The pursuit of stability in halide perovskites: the monovalent cation and the key for surface and bulk self-healing

We find significant differences between degradation and healing at the surface or in the bulk for each of the different APbBr₃ single crystals (A = CH₃NH₃⁺, methylammonium (MA); HC(NH₂)₂⁺, formamidinium (FA); and cesium, Cs⁺). Using 1- and 2-photon microscopy and photobleaching we conclude that kinetics dominate the surface and thermodynamics the bulk stability. Fluorescence-lifetime imaging microscopy, as well as results from several other methods, relate the (damaged) state of the halide perovskite (HaP) after photobleaching to its modified optical and electronic properties. The A cation type strongly influences both the kinetics and the thermodynamics of recovery and degradation: FA heals best the bulk material with faster self-healing; Cs⁺ protects the surface best, being the least volatile of the A cations and possibly through O-passivation; MA passivates defects via methylamine from photo-dissociation, which binds to Pb²⁺. DFT simulations provide insight into the passivating role of MA, and also indicate the importance of the Br₃^- defect as well as predicts its stability. The occurrence and rate of self-healing are suggested to explain the low effective defect density in the HaPs and through this, their excellent performance. These results rationalize the use of mixed A-cation materials for optimizing both solar cell stability and overall performance of HaP-based devices, and provide a basis for designing new HaP variants.

Recyclable scavengers for photo-cyclopropanation via an in situ crystallization process

The palladium(ii) cyclophane systems, constructed by previously reported proof-of-concept self-assembly, represent a crucial landmark in the field of effective and recyclable scavenging of triiodide (I₃^-) in the photo-cyclopropanation of alkenes with CH₂I₂. The scavenger's driving force behind photo-cyclopropanation is the efficient in situ crystallization of triiodide-exchanged species. The exact quantitative photoreaction yields according to the mole ratios of the cyclophane system are impressive. The recycling behavior can be ascribed to the rigidity and stability of the four-layered tripalladium(ii)cyclophane.

Crystal structure of bis(3-bromomesityl)(quinolin-1-ium-8-yl)boron(III) tribromide

The title compound, C27H26.82BBr2.18N+·Br3−, is a cationic triarylborane isolated as its tribromide salt. The aryl substituents include a protonated 8-quinolyl group and two 3-bromomesityl groups. The molecule was prepared on combination of 3:1 Br2and dimesityl(quinolin-8-yl)borane in hexanes. The refinement of the structure indicated a degree of `over-bromination' (beyond two bromine atoms) for the cation. There are two tribromide ions in the asymmetric unit, both completed by crystallographic inversion symmetry.

Raman spectroscopy study of new thia- and oxazinoquinolinium triodides

New polyiodides of thia- and oxa-zinoquinolinium derivatives were characterized using Raman spectroscopy and periodic 3D calculations of the Raman intensities. Polarized Raman spectra of the oriented crystals revealed the features of spatial organization in the polyiodide-anion chains.

The nature of the chemical bond in linear three-body systems: from i3- to mixed chalcogen/halogen and trichalcogen moieties

The 3 centre-4 electrons (3c-4e) and the donor/acceptor or charge-transfer models for the description of the chemical bond in linear three-body systems, such as I(3) (-) and related electron-rich (22 shell electrons) systems, are comparatively discussed on the grounds of structural data from a search of the Cambridge Structural Database (CSD). Both models account for a total bond order of 1 in these systems, and while the former fits better symmetric systems, the latter describes better strongly asymmetric situations. The 3c-4e MO scheme shows that any linear system formed by three aligned closed-shell species (24 shell electrons overall) has reason to exist provided that two electrons are removed from it to afford a 22 shell electrons three-body system: all combinations of three closed-shell halides and/or chalcogenides are considered here. A survey of the literature shows that most of these three-body systems exist. With some exceptions, their structural features vary continuously from the symmetric situation showing two equal bonds to very asymmetric situations in which one bond approaches to the value corresponding to a single bond and the second one to the sum of the van der Waals radii of the involved atoms. This indicates that the potential energy surface of these three-body systems is fairly flat, and that the chemical surrounding of the chalcogen/halogen atoms can play an important role in freezing different structural situations; this is well documented for the I(3) (-) anion. The existence of correlations between the two bond distances and more importantly the linearity observed for all these systems, independently on the degree of their asymmetry, support the state of hypervalency of the central atom.

Polyhalide anions in crystals. Part 2. I3−asymmetry and N—H … I bonding: triiodides of the Me2NH2+, Ph2I+, tropanium,N,N,N′,N′-Me4-1,2-ethanediammonium,N,N,N′,N′-Me4-1,3-propanediammonium,N-Me-piperazinium(2+), andN,N′-Me2-piperazinium(2+) cations, and Me2NH2I

Crystal-structure determinations are reported for Me2NH2I3, (Ph2I)I3, tropanium-I3, [Me2HN(CH2)2NHMe2](I3)2, [Me2HN(CH2)3NHMe2](I3)2, (N-Me2-piperazinium) (I3)2•H2O, (N,N′-Me2-piperazinium)(I3)2, and Me2NH2I. The features of these and relevant literature structures are used to (1) classify triiodide structures by their ion-packing types; (2) analyze the relationship between the two I—I bond lengths d and d* in the I3−anion; and (3) examine the effect of N—H(N)… I hydrogen bonding on the symmetry of the I3−anion. It is found that the d,d* relationship can be represented to a high degree of correlation by the power function d* − d0 = K(d − d0)−c(d* ≥ d,d0 = d(I—I) in I2(g)) based on the 3c4e model of the anion. An empirical correlation is shown to exist between the H(N)… I and N … I distances both for unbranched and branched N—H(N)… I bonds. Comparison of the degree of asymmetry of I3−in two samples, one containing H-bonded I3−anions, the other with H-bonding absent, leads to the conclusion that while H-bonding is a factor affecting I3−symmetry, it is not a preferential factor. The four 1:2 title triiodides have structures of, or related to, the CdI2type, in which the anions form infinite pseudo-hexagonal channels. The positioning of the divalent cations on the axes of these channels gives rise to an interesting "vernier" effect governed by the cation length and H-bonding ability. Bonding in centrosymmetric I4rings in (Ph2I)I3and (Ph2I)I is examined. Key words: crystal structures, hydrogen bond, iodine–iodine bonds, polyhalide anions, triiodides.