*Article* **New Low-Dimensional Organic–Inorganic Lead Halide Hybrid Systems Directed by Imidazo[1,5-***a***]pyridinium-Based Cation or Imines: Synthesis, Structures, Non-Covalent Interactions and Optical Properties**

**Olga Yu. Vassilyeva 1,\*, Elena A. Buvaylo 1, Oksana V. Nesterova 2, Alexandre N. Sobolev <sup>3</sup> and Dmytro S. Nesterov 2,\***


**Abstract:** The organic–inorganic lead halide hybrids comprising semiconducting perovskite components and organic modules have proven to be promising candidates for optoelectronic applications. The modulation of the inorganic components as optical centres by diverse organic cationic templates is under intense investigation. Herein, we successfully prepared new one-dimensional lead halide hybrid perovskites [L1]2n[Pb2Cl6]n∞·nH2O (**1**) and [PbBr2(L2)]n∞·0.5nH2O (**2**), and the dimeric complex [PbBr2(L3)]2 (**3**) in water media. In **1**, 2-(2-hydroxyethyl)-2H-imidazo[1,5-*a*]pyridinium cation [L1]+ resulted from the oxidative condensation–cyclization between formaldehyde, ethanolamine and 2-pyridinecarbaldehyde (2-PCA); the polydentate Schiff base ligands L2 and L3 formed in the in situ condensation of 2-PCA and ethanolamine or ethylenediamine, respectively. The lead chloride hybrid **1** contains the previously unreported type of a [Pb2Cl6]<sup>∞</sup> double chain constructed from three-edgeand five-edge-sharing PbCl6 octahedra, and cations forming π-bonded stacks aligned along the inorganic wires. In the crystal of **2**, pairs of the double-side organically decorated [PbBr2(L2)]<sup>∞</sup> chains built of corner-sharing PbBr3N2O octahedra arrange hydrophilic channels to host water molecules. In the solid state, the identically stacked dimers of **3** form columns parallel to the *ab* plane with the Pb2Br4 moieties in the column being strictly coplanar. Hirshfeld surface analysis was used to rationalize the packing patterns through hydrogen bonds of O−H···O/Cl and C−H···O/Cl types with the involvement of OH groups of [L1]+, L2 and water molecules in **<sup>1</sup>** and **<sup>2</sup>**, as well as C–H···Br hydrogen bonding in **2** and **3**. The QTAIM analysis of non-covalent interactions in **1**–**3** was performed. According to the analysis of the solid-state UV–visible reflectance spectra by a Tauc plot, the optical band gap values of **1**, **2** and **3** as direct gap semiconductors were estimated to be 3.36, 3.13 and 2.96 eV, respectively.

**Keywords:** crystal structure; organic–inorganic hybrid; lead(II) perovskite; Schiff-base ligand; Hirshfeld surface analysis; QTAIM analysis; DFT calculations; reflectance spectra; band gap

## **1. Introduction**

The versatile optoelectronic properties of organic–inorganic hybrid metal halides make them an attractive alternative to photovoltaic devices utilizing a conventional crystalline silicon solar cell or emerging dye-sensitized solar cells, organic tandem cells and quantum dot cells [1]. In addition, due to their structural richness, semiconducting, electrical and optical properties, as well as processability in solution using low temperature techniques, perovskite-based hybrids are promising materials for use in other optoelectronic devices

**Citation:** Vassilyeva, O.Y.; Buvaylo, E.A.; Nesterova, O.V.; Sobolev, A.N.; Nesterov, D.S. New Low-Dimensional Organic–Inorganic Lead Halide Hybrid Systems Directed by Imidazo[1,5-*a*]pyridinium-Based Cation or Imines: Synthesis, Structures, Non-Covalent Interactions and Optical Properties. *Crystals* **2023**, *13*, 307. https:// doi.org/10.3390/cryst13020307

Academic Editors: Helmut Cölfen and Leonid Kustov

Received: 28 January 2023 Revised: 6 February 2023 Accepted: 10 February 2023 Published: 13 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

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such as light-emitting diodes (LEDs), photodetectors, semiconductor optical amplifiers and lasers [2,3].

Considering the solid-state arrangement, lead halide hybrid perovskites are built of PbHal6 octahedra that share corners, edges or faces to form structures with dimensionality varying from zero (0D) to three (3D), templated by organic cations. The dimensionality and geometry of the inorganic framework determine the optical and electrical properties of the material, while the organic cation can alter/tune its optoelectronic characteristics by modifying this framework. The broadband emission of lead halide hybrid perovskites was proposed to originate from the intrinsic excited electron–hole pairs bound to a deformable lattice (self-trapped excitons), rather than from emissive extrinsic dopants or surface defect sites [4]. There is growing evidence that 1D systems—which often combine several modes of connectivity within a single lead(II) halide chain, depending on the number of bridging halides—create the strong quantum confinement to enable easier exciton self-trapping [5,6].

Recently, we proposed an advantageous synthetic procedure for preparing organic– inorganic hybrid halometalate salts with substituted imidazo[1,5-*a*]pyridinium cations [7–12]. Imidazo[1,5-*a*]pyridines are fused nitrogen-containing bicyclic systems of interest in many research areas, e.g., material science and pharmaceuticals [13,14]. They also demonstrate enhanced fluorescence intensity combined with high quantum yield [15,16]. The organic cation formation occurs in the interaction of equimolar amounts of two aldehydes—formaldehyde (FA) and 2-pyridinecarbaldehyde (2-PCA)—with amine in aqueous media [8,11,12]. The oxidative cyclocondensation is catalyzed by acid that is conveniently introduced as the amine adduct; the cation prepared in situ does not require isolation. The reaction of the preformed heterocyclic cation synthesized using methylamine hydrochloride and metal halides yielded hybrid compounds [L]n[PbCl3]n∞, [L]2[ZnCl4] [8] and [L]2[CdCl4] [11], as well as mixed-halide analogues of the latter [12], where L+ is 2-methyl-imidazo[1,5-*a*]pyridinium cation. The photophysical properties of the organic– inorganic 1D perovskite [L]n[PbCl3]n<sup>∞</sup> and 0D pseudo-layered hybrid [L]2[ZnCl4] were presumed to originate from the synergistic effects of the electronic structure of the cation and the solid-state architectures.

In the present work, the developed approach was extended to replace CH3NH2·HCl with ethanolamine (Ea) hydrochloride, as summarized in Scheme 1, to obtain lead halide hybrid compounds with another electron-deficient cation, 2-(2-hydroxyethyl)-2H-imidazo[1,5 *a*]pyridinium [L1]+. Given that hydrogen bonds may impact important properties of the material due to their directionality and collective strength, Ea was chosen to examine the effect of introducing hydroxyl (–OH) functionality in the cation onto the resulting hybrid structure. The [L1]<sup>+</sup> cation templated the formation of a 1D chloroplumbate(II) wire in [L1]2n[Pb2Cl6]n∞·nH2O (**1**), the hybrid perovskite structure of which was confirmed by X-ray crystal structure analysis. Further replacement of chloride with bromide anion was realized by the addition of the necessary acid component, HBr, directly to the reaction media. For comparison reasons, the study was supplemented with an analogous synthesis involving the 'NH2' alternative of Ea, ethylenediamine (En), to probe hydrogen bonding with amino group. In contrast, the isolated Pb(II) bromides appeared to bear Schiff-base ligands *N*-(2-pyridylmethylene)ethanolamine, L2, and *N*,*N* bis(2-pyridylmethylene)ethylenediamine, L3, produced in the amine-aldehyde condensation reactions between 2-PCA and Ea or En, respectively. Herein, we report the preparation, crystal structures, spectroscopic characterization and optical study of the new 1D hybrid lead halide perovskites [L1]2n[Pb2Cl6]n∞·nH2O (**1**) and [PbBr2(L2)]n∞·0.5nH2O (**2**), and the dimeric Schiff-base complex [PbBr2(L3)]2 (**3**). The Hirshfeld surface (HS) analysis was used to examine the packing patterns through non-covalent interactions. The quantum theory of atoms in molecules (QTAIM) was employed to perform the topological analysis of the electron density.

**Scheme 1.** Reaction schemes and structural formulas of [L]n[PbCl3]n<sup>∞</sup> [8], [L1]2n[Pb2Cl6]n∞·nH2O (**1**), [PbBr2(L2)]n∞·0.5nH2O (**2**) and [PbBr2(L3)]2 (**3**).

#### **2. Experimental Section**

#### *2.1. Materials and General Methods*

For the synthesis of lead compounds, 2-PCA (Merck) was used as received; all other chemicals were purchased from local suppliers and used without further purification. All solvents were of AP-grade; all the experiments were carried out in air. Elemental analyses for C, H and N were performed with a Perkin–Elmer 2400 analyzer. The 1H NMR spectra of **1**–**3** in DMSO-*d*<sup>6</sup> were measured using a Mercury 400 Varian spectrometer at 400 MHz at r.t. The chemical shifts (*δ*) values are given in ppm downfield from internal Me4Si. *J* values are in hertz. The FT–IR spectra were recorded on a PerkinElmer 1600 FT–IR instrument from KBr pellets in the 400–4000 cm−<sup>1</sup> region. Optical diffuse reflectance measurements were performed using a Shimadzu UV-2600i spectrophotometer equipped with the 60 mm integrating sphere operating in the 220–1400 nm region. Ground powder samples were placed in a powder sample holder; BaSO4 was used as the reference of 100% reflectance. The reflectance data were converted to absorption according to Kubelka–Munk function α/*S* = (1 – *R*) <sup>2</sup> (2*R*) <sup>−</sup>1, where *R* represents the reflectance and α and *S* are the absorption and scattering coefficients, respectively, from which the band gap values were estimated by a Tauc plot [17,18]. HS and fingerprint plots were generated by *CrystalExplorer* 21.5 program (revision 608bb32) [19].
