirect band gap materials. In detail, the VB maximum are both situated at Γ point, while the CB minimum appear at the D for 2 and C points for 3, respectively (Figure 7b,c). Specially, compounds 2 and 3 exhibit obvious band dispersions, generally meaning the small effective mass and the good carrier transport. According to theoretical calculations, the band gap values of compounds 1-3 are found to be 1.29, 1.82, and 1.06 eV, respectively. However, upon comparison with the experimental results of 2.24, 2.78, and 1.59 eV, it is evident that the theoretical values underestimate the true values. This underestimation is a common phenomenon and can be attributed to the limitations of DFT calculations [40,41].
Ag, while the CB minimum is mostly constituted by Ni-3d with small amounts of N-2p states (Figure 7e and Figure S16). For compound 3, the mixing of I-5p state makes up the bottom of the CB, and the VB is dominated by Co-3d, C-2p and N-2p states (Figures 7f  and S17). The results of this study indicate that the optical properties of the compounds mentioned are influenced by both organic and inorganic components, particularly the photosensitive metal complex cations.  [18][19][20][21]29,42,43]. By examining the DOS and PDOS diagrams of compound 1, it can be observed that the bottom of CB is primarily influenced by the Co-3d, N-2p and C-2p orbitals through local interactions (Figures 7d and S15). The VB near the Fermi level is mainly contributed by the 3d states of Co and Ag mixed with the 5p state of I. Note that the contribution of Pb 2+ ions to the front orbitals near the Fermi level is negligible. In addition, the contribution in the region of −4 to −15 eV is almost from the C-2p and I-5s states. For compound 2, the top part of VB mainly originates from 5p state of I and 3d states of Co and Ag, while the CB minimum is mostly constituted by Ni-3d with small amounts of N-2p states (Figures 7e and S16). For compound 3, the mixing of I-5p state makes up the bottom of the CB, and the VB is dominated by Co-3d, C-2p and N-2p states (Figures 7f and S17). The results of this study indicate that the optical properties of the compounds mentioned are influenced by both organic and inorganic components, particularly the photosensitive metal complex cations. This discovery is consistent with previous studies on metal halides with optical activities, such as [Ni(phen) 3 [18][19][20][21]29,42,43].
Purity identifications of samples were conducted by a SmartLab diffractometer. Thermogravimeric curves of title compounds were obtained using a NETZSCH STA449C unit (N 2 atmosphere, 10 K/min). A Thermo Fisher GX4 scanning electron microscope and a 3600 SHIMADZU spectrometer were utilized to acquire the energy-dispersive X-ray (EDX) spectra and the solid optical diffuse reflectance data, respectively.

X-ray Crystallography
Intensity data of title compounds were gathered on a Bruker SM (1, 3) with an APEX II CCD detector and an Xcalibur E Oxford diffrac Atlas CCD detector using the graphite monochromatic Mo-Kα radia Their structures were analyzed by a direct method and optimized o least-squares technique using the SHELXTL-2014 program [44]. All n were arranged anisotropically, and the hydrogen linked to carbon ato geometrically and refined isotropically under a fixed thermal factor and structure refinement details are summarized in Table 1. The se and angles, hydrogen bond data, C-H⋯π interactions and π⋯π inter Tables S1-S9.

X-ray Crystallography
Intensity data of title compounds were gathered on a Bruker SMART diffractometer (1, 3) with an APEX II CCD detector and an Xcalibur E Oxford diffractometer (2) with an Atlas CCD detector using the graphite monochromatic Mo-Kα radiation (λ = 0.71073Å). Their structures were analyzed by a direct method and optimized on F 2 by full-matrix least-squares technique using the SHELXTL-2014 program [44]. All non-hydrogen atoms were arranged anisotropically, and the hydrogen linked to carbon atoms were presented geometrically and refined isotropically under a fixed thermal factor. Their crystal data and structure refinement details are summarized in Table 1. The selected bond lengths and angles, hydrogen bond data, C-H· · · π interactions and π· · · π interactions are listed in Tables S1-S9. Their structures were analyzed by a direct method and optimized on F by full-matrix least-squares technique using the SHELXTL-2014 program [44]. All non-hydrogen atoms were arranged anisotropically, and the hydrogen linked to carbon atoms were presented geometrically and refined isotropically under a fixed thermal factor. Their crystal data and structure refinement details are summarized in Table 1. The selected bond lengths and angles, hydrogen bond data, C-H⋯π interactions and π⋯π interactions are listed in Tables S1-S9.

Photocurrent Measurements
The photocurrent experiments of title compounds were carried out on a CHI 660E electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China). In this study, we used a typical three-electrode configuration (the sample-coated ITO 12.0376 (11) 17.9597 (8) 22.867 (2) b/ Atlas CCD detector using the graphite monochromatic Mo-Kα radiation (λ = 0.71073 Å). Their structures were analyzed by a direct method and optimized on F 2 by full-matrix least-squares technique using the SHELXTL-2014 program [44]. All non-hydrogen atoms were arranged anisotropically, and the hydrogen linked to carbon atoms were presented geometrically and refined isotropically under a fixed thermal factor. Their crystal data and structure refinement details are summarized in Table 1. The selected bond lengths and angles, hydrogen bond data, C-H⋯π interactions and π⋯π interactions are listed in Tables S1-S9.

Photocurrent Measurements
The photocurrent experiments of title compounds were carried out on a CHI 660E electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China). In this study, we used a typical three-electrode configuration (the sample-coated ITO 13.8472 (12) 13.1590 (5) 13.4056 (14) c/ (1, 3) with an APEX II CCD detector and an Xcalibur E Oxford diffractometer (2) with an Atlas CCD detector using the graphite monochromatic Mo-Kα radiation (λ = 0.71073 Å). Their structures were analyzed by a direct method and optimized on F 2 by full-matrix least-squares technique using the SHELXTL-2014 program [44]. All non-hydrogen atoms were arranged anisotropically, and the hydrogen linked to carbon atoms were presented geometrically and refined isotropically under a fixed thermal factor. Their crystal data and structure refinement details are summarized in Table 1. The selected bond lengths and angles, hydrogen bond data, C-H⋯π interactions and π⋯π interactions are listed in Tables S1-S9. least-squares technique using the SHELXTL-2014 program [44]. All non-hydrogen atoms were arranged anisotropically, and the hydrogen linked to carbon atoms were presented geometrically and refined isotropically under a fixed thermal factor. Their crystal data and structure refinement details are summarized in Table 1. The selected bond lengths and angles, hydrogen bond data, C-H⋯π interactions and π⋯π interactions are listed in Tables S1-S9.

X-ray Crystallography
Intensity data of title compounds were gathered on a Bruker SMART diffractometer (1, 3) with an APEX II CCD detector and an Xcalibur E Oxford diffractometer (2) with an Atlas CCD detector using the graphite monochromatic Mo-Kα radiation (λ = 0.71073 Å). Their structures were analyzed by a direct method and optimized on F 2 by full-matrix least-squares technique using the SHELXTL-2014 program [44]. All non-hydrogen atoms were arranged anisotropically, and the hydrogen linked to carbon atoms were presented geometrically and refined isotropically under a fixed thermal factor. Their crystal data and structure refinement details are summarized in Table 1. The selected bond lengths and angles, hydrogen bond data, C-H⋯π interactions and π⋯π interactions are listed in Tables S1-S9.

Photocurrent Measurements
The photocurrent experiments of title compounds were carried out on a CHI 660E electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China). In this study, we used a typical three-electrode configuration (the sample-coated ITO glass was the working electrode, the platinum wire was the counter electrode, and Ag/AgCl was the reference electrode). The sample/ITO electrode was prepared by the solution coating method. First, 5 mg of powder samples were added into a mixed solvent containing 475 µL ethanol and 25 µL Nafion. Ultrasonic treatment was carried out for 2 h, and the obtained solution was dropped on the surface of the pre-polished ITO glass, and then dried at room temperature. A 300-W Xenon lamp equipped with/without a 420-nm cut-off filter was used for the visible light and full spectrum light source. The electrolyte solution is the KCl solution (0.1 M).

Conclusions
In summary, the solvothermal syntheses, crystal structures, optical/photoelectric properties and theoretical studies of three new iodoargentate-based hybrids with metal complex cations were reported here. Compounds 1-3 possess chain-like structures, exhibiting semiconductor behaviors with the band gaps of 1.59-2.78 eV. Furthermore, the title compounds display interesting photoelectric switching performances upon the alternate light irradia-tions. Further work will focus on the fabrications of more metal complex-directed halide analogues and the in-depth understandings of their structure-activity relationships.