Porous crystalline frameworks including zeolites, metal-organic frameworks (MOFs), covalent organic frameworks (COFs) and hydrogen-bonded organic frameworks (HOFs) have attracted great research ...interest in recent years. In addition to their assembly in the solid-state being fundamentally interesting and aesthetically pleasing, their potential applications have now pervaded in different areas of chemistry, biology and materials science. When framework materials are endowed with 'flexibility', they exhibit some properties (e.g., stimuli-induced pore breathing and reversible phase transformations) that are distinct from their rigid counterparts. Benefiting from flexibility and porosity, these framework materials have shown promise in applications that include separation of toxic chemicals, isotopes and hydrocarbons, sensing, and targeted delivery of chemicals. While flexibility in MOFs has been widely appreciated, recent developments of COFs and HOFs have established that flexibility is not just limited to MOFs. In fact, zeolites-that are considered rigid when compared with MOFs-are also known to exhibit dynamic modes. Despite flexibility may be conceived as being detrimental to the formation and stability of periodic structures, the landscape of flexible framework structures continues to expand with discovery of new materials with promising applications. In this review, we make an account of different flexible framework materials based on their framework types with a more focus on recent examples and delve into the origin of flexibility in each case. This systematic analysis of different flexibility types based on their origins enables understanding of structure-property relationships, which should help guide future development of flexible framework materials based on appropriate monomer design and tailoring their properties by bottom-up approach. In essence, this review provides a summary of different flexibility types extant to framework materials and critical analysis of importance of flexibility in emerging applications.
Here we report that a covalent organic framework (COF), which contains 2,5-di(imine)-substituted 1,4-dihydroxybenzene (diiminol) moieties, undergoes color changes in the presence of solvents or ...solvent vapor that are rapid, passive, reversible, and easily detectable by the naked eye. A new visible absorption band appears in the presence of polar solvents, especially water, suggesting reversible conversion to another species. This reversibility is attributed to the ability of the diiminol to rapidly tautomerize to an iminol/cis-ketoenamine and its inability to doubly tautomerize to a diketoenamine. Density functional theory (DFT) calculations suggest similar energies for the two tautomers in the presence of water, but the diiminol is much more stable in its absence. Time-dependent DFT calculations confirm that the iminol/cis-ketoenamine absorbs at longer wavelength than the diiminol and indicate that this absorption has significant charge-transfer character. A colorimetric humidity sensing device constructed from an oriented thin film of the COF responded quickly to water vapor and was stable for months. These results suggest that tautomerization-induced electronic structure changes can be exploited in COF platforms to give rapid, reversible sensing in systems that exhibit long-term stability.
2D polymers (2DPs) are promising as structurally well‐defined, permanently porous, organic semiconductors. However, 2DPs are nearly always isolated as closed shell organic species with limited charge ...carriers, which leads to low bulk conductivities. Here, the bulk conductivity of two naphthalene diimide (NDI)‐containing 2DP semiconductors is enhanced by controllably n‐doping the NDI units using cobaltocene (CoCp2). Optical and transient microwave spectroscopy reveal that both as‐prepared NDI‐containing 2DPs are semiconducting with sub‐2 eV optical bandgaps and photoexcited charge‐carrier lifetimes of tens of nanoseconds. Following reduction with CoCp2, both 2DPs largely retain their periodic structures and exhibit optical and electron‐spin resonance spectroscopic features consistent with the presence of NDI‐radical anions. While the native NDI‐based 2DPs are electronically insulating, maximum bulk conductivities of >10−4 S cm−1 are achieved by substoichiometric levels of n‐doping. Density functional theory calculations show that the strongest electronic couplings in these 2DPs exist in the out‐of‐plane (π‐stacking) crystallographic directions, which indicates that cross‐plane electronic transport through NDI stacks is primarily responsible for the observed electronic conductivity. Taken together, the controlled molecular doping is a useful approach to access structurally well‐defined, paramagnetic, 2DP n‐type semiconductors with measurable bulk electronic conductivities of interest for electronic or spintronic devices.
The bulk conductivity of naphthalene‐diimide‐based 2D polymers is increased by controlled stoichiometric n‐doping with cobaltocene. Following single‐electron reduction, these 2DPs retain their periodic structure and become paramagnetic. Substoichiometric doping leads to the highest bulk electronic conductivities, which is found to proceed through a hopping‐mechanism.
The domino oxidation of diols to lactones is an important transformation, and catalytic protocols that allow this conversion smoothly are scarce. Capitalizing on the established reactivity of ...tetramethyl-IBX (TetMe-IBX) and its in situ generation in the presence of a co-oxidant, such as oxone, we have shown that a variety of diols can be converted to the corresponding lactones in respectable yields by employing the precursor of TetMe-IBX, namely, tetramethyl-o-iodobenzoic acid (TetMe-IA), as a catalyst in 5 mol % in the presence of 2 equiv of oxone.
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Tröger's bases (TBs) functionalized with carbazoles (TB-Czs) and phosphine oxides (TB-POs) were designed and synthesized as host materials for application in phosphorescent organic light-emitting ...diodes. The TB scaffold is shown to impart thermal stability with high Tg values (171-211 °C) as well as high triplet energies in the range of 2.9-3.0 eV. With a limited experimentation of the devices, it is shown that the TBs doped with a green phosphor, namely, Ir(ppy)3, permit impressive external efficiencies on the order of ca. 16% with a high brightness of ca. 3000-4000 cd/m2. Better device performance results are demonstrated by a small structural manipulation of the TB scaffold involving substitution of methyl groups in the core scaffold.
1,3-Dimethyl-2,3-dihydrobenzo
d
imidazoles,
1H
, and 1,1',3,3'-tetramethyl-2,2',3,3'-tetrahydro-2,2'-bibenzo
d
imidazoles,
1
2
, are of interest as n-dopants for organic electron-transport materials. ...Salts of 2-(4-(dimethylamino)phenyl)-4,7-dimethoxy-, 2-cyclohexyl-4,7-dimethoxy-, and 2-(5-(dimethylamino)thiophen-2-yl)benzo
d
imidazolium (
1g–i
+
, respectively) have been synthesized and reduced with NaBH
4
to
1gH
,
1hH
, and
1iH
, and with Na:Hg to
1g
2
and
1h
2
. Their electrochemistry and reactivity were compared to those derived from 2-(4-(dimethylamino)phenyl)- (
1b
+
) and 2-cyclohexylbenzo
d
imidazolium (
1e
+
) salts.
E
(
1
+
/
1
•
) values for 2-aryl species are less reducing than for 2-alkyl analogues, i.e., the radicals are stabilized more by aryl groups than the cations, while 4,7-dimethoxy substitution leads to more reducing
E
(
1
+
/
1
•
) values, as well as cathodic shifts in
E
(
1
2
•+
/
1
2
) and
E
(
1H
•+
/
1H
) values. Both the use of 3,4-dimethoxy and 2-aryl substituents accelerates the reaction of the
1H
species with PC
61
BM. Because 2-aryl groups stabilize radicals,
1b
2
and
1g
2
exhibit weaker bonds than
1e
2
and
1h
2
and thus react with 6,13-bis(triisopropylsilylethynyl)pentacene (
VII
) via a “cleavage-first” pathway, while
1e
2
and
1h
2
react only via “electron-transfer-first”.
1h
2
exhibits the most cathodic
E
(
1
2
•+
/
1
2
) value of the dimers considered here and, therefore, reacts more rapidly than any of the other dimers with
VII
via “electron-transfer-first”. Crystal structures show rather long central C–C bonds for
1b
2
(1.5899(11) and 1.6194(8) Å) and
1h
2
(1.6299(13) Å).
Abstract
Control of electrical doping is indispensable in any semiconductor device, and both efficient hole and electron doping are required for many devices. In organic semiconductors, however, ...electron doping has been essentially more problematic compared to hole doping because in general organic semiconductors have low electron affinities and require dopants with low ionization potentials that are often air-sensitive. Here, we adapt an efficient molecular doping method, so-called ion-exchange doping, to dope electrons in a polymeric semiconductor. We initially reduce the polymeric semiconductor using one electron transfer from molecular dopants, and then the ionized dopants in the resulting air-unstable films are replaced with secondary ions via cation exchange. Improved ambient stability and crystallinity of the doped polymeric semiconductors are achieved when a specific bulky molecular cation was chosen as the secondary ion, compared to conventional methods. The presented strategy can overcome the trade-off relationship between reducing capability and ambient stability in molecular dopants, and a wider selection of dopant ions will help to realize ambient-stable electron conductors.
Safety concerns of traditional liquid electrolytes, especially when paired with lithium (Li) metal anodes, have stimulated research of solid polymer electrolytes (SPEs) to exploit the superior ...thermal and mechanical properties of polymers. Polyphosphazenes are primarily known for their use as flame retardant materials and have demonstrated high Li-ion conductivity owing to their highly flexible P = N backbone which promotes Li-ion conduction via inter- and intrachain hopping along the polymer backbone. While polyphosphazenes are largely unexplored as SPEs in the literature, a few existing examples showed promising ionic conductivity. By anchoring the anion to the polymer backbone, one may primarily allow the movement of Li ions, alleviating the detrimental effects of polarization that are common in conventional dual-ion conducting SPEs. Anion-anchored SPEs, known as single Li-ion conducting solid polymer electrolytes (SLiC-SPEs), exhibit high Li-ion transference numbers (t Li+ ), which limits Li dendrite growth, thus further increasing the safety of SPEs. However, previously reported SLiC-SPEs suffer from inadequate ionic conductivity, small electrochemical stability windows (ESWs), and limited cycling stability. Herein, we report three polyphosphazene-based SLiC-SPEs comprising lithiated polyphosphazenes. The SLiC polyphosphazenes were prepared through a facile synthesis route, opening the door for enhanced tunability of polymer properties via facile macromolecular nucleophilic substitution and subsequent lithiation. State-of-the-art characterization techniques, such as differential scanning calorimetry (DSC), electrochemical impedance spectroscopy (EIS), and solid-state nuclear magnetic resonance spectroscopy (ssNMR) were employed to probe the effect of the polymer structure on Li-ion dynamics and other electrochemical properties. Produced SPEs showed thermal stability up to ∼208 °C with ionic conductivities comparable to that of the best-reported SLiC-SPEs that definitively comprise no solvents or plasticizers. Among the three lithiated polyphosphazenes, the SPE containing dilithium polybis(trifluoroethylamino)phosphazene (pTFAP2Li) exhibited the most promising electrochemical characteristics with t Li+ of 0.76 and compatibility with both Li metal anodes and LiFePO4 (LFP) cathodes; through 40 cycles at 100 °C, the PEO-pTFAP2Li blend showed 81.2% capacity utilization and 86.8% capacity retention. This work constitutes one of the first successful demonstrations of the cycling performance of a true all-solid-state Li-metal battery using SLiC polyphosphazene SPEs.
Magnéli phase Ti4O7 with high electronic conductivity showed great promise as an inactive component of S cathodes that mitigates some intrinsic limitations of ultra-lightweight lithium-sulfur ...batteries (LSBs) based on low-cost, high specific capacity and largely environmentally benign sulfur (S). Here we report for the first time a straightforward and economic approach to synthesize mesoporous Magnéli phase Ti4O7 microspheres by simply heating a mixture of elemental titanium and TiO2 in argon. The phase-controlled method utilizes the mildest synthesis conditions to-date with lower temperature and exclusion of flammable hydrogen gas traditionally used, and also avoids generation of exhaust greenhouse gases during carbothermal synthesis using sacrificial polymers. The produced S/Ti4O7 composite cathode with 70 wt% S loading efficiently mitigated the shuttle effect by both chemical adsorption and physical trapping of polysulfides. Consequently, this cathode showed high (88%) first cycle columbic efficiency, excellent cycling stability (capacity decay of ~ 0.09% per cycle for 500 cycles), small polarization and good rate performance, significantly exceeding the performance of S/TiO2 and S/activated carbon cathodes with similar/smaller S loading. We anticipate that this work will provide new avenues for more economic and environmentally friendly synthesis of various Magnéli phase metal oxides, and their exploration in LSBs and other fields.
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•Precise Magnéli Ti4O7 microsphere synthesis by atom-economic and safe comproportionation reaction.•New mechanistic insights in the prevention of LiPS dissolution.•Outstanding performance of the Ti4O7/S cathodes with high S loading (~ 70%).•Generality of the synthesis is shown for other Magnéli phases.
Hierarchically porous TiO2–x /C nanofibers (NFs) with axially aligned cylindrical tunnel pore channels were synthesized as a sulfur (S) host for lithium–sulfur batteries (LSBs) by a microemulsion ...electrospinning method. We explored a synergistic chemical trapping reinforced by coordinatively unsaturated Ti3+ nuclei with oxygen deficiency (or more broadly via polar O–Ti–O units) in combination with physical trapping in both narrow pores (<5 nm) and larger ordered pore tunnels (20–100 nm) separated by thin walls to allow for a large volume of active material and rapid diffusion within the channels while effectively blocking out the diffusion of soluble lithium polysulfides. Due to this unique architecture and enhanced conductivity, the prepared materials enabled a high S loading (∼72 wt %) and significantly reduced the shuttle effect. Hence, the composite TiO2–x /C@S cathodes exhibited a high utilization of active materials, excellent rate performance, and promising cycling stability (retention of up to ∼1010 mAh g–1 after 150 cycles for the aerial capacity of 1.5 mAh cm–2, with very stable performance even for the high S loading of 2.5 mg cm–2). By designing control nanomaterials that lack either the pore tunnels or the desired chemical compositions, we elucidated the importance of the synergistic effect of both factors. This work demonstrates a successful exploration of oxide NFs with tunnel pores via a simple single-needle microemulsion electrospinning method, which should pave the way for similar nanomaterials engineering with other chemistries for improved LSB performance.
