Postsynthetic Crystalline Transformation in Two-Dimensional Perovskites via Organothiol-Based Chemistry

Open AccessCCS ChemistryCOMMUNICATION11 May 2021Postsynthetic Crystalline Transformation in Two-Dimensional Perovskites via Organothiol-Based Chemistry Zilong Yuan†, Liang Zhao†, Ekadashi Pradhan, Ming Lai, Tao Zeng and Zhenyu Yang Zilong Yuan† MOE Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry, Sun Yat-sen University, Guangzhou, Guangdong 510275 , Liang Zhao† MOE Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry, Sun Yat-sen University, Guangzhou, Guangdong 510275 , Ekadashi Pradhan Department of Chemistry, York University, Toronto, ON M3J1P3 , Ming Lai MOE Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry, Sun Yat-sen University, Guangzhou, Guangdong 510275 , Tao Zeng Department of Chemistry, York University, Toronto, ON M3J1P3 and Zhenyu Yang *Corresponding author: E-mail Address: [email protected] MOE Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry, Sun Yat-sen University, Guangzhou, Guangdong 510275 https://doi.org/10.31635/ccschem.021.202100929 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail The first postsynthetic solution-based crystal transformation of two-dimensional metal halide perovskites (2D MHPs) through organothiol-based reactions is reported. It is well-established that the crystal formation from a solution containing predesigned metal ion and organic cation precursors produces well-defined 2D MHPs with various intercalating organic cations. However, few reports outlining the postsynthetic crystal transformation of 2D MHPs have appeared. Here, we report that, upon organothiol-based redox or condensation reactions, large organic cations in three types of 2D MHPs can interconvert under ambient conditions without damage to the layered inorganic framework. The swift and complete crystal interconversion has been confirmed using combined techniques including X-ray diffraction and 13C nuclear magnetic resonance (13C NMR) spectroscopy. Electronic structures of the MHPs were investigated using computational chemistry. Download figure Download PowerPoint Introduction Metal halide perovskites (MHPs) have been acclaimed as a class of promising semiconductor materials in the past decade because of their compositionally tunable optoelectronic properties, high carrier mobility, and simplicity of fabrication.1–4 Advances in managing the ion compositions, surface passivating ligands and adatoms, as well as the progress in device architecture and encapsulation techniques, have led to rapid improvements in the performance of perovskite-based optoelectronics.5–7 Conventional MHPs share a three-dimensional (3D) pseudocubic crystal structure (general chemical formula of ABX3) in which monovalent cations A (e.g., CH3NH3+ and Cs+) are located in the voids created by a corner-sharing metal halide [BX6]4− octahedral framework. When small A-site cations are substituted by large organoammonium cations, the Goldschmidt tolerance factor is no longer within the allowable regime and thus metal halide octahedra are isolated to form two-dimensional (2D), layered inorganic structures. The reduced dimensionality and the incorporation of intercalating organic cations lead to unrivaled tunability in lattice structures and photophysical properties of 2D MHPs.8–10 Recent reports have demonstrated the modulation of optoelectronic properties of 2D MHPs via the design of intercalating cations.11–14 Postsynthetic crystal transformation through ion or ligand engineering offers additional structural flexibility of materials and allows rational design and tailoring of material characteristics. It has high versatility and has been widely applied in various classes of materials such as quantum dots and metal–organic frameworks.15,16 Unfortunately, postsynthetic treatment methods are not directly transferrable to MHPs due to the weak van der Waals interaction between the intercalating organic groups and the low formation energy of the liquid-like ionic crystal structures.8 Till now, the majority of 2D MHPs, regardless of the structural type and material form (e.g., thin film, powder, and single crystal), are prepared using direct crystal growth from a predesigned solution containing large organic cations, halides, and metal precursors (Figure 1).17–19 While there are few examples of the postsynthetic reactions for the 2D MHP structural transformation,20,21 the key contribution of this work is to realize crystal interconversion of 2D MHPs using solution-based postsynthetic methods. Additionally, the crystal structures are well characterized. Figure 1 | Schematic illustration of the preparation of 2D MHPs. In this work, we propose a series of solution-based reactions to fulfill postsynthetic crystal transformation between three types of 2D MHPs. The transformation is achieved via the interconversion between thiol, disulfide, and dithioketal functional groups on the large organic cations. For brevity, the alkyl fragments that connect thiol and NH3+ are not shown. Download figure Download PowerPoint Noting the high reactivity of organothiols, which can readily convert to other organic functional groups, we developed a series of solution-based reactions of replacing the A-site cation mercaptoethylammonium (MEA) on 2D MHPs. This facilitated efficient in situ crystal transformation between three types of 2D MHPs without damaging the original layered lattice framework (Figure 1). The A-site reaction process relied on the interconversion from thiols to disulfide and dithioketal structures when interacting with oxidative reagents and acetone, respectively. We also demonstrated the successful conversion of the dithioketal cations back to thiol and disulfide groups by solution-based reactions at room temperature. We investigated these crystal transformations of 2D MHPs using combined spectroscopic techniques including X-ray diffraction (XRD) and 13C nuclear magnetic resonance (13C NMR) spectroscopy. Results and Discussion Initially, we applied an optimized temperature-lowering spontaneous nucleation method to prepare three types of 2D MHP single crystals: (MEA)2PbI4, (DSBA)PbI4, and (PDSBA)PbI4, where MEA = SHCH2CH2NH3+, DSBA = H3N+CH2CH2SSCH2CH2NH3+, and PDSBA = NH3+CH2CH2SC(CH3)2SCH2CH2NH3+ (Figure 2a). The structures of (MEA)2PbI4 and (PDSBA)PbI4 are reported for the first time in this study (Figure 2a and Supporting Information Figures S1 and S3). As for (DSBA)PbI4, Louvain et al.22 reported two polymorphs, and in this study we only synthesized the α-conformation polymorph, which is believed to be more stable at room temperature. In the synthesis of (MEA)2PbI4, we noticed that the use of the reductant hypophosphorous acid (H3PO2) is necessary to avoid the undesired, spontaneous oxidation of MEA (see Supporting Information Experimental Section). These 2D MHPs have various crystal morphologies and colors (Figure 2b), and their crystal and geometrical parameters are summarized in Table 1. Figure 2 | Crystal structures and morphologies of three 2D MHPs. (a) Single-crystal unit cells of (MEA)2PbI4, (DSBA)PbI4, and (PDSBA)PbI4. Only half of the unit cell of (PDSBA)PbI4 is shown for a clearer demonstration of the spatial configuration of the organic cations. (b) Stereo Fluorescence Microscope (SFM) images of single crystals under the illumination of visible and 365-nm UV light. Download figure Download PowerPoint Table 1 | Single-Crystal Parameters of (MEA)2PbI4, (DSBA)PbI4, and (PDSBA)PbI4 Formula (MEA)2PbI4 (DSBA)PbI4a (PDSBA)PbI4 Crystal system Orthorhombic Monoclinic Orthorhombic Color Red Orange Orange Space group Pnma P21/n Pnma Unit cell dimensions a = 12.9639 (3) Å a = 17.7855 (10) Å a = 8.5801 (17) Å b = 20.5958 (5) Å b = 8.5500 (4) Å b = 54.936 (11) Å c = 6.4283 (2) Å c = 23.270 (2) Å c = 8.6960 (17) Å α = β = γ = 90° α = γ = 90°, β = 98.78 (1)° α = β = γ = 90° Volume (Å3) 1716.37 (8) 3497.11 (41) 4098.9 (14) Z 4 4 8 Density (g/cm3) 3.3371 3.301 2.953 aThe parameters of (DSBA)PbI4 perovskites are reported in the CCDC no.: 724584.22 All three types of 2D MHPs share similar layered structures with a corner-sharing inorganic framework ( Supporting Information Figures S1–S3). The interaction of the inorganic–organic layers is primarily dominated by the H atoms of the ammonium end groups and I atoms on the surface of the [PbI6]4− octahedra. The average H···I bond lengths of (MEA)2PbI4, (DSBA)PbI4, and (PDSBA)PbI4 are 2.60, 2.50, and 2.64 Å ( Supporting Information Figures S1–S3), respectively, which are shorter than the summation of the van der Walls radii of H and I atoms (∼3.1 Å), reflecting the formation of hydrogen bonds. The distortion degree of the inorganic layers, however, varies depending on the organic cations. (MEA)2PbI4 has 178.5° I–Pb–I equatorial angles and 175.9° Pb–I–Pb angles. For comparison, (PDSBA)PbI4 has 180° I–Pb–I equatorial angles and smaller 145.3° Pb–I–Pb angles, which is similar to those of (DSBA)PbI4 (I–Pb–I: 171.5° and Pb–I–Pb 139.2°). The larger lattice distortions of the latter two are attributed to the hydrogen bonds between the ammonium end groups and the equatorial I atoms. This difference arises from the orientations of the ligands. The MEA chains are oriented in parallel to the inorganic layers, while the PDSBA and DSBA chains are vertical to the layers, inserting more toward the equatorial I atoms. Because both the –NH3+ "head" and –SH "tail" groups of MEA point toward the interlayer space, the distance between adjacent inorganic layers of (MEA)2PbI4 is reduced to 3.93 Å ( Supporting Information Figure S1). The distance between the S atoms of two adjacent MEA cations is 3.66 Å ( Supporting Information Figure S1), which is longer than the length of a disulfide bond in organothiol molecules (1.8 Å∼3.0 Å)23 and matches two times the van der Waals radius (∼3.6 Å) of S. The reduced, yet relatively large interlayer distance, together with the weak van der Waals interactions between the thiol end groups hint at the incorporation of atomic and small molecular dopants into the lattice. We next induced the crystal transformation between these three types of 2D MHPs. Inspired by the formation of disulfide linkage by the oxidation of organothiols,24 we first investigated the conversion of (MEA)2PbI4 to (DSBA)PbI4 using N-iodosuccinimide (NIS) as the oxidant (Path A in Figure 3a). The crystal transformation was monitored using powder XRD (PXRD). Prior to the reaction, the PXRD spectrum of (MEA)2PbI4 shows distinct patterns of (0k0) orientation of 2D MHPs (Figure 3c). After the treatment with NIS, the (MEA)2PbI4 signals disappear and new layered structure patterns are found, which are attributed to the (002) orientation of (DSBA)PbI4 (Figure 3c and Supporting Information Figure S4). The oxidation of MEA was further confirmed by the 13C NMR analysis of the crystals dissolved in deuterated dimethyl sulfoxide (DMSO-d6, Figures 3b and 3d). The obvious differences in the chemical shifts before and after the reaction support the hypothesis that two terminal thiol groups are oxidized by NIS and form a disulfide bond, generating new organic cation DSBA in situ. Neither additional crystalline signals nor chemical shifts are found from PXRD and NMR results, indicating the complete conversion of (MEA)2PbI4 to (DSBA)PbI4 without the formation of byproducts. Figure 3 | Postsynthetic 2D MHP crystal transformation. (a) Schematic representation of four crystal transformation routes discussed in this study. (b) Molecular structures of MEA, DSBA, and PDSBA groups with the various types of carbon atoms labeled. (c–f) PXRD and 13C NMR spectra of (MEA)2PbI4 before and after the crystal transformation reactions following Paths A–D. The XRD signals from the unreacted crystals are marked by triangles. NMR signals from the solvent DMSO-d6 are marked by asterisks. The full 13C NMR spectra are presented in Supporting Information Figures S5–S10. Download figure Download PowerPoint Motivated by the effective disulfide linkage formation from the thiol groups in (MEA)2PbI4, we further investigated whether the thiol end groups can interact with small molecules within the 2D MHP lattice. Ketone is readily condensed with organothiols and forms dithioketal linkages under mild conditions.25 Therefore, we explored the potential interaction between (MEA)2PbI4 and acetone (Path B in Figure 3a, see Supporting Information Experimental Section for details). A new series of layered crystalline features emerge in the PXRD analysis of the acetone-treated (MEA)2PbI4 with the diffraction angles consistent with those of (PDSBA)PbI4 (Figure 3c and Supporting Information Figure S4). In the 13C NMR spectrum, new chemical shifts at 27.0, 30.6, 39.2, and 56.7 ppm indicate the successful formation of a dithioketal linkage (Figure 3d). As only weak signals originating from (MEA)2PbI4 can still be found in the PXRD of the acetone-treated sample (Figure 3c), we conclude that most of the thiol end groups in (MEA)2PbI4 have been transformed into dithioketal structures through the condensation route, yielding (PDSBA)PbI4 crystal. Dithioketals are commonly used protecting groups for carbonyl-based reactions because they can be readily cleaved to thiols and ketones.26 Similarly, (PDSBA)PbI4 crystals can be converted back to (MEA)2PbI4 after soaking in a I2–dodecanethiol (1-DDT) mixed solution at room temperature for 24 h (Path C in Figures 3a, 3e, and 3f, also see Supporting Information Experimental Section). We also achieved the complete transformation from (PDSBA)PbI4 to (DSBA)PbI4 using NIS that effectively cleaves the C–S bonds (Figures 3e and 3f). We speculate that the mechanism of dithioketal-to-disulfide conversion may involve the dissociation of C–S bond promoted by NIS27 and subsequent S–S bond formation from a thiol-intermediate-driven condensation.28 To determine the generality of the postsynthetic reaction routes, we further tested the effectiveness of the crystal transformation reactions on bromide 2D MHPs. Single crystals of (MEA)2PbBr4, (DSBA)PbBr4, and (PDSBA)PbBr4 were prepared, characterized, and used as the standard samples to verify the effectiveness of the MHP conversion ( Supporting Information Figure S11 and Table S1). However, we only observed the effective transformation from (PDSBA)PbBr4 to (DSBA)PbBr4 ( Supporting Information Figure S12). We explain this lack of transformation as a result of the weak SH···Br interaction in (MEA)2PbBr4 (Figure 4). The SH···X distances are 2.94 and 2.73 Å in (MEA)2PbBr4 and (MEA)2PbI4, respectively. The former is 0.21 Å larger, while the van der Waals radius of Br atom is 0.13 Å smaller than that of I. Actually, the 2.94 Å is just within the summation of the H and Br van der Waals radii (3.05 Å). The weak interaction is evident, and a consequence of this weak hydrogen bond is less activation of the S–H bond, which is then more difficult to break for forming the S–S and S–C bonds in (DSBA)PbBr4 and (PDSBA)PbBr4. Figure 4 | Comparison of the distance between hydrogen atom of the thiol group and closest bromine or iodine atom in the crystal structure of (MEA)2PbBr4 and (MEA)2PbI4. Download figure Download PowerPoint We further investigated the optical properties of these 2D MHPs using UV–vis absorption and photoluminescence (PL) spectroscopy. The optical band gaps of three MHPs are 2.02, 2.09, and 2.18 eV, respectively (Figure 5a), slightly smaller than those of conventional 2D MHPs such as butylammonium lead iodide [(BA)2PbI4] and phenylethylammonium lead iodide [(PEA)2PbI4].29 This is not unexpected because of the reduced distance between two inorganic layers in all three perovskites ( Supporting Information Figures S1–S3). This band gap narrowing is also reflected by the steady-state PL maxima, which are found in the lower energy regime compared with (BA)2PbI4 and (PEA)2PbI4 ( Supporting Information Figure S13).19,28–31 It is interesting to note that only (PDSBA)PbI4 crystals show strong PL with the longest average lifetime decay value (τavg = 3.29 ns, Supporting Information Figure S14 and Table S2), suggesting lower trap density and slower charge-carrier recombination in (PDSBA)PbI4. Figure 5 | Photophysical and electronic properties of 2D MHPs. (a and b) Normalized absorption and normalized static-state PL spectra of (MEA)2PbI4, (DSBA)PbI4, and (PDSBA)PbI4. (c) The Fermi energies of the three systems have been shifted to 0 eV, to have a better comparison of DOS and pDOS in the three panels in (c). Download figure Download PowerPoint To gain insight into the electronic structures of the three 2D MHPs, we turned our focus to computational chemistry. The calculated density of states (DOS) and atomic projected DOS (pDOS) are shown in Figure 5c. The calculated band gaps are 1.95, 2.15, and 2.20 eV for (MEA)2PbI4, (DSBA)PbI4, and (PDSBA)PbI4, respectively. They are in excellent agreement with the aforementioned optical bandgaps (Figure 5a), in both magnitudes and sequence of magnitudes. The pDOS results clearly show that all three compounds have their conduction band minimum (CBM) dominated by Pb. While the valence band maximum (VBM) of (MEA)2PbI4 is dominated by I, those of (DSBA)PbI4 and (PDSBA)PbI4 feature a mixture of S and I contributions, especially in (PDSBA)PbI4. All the band edge absorptions must occur as charge-transfer excitations from I to Pb. The comparison of the absorption and PL spectra in Figures 5a and5b clearly shows a regular Stokes shift in the emission of (MEA)2PbI4. This shift is not obvious for (DSBA)PbI4, and an anti-Stokes shift is even observed for (PDSBA)PbI4. It is logical to associate the Stokes-to-anti-Stokes change to the more significant S contribution in VBM. The degenerate S lone pair and I 5p orbitals in the VBM of (DSBA)PbI4 and (PDSBA)PbI4 facilitate hole transfer from I to S. Despite the far distance between S and I, first the hole can be transferred from I to N through H atom migration from N to I; and then the hole is transferred from N to S through their cis 1–4 orbital overlap. Such a separation of hole and electron quenches the low-energy emissions from CBM to VBM. PL in (DSBA)PbI4 and (PDSBA)PbI4 thus occurs at shorter wavelength than absorption. Since the mixture of S and I pDOS is more significant in the (PDSBA)PbI4, a more significant quench of low-energy emission ensues. Overall, the separation of hole and electron into the ligands and inorganic frameworks is likely to extend the exciton lifetime and enhance conductivity. The so-elongated exciton lifetime explains the (PDSBA)PbI4 > (DSBA)PbI4 > (MEA)2PbI4 sequence of PL intensity ( Supporting Information Figure S13). Such a change in optoelectronic property is now realized by postsynthetic transformation. Moreover, the overlaps of pDOS of different atoms can be used to further elucidate the details in the absorption and PL spectra. The two PL peaks of (DSBA)PbI4 are consistent with the two absorption peaks in Figure 5a. While the absorption extends to shorter wave length, the PL emission disappears beyond 460 nm. This is consistent with an overlap between the Pb and S pDOS at 0.5 eV above CBM and the overlap between the I and S pDOS at 0.3 eV below VBM (middle panel of Figure 5c). These overlaps facilitate electron and hole transfers in excited states, respectively, from Pb to S and from I to S, and consequently shorten the lifetimes of those high-energy states and eliminate the corresponding PL emissions. In comparison, in (MEA)2PbI4, the Pb pDOS does not overlap with any other pDOS until 1 eV above CBM, and the I pDOS does not overlap with the S pDOS until 0.4 eV below VBM (top panel of Figure 5c). The electron and hole in excited states in a larger range of energy remain mostly on Pb and I, which readily recombine radiatively and give the <500 nm PL emissions of (MEA)2PbI4. A more in-depth insight of the exciton dynamics requires nonadiabatic dynamics simulations for the three compounds, which will be addressed in a future study. Conclusion We characterized the single-crystal structures of two novel 2D MHPs, (MEA)2PbI4 and (PDSBA)PbI4, and we achieved postsynthetic switching between the perovskite crystals entirely by solution-processed treatments without destroying the perovskite framework or generating any byproduct. The crystalline structure and chemical composition characterized by XRD and 13C NMR together confirm the transformation process. Furthermore, we demonstrated the changes of optical properties resulting from the conversion of the ligands by photophysical characterizations and computational simulations. This work demonstrates the great potential of 2D MHPs: their optoelectronic performance can be tuned by postsynthetic transformation while maintaining the crystal structure skeleton. Quantum chemistry calculations revealed different extents of pDOS overlaps at band edges, which explain the different PL spectra and excited states dynamics of the three 2D MHPs. Supporting Information Supporting Information is available and includes full details for experimental sections, additional crystallographic data, full 13C NMR spectra, single crystal structures and parameters of bromide-based MHPs, transformation XRD of bromide-based MHPs, PL spectra, and PL lifetime decay results (PDF), as well as crystallographic data of (MEA)2PbI4, (PDSBA)PbI4, (MEA)2PbBr4, and (PDSBA)PbBr4 (CIF). Conflict of Interest Competing interests: A provisional patent application CN 202110140411.0 was filed on February 2, 2021 by the Sun Yat-sen University. Notes Complete crystallographic data of (MEA)2PbI4, (PDSBA)PbI4, (MEA)2PbBr4, and (PDSBA)PbBr4 have been deposited to the Cambridge Crystallographic Data Centre (with CCDC numbers of 2059317, 2059319, 2059316 and 2059318). It can be obtained free of change from Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif Acknowledgments All calculations were carried out using the Tianhe-2 (TH-2) super computer clusters (Guangzhou, Guangdong, China). Z.Y. would like to acknowledge financial support from the National Natural Science Foundation of China (no. 21905316), the Department of Science and Technology of Guangdong Province (no. 2019QN01C108), and Sun Yat-sen University. Profs. P. Hu, T. Zhu, and Z. W. Wei are thanked for valuable suggestions. T.Z. thanks York University for the start-up grant (no. 481333) and the Natural Sciences and Engineering Research Council (NSERC) of Canada (no. RGPIN-2016-06276) for financial support. References 1. Cao D. H.; Stoumpos C. C.; Farha O. K.; Hupp J. Perovskites as for J. J. P. of Metal and for D. H.; S. for J. J. H.; Department of May D. for Functional Stoumpos C. C.; and H.; J. C.; in Two-Dimensional C.; Yang Wei Liang H.; in C.; C.; Yang S. J. Liang C.; S. C.; Engineering of Perovskites Quantum S. C.; Lai Yang K.; S. of C. H.; D. from Quantum D. and Engineering in Lai S. C. Yang Two-Dimensional C. D. D. J. C.; of of Two-Dimensional and in C.; P. in 2D Perovskites by and Louvain C.; and in the J. of as an and for the of to in or under C.; as a for and of C.; of in and with as a S. with on of and I. of Two-Dimensional Perovskites for D. Crystal and and of Stoumpos C. C.; Cao D. H.; D. J. J. Hupp J. 2D Information 2021 halide calculations were carried out using the Tianhe-2 (TH-2) super computer clusters (Guangzhou, Guangdong, China). Z.Y. would like to acknowledge financial support from the National Natural Science Foundation of China (no. 21905316), the Department of Science and Technology of Guangdong Province (no. 2019QN01C108), and Sun Yat-sen University. Profs. P. Hu, T. Zhu, and Z. W. Wei are thanked for valuable suggestions. T.Z. thanks York University for the start-up grant (no. 481333) and the Natural Sciences and Engineering Research Council (NSERC) of Canada (no. RGPIN-2016-06276) for financial support. times

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