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ABSTRACT: The controlled preparation of chiral structures is a contemporary challenge for supramolecular science because of the interesting properties that can arise from the resulting materials, and here we show that a synthetic nonamphiphilic C(3) compound containing π-functional tetrathiafulvalene units can form this kind of object. We describe the synthesis, characterization, and self-assembly properties in solution and in the solid state of the enantiopure materials. Circular dichroism (CD) measurements show optical activity resulting from the presence of twisted stacks of preferential helicity and also reveal the critical importance of fiber nucleation in their formation. Molecular mechanics (MM) and molecular dynamics (MD) simulations combined with CD theoretical calculations demonstrate that the (S) enantiomer provides the (M) helix, which is more stable than the (P) helix for this enantiomer. This relationship is for the first time established in this family of C(3) symmetric compounds. In addition, we show that introduction of the "wrong" enantiomer in a stack decreases the helical reversal barrier in a nonlinear manner, which very probably accounts for the absence of a "majority rules" effect. Mesoscopic chiral fibers, which show inverted helicity, i.e. (P) for the (S) enantiomer and (M) for the (R) one, have been obtained upon reprecipitation from dioxane and analyzed by optical and electronic microscopy. The fibers obtained with the racemic mixture present, as a remarkable feature, opposite homochiral domains within the same fiber, separated by points of helical reversal. Their formation can be explained through an "oscillating" crystallization mechanism. Although C(3) symmetric disk-shaped molecules containing a central benzene core substituted in the 1,3,5 positions with 3,3'-diamido-2,2'-bipyridine based wedges have shown peculiar self-assembly properties for amphiphilic derivatives, the present result shows the benefits of reducing the nonfunctional part of the molecule, in our case with short chiral isopentyl chains. The research reported herein represents an important step toward the preparation of functional mesostructures with controlled helical architectures.
Journal of the American Chemical Society 06/2011; 133(21):8344-53. · 9.91 Impact Factor
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Chemistry 05/2011; 17(23):6330-3. · 5.93 Impact Factor
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ABSTRACT: A presentation of the complex polarization propagator in the restricted open-shell self-consistent field approximation is given. It rests on a formulation of a resonant-convergent, first-order polarization propagator approach that makes it possible to directly calculate the X-ray absorption cross section at a particular frequency without explicitly addressing the excited states. The quality of the predicted X-ray spectra relates only to the type of density functional applied without any separate treatment of dynamical relaxation effects. The method is applied to the calculation of the near K-edge X-ray absorption fine structure spectra of allyl and copper phthalocyanine. Comparison is made between the spectra of the radicals and those of the corresponding cations and anions to assess the effect of the increase of electron charge in the frontier orbital. The method offers the possibility for unique assignment of symmetry-independent atoms. The overall excellent spectral agreement motivates the application of the method as a routine precise tool for analyzing X-ray absorption of large systems of technological interest.
The Journal of Physical Chemistry B 05/2011; 115(18):5096-102. · 3.70 Impact Factor
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ABSTRACT: ECD and NMR experiments show that the complexation of propylene oxide (PrO) within the cavity of an enantiopure water-soluble cryptophane 1 in NaOH solution is enantioselective and that the (R)-PrO@PP-1 diastereomer is more stable than the (S)-PrO@PP-1 diastereomer with a free energy difference of 1.7 kJ/mol. This result has been confirmed by molecular dynamics (MD) and ab initio calculations. The enantioselectivity is preserved in LiOH and KOH solutions even though the binding constants decrease, whereas PrO is not complexed in CsOH solution.
The Journal of Organic Chemistry 04/2011; 76(10):4178-81. · 4.45 Impact Factor
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ABSTRACT: Type-II quantum dots (QDs) were applied for QDs-sensitized solar cells for the first time and showed prominent absorbed photon to current conversion efficiency.
Chemical Communications 02/2011; 47(5):1536-8. · 6.17 Impact Factor
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ABSTRACT: In order to study the role of surface ligands in determining optical properties of colloidal quantum dots (QDs), we have selectively fabricated and studied CdSe/CdS core-shell QDs with strongly confined electron and hole states attached with commonly used surface ligands. Optical properties, viz. absorption and fluorescence of these QDs, are characterized from which salient changes have been observed for different ligand substitutions which, through theoretical analysis, can be associated with electronic structure properties of the QD-ligand composite systems, in particular localization of wave functions of electrons and holes in the QDs and the band matching of the HOMO-LUMO gap of the ligands. The findings can be utilized to facilitate the understanding and optimization of properties of QD biomarkers with functionalizing surface ligands for targeting cellular objects.
Physical Chemistry Chemical Physics 02/2011; 13(13):5848-54. · 3.57 Impact Factor
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Koji Ohta,
Satoru Yamada,
Kenji Kamada,
Aaron D Slepkov,
Frank A Hegmann,
Rik R Tykwinski,
Laura D Shirtcliff,
Michael M Haley,
Paweł Sałek,
Faris Gel'mukhanov, Hans Ågren
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ABSTRACT: The two-photon absorption (TPA) properties of four TPEB [tetrakis(phenylethynyl)benzene] derivatives (TD, para, ortho, and meta) with different donor/acceptor substitution patterns have been investigated experimentally by the femtosecond open-aperture Z-scan method and theoretically by the time-dependent density-functional theory (TDDFT) method. The four compounds show relatively large TPA cross sections, and the all-donor substituted species (TD) displays the largest TPA cross-section σ(2) = 520 ± 30 GM. On the basis of the calculated electronic structure, TD shows no TPA band in the lower energy region of the spectrum because the transition density is concentrated on particular transitions due to the high symmetry of the molecular structure. The centrosymmetric donor-acceptor TPEB para shows excitations resulting from transitions centered on D-π-D and A-π-A moieties, as well as transition between the D-π-D and A-π-A moieties; this accounts for the broad nature of the TPA bands for this compound. Calculations for two noncentrosymmetric TPEBs (ortho and meta) reveal that the diminished TPA intensities of higher-energy bands result from destructive interference between the dipolar and three-state terms. The molecular orbitals (MOs) of the TPEBs are derivable with linear combinations of the MOs of the two crossing BPEB [bis(phenylethynyl)benzene] derivatives. Overall, the characteristics of the experimental spectra are well-described based on the theoretical analysis.
The Journal of Physical Chemistry A 01/2011; 115(2):105-17. · 2.95 Impact Factor
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ABSTRACT: A new water-soluble cryptophane 1 derivative (penta-hydroxyl cryptophane-A) has been synthesized from cryptophanol-A and the chiroptical properties of its two enantiomers MM-1 and PP-1 have been studied by polarimetry, electronic circular dichroism (ECD), and vibrational circular dichroism (VCD). Cryptophane 1 shows specific circular dichroism responses upon complexation that depend on the size of the guest and on the nature of the counterion (Li(+), Na(+), K(+), Cs(+)) present in the solution. In LiOH and NaOH solutions, chiroptical changes induced by the encapsulation of guests and by the presence of cations in the vicinity of hosts can be interpreted from molecular dynamics (MD) and ab initio calculations by subtle conformational changes of the bridges. In KOH solution, the exchange dynamics is dependent on the size of the guest molecules, whereas in CsOH solution no encapsulation effect is observed whatever the size of the guest molecule. This last behavior comes from the fact that host 1 exhibits a very high affinity for cesium cations.
The Journal of Organic Chemistry 01/2011; 76(5):1372-83. · 4.45 Impact Factor
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ABSTRACT: In this paper, we have synthesized a series of core/shell quantum dots (QDs) for the purpose of enhancing the electron transfer from the dots to a TiO2 substrate. We make use of the fact that CdSe is a small-bandgap material compared with CdS; therefore, in a common CdSe/CdS core/shell QD, the photo-excited electron is confined deeply in the CdSe core. By special construction of the CdS/CdSe core/shell QDs, referred as reversed type-I, the electron wave function will distribute largely in the shell region. This facilitates the transfer of the electron from the QD to the TiO2 substrate, resulting in significantly improved electron-injection efficiency. Such an enhanced electron-injection efficiency was confirmed by fluorescence lifetime decay measurements, showing the largest lifetime reduction after that the QDs were adsorbed on the TiO2 surface. The reversed type-I CdS/CdSe QDs show a much higher photon-to-current conversion efficiency than type-I CdSe/CdS and CdSe QDs without shell. Furthermore, by chemical-bath depositing of CdS on the QD-sensitized electrode to form a quantum-well structure, the electron recombination between the QDs and the redox couple was reduced, hence further enhancing the electron-injection efficiency. The absorbed-photon-to-current efficiency of the quantum well CdS/CdSe/CdS sensitized solar cells reaches a value as high as 60%.
08/2010;
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ABSTRACT: We highlight the course of development of a project initiated 1998 by the authors and concerned with quantum modeling of optical power limiting materials. The methodological development has involved various quantum mechanical technologies for a description of multi-photon excitations within the framework of wave function and density function theories, with relativistic effects being addressed at rigorous as well as more approximate levels of theory. The method development has also involved models for vibronic and solvent contributions to the multi-photon excitations as well as pulse propagation based on the solution of the classical wave equations. Our review of these issues is completely nonmathematical. Results from sample applications are discussed in order to illus- trate different aspects of concern for choosing materials with good optical limiting capabilities. Some focus is given to organic and organometallic push-pull compounds, including metallo-porphyrin compounds and platinum complexes.
Journal of Computational and Theoretical Nanoscience 11/2004; 1(4):343-366. · 0.91 Impact Factor
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ABSTRACT: Dioxygen reduction in the oxidative half-reaction of copper amine oxidases (CAOs) has been studied quantum chemically using the hybrid density functional theory (B3LYP). The reductive activation of dioxygen is a spin-forbidden process for which substantial kinetic O-18 (but no deuterium) isotope effects have been found experimentally. The proposed mechanism was divided into three steps, and the last step was studied for two different potential energy surfaces: the quartet and the doublet surfaces. It is suggested that dioxygen reduction occurs through a spin transition that is induced by the exchange interaction between the unpaired spins of the Cu(II) ion and the O2- anion. The step involving this spin transition is suggested to be rate-limiting, which gives a rationalization for the puzzling experimental results when copper is substituted for other metals. The spin transition is triggered by the calculated vibronic perturbation of 5.4 (kcal/mol) Å-1, which leads to a very fast rate of 8 1010 s-1 for the spin transition. However, since the spin transition occurs at a calculated energy that is 18-20 kcal/mol higher than that of the reactant, this step could still be rate-limiting. The difference in the O-O bond distance between the resting state (free dioxygen) and the point of the spin transition provides an explanation for the oxygen isotope effect.
Journal of Physical Chemistry B - J PHYS CHEM B. 01/2004; 108(36):13882-13892.
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ABSTRACT: In an effort to obtain a microscopic understanding of the origin of vuv emission from inert heavy-atom gas mixtures we present results from relativistic calculations of the krypton-xenon spectrum in the 68 000 cm-1 region. Calculations carried out on the interatomic interaction in the ground and in the low-lying spin-orbit excited states [0-(3P2),0+(3P1),1(3P1),1(3P2),2(3P1)] of the krypton-xenon dimer are presented. The calculations were executed in a two-step procedure in which the CCSD(T) model for the ground state and the CCSD response theory model for the excited states were used to obtain spin-free potential curves in the first step. A perturbational treatment of the spin-orbit interaction within the atomic mean-field approximation was applied in the next step. Large-core quasirelativistic effective core potentials with richly augmented valence basis sets and midbond functions were used. All calculations were corrected for basis set superposition errors by applying the counterpoise method. For all involved states, spin-orbit perturbed potential energy curves were computed and equilibrium geometries, potential well depths, and local extrema were determined. Vibrational analysis and Franck-Condon factors for the ground and 0+(3P1),1(3P1) states were also calculated. The geometry dependence of the electronic dipole transition matrix elements between all involved excited states and the ground state was investigated. Excitation energies and potentials were obtained that are in excellent agreement with results based on fitting to experimental data. Out of some earlier disparate experimental assignments of the character of the excited states of the KrXe complex, the present results adhere most closely to the ones presented by Pibel et al. [J. Chem. Phys. 101, 10 242 (1994)]. Analyzed together with the experimental data, the theoretical results provide a clear picture of the organization of exciplex states and of the origin of strong vuv emission from these states that follows from binary collisions of the heavy inert Kr and Xe atoms.
Phys. Rev. A. 04/2003; 67(4).
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ABSTRACT: Relativistic calculations of electric dipole moments, linear polarizabilities, and first- and second-order hyperpolarizabilities have been carried out for the isovalent group VI dihydrides (O–Po) and group VII monohydrides (F–At) at three different levels: the time-dependent Dirac–Hartree–Fock approximation, the time-dependent Hartree–Fock approximation with a Douglas–Kroll transformed one-component Hamiltonian, and the time-dependent Hartree–Fock approximation with effective-core potentials. These calculations are compared with nonrelativistic time-dependent Hartree–Fock results in order to elucidate the role of relativistic effects on these properties and to investigate the extent to which the Douglas–Kroll approach and the effective-core potentials—both of which neglect spin-dependent terms but are computationally less demanding—are able to reproduce the 4-component Dirac–Hartree–Fock results. The results show that qualitatively correct relativistic corrections in most cases can be obtained with the more approximative methods, but that a quantitative agreement with 4-component calculations is often not obtained. © 2002 American Institute of Physics.
The Journal of Chemical Physics. 04/2002; 116(16):6914-6923.
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ABSTRACT: Solvent-induced two-photon absorption cross sections are calculated for a push−pull molecule in solutions using both self-consistent reaction field and internal finite field approaches. It is shown analytically and numerically that the results from the two methods can be connected through induced local reaction field factors. The two-photon cross sections of the studied push−pull polyene are found to be rather insensitive to the choice of cavity shape. The solvent dependence of the two-photon absorption displays a pattern different from that of the first hyperpolarizability.
03/2000;
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ABSTRACT: Solvent effects on the static polarizabilities and hyperpolarizabilities of conjugated polymers have been studied by means of a semiclassical solvation model using results from ab initio calculations. The solvent-induced changes of the static polarizabilities show maxima at fairly short oligomer lengths, with the main axial contribution decreasing rapidly to a zero value at the geometrically nondistorted polymer limit. Different saturation behaviors of the static polarizabilities of conjugated oligomers in gas phase and in solution are observed. It is concluded that the dipole–dipole interaction is responsible for the solvent-induced property changes of the oligomers. © 1999 American Institute of Physics.
The Journal of Chemical Physics 11/1999; 111(21):9853-9858. · 3.33 Impact Factor
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ABSTRACT: Ab initio simulations of two-photon absorption in donor–acceptor substituted cumulene-containing aromatic molecules have been carried out. The results reveal that these two-dimensional, charge-transfer species possess particularly large two-photon absorption in the visible region with possible applications for optical limiting. © 1999 American Institute of Physics.
The Journal of Chemical Physics 10/1999; 111(17):7758-7765. · 3.33 Impact Factor
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ABSTRACT: A recently proposed semiclassical model for simulating optical properties of solutions and liquids is extended to encompass vibrational polarizabilities. Taking account of both the electronic and vibrational contributions it is shown that the size of an ellipsoidal cavity of pure liquids can be determined in a way that gives consistency between the predicted microscopic and macroscopic properties. It is found that a good estimate of the vibrational polarizability of the pure liquid can be obtained from experimental data on the molecular, gas phase, infrared (IR) spectrum. The influence of the cavity shapes on the liquid susceptibilities is examined using pure liquid ethanol as a test system. © 1999 American Institute of Physics.
The Journal of Chemical Physics 04/1999; 110(16):7960-7965. · 3.33 Impact Factor
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ABSTRACT: On the grounds of results from quantum chemical calculations, we introduce spin-catalysis as a new concept, and classify different types of chemical reactions according to this concept. We propose a general definition of spin-catalysis as phenomena in which chemical reactions are promoted by substances which assist in overcoming spin-prohibition or in which the activation barrier is lowered through spin uncoupling induced by a paramagnetic catalyst. A number of known phenomena fall into this definition and can be classified according to two main categories: (1) spin-orbit coupling induced and (2) paramagnetic-exchange induced spin-catalysis. Other types of spin-catalysis are also discussed: (3) processes with participation of substances which assist in (a) photochemical and (b) thermal or electrochemical generation of active particles by energy and—by electron transfer [radicals, diradicals, O2(1Δg), etc.], reacting further without spin-prohibition: (4) processes induced by an external magnetic field. Processes (3) and (4) are quite general and well known; their inclusion in the spin-catalysis classification does not introduce any new findings for the chemical kinetics, but the general features, which unite them with the important catalytic processes of the first two types, serve as a useful guide in catalysis theory. Few models of spin-catalytic processes have been simulated for the purpose of illuminating the principles of spin-catalysis; the cis-trans isomerization of ethylene catalyzed by molecular oxygen, the external heavy atom effect in ethylene photochemistry, and some others. © 1996 John Wiley & Sons, Inc.
International Journal of Quantum Chemistry 12/1998; 57(3):519 - 532. · 1.36 Impact Factor
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ABSTRACT: We propose a simple semiclassical approximation model for frequency-independent properties of molecules in solution using the cavity/dielectric approach. The model is applied to test systems comprising nonpolar, polar, and charged solutes, and is compared with the perturbation expansion method and the self-consistent reaction field theory method. Considering the simplicity of the model, the results compare well with the full self-consistent reaction field theory results, especially for the linear properties. Since the model relies only on gas phase calculations, it can be used for any electronic structure method that is implemented for static properties. © 1998 American Institute of Physics.
The Journal of Chemical Physics 08/1998; 109(9):3589-3595. · 3.33 Impact Factor
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ABSTRACT: The methanol molecule is used as a test case for studying the simultaneous action of solvation and vibrational effects on static and dynamic hyperpolarizabilities within the reaction field model. The calculations indicate that the medium can play a significant role for the vibrational correction. The calculated third-order optical nonlinear susceptibilities of methanol in gas and liquid phases are in fair to good agreement with their experimental counterparts. © 1998 American Institute of Physics.
The Journal of Chemical Physics 08/1998; 109(9):3580-3588. · 3.33 Impact Factor
