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Benoît Champagne

Publications and source records attributed to Benoît Champagne.

At least 19 recordsLinked to original sources
PubMed2006

Investigation of the UV/visible absorption spectra of merocyanine dyes using time-dependent density functional theory.

The visible spectral properties of a set of merocyanine dyes have been determined experimentally and compared to theoretical estimates calculated using time-dependent density functional theory. Three families of merocyanines have been investigated. The merocyanines of the first class contain the 4-thiazolidinone, 2-thioxo group in resonance with different donor groups. In the second family, they present a common pyrrolo[2,3-c]pyrazole-6-(2H)-acetic acid, the 4-[4-(dialkylamino)phenyl]-3,5-dihydro-3,5-dioxo-2-phenyl group with different substituents on the two phenyl rings, while in the third family, they are constituted of an isoxazolone group linked via a conjugated polyenic segment to an aminophenyl group. Two hybrid exchange-correlation functionals have been used, whereas solvent effects have been included using the integral equation formalism of the polarizable continuum model. Moreover, for one compound, the vibrational structure of the electronic transitions has been determined by calculating the Franck-Condon factors within the Born-Oppenheimer approximation. The calculations show that the dominant transitions present a pi-pi(*) character. This approach reproduces nicely the variations of excitation energies with the nature of the merocyanine so that least-squares fits can be used to correct for the major systematic errors of the computational scheme and to reach an accuracy of 0.09 eV or better. Part of these systematic errors is shown to originate from the description of the solvent effects, whereas vibronic effects can account for most of the remaining differences.

Journal Article↗
PubMed2006

Theoretical investigation of the second-order nonlinear optical properties of helical pyridine-pyrimidine oligomers.

The structure and second-order nonlinear optical properties of a series of helical pyridine-pyrimidine oligomers recently synthesized by Barboiu and Lehn have been investigated theoretically by combining molecular mechanics and quantum chemistry approaches. In the absence of substituents, the hyper-Rayleigh scattering response (betaHRS) and the projection of the first hyperpolarizability on the dipole moment (beta||) exhibit periodic variations with chain length, but these nonlinear optical responses remain small. The first hyperpolarizabilities can, however, be enhanced by adding substituents. The greatest enhancement is obtained by substituting the pyrimidine groups by donor groups. Moreover, regular distributions of the donor groups around the helices enable the design of supramolecular structures exhibiting dipolar, octupolar or Lambda-shaped nonlinear optical characters, evident from the values of the depolarization ratios in hyper-Rayleigh scattering. Therefore this theoretical investigation demonstrates the potential of helical structures for the organization of chromophores in such a way that they exhibit large and specific second-order nonlinear optical responses.

Journal Article↗
PubMed2006

Density functional theory investigation of the polarizability and second hyperpolarizability of polydiacetylene and polybutatriene chains: treatment of exact exchange and role of correlation.

The static polarizability and second hyperpolarizability of increasingly large polydiacetylene and polybutatriene (PBT) chains have been evaluated using the optimized effective potential for exact exchange (OEP-EXX) method developed by Yang and Wu [Phys. Rev. Lett. 89, 143002 (2002)], where the unknown part of the effective potential is expressed as a linear combination of Gaussian functions. Various conventional atomic orbital basis sets were employed for the exchange potential (X basis) as well as for the Kohn-Sham orbitals [molecular orbital (MO) basis]. Our results were compared to coupled-perturbed Hartree-Fock (CPHF) calculations and to ab initio correlated values obtained at various levels of approximation. It turns out that (a) small conventional basis sets are, in general, unsatisfactory for the X basis; (b) the performance of a given X basis depends on the MO basis and is generally improved when using a larger MO basis; (c) these effects are exaggerated for the second hyperpolarizability compared to the polarizability; (d) except for the second hyperpolarizability of PBT chains, using 6-311++G** for the X basis gives reasonable agreement with the CPHF results for all MO basis sets; (e) our results suggest that in the limit of a complete X basis the OEP-EXX values may approach the CPHF data; and (f) in general, the quality of a given conventional X basis degrades with the length of the oligomer, which correlates with the fact that the number of X basis functions becomes a smaller fraction of the number required to reproduce exactly the finite-basis-set Hartree-Fock energies. Linear and especially nonlinear electric field responses constitute a very stringent test for assessing the quality of functionals and potentials; appropriately tailored basis sets are needed to describe the latter. Finally, this study further highlights the importance of electron correlation effects on linear and nonlinear responses, for which correlated functionals with OEP are required.

Journal Article↗
PubMed2006

Theoretical determination of the vibrational Raman optical activity signatures of helical polypropylene chains.

Raman and vibrational Raman optical activity (VROA) spectra of helical conformers of polypropylene chains are simulated using ab initio methods to unravel the relationships between the vibrational signatures and the primary and secondary structures of the chains. For a polypropylene chain containing three units, conformational effects are shown to lead to more acute signatures for VROA than for Raman spectra. In addition to regular polypropylene chains, which can display right and left helicities with the same probability, chirality and therefore helicity are enforced by substituting one chain end with a phenyl group. The simulations predict that the threefold helical structures, which correspond to (TG)(N) conformations of the backbone, have a specific VROA backward signature in the form of an intense couplet around 1100 cm(-1). This couplet is associated with collective wagging and twisting motions, while most of its intensity comes from the anisotropic invariants combining normal coordinate derivatives of the electric dipole-electric dipole polarizability and of the electric dipole-magnetic dipole polarizability. A similar signature has already been found in model helical polyethylene chains, whereas it is very weak in forward VROA.

Journal Article↗
PubMed2006

Theoretical investigation on 1H and 13C NMR chemical shifts of small alkanes and chloroalkanes.

Using density functional theory (DFT) with the B3LYP, PBE, and PBE0 exchange-correlation functionals as well as the Moller-Plesset second-order perturbation theory (MP2) combined with a series of rather extended basis sets, 1H and 13C chemical shifts of small alkanes and chloroalkanes (with different numbers of chlorine atoms on specific positions) have been simulated and compared to experimental data. For the 1H chemical shifts, theory tends to reproduce experiment within the limits of the experimental errors. In the case of 13C chemical shift, the differences between theory and experiment increase monotonically with the number of chlorine atoms and exhibit a deviation from additivity. This behavior is related to the saturation of the experimental 13C chemical shifts with the number of chlorine atoms, whereas the evolution is mostly linear at both DFT and MP2 levels of approximation. This difference has been traced back to the relativistic spin-orbit coupling effects, which are exalted as a result of the enhancement of the s character of the C atom when increasing the number of linked Cl atoms. Thus, it was demonstrated that not only electron correlation but also relativistic effects have to be considered for estimating the 13C chemical shifts when several Cl atoms are directly attached to the C atom. Linear (theory/experiment) regressions have then been performed for the different types of C atoms, i.e., bearing one, two, and three Cl atoms, with excellent correlation coefficients. The linear correlation relationships so obtained can then serve to predict and facilitate the interpretation of the nuclear magnetic resonance spectra of more complex compounds. Furthermore, by investigating the basis set effects, the correlation between the chemical shifts calculated using the 6-311 + G(2d,p) basis set and the more extended 6-311 + G(2df,p) and aug-cc-pvtz basis sets is excellent, demonstrating that the choice of the 6-311 + G(2d,p) basis set for calculating the 1H and 13C chemical shifts is relevant.

Journal Article↗
PubMed2006

Origin of the enhancement of the second hyperpolarizability of singlet diradical systems with intermediate diradical character.

The origin of the diradical character dependence of the second hyperpolarizability (gamma) of neutral singlet diradical systems is clarified based on the perturbation formula of gamma using the simplest diradical molecular model with different diradical characters, i.e., H2 under bond dissociation. The enhancement of gamma in the intermediate diradical character region turns out to originate from the increasing magnitude of the transition moment between the first and second excited states and the decrease of that between the ground and first excited states, respectively, with the increase in diradical character. This feature confirms that open-shell singlet conjugated molecules with intermediate diradical characters constitute a new class of third-order nonlinear optical systems, whose gamma values can be controlled by the diradical character in addition to the conjugation length.

Journal Article↗
PubMed2006

Evaluation of alternative sum-over-states expressions for the first hyperpolarizability of push-pull pi-conjugated systems.

A dipolar-free sum-over-states expression for the diagonal components of the first hyperpolarizability (beta) tensor has been proposed by Kuzyk [Phys. Rev. A 72, 053819 (2005)] as an alternative to the traditional expression. We examine both alternatives for the longitudinal beta of four typical push-pull pi-conjugated systems using the ab initio CIS and CIS(D) schemes to approximate the excited state properties. Since they are each evaluated approximately the two SOS expressions yield different values for beta and it is found that (i) they evolve symmetrically as the number of excited states is increased so that their average is nearly constant; (ii) in the static limit, the two values agree better with one another when their average is close to the "exact" correlated result; and (iii) frequency dispersion can affect the agreement between the alternative expressions. On the basis of (i) and (ii) it appears best for typical push-pull pi-conjugated systems to estimate the static beta, and the error in the value so obtained, by averaging the Kuzyk and traditional results.

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PubMed2006

Experimental and theoretical investigation of the Raman and hyper-Raman spectra of acetonitrile and its derivatives.

The Raman and hyper-Raman spectra of acetonitrile and its deuterated analog have been investigated by combining experimental analysis and theoretical interpretation. It has been observed that the Raman spectra can easily be reproduced at both the Hartree-Fock and Moller-Plesset second-order levels of approximation and that for these fundamental transitions, inclusion of anharmonicity effects is not essential. On the other hand, the hyper-Raman spectra are more difficult to simulate and interpret. In particular, electron correlation has to be included in order to describe properly the intensity of the CN stretching mode. Then, a pseudo-C(infinity v) symmetry was assumed to better fit the experimental observations. This accounts for the fact that the a1- and e-symmetry modes correspond to time-decoupled vibrations. The e-symmetry modes, associated with nuclear motions perpendicular to the molecular axis are indeed subject to relaxation processes and, except the CCN bending mode, not visible in the hyper-Raman spectra of acetonitrile or of its deuterated analog. This assumption is supported by the gradual decrease of the phenomenon when going from acetonitrile to trichloroacetonitrile, where the presence of the heavier chlorine atoms in the latter reduces the relaxation processes.

Journal Article↗
PubMed2006

Acidoswitchable NLO-phores: benzimidazolo[2,3-b]oxazolidines.

This paper presents a series of acidoswitchable NLO-phores combining the 9-methylbenzimidazolo[2,3-b]oxazolidine core with various pi systems such as phenylethenyl, phenylethynyl, and naphthylethenyl. All the prepared derivatives are shown to display acidochromic behavior at ambient temperature. The remarkable contrast in the NLO response along the reversible transformations observed in HRS experiments is rationalized by high level theoretical calculations.

Journal Article↗
PubMed2006

Second hyperpolarizabilities (gamma) of bisimidazole and bistriazole benzenes: diradical character, charged state, and spin state dependences.

The second hyperpolarizabilities of bisimidazole- and bistriazole-benzene compounds have been calculated at different levels of approximation to unravel the effects of diradical character as well as of charge and spin multiplicity. The largest second hyperpolarizabilities are associated with intermediate diradical character, provided positive charging does not compensate for this effect. For the neutral diradical bisimidazole compound, the singlet diradical species possesses a second hyperpolarizability two to three times larger than the corresponding triplet, demonstrating the possibility of spin state control of the third-order NLO responses for diradical species.

Benzene Derivatives↗
PubMed2006

Facile synthesis and characterization of naphthidines as a new class of highly nonplanar electron donors giving robust radical cations.

Naphthidines 2 were prepared by nickel-catalyzed amination of 1-chloronaphthalene followed by oxidative homocoupling of 1-naphthalene amines 1 using titanium(IV) tetrachloride. The electronic and magnetic properties of materials 2 were investigated by cyclic voltammetry and other electrochemical techniques, EPR and UV-visible spectroscopies, and magnetic susceptibility. It was demonstrated that compounds 2 could be easily and reversibly oxidized via a two-electron-transfer reaction into their bis(radical cation) 2(2.2+), which displays a substantial stability at room temperature (the half-life of 2(2.2+) estimated by EPR at 25 degrees C was 10 days). B3LYP/6-31G optimized structures of N,N'-bis(4-methoxyphenyl)-(1,1'-binaphthyl)-4,4'-diamine 2g shows significant differences in the torsion angle between the naphthalene moieties depending on its oxidation state. Twisted structures are preferred for neutral compounds, whereas more planar are favored for the oxidized forms 2g*+ and 2g(2.2+) to realize spin and/or charge delocalizations over the whole pi-system. Such conformation changes concerted with the electron transfers contribute to explain the unusual two-electron process observed in the electrochemical behavior of 2g instead of the two single-electron transfers that would have been expected in the case of two successive oxidations. It is finally shown that the oxidation of 2g in CH2Cl2 with thianthrenium perchlorate (ThClO4) generates the dication 2g(2.2+) with singlet spin-multiplicity.

Journal Article↗
PubMed2005

Modeling the electric field third-order nonlinear responses of an infinite aggregate of hexatriene chains using the electrostatic interaction model.

A classical electrostatic polarization scheme using the additive distribution procedure has been applied to determine the static longitudinal polarizability and second hyperpolarizability of a all-trans hexatriene molecule in an infinite stretched fiber. The parameters have been derived from ab initio coupled-perturbed Hartree-Fock calculations and the electrostatic scheme has been validated via comparison with ab initio results on small clusters. Upon packing, the polarizability of all-trans hexatriene increases by 7% whereas the second hyperpolarizability increases by as much as 61%. These increases result from the balance between the enhancement of the (hyper)polarizability due to collinear packing and the reduction associated with lateral packing.

Algorithms↗
PubMed2005

Density-functional theory (hyper)polarizabilities of push-pull pi-conjugated systems: treatment of exact exchange and role of correlation.

The performance of the optimized effective potential procedure for exact exchange in calculating static electric-field response properties of push-pull pi-conjugated systems has been studied, with an emphasis on NO2-(CH=CH)n-NH2 chains. Good agreement with Hartree-Fock dipole moments and (hyper)polarizabilities is obtained; particularly noteworthy is the chain length dependence for beta/n. Thus, the problem that conventional density-functional theory functionals dramatically overestimate these properties is largely solved, although there remains a significant correlation contribution that cannot be accounted for with current correlation functionals.

Journal Article↗
PubMed2005

Mixed electric-magnetic second-order nonlinear optical response of helicenes.

The mixed electric-magnetic second-order nonlinear optical responses of oriented films of helicenes have been computed ab initio using the random phase approximation method and compared to the pure electric-dipole counterpart. It turns out that the mixed electric-magnetic responses can be of the same order of magnitude as the pure electric-dipole counterpart when there is no donor/acceptor (D/A) substituent or these D/A pairs are weak, i.e., when the pure electric-dipole response is small. When adding strong D/A substituents, the pure electric-dipole response increases substantially and much more than its mixed electric-magnetic counterpart. Consequently, the ratio between the mixed electric-magnetic and pure electric responses decreases. Although there is no general rule, the mixed responses evolve as a function of substitution quasi similarly to the pure electric contribution. This study confirms therefore the possibility of tuning the mixed electric-magnetic response by employing appropriate chiral molecules.

Journal Article↗
PubMed2005

Acido- and phototriggered NLO properties enhancement.

In this paper, acido- and phototriggered enhancement of the nonlinear optical (NLO) properties are achieved by combining the 10-(2-arylethenyl)indolino[2,1-b]oxazolidine unit with various styrylic residues. The synthetic pathway allows the preparation of phenyl, naphthyl, anthryl, and phenanthryl derivatives, which have been shown to display photochromic and acidochromic behavior at ambient temperature. The multiaddressable molecular switches are characterized by a large contrast of the NLO response along the reversible transformations. Hyper-Rayleigh scattering (HRS) experiments clearly indicate the strong influence of the electrodonating substituent borne by the aromatic system. Theoretical calculations have been performed for representative molecules showing a good agreement with the experimental hyperpolarizabilities. This enables a better understanding of the NLO responses in terms of structural and electronic parameters for both closed and protonated open forms.

Journal Article↗
PubMed2005

Vibrational raman optical activity as a mean for revealing the helicity of oligosilanes: a quantum chemical investigation.

Using theoretical simulations based on Hartree-Fock and density-functional theory calculations, the simulated vibrational Raman optical activity spectra of helical conformers of heptasilane are shown to present signatures sensitive to the helicity. These signatures are associated with collective wagging, twisting, and rocking motions. These simulated spectra have been obtained by combining analytical and numerical differentiation procedures to evaluate the geometry derivatives of the optical tensors entering into the expressions of the vibrational Raman optical activity intensities. From an investigation of basis set and electron correlation effects, it is shown that, like for local vibrations, diffuse functions are compulsory for evaluating the vibrational Raman optical activity intensities of collective vibrational motions.

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