Using steric hindrance to manipulate and stabilize metal halide perovskites for optoelectronics

The chemical instability of metal halide perovskite materials can be ascribed to their unique properties of softness, in which the chemical bonding between metal halide octahedral frameworks and cations is the weak ionic and hydrogen bonding as in most perovskite structures. Therefore, various strategies have been developed to stabilize the cations and metal halide frameworks, which include incorporating additives, developing two-dimensional perovskites and perovskite nanocrystals, etc. Recently, the important role of utilizing steric hindrance for stabilizing and passivating perovskites has been demonstrated. In this perspective, we summarize the applications of steric hindrance in manipulating and stabilizing perovskites. We will also discuss how steric hindrance influences the fundamental kinetics of perovskite crystallization and film formation processes. The similarities and differences of the steric hindrance between perovskite solar cells and perovskite light emission diodes are also discussed. In all, utilizing steric hindrance is a promising strategy to manipulate and stabilize metal halide perovskites for optoelectronics.

Manipulation on steric hindrance can influence the fundamental kinetics of perovskite crystallization and film formation, therefore stabilizing and passivating perovskite structures, and promoting the commercialization of stable perovskite devices.     

Hyperpolarizability of donor-acceptor azines subject to push-pull character and steric hindrance

The push-pull character of two series of donor-acceptor azines has been quantified by ¹³C, ¹⁵N chemical shift differences of the partial C(1)N(1) and N(2)C(2) double bonds in the central linking C(1)N(1)-N(2)C(2) unit and by the quotient of the occupations of the bonding π and anti-bonding π^∗ orbitals of these bonds. Excellent correlation of the latter push-pull parameter with the corresponding bond lengths d_CN strongly recommend both the occupation quotients π^∗/π and the corresponding bond lengths as reasonable sensors for quantifying the push, pull character along the CN-NC linking unit, for the donor-acceptor quality of the two series of azines and for the molecular hyperpolarizability ß₀ of these compounds. Within this context, reasonable conclusions concerning the interplay of steric hindrance in the chromophore, push-pull character and hyperpolarizability of the azines and their application as NLO materials will be drawn.     

位阻型有机电致磷光材料的研究进展

重金属旋轨耦合作用使磷光有机发光二极管的内量子效率在理论上可达100%,突破了传统有机荧光二极管内量子效率为25%的限制,是目前最有潜力的第三代显示器.但是,磷光材料常因浓度猝灭、三线态湮灭、二聚体发光等因素影响器件性能.位阻型磷光材料能够抑制分子间的强相互作用,从而解决了上述问题.本文作者综述了近年来具有大体积空间位阻效应的铱(III)、铂(II)、锇(II)等有机小分子磷光材料的研究进展,讨论了其目前存在的问题和未来的发展趋势.     

Radical-carbanion cyclo-coupling in armed aromatics: overriding steric hindrance to ring closure

Omega-(2-Halophenyl)alkyl-2-oxazolines were prepared and reacted via base promoted intramolecular coupling of radical with carbanionic centres to yield 1-phenyl-1-oxazolino-indan and -tetralin derivatives containing quaternary C-atoms.     

Conjugation, hybridization, and steric hindrance in relation to bond lengths

Three theories which have been proposed to explain the observed shortening of a single bond when it is adjacent to a double bond are discussed. Possible predictions from these theories are examined, especially for comparison with various quantities measurable by microwave spectroscopy. It is concluded that the steric theory is probably untenable, that some conjugation appears necessary to explain observed barries to internal rotation, and that it is difficult to find testable predictions from the hybridization theory.     

Na_4-CyDTA空间位阻的动态核磁共振研究

用动态核磁共振(DNMR)的方法研究了Na4CyDTA乙羧基质子1H NMR谱随温度的变化关系,结果表明,常温下由于乙羧基旋转受到空间阻碍导致其1H NMR谱分裂为1组AB谱,随温度升高,AB谱的化学位移差减小.通过拟和化学位移差与温度的关系,计算出了Na4CyDTA中阻碍乙羧基旋转的能垒为16.95 kJ/mol.     

Polyoxometalate steric hindrance driven chirality-selective separation of subnanometer carbon nanotubes

Subnanometer single-chirality single-walled carbon nanotubes (SWCNTs) are of particular interest in multiple applications. Inspired by the interdisciplinary combination of redox active polyoxometalates and SWCNTs, here we report a cluster steric hindrance strategy by assembling polyoxometalates on the outer surface of subnanometer SWCNTs via electron transfer and demonstrate the selective separation of monochiral (6,5) SWCNTs with a diameter of 0.75 nm by a commercially available conjugated polymer. The combined use of DFT calculations, TEM, and XPS unveils the mechanism that selective separation is associated with tube diameter-dependent interactions between the tube and clusters. Sonication drives the preferential detachment of polyoxometalate clusters from small-diameter (6,5) SWCNTs, attributable to weak tube–cluster interactions, which enables the polymer wrapping and separation of the released SWCNTs, while strong binding clusters with large-diameter SWCNTs provide steric hindrance and block the polymer wrapping. The polyoxometalate-assisted modulation, which can be rationally customized, provides a universal and robust pathway for the separation of SWCNTs.

We develop a cluster steric hindrance strategy by assembling polyoxometalates on subnanometer single-walled carbon nanotubes (SWCNTs) and demonstrate the selective separation of single-chirality (6,5) SWCNTs via polymer extraction.     

Na4-CyDTA空间位阻的动态核磁共振研究

用动态核磁共振（DNMR）的方法研究了Na4-CyDTA乙羧基质子^1H—NMR谱随温度的变化关系，结果表明，常温下由于乙羧基旋转受到空间阻碍导致其^1H—NMR谱分裂为1组AB谱，随温度升高，AB谱的化学位移差减小．通过拟和化学位移差与温度的关系，计算出了Na4-CyDTA中阻碍乙羧基旋转的能垒为16．95kJ／mol．     

Observation of steric hindrance in heteraferrocenophanes by carbon-13 NMR

The ¹³C NMR spectra of stereoisomeric trans- and cis-1,3-dimethyl-2-oxa[3](1, 1′)ferrocenophanes, 1,2,3-trithia[3](1, 1′)ferrocenophane and 1,3-dithia[3](1, 1′)ferrocenophane have been recorded and analysed. The presence of steric hindrance in some of these compounds was confirmed and the conformations of the molecules were determined.     

Anomeric effects versus steric hindrance to ionic solvation in protonated glucosylanilines and cyclohexylanilines

The so-called reverse anomeric effect is the preference of cationic substituents for the equatorial position on a pyranose ring, but it is not consistent with current theories of molecular structure or with previous studies designed to test it. To probe this further, the N-protonation-induced shifts of the anomeric equilibrium in a series of N-(tetra-O-methylglucopyranosyl)anilines have been measured with high precision through an NMR titration method that compares basicities of alpha and beta anomers in a mixture of the two. For comparison, the N-protonation-induced shifts of the cis/trans equilibrium in N-(4-tert-butylcyclohexyl)anilines have also been measured by this same method. In both series, there is a shift of the equilibrium toward equatorial upon N-protonation, consistent with steric hindrance to ionic solvation. This shift is smaller for the glucosylanilines than for the cyclohexylanilines, consistent with an enhancement of the normal anomeric effect that counters the steric hindrance and reduces the shift toward the equatorial beta anomer. Moreover, the shift toward equatorial increases slightly but detectably with electron-withdrawing substituents on the cyclohexylaniline, which fine-tune the steric hindrance to ionic solvation. In contrast, the shift decreases for the glucosylanilines. This is consistent with an enhancement of the normal anomeric effect due to a more localized positive charge, rather than with a reverse anomeric effect. These results thus define the substituent dependence of the anomeric effect.     

Improving the yield of mono-DNA-functionalized gold nanoparticles through dual steric hindrance

A novel strategy of dual steric hindrance, which was obtained by Janus modification of gold nanoparticles (Au NPs) and volume exclusion of DNA, was adopted to prepare mono-DNA-modified Au NPs. The yield of mono-DNA-functionalized Au NPs significantly improved from 44 to 70% in the reaction between Au NPs and thiolated DNA. Furthermore, the specificity of mono-DNA-functionalized Au NPs was enhanced from 57 to 95%. The as-prepared Au NPs without postsynthetic treatment showed good controllability in self-assembly fabrication of complex nanostructures.     

Topology control of three-dimensional covalent organic frameworks by adjusting steric hindrance effect

Since the intrinsic topological network determines their pore characteristics and functional applications, it is important to construct 3D COFs with target topology from predesigned functional building blocks. However, when starting from precursors with same connectivity but different bulky groups, the structure and topology of 3D COFs may change, which will greatly increase the complexity of the crystal structure determination. Therefore, it is essential to understand how to control the steric ...     

Role of steric hindrance in photoinduced proton transfer from solvent to excited molecules of 1,2-dihydroquinolines

It was shown that the photolysis of 1,2,6-trimethyl-1,2-dihydroquinoline (126TMDHQ) in water, methanol, ethanol, and isopropanol affords the corresponding adducts of water and the alcohols, unlike the case of 2,2,4-trimethyl-1,2-dihydroquinolines bearing the methyl, alkoxyl, and hydroxyl substituents in the 1-, 6-, and 8-positions, which were previously found to form adducts only in the presence of water and MeOH. The quantum yield of the 126TMDQ photolysis (Φ) in this solvent series changes as Φ_MeOH:Φ_EtOH:Φ_PrOH = 10:3:1. The results were rationalized in terms of the effect of steric hindrance caused by substituents on the heterocycle and increasing size of the alcohol alkyl group on proton transfer from the solvent to the 1,2-dihydroquinoline molecule in the excited singlet state. The existence of two adduct isomers was revealed. The preferential formation of one of the isomers was considered from the standpoint of carbocation accessibility to the solvent by nucleophilic attack.     

Reaction of alkylcarbonyloxymethyl halides with phenols: reevaluating the influence of steric hindrance

Evidence is presented that contradicts an earlier finding that, in the absence of steric hindrance, the coupling reaction of alkylcarbonyloxymethyl (ACOM) halides with phenols favors acylated product. A one-step synthesis is used to generate sterically unhindered ACOM iodides, which are then reacted with several phenols to give mainly alkylated phenol.