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

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 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 the σ value increased by about 54 times to × S·cm−1. This is the hitherto largest for light-responsive proton Thus, the above results confirmed that a light-responsive proton could be by the PIET Figure 3 | (a) Nyquist plot of the impedance of 1A and 1B under 303 K and 95% an of (b) proton conductivities of 1A and 1B at 95% of the single-crystal samples Download figure Download PowerPoint gain insight into the electron transfer an increase in proton proton conductivities were TGA revealed that compound 1 could be stable to this the free water molecules to (Supporting Information Figure 95% the σ of 1A and 1B increased from 2. 83 × 10−5 to × and from × to × respectively, with the temperature from 303 to 323 K (Figure The = based on the along with the has often been applied to the proton conduction in The calculated energy of 1A and 1B are around and (Figure and Supporting Information Figure The decrease of after irradiation indicated a in the proton conduction to an increase in the σ further understand the for increased proton conduction after irradiation FT-IR spectra were recorded (Figure As the irradiation the relative of around cm−1 for and groups while the of peaks remained unchanged. Moreover, and positions of other peaks not of a mode on the change in dipole moment by the where and were the dipole moment and the respectively. The the in of atoms connected at both ends of one chemical the the change in during and the the one electron transferred from the Ge complex to the electron density of the Ge complex The decrease in electron density the in between and H atoms in the hydroxyl group of the Ge complex, which tended to the value of the hydroxyl with a decrease in the The calculated of the O (9) hydroxyl fragment (Supporting Information showed that the of the dipole moment reduced from to Debye after electron consistent with the above Therefore, the decrease in the of the hydroxyl group reduced the of the hydrogen bonds between the hydroxyl group and the free water molecule, in increased the proton on the hydroxyl and water groups and the proton conduction Figure | (a) of the of 1 in the upon irradiation. (b) of complex and with green and set at respectively. Download figure Download PowerPoint provides a insight into the electron influence on proton conduction from an electronic structural As shown in Figure the of orbital the overlap region between the hydroxyl group and the water molecule with a large positive r value which indicated a bonding In a in the overlap region in orbital with a large negative r value an electron repulsive (Figure Supporting Information S2 and and Figure The typical bonding and the hydrogen-bonding relationship within the hydroxyl group and water the hydrogen-bonding relationship mainly contributed to charge transfer between the Ge complex and water As shown in the electrons were transferred from the Ge complex to the water molecule due to the corresponding complex orbital This is in with our chemical in that the hydroxyl atom provided electrons to form a hydrogen with the proton in the water molecule, to the formation The decrease in from to between the Ge complex and the water molecule after coloration resulted in hydrogen-bonding the H atoms in the water molecule high and increased proton 1 | Results for a of One Ge and One in 1 d b r state state The term d denotes the number of electrons donated from the Ge complex to the water The term b denotes the electrons donated back from the water molecule to the Ge complex. The term that the Ge complex provides its electrons from occupied FOs to virtual FOs of the water The term r reveals closed-shell interaction between two occupied FOs in different We have shown in a first to modify proton conduction by the PIET Through functional structural we were able to one proof-of-concept crystalline photochromic viologen-based H-bonded supramolecule from the For a real high-contrast enhancement of proton conductivity after light irradiation is highly the of our only one known has shown proton conductivity upon irradiation and the observed of ca. one time, which is In this the proton conductivity for the proof-of-concept compound increased to ca. 54 times after a for light-responsive proton The increased proton conductivity was derived from a decrease in the energy of the proton transport by the weakening of the hydrogen-bonding interaction after A of the relationship between the electron-transfer process and proton conduction and the of high for the PIET method inspire the exploration of photon conductors with higher proton conductivities or switchable smart systems with high contrasts. 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