A New 3D Iodoargentate Hybrid: Structure, Optical/Photoelectric Performance and Theoretical Research

The explorations of new three-dimensional (3D) microporous metal halides, especially the iodoargentate-based hybrids, and understanding of their structure-activity relationships are still quite essential but full of great challenges. Herein, with the aromatic 4,4′-dpa (4,4′-dpa = 4,4′-dipyridylamine) ligands as the structural directing agents, we solvothermal synthesized and structurally characterized a novel member of microporous iodoargentate family, namely [H2-4,4′-dpa]Ag6I8 (1). Compound 1 possesses a unique and complicated 3D [Ag6I8]n2n− anionic architecture that was built up from the unusual hexameric [Ag6I13] secondary building units (SBUs). Research on optical properties indicated that compound 1 exhibited semiconductor behavior, with an optical band gap of 2.50 eV. Under the alternate irradiation of light, prominent photoelectric switching abilities could be achieved by compound [H2-4,4′-dpa]Ag6I8, whose photocurrent densities (0.37 μA·cm−2 for visible light and 1.23 μA·cm−2 for full-spectrum) compared well with or exceeded those of some high-performance halide counterparts. Further theoretical calculations revealed that the relatively dispersed conduction bands (CBs) structures in compound 1 induced higher electron mobilities, which may be responsible for its good photoelectricity. Presented in this work also comprised the analyses of Hirshfeld surface, powder X-ray diffractometer (PXRD), thermogravimetric measurement, energy-dispersive X-ray spectrum (EDX) along with X-ray photoelectron spectroscopy (XPS).


Introduction
Iodoargentate-based hybrids, combining the advantages of both organic and inorganic components, continue to captivate researchers by virtue of their rich structural chemistry and distinctive photophysical properties [1,2].Among them, 3D microporous architecture characteristics of regular holes or channels are of special importance and have drawn increasing attention in recent years, mainly benefiting from their immense breakthrough in the domains of semiconductor, photocatalysis, adsorption, nonlinear optics, white-light emission, photochromism/thermochromism and piezoelectricity/ferroelectricity [3][4][5][6][7][8][9].
On the basis of the above consideration, we undertook systematic studies for exploring new 3D microporous derivatives under the solvothermal condition.Fortunately, by employing the aromatic 4,4 ′ -dpa ligands as the structural modifiers, we successfully constructed a novel and complicated hybrid iodoargentate, namely [H 2 -4,4 ′ -dpa]Ag 6 I 8 (1).The optical study showed that compound 1 has an optical band gap of 2.50 eV, implying the nature of semiconductor properties.More attractively, compound [H 2 -4,4 ′ -dpa]Ag 6 I 8 exhibited good photoelectric conversion abilities, with the visible light and full-spectrum on/off current density of 0.37 µA•cm −2 and 1.23 µA•cm −2 , respectively.Particularly, such photocurrent densities were comparable with or largely outperformed those of some high-performance halide counterparts that may be ascribed to the stronger mobility of photo-induced electrons.Given here also consists of the analyses of the Hirshfeld surface, thermogravimetric test, PXRD, EDX and XPS.

Hirshfeld Surface Analyses of Compound 1
As a complementary to X-ray crystallography studies, we further performed the Hirshfeld surface analyses for compound [H 2 -4,4 ′ -dpa]Ag 6 I 8 , which helped us to clearly recognize and quantitatively identify the noncovalent interactions across the molecular structure (Figure 3).The full 2D fingerprint plots, depicted in Figure 3a, covered the region of 2.0 ≤ d e + d i ≤ 6.0 Å, showing the contours bounding the effective electron density.Analyses of decomposed fingerprint plots revealed that the major contributor is due to the H•••I interactions, with the sky-blue points scattering in the range of 2.8 ≤ d e + d i ≤ 5.4 Å (Figure 3b).They emerged like a pair of pseudo-symmetric wings, accounting for 38.3% of the total Hirshfeld surface (top left: 15.5%; bottom right: 22.8%).A high occupying ratio indicated that hydrogen bond interactions may play an important role in the stabilization of crystal packing.This has also appeared in the cases of some hydrogenplentiful metal halides, such as [Co( 5 3f).

Hirshfeld Surface Analyses of Compound 1
As a complementary to X-ray crystallography studies, we further performed the Hirshfeld surface analyses for compound [H2-4,4′-dpa]Ag6I8, which helped us to clearly recognize and quantitatively identify the noncovalent interactions across the molecular structure (Figure 3).The full 2D fingerprint plots, depicted in Figure 3a, covered the region of 2.0 ≤ de + di ≤ 6.0 Å , showing the contours bounding the effective electron density.Analyses of decomposed fingerprint plots revealed that the major contributor is due to the H•••I interactions, with the sky-blue points scattering in the range of 2.8 ≤ de + di ≤ 5.4 Å (Figure 3b).They emerged like a pair of pseudo-symmetric wings, accounting for 38.3% of the total Hirshfeld surface (top left: 15.5%; bottom right: 22.8%).A high occupying ratio indicated that hydrogen bond interactions may play an important role in the stabilization of crystal packing.This has also appeared in the cases of some hydrogen-plentiful metal halides, such as [Co( 5

Characterizations and Optical Behaviors of Compound 1
As displayed in Figure S7, the experimental powder X-ray diffraction (PXRD) pattern matched well with the simulated result derived from single-crystal X-ray diffraction data, indicating the high phase purity of our as-grown crystals.Under the nitrogen environment, we next checked the thermal behavior of compound [H 2 -4,4 ′ -dpa]Ag 6 I 8 , which was depicted in Figure 4a S10 and 4b).Analyses of the high-resolution Ag−3d spectrum showed that it contained two single peaks centered at binding energies of 373.9 and 367.9 eV, corresponding to the 3d 3/2 and 3d 5/2 states of Ag + ions (Figure S11).The high-resolution I−3d spectrum was also characterized by two single peaks, with the binding energies located at 630.1 (3d 3/2 ) and 618.6 (3d 5/2 ) eV, respectively (Figure S12).These observed values are reasonable and are very close to some reported results (e.As displayed in Figure S7, the experimental powder X-ray diffraction (PXRD) pattern matched well with the simulated result derived from single-crystal X-ray diffraction data, indicating the high phase purity of our as-grown crystals.Under the nitrogen environment, we next checked the thermal behavior of compound [H2-4,4′-dpa]Ag6I8, which was depicted in Figure 4a [4,13,15,20,[30][31][32].The chemical compositions of compound [H2-4,4′-dpa]Ag6I8, i.e., C, N, Ag and I elements, were further confirmed by the EDX and XPS results (Figures S10 and 4b).Analyses of the high-resolution Ag−3d spectrum showed that it contained two single peaks centered at binding energies of 373.9 and 367.9 eV, corresponding to the 3d3/2 and 3d5/2 states of Ag + ions (Figure S11).The high-resolution I−3d spectrum was also characterized by two single peaks, with the binding energies located at 630.1 (3d3/2) and 618.6 (3d5/2) eV, respectively (Figure S12).These observed values are reasonable and are very close to some reported results (e.To appreciate the semiconductor behavior of compound [H2-4,4′-dpa]Ag6I8, we then measured its UV-Vis diffuse reflectance and absorption spectra using a powdered sample at room temperature.As presented in the inset of Figure 4c, compound 1 has an absorption at 400-600 nm, with a steep absorption edge at about 496 nm.According to the Kubelka−Munk function, the optical band gap of compound 1 was estimated to be 2.50 eV (Figure 4c), suggesting its semiconducting nature.This energy gap was well comparable to the values of [V(DMSO)5(H2O)]Ag6I8 (2.61 eV), [MCP][Ag4I5] (2.63 eV), To appreciate the semiconductor behavior of compound [H 2 -4,4 ′ -dpa]Ag 6 I 8 , we then measured its UV-Vis diffuse reflectance and absorption spectra using a powdered sample at room temperature.As presented in the inset of Figure 4c, compound 1 has an absorption at 400-600 nm, with a steep absorption edge at about 496 nm.According to the Kubelka−Munk function, the optical band gap of compound 1 was estimated to be 2.50 eV (Figure 4c), suggesting its semiconducting nature.This energy gap was well comparable to the values of [V(DMSO) 5 [12,16,18,[35][36][37].
Mott-Schottky plots depicted in Figure 4d were tested to further know the optical behavior of compound [H 2 -4,4 ′ -dpa]Ag 6 I 8 .Evidently, the positive slope indicated the n-type semiconducting character of compound 1, with the flat-band position of −0.45 eV versus Ag/AgCl.As is known to all, for n-type semiconductors, the flat band potential is customarily 0.20 V lower than the conduction band.Therefore, the conduction band of compound [H 2 -4,4 ′ -dpa]Ag 6 I 8 was determined to be −0.25 V vs. normal hydrogen electrode (NHE).Correspondingly, the valence band was evaluated as approximately 2.25 eV vs. NHE.

Photoelectric Performances of Compound 1
Considering that the photoinduced current generation can be potentially applied in the fields of intelligent switch, information storage and communication transmission, we further executed the photoelectrochemical measurement for compound [H 2 -4,4 ′ -dpa]Ag 6 I 8 .The photocurrent-time curves with the on/off status were recorded in Figure 5a,b.The rapid circuit photocurrent responses with negligible decays could be monitored under the periodic irradiation achieved by a manual shutter.Despite the illumination after several cycles, the photoelectric switching performance could be well preserved, indicating good moisture stability and performance durability.This is markedly different from some iodoplumbate-based hybrid materials (e.g., [CH 3 NH 3 ]PbI 3 ), which usually performed poorly due to the long-term instabilities [38].Under the visible light condition, the average photocurrent density reached to 0.37 µA•cm −2 (Figure 5a), which is comparable to those of some highly reactive iodoargentate-based hybrids, such as AgI(bpt), Ag 2 I 2 (phen), [Co(5,5-dmpy) 3 ]Ag 5 I 8 , [La(dpdo)(DMF) 14 ]Ag 12 I 18 (dpdo = 4,4 ′ -bipyridine N,N'-dioxide) and [Co(bipy) 3 ] 2 Ag 4 Bi 2 I 16 [21,29,35,39].Moreover, the on/off current density of compound [H 2 -4,4 ′ -dpa]Ag 6 I 8 could be further enhanced by exposing the designed photoelectrode to the full-spectrum circumstance, with the observed photocurrent density of 1.23 µA•cm −2 (Figure 5b).Such an enhancement can be attributed to the increased amount of photoexcited electrons and holes owing to the expanded spectral range, which has also appeared in some previous studies [27][28][29].These results mean that compound 1 may serve as a promising light-harvesting and light-detecting candidate.In addition, the comparisons of photocurrent density between the title compound and some representative analogs in literature are illustrated in Figure 5c.

Theoretical Studies of Compound 1
To gain a deeper correlation of the structure-property relationships, we performed the density functional theory (DFT) calculations for compound [H2-4,4′-dpa]Ag6I8 with the assistance of a first-principle approach.The electronic band structure, energy gap and density of states (DOS) were provided.As shown in Figure 6a, the band gap was found to be 2.23 eV under the GGA + U method, which agreed well with the experi-

Theoretical Studies of Compound 1
To gain a deeper correlation of the structure-property relationships, we performed the density functional theory (DFT) calculations for compound [H 2 -4,4 ′ -dpa]Ag 6 I 8 with the assistance of a first-principle approach.The electronic band structure, energy gap and density of states (DOS) were provided.As shown in Figure 6a, the band gap was found to be 2.23 eV under the GGA + U method, which agreed well with the experimental value of 2.50 eV.Noteworthily, this obviously distinguished the results obtained through conventional GGA, which generally significantly underestimated the true value.Analyses of band structures showed that the valence bands (VBs) maximum and conduction bands (CBs) minimum were both located at Γ points.Hence, the title compound can be considered as the quasi-direct bandgap semiconductor.In addition, the dispersive band seemed like a pocket at the CBs minimum, which may largely be benefiting the transport of photogenerated electrons.

Theoretical Studies of Compound 1
To gain a deeper correlation of the structure-property relationships, we performed the density functional theory (DFT) calculations for compound [H2-4,4′-dpa]Ag6I8 with the assistance of a first-principle approach.The electronic band structure, energy gap and density of states (DOS) were provided.As shown in Figure 6a, the band gap was found to be 2.23 eV under the GGA + U method, which agreed well with the experimental value of 2.50 eV.Noteworthily, this obviously distinguished the results obtained through conventional GGA, which generally significantly underestimated the true value.Analyses of band structures showed that the valence bands (VBs) maximum and conduction bands (CBs) minimum were both located at Γ points.Hence, the title compound can be considered as the quasi-direct bandgap semiconductor.In addition, the dispersive band seemed like a pocket at the CBs minimum, which may largely be benefiting the transport of photo-generated electrons.Whereas the 4s states of Ag atoms and 5p states of I atoms dominated the DOS around the bottom of CBs.The s-p hybridization may be responsible for the CB dispersion, which was found to promote the photo-excited carrier transfers.Thus, from the above-mentioned calculated results, we can conclude that the electrons in the title compound behaved with higher carrier mobility than holes.

Instruments and Measurements
The purity identifications of the title compound were completed by the powder X-ray diffractometer (Bruker D8, CuKα radiation) and elemental analyzer (German Elementar Vario EL Cube apparatus).The thermogravimetric behavior was evaluated by a NETZSCH STA449F3 unit with a heat rate of 10 K/min (N 2 atmosphere).The UV-Vis absorption and diffuse reflectance patterns were monitored by a SHIMADZU UV-3600 spectrometer.Energy-dispersive X-ray spectrum studies were performed on a Thermo Fisher GX4 scanning electron microscope.A Thermo Scientific ESCLAB 250Xi spectrometer was used to acquire X-ray photoelectron spectroscopy diagrams.

Preparation of Compound 1
117 mg of AgI (0.5 mmol), 166 mg of KI (1.0 mmol), and 34 mg of 4,4 ′ -dpa (0.2 mmol) were put in 0.5 mL HI, 2 mL H 2 O and 4 mL CH 3 CN.Afterward, the resulting solution was sealed in a 23 mL polytetrafluoroethylene-lined container and kept heating for 5 days at 140 • C. Through the filtration and the ethanol washing, yellow sheet-type crystals were harvested by manual separation, with a yield of around 21% based on AgI.It is emphasized that the mixed HI/H 2 O/CH 3 CN solvent may have a significant impact on the crystallization of the title compound.Our extensive syntheses studies have shown that other reaction medium (such as methanol, ethanol, acetone, DMF and DMSO) was found to be unfavorable, often leading to a failure to obtain the targeted product.Elemental analysis (EA) calculated for compound 1: C, 6.54%; H, 0.60%; N, 2.29%.Found: C, 6.72%; H, 0.65%; N, 2.30%.

X-ray Crystallography
Single-crystal X-ray diffraction data collection of 1 was accomplished on an Xcalibur E Oxford diffractometer using Mo-Kα radiation (λ = 0.71073 Å) at 298(2) K. Its structure obtained by direct methods was then refined by the SHELXL-2018 program based on full matrix least-squares routines against F 2 [40].During the refinement, all non-hydrogen atoms were treated anisotropically, while the hydrogen atoms were positioned geometrically with fixed thermal factors.In compound 1, the Ag atoms except Ag(6) behave the disorder: Ag(1)/Ag(1B), Ag(3)/Ag(3B) and Ag(5)/Ag(5B) exist two statistical distributions; while the Ag(2)/Ag(2B)/Ag(2C) and Ag(4)/Ag(4B)/Ag(4C) are three statistical distributions.The empirical formula was further verified by the element analyses and thermogravimetric results.More structural refinement parameters are listed in Table 3. CCDC number 2,301,354 corresponds to compound 1, which was acquired free of charge from the Cambridge Crystallographic Data Centre.Some important bond distances and angles are supplied in Table S1.

Photoelectric Examinations
Utilizing a CHI660E electrochemical workstation (Chenhua, Shanghai, China) equipped with the three-electrode configuration, we examined the photoelectric switching performance of the title compound.The working electrode was prepared by a typical solution coating method that stated as follows: 5 mg of microcrystalline powder 1 was added into a mixed Nafion/ethanol solution, suffering from the ultrasonic treatment.After lasting for 30 min, the gained suspension was deposited on the clean surface of ITO glass and then dried in the air.The effective area was 1.0 × 1.0 cm 2 .In this study, Ag/AgCl and platinum wire served as the reference electrode and the counter electrode, respectively.A 300 W Xenon lamp was used as the irradiation source, while the visible light was realized with the help of a 420 nm cut-off filter.The supporting electrolyte is the 0.1 M KCl solution.

Computational Details
The electronic structure calculations of compound 1 were conducted by the density functional theory (DFT) framework implemented in the Vienna Ab initio Simulation Package (VASP) [41].The Perdew-Burke-Ernzerhof (PBE) form of generalized gradient approximation (GGA) exchange-correlation functionals have been employed, utilizing projector augmented wave (PAW) potentials [42,43].The H−1s, C−2s, C−2p, N−2s, N−2p, Ag−4d, Ag−5s, I−5s and I−5p were treated as its valence states.In order to escape the well-known bandgap underestimate of GGA, the Coulomb self-interaction potential was considered.Within the GGA + U approximation, the onsite Coulomb term U value was used for the Ag−4d states.The energy cut-off for the plane wave basis set was kept at 500 eV.The reciprocal space sampling was completed with k-point Monckhorst-Pack grids of 5 × 5 × 3 for the title compound.

Conclusions
In summary, using the solvothermal method, we successfully fabricated a new iodoargentate microporous material, which was subsequently structurally analyzed and characterized by means of multiple techniques.The title compound featured a novel and complicated 3D [Ag 6 I 8 ] n 2n− anionic framework based on hexameric [Ag 6 I 13 ] SBUs, containing the peanut-shaped 1D channels where the [H 2 -4,4 ′ -dpa] 2+ template cations reside.Further research showed that the obtained material exhibited semiconductive behavior, rendering it with good photoelectric conversion properties upon alternate light illumination.Of note, its photocurrent density competed well with or even surpassed those of some high-performance halide analogs, which was mainly attributed to the high mobility of electrons as revealed by theoretical calculations.Future work will focus on the exploratory syntheses of more new numbers and the deep investigation of their structure-property relationships.

Figure 2 .
Figure 2. (a) Perspective view of 3D anionic framework of compound 1 along the a axis.(b) Detailed view of the peanut-typed window in compound 1.(c) A diagram for showing the C−H•••I hydrogen bonds between the [H 2 -4,4 ′ -dpa] 2+ cations and the [Ag 6 I 8 ] n 2n− anions.

Molecules 2023 , 13 Figure 5 .
Figure 5.The photocurrent curves of compound 1: Visible light (a) and full-spectrum (b).(c) The comparisons of photocurrent density between title compound and some representative analogues in literature.

Figure 5 .
Figure 5.The photocurrent curves of compound 1: Visible light (a) and full-spectrum (b).(c) The comparisons of photocurrent density between title compound and some representative analogues in literature.

Figure 5 .
Figure 5.The photocurrent curves of compound 1: Visible light (a) and full-spectrum (b).(c) The comparisons of photocurrent density between title compound and some representative analogues in literature.

Figure 6 .
Figure 6.(a) The band structure of compound 1.(b) DOS and partial DOS of compound 1.

Figure 6 .
Figure 6.(a) The band structure of compound 1.(b) DOS and partial DOS of compound 1.The total DOS of compound [H 2 -4,4 ′ -dpa]Ag 6 I 8 , as well as the partial DOS of C, N, H, Ag and I atoms, were given in Figures 6b and S15.It can be seen that the VBs maximum predominantly corresponds to the 2p levels of N and C atoms, suggesting the obvious covalent interactions of C−N bonds.Furthermore, the contributions of N and C induced a few isolated bands of the upper VBs near 0 eV.The broad intensity peak near the top of the VBs was mainly dominated by I−5p and Ag−4d states, which revealed the presence of strong interactions between Ag and I atoms.The deeper VBs, for example, with energy less than −4 eV, were mainly contributed by Ag−4d orbits.Taking into account the strong localization of the d states of Ag, the flat valence band structures were further verified.Whereas the 4s states of Ag atoms and 5p states of I atoms dominated the DOS around the bottom of CBs.The s-p hybridization may be responsible for the CB dispersion, which was found to promote the photo-excited carrier transfers.Thus, from the above-mentioned calculated results, we can conclude that the electrons in the title compound behaved with higher carrier mobility than holes.

Table 2 .
The structural comparisons of compound 1 with some related iodoargentate derivatives.

Table 3 .
Crystallographic data and structural refinement details of compound 1.