A 2D coordination polymer, {[Fe(L) 2 (NCSe) 2 ]•6MeOH•14H 2 O} n (1; L = 2,5-dipyridylethynylene-3,4-ethylenedioxythiophene), has been synthesized based on a redox active luminescence ligand. 1 possesses a 2D [4 × 4] square-grid network where the iron(II) center is in a FeN 6 octahedral coordination environment. 1 displays reversible thermoinduced high-spin (HS; S = 2) to diamagnetic low-spin (LS; S = 0) ON/OFF spin-state switching with a T 1/2 value of 150 K. Interestingly, optical reflectivity and photomagnetic studies at 10 K under light irradiation revealed an efficient conversion to a photoinduced metastable HS excited state from a LS ground state. Remarkably, the photoexcited HS state can be reversibly switched ON and OFF by using 625 and 850 nm light-emitting-diode lights. Intriguingly, the thermal dependence of the luminescence intensity of the maximum emission at 524 nm for 1 shows a minimum at around the spin-crossover (SCO) temperature, indicating a cooperative nature between the SCO and luminescence properties. Theoretical calculations confirmed the above findings.

Three iron(II) complexes, [Fe(L1)2(NCS)2(MeOH)2].2MeOH.3H2O (1), [Fe(L1)2(NCSe)2(MeOH)2].4MeOH (2), and {[Fe(L2)2(NCS)2].4EtOH.4H2O}n (3) (L1 = 2,5-dipyridyl-3,4,-ethylenedioxylthiophene and L2 = 2,5-diethynylpyridinyl-3,4-ethylenedioxythiophene), have been synthesized using redox-active luminescent ethylenedioxythiophene (EDOT) - based ligands, and characterized...

The isomeric compounds, 4-bromo-2-chloro benzoic acid (4Br) and 2-bromo-4-chlorobenzoic acid (2Br), crystallize in entirely different space groups, P2/n and P1[combining macron] respectively. Both structures are stabilized by a strong O-HO hydrogen bonds generating a carboxylic acid dimer along with an unusual triangular halogen bonded motif in the former and a well-defined halogen bond in the latter. Charge density analysis establishes the nature of halogen bonds by bringing out significant changes in the packing features of the two structures as well as the quantification of the interaction energies involved in the formation of the motifs. Cocrystallization efforts lead to the formation of solid solutions of varied stoichiometric ratios among the two entirely different crystalline forms, a feature which is observed for the first time, and depends on the nature of the halogen bonds. Despite the significant variations in the charge density distribution in intermolecular space, the triangular motif, with two type II BrCl and ClBr and one type I BrBr contact in the structure of 4Br dictates the packing preferences in the solid solution as established by accurate single crystal diffraction studies supported by cognate powder diffraction analysis (PXRD) and differential scanning calorimetric (DSC) studies. A systematic study of the solid solution by varying the stoichiometric ratios establishes the hierarchy in halogen bonded motifs and consequently its directional influence to form the resultant supramolecular assembly.

Finding stable analogues of three-dimensional (3D) lead halide perovskites has motivated the exploration of an ever-expanding repertoire of two-dimensional (2D) counterparts. However, the bandgap and exciton binding energy in these 2D systems are generally considerably higher than those in 3D analogues due to size and dielectric confinement. Such quantum confinements are most prominently manifested in the extreme 2D realization in (A) m PbI 4 (m = 1 or 2) series of compounds with a single inorganic layer repeat unit. Here, we explore a new A-site cation, 4,4′-azopyridine (APD), whose size and hydrogen bonding properties endow the corresponding (APD)PbI 4 2D compound with the lowest bandgap and exciton binding energy of all such compounds, 2.19 eV and 48 meV, respectively. (APD)PbI 4 presents the first example of the ideal Pb−I− Pb bond angle of 180°, maximizing the valence and conduction bandwidths and minimizing the electron and hole effective masses. These effects coupled with a significant increase in the dielectric constant provide an explanation for the unique bandgap and exciton binding energies in this system. Our theoretical results further reveal that the requirement of optimizing the hydrogen bonding interactions between the organic and the inorganic units provides the driving force for achieving the structural uniqueness and the associated optoelectronic properties in this system. Our preliminary investigations in characterizing photovoltaic solar cells in the presence of APD show encouraging improvements in performances and stability.

Anisotropic mechanical response of a material to an applied external stress results in bending of organic crystals in particular directions. This phenomenon is essentially dictated by intermolecular interactions. In this work, we have tactically designed solid solutions of two non-isostructural molecular crystalline phases, 4-bromo-3-chlorophenol (4BR, I4 1 /a) and 3bromo-4-chlorophenol (3BR, P2 1 /c)an exception to the Kitaigorodsky rule. Single crystals of 4BR show elastic bending, whereas 3BR crystals are brittle in nature. The solid solutions of 4BR and 3BR in 1:1 and 1:2 stoichiometric ratios attain a unique solid solution crystal structure (P2 1 /c, Z′ = 2). In response to mechanical stimuli, we observed elastic bending in 1:1 solid solution crystals and plastic bending in 1:2 solid soltion crystals. A detailed investigation combining laboratory single crystal X-ray diffraction, powder X-ray diffraction, thermal analysis, and periodic DFT calculations reveals intriguing structural features of these solid solutions and explores the role of halogen bonding in modulating the mechanical property from elastic (1:1) to plastic (1:2) with stoichiometric variation.