Realizing the Lowest Bandgap and Exciton Binding Energy in a Two-Dimensional Lead Halide System.

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)mPbI4 (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)PbI4 2D compound with the lowest bandgap and exciton binding energy of all such compounds, 2.19 eV and 48 meV, respectively. (APD)PbI4 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.

Cyclic Disaccharide Formation Enforced by a Ring Contraction: 2,3-Dideoxy Pyranoside Glycoside Donor to a Furanoside Macrocycle.

The synthesis of a disaccharide macrocycle through 2,3-dideoxy glucopyranosyl monosaccharide is reported. 2,3-Dideoxy-erythro-hexopyranosyl thioglycoside possessing a free hydroxy functionality at the C-4 carbon is prepared, and cycloglycosylation is conducted. In the event, the cycloglycosylation occurs with a ring contraction of the monosaccharide moiety and affords the cyclic furanoside disaccharide. Solution-phase and single-crystal X-ray diffraction structural characterizations permit the features of the macrocycle to be uncovered. The solubilization and encapsulation properties of the macrocycle are studied in aqueous solutions with 1-aminoadamantane.

Enhanced proton conductivity in amino acid based self-assembled non-porous hydrogen-bonded organic frameworks.

ß-Alaninium oxalate hemihydrate, glycinium oxalate, and L-leucinium oxalate salt-cocrystals as non-porous self-assembled hydrogen-bonded organic frameworks afforded proton conductivity of 2.43 × 10-2 S cm-1 (60 °C, 95% RH), 3.03 × 10-2 S cm-1 (60 °C, 95% RH), and 1.19 × 10-2 S cm-1 (80 °C, 95% RH), respectively. These materials possess an extensive 3-dimensional network of hydrogen bonds in their crystal structures, making them efficient proton conducting membranes. The reduction in conductivity values over 10-1 S cm-1 order upon exposure of the samples to a D2O atmosphere in extreme conditions ratified the role of humidity for the conduction of protons. This work explores the relationship between structural features and proton conductivity for the design of proton conducting membranes that are easy to synthesize, eco-friendly, and cheap with potential for futuristic applications.

Thermo- and photoinduced spin state switching in an iron(II) 2D coordination network associated with large light-induced thermal hysteresis and tuning of dimensionality via ligand modulation.

Three iron(ii) complexes, [Fe(L1)2(NCS)2(MeOH)2] (1), [Fe(L1)2(NCSe)2(MeOH)2] (2), and [Fe(L2)2(NCS)2]n (3) (L1 = 2,5-dipyridyl-3,4-ethylenedioxythiophene and L2 = 2,5-diethynylpyridinyl-3,4-ethylenedioxythiophene), have been synthesized using redox-active luminescent ethylenedioxythiophene (EDOT)-based ligands, and characterized by variable temperature single-crystal X-ray diffraction, (photo)magnetic, optical reflectivity, and spectroscopy studies. Magneto-structural investigations revealed that 1 and 2 are mononuclear with a FeN4O2 octahedral coordination geometry and remain in a high-spin (HS) (S = 2) state in a temperature range of 2-280 K. Interestingly, a 2D coordination network structure with FeN6 surrounding each iron center was observed for 3, which exhibits reversible thermo-induced spin-state switching between the paramagnetic high-spin (HS) (S = 2) and diamagnetic low-spin (LS) (S = 0) states at around 105 K (T1/2). Furthermore, optical reflectivity and photomagnetic measurements at low temperature confirmed that 3 shows reversible ON/OFF switching between the photoinduced excited paramagnetic HS metastable state and diamagnetic LS state under light irradiation (ON mode using red light and OFF mode using green light). Finally, the photoinduced excited HS state can be reversibly relaxed back to the diamagnetic ground LS state by heating the system at ca. 88 K (TLIESST = 88 K) (light-induced excited spin state trapping (LIESST) effect). Furthermore, 3 also showed an exciting and unique 18 K wide light-induced thermal hysteresis (LITH) effect above liquid nitrogen temperature (100 K). DFT and CASSCF level theoretical calculations were utilized to better understand the magneto-structural correlations of these complexes.

ON/OFF Photoswitching and Thermoinduced Spin Crossover with Cooperative Luminescence in a 2D Iron(II) Coordination Polymer.

A 2D coordination polymer, {[Fe(L)2(NCSe)2]·6MeOH·14H2O}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 FeN6 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 T1/2 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.

Do halogen bonds dictate the packing preferences in solid solutions?

The isomeric compounds, 4-bromo-2-chloro benzoic acid (4Br) and 2-bromo-4-chlorobenzoic acid (2Br), crystallize in entirely different space groups, P21/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.