Light-Responsive Proton Conductor: Record High Gain of Proton Conductivity Achieved by Photoinduced Electron-Transfer Strategy

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Nov 2021Light-Responsive Proton Conductor: Record High Gain of Proton Conductivity Achieved by Photoinduced Electron-Transfer Strategy Xiu-Shuang Xing†, Cai Sun†, Lu Liu, Ming-Sheng Wang and Guo-Cong Guo Xiu-Shuang Xing† State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Henan Key Laboratory of New Optoelectronic Functional Materials, College of Chemistry and Chemical Engineering, Anyang Normal University, Anyang 455000 †X.-S. Xing and C. Sun contributed equally to this work.Google Scholar More articles by this author , Cai Sun† State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou, Fujian 350108 Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou, Fujian 350108 †X.-S. Xing and C. Sun contributed equally to this work.Google Scholar More articles by this author , Lu Liu State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Google Scholar More articles by this author , Ming-Sheng Wang *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou, Fujian 350108 Google Scholar More articles by this author and Guo-Cong Guo State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou, Fujian 350108 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202000610 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Light-responsive proton conductors might find applications in both traditional fields (fuel cells, chemical sensors, bio-ionic functions, etc.) and modern high-speed switchable smart systems (Internet of things, robotics, etc.). Previous synthetic methods resulted in low switching contrasts (<two times) or they tended to be limited significantly in solid matrixes due to large structural changes. The photoinduced electron-transfer (PIET) method avoids the influence of stereo space in solid matrixes and capable of achieving high switching contrasts. For the first time, we applied the PIET strategy to design light-responsive proton conductors to achieve the hitherto largest gain of proton conductivity (ca. 54 times) for light-responsive proton conductors in one crystalline photochromic viologen-based H-bonded supramolecule. The weakening of hydrogen-bonding interactions in the proton-transport path after PIET accounted for an increased proton conductivity. These findings would inspire the exploration of photon conductors that display higher proton conductivities or switchable smart systems with high contrasts. Download figure Download PowerPoint Introduction Proton conductors attract extensive attention for applications in fuel cells, chemical sensors, and bio-ionic functions.1–3 Current research endeavors mainly focus on achieving high conductivity in crystalline materials with H-bonded networks such as coordination polymers (CPs) and metal–organic frameworks (MOFs) by encapsulation of proton carriers, pore surface functionalization, defect introduction, and so on.4–12 However, due to the development and stimulation of high-tech industries such as the Internet of things and robotics, regular proton conductors cannot meet the new stimuli-responsive smart systems’ requirements. The light mode has advantages of noninvasiveness, high spatial resolution, and easy and quick modulation, and thus, light-responsive proton conductors and related systems have attracted close attention. For example, Wen et al.13 fabricated a bio-inspired photoelectric conversion system based on a cross-membrane pump of proton originating from the light-driven dissociation of a photoacid; the Kitagawa group14 provided a photoacid (pyranine)-doping strategy to increase mobile acidic protons and local defects upon irradiation, and thus, realized the enhancement of proton conductivity (contrast: ca. one time) in a melted CP. Additionally, the Heinke group observed a photoinduced decrease of proton conductivity due to an enhancement of hydrogen-bonding interactions between framework and proton-conducting guests when photochromic azobenzene (contrast: ca. one time)15 or spiropyran (contrast: ca. 100 times)16 moieties were anchored to the linker of a MOF. Further, very recently, Chen’s group17 achieved a dramatic decrease in proton conductivity (contrast: ca. 10,000 times) by encapsulating sulfonated spiropyran into MOF pores, where photoinduced ring open of photochromic spiropyran blocked hydrogen-bonding network and consequently reduced proton-conduction mobility. These studies explored two applicable methods to modify proton conductivities successfully: (1) light-driven dissociation of protons and (2) photoisomerization of photochromic molecules. The former had few stereo space requirements in the solid matrixes but usually resulted in low switching contrasts (<two times). The latter might yield high switching contrasts but are significantly limited in solid matrixes due to extensive structural changes. Thus, it is highly desirable to explore a new method that can combine the advantages of the two known methods. Additionally, high-contrast enhancement efforts of proton conductivity after light irradiation, instead of weakening the process, are appealing for a real application; however, effective strategies to this aim are also lacking. Electrons and protons often work together in natural photosynthetic and enzymatic systems.18,19 Besides, electron transfer usually results in minor structural change.20,21 Inspired by these points, we presume that electron transfer or electron redistribution in a material system could affect the performance of proton conduction significantly and is well adapted to a solid matrix. Suppose one crystalline compound with an infinite hydrogen-bonding network is able to undergo photoinduced electron transfer (PIET), it can act as a good proof-of-concept model to verify our idea and further explore the regulated mechanism involving the interrelationship of electron transfer and proton transport. Diprotonated 4,4′-bipyridinium (a typical viologen; abbreviated as H2V hereafter) can accept one electron from an electron donor to yield a stable radical and generate a photochromic phenomenon after irradiation.22 The existence of N–H bonds in H2V offers an opportunity to construct a hydrogen-bonding network. If free or coordinated water molecules and/or hydroxyl groups are further included, then the formation of an H-bonded supramolecule with an infinite hydrogen-bonding network is highly possible. Furthermore, crystalline species particularly favor the study of internal structural information. Therefore, crystalline compounds with “H2V,” “free or coordinated water” and/or “hydroxyl” groups are suitable proof-of-concept models to understand the relationship between the PIET process and proton conduction. Experimental Methods Materials and instruments All chemicals of analytical grade were obtained from commercially available sources and used as received without further purification. Powder X-ray diffraction (PXRD) patterns at room temperature were acquired on a Rigaku Miniflex II Desktop X-ray diffractometer (Tokyo, Japan) using Cu Kα radiation (λ = 1.540598 Å) at 40 kV and 40 mA ranging from 5° to 50°. A simulated PXRD pattern was obtained from the Mercury Version 2020.1 software ( http://www.ccdc.cam.ac.uk/products/mercury). Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analysis experiments were carried out on a Mettler TOLEDO simultaneous TGA/DSC apparatus (Zurich, Switzerland) in N2, heating the sample in an Al2O3 crucible at a heating rate of 10 K·min−1. Fourier transform infrared (FT-IR) spectra were recorded on a PerkinElmer Spectrum One FT-IR spectrometer (Waltham, MA) using KCl pellets in the 4000–400 cm−1 range. Electron absorption spectra were measured at room temperature on a PerkinElmer Lambda 900 UV/vis/near-infrared (NIR) spectrophotometer (Waltham, MA) equipped with an integrating sphere and BaSO4 as a reference. Electron paramagnetic resonance (ESR) spectra were recorded on a Bruker-BioSpin ER-420 spectrometer (Rheinstetten, Germany) with a 100 kHz magnetic field in the X band at room temperature. Synthesis of {(H2V)[Ge(ox)2(OH)2]}·2H2O ( 1; ox = oxalate) Compound 1 was synthesized, as described previously.23 The crystal samples for all testing and characterizations were carefully picked with the aid of a microscope, checking their phase purity by PXRD ( Supporting Information Figure S1). Proton conductivity measurements Proton conductivity measurements were performed using a quasi-four-electrode alternating current (AC) impedance technique with a Solartron 1260 impedance/gain-phase analyzer. Our single-crystal measurements revealed the single-crystal shape as a triangular prism, wherein the cross-section area sizes and the length were 0.157 × 0.270 mm2 and 0.350 mm, respectively. Gold wires were connected to both ends of the longer axis of each crystal. The single crystal was measured at frequencies ranging from 107 to 1 Hz as the temperatures were varied from 303 to 323 K and the relative humidity (RH) was 95%. The resistances of the crystalline samples were deduced with the operation of a “fit cycle” from the Debye semicircle in the Nyquist plot. Computational approaches Projected band structure The calculation was based on density functional theory (DFT) in conjunction with the projector augmented wave (PAW) potential, which is implemented in the Vienna ab initio Simulation Package (VASP).24,25 The Perdew–Burke–Ernzerhof (PBE) exchange-correlation functional with an optB86b-vdW correction was used, considering the dispersion interaction between neighboring organic components.26 Single-crystal X-ray diffraction data of 1 were used to build the calculation model. Single-point energy was calculated using plane-wave cutoff energy of 400 eV and a 3 × 3 × 2 Monkhorst–Pack grid of k-points. The dipole moment of a fragment and charge decomposition analysis The calculations indicated above were derived using the Gaussian 09 D01 version and Multiwfn software.27–29 As shown in Supporting Information Figure S8, the molecular model was taken from the single-crystal structure of 1. H atoms in the Ge complex and the water molecule were optimized at the M06-2X/def2-svp level, while other atoms remained unchanged. The dipole moment of a fragment (F) based on the Hirshfeld weighting function given by the equation: D F = ∑ A ∈ F [ Z A R A − ∫ [ ω A ( r ) ρ ( r ) ] r d r ] where A is the atomic index in F, ZA, RA, and ωA(r) are nuclear charge, position, and atomic weighting function of atom A, respectively.28 The charges were set as −2 and −1 for the Ge complex in initial and colored states. The charge decomposition analysis (CDA) proposed by Dapprich and Frenking is used to provide a deep insight into how charges are transferred within fragments in a complex to achieve charge equilibrium.27 In CDA, the fragment orbital (FO) denotes the molecular orbital (MO) of a fragment in its isolated state. Three terms are defined as follows: d i = ∑ m ∈ A occ ∑ n ∈ B vir η i C m , i C n , i S m , n b i = ∑ m ∈ A vir ∑ n ∈ B occ η i C m , i C n , i S m , n r i = ∑ m ∈ A occ ∑ n ∈ B occ η i C m , i C n , i S m , n where i and η are index and occupation number of MO of complex, respectively. S m , n = ∫ f m ( r ) f n ( r ) d r is an overlap integral between FO m and FO n. Cm,i denotes the coefficient of FO m in MO i of the complex. The superscript “vir” and “occ” mean virtual (viz. unoccupied) and occupied, respectively. The term di denotes the amount of electron donated from fragment A to B via MO i of the complex; similarly, the term bi denotes the electron back donated from B to A. The term r reveals closed-shell interaction between two occupied FOs in different fragments; a positive value of ri means that owing to MO i, the electrons of the two fragments are accumulated in their overlap region and shows bonding character, while a negative value indicates that the electrons are depleted from the overlap region, and thus, reflect an electron repulsive effect. In the initial state, the charges for the Ge complex, water, and H2V were set as −2, 0, and +2, respectively, while those in the colored state were set as −1, 0, and +1, respectively. The spin multiplicities were set as 1 and 3 for initial and colored states of the model complex, respectively. Results and Discussion Through screening of the Cambridge Crystallographic Data Center (CCDC) database, 185 structures with “H2V,” “free or water,” and/or “hydroxyl” were found ( Supporting Information Figures S2 and S3). From these structures, compound 123 was chosen as a proof-of-concept model, considering its well-resolved crystal structure, the presence of an infinite hydrogen-bonding network, and potential electron-transfer photochromic property. Figure 1 shows that compound 1 was constructed by H2V ions, Ge complexes, and free water molecules through intermolecular hydrogen bonds and van der Waals interactions. In the Ge complex, each Ge atom was coordinated by six oxygen atoms from two ox ligands and two hydroxyl groups to form a distorted octahedron. The coordinated O(9) hydroxyl groups and free O(2W) water molecules form an infinite hydrogen-bonding network along a direction that provides 1D proton-transporting channels. Notably, the hydrogen atoms of the O(9) hydroxyl group and the O(2W) water molecule were found to be disordered over two distinct crystallographic positions [H(4W) and H(5W) for the water molecule, and H(9A) and H(9B) for the hydroxyl group].23 This disorder means that the active O(9) hydroxyl group and the O(2W) water molecule can reorient themselves readily to make the oxygen atoms have a proper angle to accept hydrogen atoms for proton transport through the Grotthuss mechanism.30,31 Moreover, it has been reported that the electron on the oxygen atoms of an ox could transfer to H2V after irradiation in a photochromic process.32 The nearest distance between the ox oxygen atom and the H2V nitrogen atom in 1 was about 2.811(3) Å, which met the distance of typical PIET occurrence.33,34 Therefore, compound 1 has a high probability of exhibiting electron-transfer photochromic properties. Figure 1 | Crystal structure of 1 showing an infinite hydrogen-bonding network along the a axis. Hydrogen bonds: O(2W)–H(4W)⋯O(2W, symmetry codes: −x, 1−y, −z), dO(2W)⋯O(2W) = 2.738(3) Å, ∠[O(2W)–H(4W)⋯O(2W)] = 161(4)°; O(9)–H(9B)⋯O(2W), dO(9)⋯O(2W) = 3.188(3) Å, ∠[O(9)–H(9B)⋯O(2W)] = 119(3)°; O(9)–H(9B)⋯O(9, symmetry codes: 1−x, 1−y, −z), dO(9)⋯O(9) = 3.004(4) Å, ∠[O(9)–H(9B)⋯O(9)] = 133(5)°; O(9)–H(9A)⋯O(2W, symmetry codes: 1−x, 1−y, −z), dO(9)⋯O(2W) = 2.800(3) Å, ∠[O(9)–H(9A)⋯O(2W)] = 161(6)°. Partial disordered H atoms are drawn in light green. The green arrows indicate the proton transport channel by the Grotthuss mechanism. Download figure Download PowerPoint Our experimental data could well demonstrate the above speculation. Upon continuous irradiation by a diode-pumped solid-state (DPSS) laser (355 nm, 369 mW·cm−1) for only 30 s under ambient conditions, the colorless as-synthesized crystalline sample ( 1A) underwent a rapid, apparent color change to a purple sample ( 1B) (Figure 2a). No generation of prominent new peaks or disappearance of old peaks was observed in the PXRD pattern ( Supporting Information Figure S1), indicating no evident structural change during the coloration process. Furthermore, TGA curves before and after the coloration also displayed no noticeable difference, which suggested that the free water molecules were not lost after coloration ( Supporting Information Figure S5). These results excluded the occurrence of photoinduced dissociation after the coloration. Additionally, two characteristic electron absorption bands of viologen radicals35 appeared around 386 and 596 nm after the coloration (Figure 2c). Time-dependent absorption data indicated that the coloration process occurred rapidly and reached saturation after 2 min of irradiation. The coloration–decoloration process for 1 could be cycled at least four times ( Supporting Information Figure S6), revealing its reversible photochromism character.36 An EPR study revealed no signal for 1A, but a strong, sharp single-line signal at g = 2.0025 for 1B (Figure 2b). Both electron absorption and EPR data demonstrate the occurrence of a PIET process and the formation of H2V radicals after coloration. We confirmed the electron donor by calculating the projected band structure of 1. As illustrated in Figure 2d, the electronic states near the valence band maximum (VBM) were mainly dominated by an ox, while the conduction band minimum (CBM) was exclusively contributed by H2V. These features indicated that ox and H2V were the electron donor and acceptor, respectively. This deduction was consistent with the previous discovery in the literature that ox is an effective electron donor.32 The 1B sample could be bleached by allowing to stand in the dark in air, but complete bleaching required 2 days, as monitored by the EPR study (Figures 2a and 2b). Figure 2 | Photochromism of 1: reversible color change (a) and EPR spectra (b) in a cycle (1A, as-synthesized sample; 1B, colored sample; decolored, color-bleached sample). (c) Time-dependent electron absorption spectra upon irradiation. (d) Projected band structure with the Fermi level was set to zero by default. Download figure Download PowerPoint Proton conductivities (σ) of a single crystal of 1 were investigated by impedance spectroscopy using silver paste as electrodes with resistance extracted by fitting the corresponding Nyquist plot. As shown in Figure 3a, the Nyquist plot impedance data on the 95% RH at 303 K showed that the 1A sample has a σ value of 2.83 × 10−5 S·cm−1. After irradiation for 2 min by the DPSS laser to produce the 1B sample, the σ value increased by about 54 times to 1.51 × 10−3 S·cm−1. This is the hitherto largest contrast for light-responsive proton conductor gain.13–17 Thus, the above results confirmed that a light-responsive proton conductor could be successfully designed by the PIET strategy. Figure 3 | (a) Nyquist plot of the impedance (Z) of 1A and 1B under 303 K and 95% RH. Inset: an enlarged image of 1B. (b) Temperature-dependent proton conductivities of 1A and 1B at 95% RH. Inset: photos of the single-crystal samples tested. Download figure Download PowerPoint To gain insight into why the electron transfer caused an increase in proton conductivity, temperature-dependent proton conductivities were measured. TGA revealed that compound 1 could be stable up to 60 °C (323 K). Above this temperature, the free water molecules began to lose gradually ( Supporting Information Figure S4). Around 95% RH, the σ values of 1A and 1B increased from 2.83 × 10−5 to 5.64 × 10−4 S·cm–1 and from 1.51 × 10−3 to 6.77 × 10−3 S·cm−1, respectively, with the temperature increasing from 303 to 323 K (Figure 3b). The expression σT = Aexp (–Ea/kBT), based on the Arrhenius equation along with the Nernst–Einstein relation, has often been applied to describe the proton conduction in solids.37 The calculated activation energy Ea values of 1A and 1B are around 0.51 and 0.30 eV, respectively (Figure 3b and Supporting Information Figure S7). The decrease of Ea after irradiation indicated a drop in the proton conduction barrier, leading to an increase in the σ value. To further understand the reason for increased proton conduction after PIET, irradiation time-dependent FT-IR spectra were recorded (Figure 4a). As the irradiation time increased, the relative intensity of O–H stretching vibration around 3398 cm−1 for H2O and −OH groups gradually decreased, while the position of peaks remained almost unchanged. Moreover, intensities and positions of other peaks almost did not vary. Infrared intensity (I) of a vibration mode depends on the change in dipole moment caused by the vibration: I ∼ (∂μ/∂q)2, where μ and q were the dipole moment and the normal coordinate, respectively. The greater the difference in electronegativity of atoms connected at both ends of one chemical bond, the greater the change in μ during vibration, and the stronger the vibration absorption. When one electron transferred from the Ge complex component to H2V, the overall electron density of the Ge complex decreased. The decrease in electron density created the difference in electronegativity between O and H atoms in the hydroxyl group of the Ge complex, which tended to average, thereby reducing the μ value of the hydroxyl group, with a resultant decrease in the I value. The calculated μ values of the O(9) hydroxyl fragment ( Supporting Information Table S1) showed that the magnitude of the dipole moment reduced from 2.77 to 2.40 Debye after electron transfer, consistent with the above analysis. Therefore, the decrease in the polarity of the hydroxyl group reduced the strength of the hydrogen bonds between the hydroxyl group and the free water molecule, which, in turn, increased the proton activity on the hydroxyl and water groups and the proton conduction capability. Figure 4 | (a) Evolution of the IR spectrum of 1 in the KBr matrix upon irradiation. (b) Isosurfaces of complex orbitals 101 and 115 with green and blue isosurface levels being set at ±0.05 a.u., respectively. Download figure Download PowerPoint CDA provides a deeper insight into the electron transfer’s influence on proton conduction from an electronic structural perspective.27 As shown in Figure 4b, the isosurfaces of orbital 101 cover the overlap region between the hydroxyl group and the water molecule with a large positive r value (0.0332), which indicated a bonding character. In comparison, a node existed in the overlap region in orbital 115 with a large negative r value (−0.0223), reflecting an electron repulsive effect (Figure 4b, Supporting Information Tables S2 and S3 and Figure S9). The typical bonding and antibonding orbitals denoted the hydrogen-bonding relationship within the hydroxyl group and water molecule. Hence, the hydrogen-bonding relationship mainly contributed to charge transfer between the Ge complex and water molecule. As shown in Table 1, the net electrons were transferred from the Ge complex to the lattice water molecule due to the corresponding complex orbital formation. This result is in line with our chemical intuition, in that the hydroxyl O atom provided lone pair electrons to form a hydrogen bond with the exposed proton in the water molecule, according to the classical hydrogen-bond formation theory.37,38 The decrease in net electrons’ donation from 0.0304 to 0.0258 between the Ge complex and the water molecule after coloration resulted in weak hydrogen-bonding interaction. Accordingly, the H atoms in the water molecule featured high activity and increased proton mobility (Table 1). Table 1 | Total CDA Results for a Pair of One Ge Complex and One Water Molecule in 1 d b d–b r Initial state 0.0528 0.0224 0.0304 −0.0375 Colored state 0.0488 0.0229 0.0258 −0.0369 Note: The term d denotes the number of electrons donated from the Ge complex to the water molecule. The term b denotes the electrons donated back from the water molecule to the Ge complex. The term d–b implied that the Ge complex provides its electrons from occupied FOs to virtual FOs of the water molecule. The term r reveals closed-shell interaction between two occupied FOs in different fragments. Conclusion We have shown in a first attempt to modify proton conduction by the PIET strategy. Through functional motif-oriented structural design,39 we were able to screen one proof-of-concept crystalline photochromic viologen-based H-bonded supramolecule from the CCDC database. For a real application, high-contrast enhancement of proton conductivity after light irradiation is highly desirable, rather than weakening. To the best of our knowledge, only one known case has shown improved proton conductivity upon irradiation and the observed contrast of ca. one time, which is relatively small. In this study, the proton conductivity for the proof-of-concept compound increased to ca. 54 times after PIET, representing a record for light-responsive proton conductors. The increased proton conductivity was derived from a decrease in the energy barrier of the proton transport caused by the weakening of the hydrogen-bonding interaction after PIET. A clear explanation of the relationship between the electron-transfer process and proton conduction and the evidence of high efficiency for the PIET method will inspire the exploration of photon conductors with higher proton conductivities or switchable smart systems with high contrasts. Supporting Information Supporting Information is available and includes PXRD patterns, screening condition and result, TGA analysis, impedance spectroscopy, the dipole moment of a fragment, and CDA results. Conflict of Interest There is no conflict of interest to report. Funding Information This work was supported by the National Natural Science Foundation of China (nos. 91545201, 21827813, 22001011, and 22001037), the Strategic Priority Research Program of the Chinese Academy of Sciences (nos. XDB20010100 and YJKYYQ20180006), and the Key Research Program of Frontier Science, Chinese Academy of Sciences (no. QYZDB-SSW-SLH020). The authors sincerely thank Prof. Dr. Gang Xu for his help on the measurement of proton conductivity.