Hydrogen bonding principles are at the core of supramolecular design. This overview features a discussion relating molecular structure to hydrogen bond strengths, highlighting the following ...electronic effects on hydrogen bonding: electronegativity, steric effects, electrostatic effects, π‐conjugation, and network cooperativity. Historical developments, along with experimental and computational efforts, leading up to the birth of the hydrogen bond concept, the discovery of nonclassical hydrogen bonds (CH…O, OH…π, dihydrogen bonding), and the proposal of hydrogen bond design principles (e.g., secondary electrostatic interactions, resonance‐assisted hydrogen bonding, and aromaticity effects) are outlined. Applications of hydrogen bond design principles are presented.
This article is categorized under:
Structure and Mechanism > Molecular Structures
Structure and Mechanism > Reaction Mechanisms and Catalysis
Hydrogen bonds are chemical interactions that can bind molecules and molecular fragments together to create elaborate structures and functions. In the past 30 years, many hydrogen bond design principles have emerged, and together they propel the field of supramolecular chemistry.
The Watson-Crick A·T and G·C base pairs are not only electronically complementary, but also photochemically complementary. Upon UV irradiation, DNA base pairs undergo efficient excited-state ...deactivation through electron driven proton transfer (EDPT), also known as proton-coupled electron transfer (PCET), at a rate too fast for other reactions to take place. Why this process occurs so efficiently is typically reasoned based on the oxidation and reduction potentials of the bases in their electronic ground states. Here, we show that the occurrence of EDPT can be traced to a reversal in the aromatic/antiaromatic character of the base upon photoexcitation. The Watson-Crick A·T and G·C base pairs are aromatic in the ground state, but the purines become highly antiaromatic and reactive in the first
1
ππ* state, and transferring an electron and a proton to the pyrimidine relieves this excited-state antiaromaticity. Even though proton transfer proceeds along the coordinate of breaking a N-H σ-bond, the chromophore is the π-system of the base, and EDPT is driven by the strive to alleviate antiaromaticity in the π-system of the photoexcited base. The presence and absence of alternative excited-state EDPT routes in base pairs also can be explained by sudden changes in their aromatic and antiaromatic character upon photoexcitation.
The Watson-Crick A·T and G·C base pairs are not only electronically complementary, but also photochemically complementary.
Here we report five blue‐phosphorescent platinum bis‐phenylacetylide complexes with an investigation of their photophysical and electrochemical attributes. Three of the complexes (1–3) are of the ...general formula cis‐Pt(CNR)2(C≡CPh)2, in which CNR is a variably substituted isocyanide and C≡CPh is phenylacetylide. These isocyanide complexes serve as precursors for complexes of the general formula cis‐Pt(CNR)(ADC)(C≡CPh)2 (4 and 5), in which ADC is an acyclic diaminocarbene installed by amine nucleophilic addition to one of the isocyanides. All of the complexes exhibit deep blue phosphorescence with λmax ∼430 nm in poly(methyl methacrylate) (PMMA) thin films. Whereas isocyanide complexes 1–3 exhibit modest photoluminescence quantum yields (ΦPL), incorporation of one acyclic diaminocarbene ligand results in a three‐fold to 16‐fold increase in ΦPL while still maintaining an identical deep blue color profile.
The rising of the photoluminescence quantum yield: This work describes a new design for blue‐phosphorescent platinum acetylide complexes, using strong σ‐donor acyclic diaminocarbene (ADC) auxiliary ligands. Conversion of one isocyanide in the precursor complex to and ADC, by nucleophilic addition of a secondary amine, engenders up to a 16‐fold increase in photoluminescence quantum yield.
All-inorganic perovskites nanostructures, such as CsPbCl3 nanocrystals (NCs), are promising in many applications including light-emitting diodes, photovoltaics, and photodetectors. Despite the ...impressive performance that was demonstrated, a critical issue remains due to the instability of the perovskites in ambient. Herein, we report a method of passivating crystalline CsPbCl3 NC surfaces with 3-mercaptopropionic acid (MPA), and superior ambient stability is achieved. The printing of these colloidal NCs on the channel of graphene field-effect transistors (GFETs) on solid Si/SiO2 and flexible polyethylene terephthalate substrates was carried out to obtain CsPbCl3 NCs/GFET heterojunction photodetectors for flexible and visible-blind ultraviolet detection at wavelength below 400 nm. Besides ambient stability, the additional benefits of passivating surface charge trapping by the defects on CsPbCl3 NCs and facilitating high-efficiency charge transfer between the CsPbCl3 NCs and graphene were provided by MPA. Extraordinary optoelectronic performance was obtained on the CsPbCl3 NCs/graphene devices including a high ultraviolet responsivity exceeding 106 A/W, a high detectivity of 2 × 1013 Jones, a fast photoresponse time of 0.3 s, and ambient stability with less than 10% degradation of photoresponse after 2400 h. This result demonstrates the crucial importance of the perovskite NC surface passivation not only to the performance but also to the stability of the perovskite optoelectronic devices.
Numerous surveys reveal high levels of pesticide residue contamination in honey bee comb. We conducted studies to examine possible direct and indirect effects of pesticide exposure from contaminated ...brood comb on developing worker bees and adult worker lifespan.
Worker bees were reared in brood comb containing high levels of known pesticide residues (treatment) or in relatively uncontaminated brood comb (control). Delayed development was observed in bees reared in treatment combs containing high levels of pesticides particularly in the early stages (day 4 and 8) of worker bee development. Adult longevity was reduced by 4 days in bees exposed to pesticide residues in contaminated brood comb during development. Pesticide residue migration from comb containing high pesticide residues caused contamination of control comb after multiple brood cycles and provided insight on how quickly residues move through wax. Higher brood mortality and delayed adult emergence occurred after multiple brood cycles in contaminated control combs. In contrast, survivability increased in bees reared in treatment comb after multiple brood cycles when pesticide residues had been reduced in treatment combs due to residue migration into uncontaminated control combs, supporting comb replacement efforts. Chemical analysis after the experiment confirmed the migration of pesticide residues from treatment combs into previously uncontaminated control comb.
This study is the first to demonstrate sub-lethal effects on worker honey bees from pesticide residue exposure from contaminated brood comb. Sub-lethal effects, including delayed larval development and adult emergence or shortened adult longevity, can have indirect effects on the colony such as premature shifts in hive roles and foraging activity. In addition, longer development time for bees may provide a reproductive advantage for parasitic Varroa destructor mites. The impact of delayed development in bees on Varroa mite fecundity should be examined further.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Many popular organic chromophores that catalyze photoinduced proton-coupled electron transfer (PCET) reactions are aromatic in the ground state but become excited-state antiaromatic in the lowest ππ* ...state. We show that excited-state antiaromaticity makes electron transfer easier. Two representative photoinduced electron transfer processes are investigated: (1) the photolysis of phenol and (2) solar water splitting of a pyridine–water complex. In the selected reactions, the directions of electron transfer are opposite, but the net result is proton transfer following the direction of electron transfer. Nucleus-independent chemical shifts (NICS), ionization energies, electron affinities, and PCET energy profiles of selected 4n and 4n + 2 π-systems are presented, and important mechanistic implications are discussed.
Background
The COVID‐19 pandemic has posed an unprecedented threat to global mental health. Children and adolescents may be more susceptible to mental health impacts related to their vulnerable ...developmental stage, fear of infection, home confinement, suspension of regular school and extracurricular activities, physical distancing mandates, and larger scale threats such as global financial recessions and associated impacts. Our objective was to review existing evidence of the COVID‐19 pandemic’s global impact on the mental health of children and adolescents <19 years of age and to identify personal and contextual factors that may enhance risk or confer protection in relation to mental health outcomes.
Methods
We conducted a search of peer‐reviewed and preprint research published in English from January 1, 2020, to February 22, 2021. We included studies collecting primary data on COVID‐19‐related mental health impacts on children and adolescents. We graded the strength of included articles using the Oxford Centre for Evidence‐Based Medicine rating scheme.
Results
Our search and review yielded 116 articles presenting data on a total of 127,923 children and adolescents; 50,984 child and adolescent proxy reports (e.g., parents, healthcare practitioners); and >3,000 chart reviews. A high prevalence of COVID‐19‐related fear was noted among children and adolescents, as well as more depressive and anxious symptoms compared with prepandemic estimates. Older adolescents, girls, and children and adolescents living with neurodiversities and/or chronic physical conditions were more likely to experience negative mental health outcomes. Many studies reported mental health deterioration among children and adolescents due to COVID‐19 pandemic control measures. Physical exercise, access to entertainment, positive familial relationships, and social support were associated with better mental health outcomes.
Conclusions
This review highlights the urgent need for practitioners and policymakers to attend to and collaborate with children and adolescents, especially those in higher risk subgroups, to mitigate short‐ and long‐term pandemic‐associated mental health effects.
Porous molecular crystals are an emerging class of porous materials that is unique in being built from discrete molecules rather than being polymeric in nature. In this study, we examined the effects ...of molecular structure of the precursors on the formation of porous solid-state structures with a series of 16 rigid aromatics. The majority of these precursors possess pyrazole groups capable of hydrogen bonding, as well as electron-rich aromatics and electron-poor tetrafluorobenzene rings. These precursors were prepared using a combination of Pd- and Cu-catalyzed cross-couplings, careful manipulations of protecting groups on the nitrogen atoms, and solvothermal syntheses. Our study varied the geometry and dimensions of precursors, as well as the presence of groups capable of hydrogen bonding and π···π stacking. Thirteen derivatives were crystallographically characterized, and four of them were found to be porous with surface areas between 283 and 1821 m2 g–1. Common to these four porous structures were (a) rigid trigonal geometry, (b) π···π stacking of electron-poor tetrafluorobenzenes with electron-rich pyrazoles or tetrazoles, and (c) hydrogen bonding between the terminal heteroaromatic rings.
Baird’s rule explains why and when excited-state proton transfer (ESPT) reactions happen in organic compounds. Bifunctional compounds that are 4n + 2 π-aromatic in the ground state, become 4n + 2 ...π-antiaromatic in the first ¹ππ* states, and proton transfer (either inter- or intramolecularly) helps relieve excited-state antiaromaticity. Computed nucleus-independent chemical shifts (NICS) for several ESPT examples (including excited-state intramolecular proton transfers (ESIPT), biprotonic transfers, dynamic catalyzed transfers, and proton relay transfers) document the important role of excited-state antiaromaticity. o-Salicylic acid undergoes ESPT only in the “antiaromatic” S₁ (¹ππ*) state, but not in the “aromatic” S₂ (¹ππ*) state. Stokes’ shifts of structurally related compounds e.g., derivatives of 2-(2-hydroxyphenyl)benzoxazole and hydrogen-bonded complexes of 2-aminopyridine with protic substrates vary depending on the antiaromaticity of the photoinduced tautomers. Remarkably, Baird’s rule predicts the effect of light on hydrogen bond strengths; hydrogen bonds that enhance (and reduce) excited-state antiaromaticity in compounds become weakened (and strengthened) upon photoexcitation.
2D atomic sheets of transition metal dichalcogenides (TMDs) have a tremendous potential for next-generation optoelectronics since they can be stacked layer-by-layer to form van der Waals (vdW) ...heterostructures. This allows not only bypassing difficulties in heteroepitaxy of lattice-mismatched semiconductors of desired functionalities but also providing a scheme to design new optoelectronics that can surpass the fundamental limitations on their conventional semiconductor counterparts. Herein, a novel 2D h-BN/p-MoTe
/graphene/n-SnS
/h-BN p-g-n junction, fabricated by a layer-by-layer dry transfer, demonstrates high-sensitivity, broadband photodetection at room temperature. The combination of the MoTe
and SnS
of complementary bandgaps, and the graphene interlayer provides a unique vdW heterostructure with a vertical built-in electric field for high-efficiency broadband light absorption, exciton dissociation, and carrier transfer. The graphene interlayer plays a critical role in enhancing sensitivity and broadening the spectral range. An optimized device containing 5-7-layer graphene has been achieved and shows an extraordinary responsivity exceeding 2600 A W
with fast photoresponse and specific detectivity up to ≈10
Jones in the ultraviolet-visible-near-infrared spectrum. This result suggests that the vdW p-g-n junctions containing multiple photoactive TMDs can provide a viable approach toward future ultrahigh-sensitivity and broadband photonic detectors.
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