| 1. |
- DeFrancisco, Justin R., et al.
(författare)
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Torsional Bias as a Strategy To Tune Single-triplet Gaps in Organic Diradicals
- 2018
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Ingår i: The Journal of Physical Chemistry C. - : AMER CHEMICAL SOC. - 1932-7447 .- 1932-7455. ; 122:23, s. 12148-12157
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Tidskriftsartikel (refereegranskat)abstract
- Quinoidal compounds with proaromatic structures possess differing degrees of diradical character, where the open-shell diradical resonance form has restored aromaticity throughout the compound. Methods to tune the diradical character of these compounds have traditionally focused on altering the length and the molecular composition of the pi-conjugated backbones. However, other molecular design strategies to tune the singlet-triplet gap of pi-conjugated quinoidal molecules have not been extensively explored. We previously reported a strikingly small energy gap between the quinoidal and diradical states of a quinoidal small molecule containing methano[10]annulene (TMTQ) that was dictated in large part by the unusual aromaticity of the central annulene ring. Here, we report on two alkylated derivatives of TMTQthat present substantially different torsional biases to the planarity of the TMTQ pi-system. Using a combination of electronic and vibrational spectroscopies, magnetic measurements, and quantum chemical calculations, we demonstrate here how a steric effect rather than pi-electron compositional molecular engineering can dramatically narrow the singlet-triplet gap of a quinoidal compound to as small as -0.52 kcal/mol, determined experimentally. This study offers important insight for the continued development of open-shell diradical molecules that need not rely exclusively on the design of synthesis of new and complex conjugated systems.
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| 2. |
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| 3. |
- Messersmith, Reid E., et al.
(författare)
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Benzo[b]thiophene Fusion Enhances Local Borepin Aromaticity in Polycyclic Heteroaromatic Compounds
- 2017
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Ingår i: Journal of Organic Chemistry. - : American Chemical Society (ACS). - 0022-3263 .- 1520-6904. ; 82:24, s. 13440-13448
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Tidskriftsartikel (refereegranskat)abstract
- This report documents the synthesis, characterization, and computational evaluation of two isomeric borepin-containing polycyclic aromatics. The syntheses of these two isomers involved symmetrical disubstituted alkynes that were reduced to Z-olefins followed by borepin formation either through an isolable stannocycle intermediate or directly from the alkene via the trapping of a transient dilithio intermediate. Comparisons of their magnetic, crystallographic, and computational characterization to literature compounds gave valuable insights about the aromaticity of these symmetrically fused [b,f]borepins. The fusion of benzo[b]thiophene units to the central borepin cores forced a high degree of local aromaticity within the borepin moieties relative to other known borepin-based polycyclic aromatics. Each isomer had unique electronic responses in the presence of fluoride anions. The experimental data demonstrate that the local borepin rings in these two compounds have a relatively high amount of aromatic character. Results from quantum chemical calculations provide a more comprehensive understanding of local and global aromatic characters of various rings in fused ring systems built upon boron heterocycles.
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| 4. |
- Yadav, Sangeeta, et al.
(författare)
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Bimetallic Hollow Nanostructures for Colorimetric Detection of Picomolar Level of Mercury.
- 2020
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Ingår i: Journal of Nanoscience and Nanotechnology. - : Ingenta. - 1533-4880 .- 1533-4899. ; 20:2, s. 991-998
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we report the use of bimetallic hollow nanostructures (BHNS), consisting of gold and silver metals, for colorimetric detection of mercury. The sodium dodecyl sulphate (SDS)-capped BHNS were prepared by galvanic etching of silver nanoparticles (AgNPs) using gold chloride resulting in a partially hollow AgNPs with the gold layer at its surface. These BHNS were interacted with an aqueous solution of mercury ions (Hg2+) in the concentration range of 10 pM-10 mM. Interestingly, at higher concentration range (10 μM-10 mM), a noticeable change in the solution color was observed with a prominent decrease in the absorption intensity and blue-shift in the peak plasmonic wavelength. This could be attributed to (i) complexation reaction between the anionic BHNS (due to the negatively charged SDS capping) and cationic Hg2+ and (ii) oxidative etching of silver from BHNS causing its depletion and resulting into Ag-Hg amalgam and/or aggregation of the nanostructures. In contrast, at lower concentration range (i.e., 10 pM-10 nM), an increase in the absorption intensity was observed, which was possibly due to the oxidative etching of silver from BHNS without aggregation of the nanostructures. The low amount of Hg2+ was not sufficient enough to interact with SDS capping layer present on the BHNS surface, unlike the higher concentrations of mercury and therefore, did not cause any aggregation. The developed colorimetric sensor showed high sensitivity and selectivity towards Hg2+ detection with a limit of detection of 10 pM and good linearity (R² = 0.97) in the concentration range of 10 pM-10 nM.
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| 5. |
- Yadav, Sangeeta, et al.
(författare)
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Exploiting the Aromatic Chameleon Character of Fulvenes for Computational Design of Baird-Aromatic Triplet Ground State Compounds
- 2019
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Ingår i: Chemistry - An Asian Journal. - : Wiley-VCH Verlagsgesellschaft. - 1861-4728 .- 1861-471X. ; 14:10, s. 1870-1878
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Tidskriftsartikel (refereegranskat)abstract
- Due to the reversal in electron counts for aromaticity and antiaromaticity in the closed-shell singlet state (normally ground state, S-0) and lowest * triplet state (T-1 or T-0), as given by Huckel's and Baird's rules, respectively, fulvenes are influenced by their substituents in the opposite manner in the T-1 and S-0 states. This effect is caused by a reversal in the dipole moment when going from S-0 to T-1 as fulvenes adapt to the difference in electron counts for aromaticity in various states; they are aromatic chameleons. Thus, a substituent pattern that enhances (reduces) fulvene aromaticity in S-0 reduces (enhances) aromaticity in T-1, allowing for rationalizations of the triplet state energies (E-T) of substituted fulvenes. Through quantum chemical calculations, we now assess which substituents and which positions on the pentafulvene core are the most powerful for designing compounds with low or inverted E-T. As a means to increase the -electron withdrawing capacity of cyano groups, we found that protonation at the cyano N atoms of 6,6-dicyanopentafulvenes can be a route to on-demand formation of a fulvenium dication with a triplet ground state (T-0). The five-membered ring of this species is markedly Baird-aromatic, although less than the cyclopentadienyl cation known to have a Baird-aromatic T-0 state.
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