Leveraging ordered voids in microporous perovskites for intercalation and post-synthetic modification

Abstract

We report the use of porous organic layers in two-dimensional hybrid organic-inorganic perovskites (HOIPs) to facilitate permanent small molecule intercalation and new post-synthetic modifications. While HOIPs are well-studied for a variety of optoelectronic applications, the ability to manipulate their structure after synthesis is another handle for control of physical properties and could even enable use in future applications. If designed properly, a porous interlayer could facilitate these post-synthetic transformations. We show that for a series of copper-halide perovskites, a crystalline arrangement of designer ammonium groups allows for permanently porous interlayer space to be accessed at room temperature. Intercalation of the electroactive molecules ferrocene and tetracyanoethylene into this void space can be performed with tunable loadings, and these intercalated perovskites are stable for months. The porosity also enables reactivity at the copper-halide layer, allowing for facile halide replacement. Through this, we access previously unobserved reactivity with halogens to perform halide substitution, and even replace halides with pseudohalides. In the latter case, the porous structure allows for stabilization of new phases, specifically a novel copper-thiocyanate perovskite phase, only accessible through post-synthetic modification. We envision that this broad design strategy can be expanded to other industrially relevant HOIPs to create a new class of highly adjustable perovskites.

Fig. 1. (a) Portion of the structure of (APOSS)[CuCl₄]₄ as determined by analysis of single crystal X-ray diffraction data. (b) Detail of the arrangement of APOSS groups within organic interlayer, with selected pore dimensions highlighted. Yellow, green, gray, blue, red, and teal spheres represent Cu, Cl, C, N, O, and Si, respectively. H atoms and disorder are omitted for clarity.

Fig. 2. (a) Scheme of gas-phase or liquid-phase intercalation in (APOSS)[CuCl₄]₄ to make (APOSS)[CuCl₄]₄·1.22Fc and (APOSS)[CuCl₄]₄·xTCNE, respectively. (b and c) EDX mapping data for Fe (b, orange dots) and Cu (c, yellow dots) of representative flake of Fc-intercalated (APOSS)[CuCl₄]₄, showing even dispersion of Fe throughout the material. White bars are 25 μm. (d) Powder X-ray diffraction patterns for (APOSS)[CuCl₄]₄ (black), (APOSS)[CuCl₄]₄·xTCNE (green), and (APOSS)[CuCl₄]₄·1.22Fc (orange), showing that perovskite phase remains intact during intercalation. (e) Infrared spectra of (APOSS)[CuCl₄]₄ (black), TCNE (purple), and (APOSS)[CuCl₄]₄·xTCNE (green), showing the presence of intercalated TCNE.

Fig. 3. (a) Scheme of post-synthetic halide exchange using Br₂, either in the liquid or gas phase. The specific bromide content, as determined by EDX, for these 14 samples are 0.0, 5.8, 10.4, 11.0, 21.7, 27.8, 39.5, 42.3, 43.3, 55.8, 59.2, 79.8, 90.0, and 100.0% (expressed as a gradient). Black lines on either side represent x = 0 or x = 4. (b) Powder X-ray diffraction patterns for (APOSS)[CuCl_4−xBr_x]₄. (c) Infrared spectra of (APOSS)[CuCl_4−xBr_x]₄, highlighting the symmetric N–H vibration around 1490 cm⁻¹. (d and e) Diffuse reflectance spectra of (APOSS)[CuCl_4−xBr_x]₄, highlighting two LMCT transitions.

Fig. 4. (a) Scheme of thiocyanate exchange to make (APOSS)[CuCl_4−x(SCN)_x]₄ with variable thiocyanate loadings. The specific thiocyanate loading, as determined through sulfur composition in EDX, for these 8 samples are x = 0.03, 0.05, 0.11, 0.16, 0. 21, 0.27, 0.34, and 0.44 (expressed as a gradient). (b) Infrared spectra of (APOSS)[CuCl_4−x(SCN)_x]₄, overlaid with that of NH₄SCN. (c) X-ray photoelectron spectroscopy of S 2p_1/2 and 2p_3/2 in (APOSS)[CuCl_3.79(SCN)_0.21]₄. Black crosses are experimental data, and the lines represent the total fit (teal) and background (gray). The 2p_1/2 fits (dashed lines) and 2p_3/2 fits (solid lines) for two sites (purple and yellow lines) are shown. (d) Powder X-ray diffraction patterns for (APOSS)[CuCl_4−x(SCN)_x]₄, showing the perovskite structure is retained.