| 1. |
- Aski, Sahar Nikkhou, et al.
(författare)
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Inclusion complexes of cryptophane–E with dichloromethane and chloroform : A thermodynamic and kinetic study using the 1D-EXSY NMR method
- 2008
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Ingår i: Magnetic Resonance in Chemistry. - : Wiley. - 0749-1581 .- 1097-458X. ; 46:12, s. 1135-1140
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Tidskriftsartikel (refereegranskat)abstract
- Complexation equilibria and kinetics of exchange of chloroform and dichloromethane molecules between the cavity of cryptophane-E and bulk solution were investigated using NMR methods. Using one dimensional magnetization transfer (1D-EXSY type sequence), chemical exchange rates were measured in different temperature ranges, limited by the equilibrium constant values of the complexes and the boiling points of the guest substances. From the kinetic data, activation energies were calculated using the Arrhenius equation. From the temperature dependence of the association constant data, the enthalpy and entropy of complexation were estimated and compared with values for similar complexes of other cryptophanes.
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| 2. |
- Cravcenco, Alexei, et al.
(författare)
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Exciton Delocalization Counteracts the Energy Gap: A New Pathway toward NIR-Emissive Dyes
- 2021
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 143:45, s. 19232-19239
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Tidskriftsartikel (refereegranskat)abstract
- Exciton coupling between the transition dipole moments of ordered dyes in supramolecular assemblies, so-called J/H-aggregates, leads to shifted electronic transitions. This can lower the excited state energy, allowing for emission well into the near-infrared regime. However, as we show here, it is not only the excited state energy modifications that J-aggregates can provide. A bay-alkylated quaterrylene was synthesized, which was found to form J-aggregates in 1,1,2,2-tetrachloroethane. A combination of superradiance and a decreased nonradiative relaxation rate made the J-aggregate four times more emissive than the monomeric counterpart. A reduced nonradiative relaxation rate is a nonintuitive consequence following the 180 nm (3300 cm(-1)) red-shift of the J-aggregate in comparison to the monomeric absorption. However, the energy gap law, which is commonly invoked to rationalize increased nonradiative relaxation rates with increasing emission wavelength, also contains a reorganization energy term. The reorganization energy is highly suppressed in J-aggregates due to exciton delocalization, and the framework of the energy gap law could therefore reproduce our experimental observations. J-Aggregates can thus circumvent the common belief that lowering the excited state energies results in large nonradiative relaxation rates and are thus a pathway toward highly emissive organic dyes in the NIR regime.
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| 3. |
- Takacs, Zoltan, 1982-
(författare)
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Chloromethane Complexation by Cryptophanes : Host-Guest Chemistry Investigated by NMR and Quantum Chemical Calculations
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Host–guest complexes are widely investigated because of their importance in many industrial applications. The investigation of their physico–chemical properties helps understanding the inclusion phenomenon. The hosts investigated in this work are cryptophane molecules possessing a hydrophobic cavity. They can encapsulate small organic guests such as halo–methanes (CH2Cl2, CHCl3). The encapsulation process was investigated from both the guest and the host point of view. With the help of Nuclear Magnetic Resonance (NMR), the kinetics of complex formation was determined. The information was further used to obtain the activation energies of the processes. Having done this on five different cryptophanes, it is possible to relate the energies to structural differences between the hosts. Via the dipolar interaction between the guest’s and host’s protons, one can get information on the orientation of the guest inside the cavity. Moreover, the dynamics of the guest can be further investigated by its relaxation properties. This revealed restricted motion of the guest inside the host cavity. Not only the nature of the guest plays an important role. The host is also changing its properties upon encapsulation. All the cryptophanes investigated here can exchange rapidly between many conformers. These conformers have different–sized cavities. Quantum chemical optimization of the structure of the conformers makes volume estimation possible. Not only the cavity volumes, but also the quantum-chemically obtained energies and the calculated chemical shifts of the carbon–13 atoms can be helpful to follow the changes of the host upon complex formation. The host cannot be considered as a rigid entity. Analysis of variable temperature proton and carbon-13 spectra shows that the encapsulation can be considered as a mixture of conformational selection and induced fit. The structures of the formed complexes are further investigated by means of two-dimensional nuclear Overhauser spectroscopy (NOESY). The complex formation, its kinetics and thermodynamics are found to be a complicated function of structure elements of the host, the cavity size and the guest size and properties.
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| 4. |
- Takacs, Zoltan, 1982-, et al.
(författare)
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Chloromethane complexes of cryptophane-A and its analogue with butoxy groups
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Host-guest complexes between cryptophane-A as host and dichloromethane and chloroformas guests are investigated using 1H and13C NMR spectroscopy. Moreover, a relatedcryptophane, with the methoxy groups replaced by butoxy units (cryptophane-But), and itscomplexes with the same guests were also studied. Variable temperature spectra showedeffects of chemical exchange between the free and bound guests, as well as of conformationalexchange of the host. The guest exchange was studied quantitatively by exchangespectroscopy or line shape analysis. In the case of CHCl3@cryptophane-A, carbon-13relaxation of the guest was also investigated. Structural information was obtained throughmeasurements of cross-relaxation rates, both within the host and between the host and guestprotons. The NMR results were supported by DFT calculations.1
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| 5. |
- Takács, Zoltan, 1982-, et al.
(författare)
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Inclusion of Chloromethane Guests Affects Conformation and Internal Dynamics of Cryptophane-D Host
- 2012
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 116:27, s. 7898-7913
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Tidskriftsartikel (refereegranskat)abstract
- Cryptophane-D is composed of two nonequivalent cyclotribenzylene caps bound together by three OCH2CH2O bridges in a syn arrangement. Host-guest complexes with chloroform and dichloromethane were investigated in solution by NMR spectroscopy. Variable temperature NMR. H-1 and C-13 spectra showed effects of chemical exchange between the free and bound guest and of conformational exchange for the host, strongly and specifically affected by guest binding. We found in particular that the carbon-13 chemical shifts for the linkers connecting the two cyclotribenzylene units are very informative. The NMR results were supported by DFT calculations. The guest exchange was also studied quantitatively, either by EXSY measurements (for chloroform as guest) or by line-shape analysis (for dichloromethane as guest). In the case of chloroform guest, we also investigated cross relaxation between the guest and host protons, as well as carbon 13 longitudinal relaxation and heteronuclear NOE at three different fields. The results were interpreted in terms of orientation and dynamics of the guest inside the host cavity. Putting together various types of evidence resulted in remarkably detailed insight into the process of molecular recognition of the two guests by cryptophane-D host.
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| 6. |
- Takacs, Zoltan, 1982-, et al.
(författare)
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NMR Investigation of Guest–Host Complex between Chloroform and Cryptophane C
- 2010
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Ingår i: Magnetic Resonance in Chemistry. - : Wiley. - 0749-1581 .- 1097-458X. ; 48:8, s. 623-629
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Tidskriftsartikel (refereegranskat)abstract
- Guest–host complex between cryptophane C, possessing two non-equivalent caps, and chloroform is investigated by NMR spectroscopy. The kinetics of the chloroform exchange between the bound and free sites is determined by 1H exchange spectroscopy. Moreover, the preferential orientation of chloroform molecule with respect to the cryptophane C frame is examined by the NOESY and ROESY experiments. The experimental findings are compared to the results of quantum chemical calculations.
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