ONIOM Investigation of the Second-Order Nonlinear Optical Responses of Fluorescent Proteins.

The first hyperpolarizability (ß) of six fluorescent proteins (FPs), namely, enhanced green fluorescent protein, enhanced yellow fluorescent protein, SHardonnay, ZsYellow, DsRed, and mCherry, has been calculated to unravel the structure-property relationships on their second-order nonlinear optical properties, owing to their potential for multidimensional biomedical imaging. The ONIOM scheme has been employed and several of its refinements have been addressed to incorporate efficiently the effects of the microenvironment on the nonlinear optical responses of the FP chromophore that is embedded in a protective ß-barrel protein cage. In the ONIOM scheme, the system is decomposed into several layers (here two) treated at different levels of approximation (method1/method2), from the most elaborated method (method1) for its core (called the high layer) to the most approximate one (method2) for the outer surrounding (called the low layer). We observe that a small high layer can already account for the variations of ß as a function of the nature of the FP, provided the low layer is treated at an ab initio level to describe properly the effects of key H-bonds. Then, for semiquantitative reproduction of the experimental values obtained from hyper-Rayleigh scattering experiments, it is necessary to incorporate electron correlation as described at the second-order Møller-Plesset perturbation theory (MP2) level as well as implicit solvent effects accounted for using the polarizable continuum model (PCM). This led us to define the MP2/6-31+G(d):HF/6-31+G(d)/IEFPCM scheme as an efficient ONIOM approach and the MP2/6-31+G(d):HF/6-31G(d)/IEFPCM as a better compromise between accuracy and computational needs. Using these methods, we demonstrate that many parameters play a role on the ß response of FPs, including the length of the π-conjugated segment, the variation of the bond length alternation, and the presence of π-stacking interactions. Then, noticing the small diversity of the FP chromophores, these results highlight the key role of the ß-barrel and surrounding residues on ß, not only because they can locally break the noncentrosymmetry vital to a ß response but also because it can impose geometrical constraints on the chromophore.

Open-shell characters and second hyperpolarizabilities of one-dimensional graphene nanoflakes composed of trigonal graphene units.

The impact of topology on the open-shell characters and the second hyperpolarizabilities (γ) has been addressed for one-dimensional graphene nanoflakes (GNFs) composed of the smallest trigonal graphene (phenalenyl) units. The main results are: 1) These GNFs show not only diradical but also multiradical characters when increasing the number of linked units. 2) GNFs composed of an equivalent number of units can exhibit a wide range of open-shell characters-from nearly closed-shell to pure multiradical characters-depending on the linking pattern of the trigonal units. 3) This wide variation in open-shell characters is explained by their resonance structures and/or by their (HOMO-i)-(LUMO+i) gaps deduced from the orbital correlations. 4) The change in the linking structure of the units can effectively control their open-shell characters as well as their γ values, of which the longitudinal components are significantly enhanced for the singlet GNFs having intermediate open-shell characters. 5) Singlet alternately linked (AL) systems present intermediate multiradical characters even in the case of a large number of units, which creates a significant enhancement of γ with increasing the size, whereas nonalternately linked (NAL) systems, which present pure multiradical characters, possess much smaller γ values. Finally 6) by switching from the singlet to the highest spin states, the γ values of NAL systems hardly change, whereas those of AL systems exhibit large reductions. These fascinating structure-property relationships between the topology of the GNFs, their open-shell characters, and their γ values not only deepen the understanding of open-shell characters of GNFs but aim also at stimulating further design studies to achieve giant NLO responses based on open-shell graphene-like materials.

From molecular to macroscopic engineering: shaping hydrogen-bonded organic nanomaterials.

The self-assembly and self-organization behavior of chromophoric acetylenic scaffolds bearing 2,6-bis(acetylamino)pyridine (1, 2) or uracyl-type (3-9) terminal groups has been investigated by photophysical and microscopic methods. Systematic absorption and luminescence studies show that 1 and 2, thanks to a combination of solvophilic/solvophobic forces and π-π stacking interactions, undergo self-organization in apolar solvents (i.e., cyclohexane) and form spherical nanoparticles, as evidenced by wide-field optical microscopy, TEM, and AFM analysis. For the longer molecular module, 2, a more uniform size distribution is found (80-200 nm) compared to 1 (20-1000 nm). Temperature scans in the range 283-353 K show that the self-organized nanoparticles are reversibly formed and destroyed, being stable at lower temperatures. Molecular modules 1 and 2 were then thoroughly mixed with the complementary triply hydrogen-bonding units 3-9. Depending on the specific geometrical structure of 3-9, different nanostructures are evidenced by microscopic investigations. Combination of modules 1 or 2 with 3, which bears only one terminal uracyl unit, leads to the formation of vesicular structures; instead, when 1 is combined with bis-uracyl derivative 4 or 5, a structural evolution from nanoparticles to nanowires is observed. The length of the wires obtained by mixing 1 and 4 or 1 and 5 can be controlled by addition of 3, which prompts transformation of the wires into shorter rods. The replacement of linear system 5 with the related angular modules 6 and 7 enables formation of helical nanostructures, unambiguously evidenced by AFM. Finally, thermally induced self-assembly was studied in parallel with modules 8 and 9, in which the uracyl recognition sites are protected with tert-butyloxycarbonyl (BOC) groups. This strategy allows further control of the self-assembly/self-organization process by temperature, since the BOC group is completely removed on heating. Microscopy studies show that the BOC-protected ditopic modules 8 self-assemble and self-organize with 1 into ordered linear nanostructures, whereas BOC-protected tritopic system 9 gives rise to extended domains of circular nano-objects in combination with 1.

Enhancement of second hyperpolarizabilities in open-shell singlet slipped-stack dimers composed of square planar nickel complexes involving o-semiquinonato type ligands.

Using the spin-unrestricted hybrid density functional theory method, we have investigated the intermolecular interaction effects on the longitudinal static second hyperpolarizability (γ) of open-shell singlet slipped-stack dimers composed of singlet diradical square planar nickel complexes involving o-semiquinonato type ligands, Ni(o-C(6)H(4)X(2))(2) (where X = O, NH, S, Se, PH). For comparison, we have also examined the γ values of a closed-shell singlet slipped-stack dimer composed of closed-shell monomers Ni[o-C(6)H(4)S(NH(2))](2). It is found that for interplanar distance ranging between 3.0 and 5.0 Šthe slipped-stack dimers with intermediate monomer diradical characters exhibit larger γ values per monomer (γ(dimer)/2) than those with large monomer diradical characters or than the closed-shell dimer. These results extend the domain of validity of the relationship found between γ and the diradical character for individual molecules. It also turns out that the ratio R = (γ(dimer)/2)/γ(monomer) increases upon decreasing the interplanar distance and that this increase is larger for intermediate diradical character than for the other cases. These phenomena have been analyzed by considering the γ density distributions of the dimers, demonstrating a significant field-induced third-order charge transfer between the monomers in the case of intermediate diradical character. The present results indicate that open-shell singlet slipped-stack aggregates composed of monomers with intermediate diradical characters constitute another mean for achieving highly efficient and tunable third-order nonlinear optical materials.

Theoretical and experimental investigation of the structural and spectroscopic properties of coumarin 343 fluoroionophores.

A joint theoretical-experimental investigation has been carried out to unravel the details of the complexation of cations by fluoroionophores based on coumarin 343 and to interpret the modifications in the ligand and also in the coumarin structural, electronic, magnetic, and vibrational properties. It is confirmed that C343-dea (1) complexes the cations by both the lactone and the amide oxygen atoms whereas for C343-crown (2) and C343-dibenzocrown (3), the cations are complexed by the oxygen atoms of the lactone as well as those of the crown ligand. These complexations induce geometric modifications, which are delocalized over the coumarin backbone and are related to electronic reorganizations that modify the spectroscopic signatures. This paper analyzes these signatures and shows how they are related as well as how they can be used to monitor the complexation process. Upon complexation, the UV-visible absorption spectra display a bathochromic shift of the most intense electronic transition; this shift is generally larger for the most flexible compound 1 as well as when complexing divalent cations. NMR spectra bear many signatures of the complexation, of which the most remarkable ones are the large shielding of C(1) and the large deshieldings of C(9) and C(16). Additional makers of complexation are highlighted in the IR vibrational spectra, in particular the bands associated with the lactone and amide CO vibrations, which are downshifted when the corresponding CO is involved in the complexation mode and, otherwise, upshifted. A high degree of consistency characterizes the different geometrical, electronic, magnetic, and vibrational signatures, which substantiates the assignment of the modes of complexation in 1-3. In addition, the agreement between the experimental data and the theoretical values is rather satisfactory, in that it at least enables us to interpret the spectral signatures.

A three-step synthetic approach to asymmetrically functionalized 4H-cyclopenta[2,1-b:3,4-b']dithiophenes.

A convenient and efficient three-step route toward both symmetrically and asymmetrically functionalized 4H-cyclopenta[2,1-b:3,4-b']dithiophenes has been developed. Using this method a broad collection of functionalized bridged bithiophenes can smoothly be accessed. Starting from 3-bromo-2,2'-bithiophene, prepared by Kumada coupling of 2-thienylmagnesium bromide with 2,3-dibromothiophene under Pd(dppf)Cl(2) catalysis, lithiation and subsequent reaction with dialkyl ketones afforded (a)symmetrically dialkylated tertiary alcohol derivatives. By means of final Friedel-Crafts dehydration cyclization in sulfuric acid medium, these derivatives were converted to 4,4-dialkyl-4H-cyclopenta[2,1-b:3,4-b']dithiophenes. Upon replacement of the dialkyl ketone reagent by ethyl levulinate, an ester-functionalized 4H-cyclopenta[2,1-b:3,4-b']dithiophene was prepared, representing an attractive precursor for variously functionalized cyclopentadithiophene compounds.

Giant electric field effect on the second hyperpolarizability of symmetric singlet diradical molecules.

This contribution reveals the effects of a static electric field on the static second hyperpolarizability γ of symmetric singlet diradical molecules using the valence configuration interaction scheme. It is found that under the effect of a field, the component of γ along the axis joining the two radical sites can be gigantically (approximately two to three orders) enhanced for symmetric diradicals having intermediate diradical characters with respect to those of closed-shell and pure diradical molecules in the absence of a field. Moreover, this electric field enhancement of γ increases as a function of the diradical character. These results and their analysis propose therefore a new strategy to design materials with exceptional nonlinear optical responses.

Synthesis and characterization of teranthene: a singlet biradical polycyclic aromatic hydrocarbon having Kekulé structures.

A teranthene derivative has been successfully isolated in a crystalline form for the first time. Geometrical considerations and physical property investigations indicate that the molecule possesses prominent biradical character in the ground state.

Long-range corrected density functional theory study on static second hyperpolarizabilities of singlet diradical systems.

Within the spin-unrestricted density functional theory (DFT) the long-range correction (LC) scheme combined with the Becke-Lee-Yang-Parr exchange-correlation functional, referred to as LC-UBLYP method, has been applied to the calculation of the second hyperpolarizability (gamma) of open-shell singlet diradical systems of increasing complexity and has demonstrated good performance: (i) for the simplest H(2) dissociation model, the gamma values calculated by the LC-UBLYP method significantly overshoot the full configuration interaction result but reproduce qualitatively the evolution of gamma as a function of the diradical character, (ii) for small singlet diradical 1,3-dipole systems, the diradical character dependence of gamma determined by the UCCSD and UCCSD(T) reference methods is reproduced semiquantitatively by the LC-UBLYP method except in the small diradical character region, where the spin-unrestricted solutions coincide with spin-restricted solutions, (iii) the LC-UBLYP method also closely reproduces the UCCSD(T) results on the diradical character dependence of gamma of the p-quinodimethane model system, particularly in the intermediate and large diradical character regions, whereas it shows an abrupt change for a diradical character (y) close to 0.2 originating from the triplet instability, (iv) the reliability of LC-UBLYP to reproduce reference coupled cluster results on open-shell singlet systems with intermediate and large diradical characters has also been substantiated in the case of gamma of 1,4-bis-(imidazol-2-ylidene)-cyclohexa-2,5-diene (BI2Y), then (v), for real systems built from a pair of phenalenyl radicals separated by a conjugated linker, the LC-UBLYP results have been found to closely match the UBHandHLYP values-which, for small systems are in good agreement with those obtained using correlated molecular orbital methods-whereas the UB3LYP results can be much different. These results are not only important from the viewpoint of an efficient determination of the nonlinear optical properties of open-shell singlet systems, but also from the viewpoint of defining new challenges for elaborating improved exchange-correlation functionals.

Remarkable two-photon absorption in open-shell singlet systems.

Remarkable enhancement of two-photon absorption (TPA) peak is theoretically predicted in symmetric open-shell singlet diradical systems with intermediate diradical character as compared to closed-shell and pure diradical systems. It is revealed that the largest TPA peak intensities occur for open-shell singlet diradicals having a ferromagnetically coupled ground state and strongly depend on the ratio between damping factors of the excited states. This result confirms that open-shell singlet conjugated molecules with intermediate diradical characters have precedence over conventional closed-shell conjugated systems in resonant third-order nonlinear optical properties.

Effect of the dynamical disorder on the second-order nonlinear optical responses of helicity-encoded polymer strands.

By combining classical samplings with quantum chemistry semiempirical time-dependent Hartree-Fock calculations, the high impact of dynamic fluctuations on the NLO properties of helical strands has been evidenced. In particular, these fluctuations are responsible for relative variations of approximately 20% in the hyper-Rayleigh responses in both pyridine-pyrimidine (py-pym) and hydrazone-pyrimidine (hy-pym) strands. Then, dynamical disorder has an even more important impact on the electric-field-induced second harmonic generation responses, whose variations can reach 2 (py-pym) or 5 (hy-pym) times their mean value. This work also highlights the relationships between the unit cell nature and helical conformation of foldamers and their second-order NLO responses. In particular, the octupolar symmetry of the hyper-Rayleigh depolarization ratios is related to the helix periodicity of three unit cells per turn in both compounds.

A joined theoretical-experimental investigation on the 1H and 13C NMR signatures of defects in poly(vinyl chloride).

1H and 13C chemical shifts of PVC chains have been evaluated using quantum chemistry methods in order to evidence and interpret the NMR signatures of chains bearing unsaturated and branched defects. The geometrical structures of the stable conformers have been determined using molecular mechanics and the OPLS force field and then density functional theory with the B3LYP functional and the 6-311G(d) basis set. The nuclear shielding tensor has been calculated at the coupled-perturbed Kohn-Sham level (B3LYP exchange-correlation functional) using the 6-311+G(2d,p) basis set. The computational scheme accounts for the large number of stable conformers of the PVC chains, and average chemical shifts are evaluated using the Maxwell-Boltzmann distribution. Moreover, the chemical shifts are corrected for the inherent and rather systematic errors of the method of calculation by employing linear regression equations, which have been deduced from comparing experimental and theoretical results on small alkane model compounds containing Cl atoms and/or unsaturations. For each type of defect, PVC segments presenting different tacticities have been considered because it is known from linear PVC chains that the racemic (meso) dyads are characterized by larger (smaller) chemical shifts. NMR signatures of unsaturations in PVC chains have been highlighted for the internal -CH=CH- and -CH=CCl- units as well as for terminal unsaturations like the chloroallylic -CH=CH-CH2Cl group. In particular, the 13C chemical shifts of the two sp2 C atoms are very close for the chloroallylic end group. The CH2 and CHCl units surrounding an unsaturation present also specific 13C chemical shifts, which allow distinguishing them from the others. In the case of the proton, the CH2 unit of the -CHCl-CH2-CCl=CH- segment presents a larger chemical shift (2.6-2.7 ppm), while some CHCl units close to the -CH=CH- unsaturations appear at rather small chemical shifts (3.7 ppm). The -CH2Cl and -CHCl-CH2Cl branches also display specific signatures, which result in large part from modifications of the equilibrium conformations and their reduced number owing to the increased steric interactions. These branches lead to the appearance of 13C peaks at lower field associated either to the CH unit linking the -CH2Cl and -CHCl-CH2Cl branches (50 ppm) or to the CHCl unit of the ethyl branches (60 ppm). The corresponding protons resonate also at specific frequencies: 3.5-4.0 ppm for the -CH2Cl branch or 3.8-4.2 ppm for the terminal unit of the -CHCl-CH2Cl branch. Several of these signatures have been detected in the experimental 1H and 13C NMR spectra and are consistent with the reaction mechanisms.

Second-harmonic generation in GFP-like proteins.

The second-order nonlinear optical properties of green fluorescent proteins (GFPs), such as the photoswitchable Dronpa and enhanced GFP (EGFP), have been studied at both the theoretical and experimental levels. In the case of Dronpa, both approaches are consistent in showing the rather counterintuitive result of a larger second-order nonlinear polarizability (or first hyperpolarizability, beta) for the protonated state, which has a higher transition energy, than for the deprotonated, fluorescent state with its absorption at lower energy. Moreover, the value of beta for the protonated form of Dronpa is among the highest reported for proteins. In addition to the pH dependence, we have found a wavelength dependence in the beta values. These properties are essential for the practical use of Dronpa or other GFP-like fluorescent proteins as second-order nonlinear fluorophores for symmetry-sensitive nonlinear microscopy imaging and as nonlinear optical sensors for electrophysiological processes. An accurate value of the first hyperpolarizability is also essential for any qualitative analysis of the nonlinear images.

Theoretical study on second hyperpolarizabilities of singlet diradical square planar nickel complexes involving o-semiquinonato type ligands.

Hybrid density functional theory method is applied for investigating the diradical character dependence of the second hyperpolarizability (gamma) of square planar nickel complexes involving several types of bidentate ligands [o-C6H4XY, where X = Y = O, NH, S, Se, and PH as well as (X, Y) = (NH, NH2) and (S, NH2)]. It is found that, as a function of the donor atoms, the diradical character of these complexes varies from 0.0 to 0.884 and is associated with substantial variations of gamma ranging from 14 x 10(3) to 819 x 10(3) au. In particular, the largest gamma values are associated with intermediate diradical characters in good agreement with the structure-property relationship obtained for pure hydrocarbon systems. Increasing the electronegativity of the X and Y donor groups of the ligands leads to larger diradical characters as a result of the enhancement of the double bond nature of the C=X(Y) bonds, which further stabilizes the diradicals on both-end benzene rings. This demonstrates that the electronegativities of the donor atoms of the ligands become a tuning parameter of the diradical character and then of the gamma values of these complexes.

Joint theoretical experimental investigation of the electron spin resonance spectra of nitroxyl radicals: application to intermediates in in situ nitroxide mediated polymerization (in situ NMP) of vinyl monomers.

Density functional theory (DFT) calculations have been performed to address the structure of nitroxide intermediates in controlled radical polymerization. In a preliminary step, the reliability of different theoretical methods has been substantiated by comparing calculated hyperfine coupling constants (HFCCs) to experimental data for a set of linear and cyclic alkylnitroxyl radicals. Considering this tested approach, the nature of different nitroxides has been predicted or confirmed for (a) the reaction of C-phenyl- N- tert-butylnitrone and AIBN, (b) N- tert-butyl-alpha-isopropylnitrone and benzoyl peroxide, (c) tert-butyl methacrylate polymerization in the presence of sodium nitrite as mediator, and (d) for the reaction of a nitroso compound with AIBN. Values of HFCC experimentally determined have been confirmed by DFT calculations.

Evaluation of the molecular polarizability using the IPPP-CLOPPA-INDO/S method. Application to molecules of biological interest.

The IPPP-CLOPPA-INDO/S method is introduced to investigate the static molecular polarizability in macromolecules. As an example of application, the polarizability of phospholipidic compounds, with and without the presence of water molecules has been estimated. The IPPP technique was employed to calculate the polarizability of the polar head and the hydrocarbon chains separately to analyze the feasibility of evaluating the total polarizability of the molecule by addition of these two projected results. INDO/S dipole moments of different fragments of the complex molecule were obtained by means of localized molecular orbitals in order to evaluate the charge transfer in the system.

Theoretical investigation on the linear and nonlinear susceptibilities of urea crystal.

The linear and second-order nonlinear susceptibilities of the urea crystal have been evaluated by applying the supermolecule approach. Calculations performed at the time-dependent Hartree-Fock (TDHF) level using the Austin model (AM1) semiempirical Hamiltonian have first demonstrated the almost additive character of the essential polarizability and first hyperpolarizability components. In fact, the only exception concerns the chi(cc) ((1)) component when stacking urea molecules along the c axis, i.e., the axis of the hydrogen bonds. This behavior has been confirmed by ab initio calculations on small clusters. The macroscopic quantities have then been determined by adopting the multiplicative scheme and by correcting the TDHF/AM1 values for missing electron correlation by means of density functional theory and coupled cluster method. The reliability of the multiplicative scheme was demonstrated for clusters as large as 3ax3bx3c. While the electron correlation correction factors are similar for a single molecule and different small clusters, the global performance of the scheme differs for the linear and nonlinear responses. For the second-order nonlinear susceptibility, our predictions are in good agreement with experiment, while for the linear susceptibility and the associated refractive index, our predictions underestimate the experimental values. The limitations of our approach may be attributed to its inability to account for more subtle cooperative effects, like those associated with a network of hydrogen bonds. Together with other works, the supermolecule calculations confirm that the sign of chi(abc) ((2)) is negative, contrary to an estimate from band structure calculation.

Circular dichroism of helical structures using semiempirical methods.

A general semiempirical scheme has been elaborated to simulate circular dichroism (CD) spectra of supramolecular systems. This approach adopts the analytical method of Beck and Hohlneicher [Theor. Chem. Acc. 101, 297 (1999).] to evaluate the one- and two-center integrals over Slater atomic orbitals. The performance of the method, employing INDO/S and CNDO/S semiempirical parametrizations, has been assessed by considering (i) the effect of the size of the singly excited states manifold, (ii) the origin invariance, and (iii) comparisons with the experimental and other theoretical spectra of several helicenes as well as pyridine-pyrimidine oligomers, which can adopt helical conformations. The main results are (i) the INDO/S parametrization with rather small excitation manifolds is able to reproduce, at low computational costs, the experimental CD spectra of several helicenes as well as CD simulations performed at ab initio and time-dependent density functional theory level of approximation; (ii) in the series of homohelicenes, the rotatory strength of the lowest-energy band increases almost linearly with the size of the helix; (iii) as evidenced by the study of tetradodecyloxy helicene bisquinone, packing effects can change the sign of remarkable CD bands, which are used to assign the structure configuration.

Relationship between third-order nonlinear optical properties and magnetic interactions in open-shell systems: a new paradigm for nonlinear optics.

Although most third-order nonlinear optical (NLO) materials are closed-shell singlet systems, this Letter theoretically shows that the third-order NLO properties are drastically enhanced in symmetric open-shell diradical systems with intermediate diradical character and further reveals that this enhancement is associated with the ferromagnetic and antiferromagnetic interactions. This new paradigm opens up a promising area for designing new third-order NLO materials.

Second hyperpolarizabilities of singlet polycyclic diphenalenyl radicals: effects of the nature of the central heterocyclic ring and substitution to diphenalenyl rings.

Adopting density functional theory and a hybrid exchange-correlation functional, the relationship between the second hyperpolarizability (gamma) and the diradical character has been investigated for diphenalenyl-based compounds containing different heterocyclic five-membered central rings (C(4)H(4)X, where X = NH, PH, O, S, CH(2), SiH(2), BH, GaH, C=O, C=S, and C=Se) or substituted by donor (NH(2))/acceptor(NO(2)) groups. It turns out that these structural modifications can tune the diradical character from 0.0 to 0.968 and lead to variations of gamma over more than 1 order of magnitude, demonstrating the controllability of gamma in this family of compounds. In particular, when the central ring is strongly aromatic, the diradical character is larger than 0.7, which is associated with pretty large gamma values except for almost the pure diradical case (y approximately 1). On the other hand, when the aromaticity decreases--or the antiaromaticity increases--the diradical character and the second hyperpolarizability get smaller. These relationships are correlated to structural (bond length alternation) and charge distribution (charge transfer between the phenalenyl rings and the central ring) properties, which account for the relative importance of the resonance diradical, zwitterionic, and quinoid forms. Therefore, the diradical character and the second hyperpolarizability can be controlled by the aromaticity of the ring while the paradigm of the enhancement of gamma for intermediate diradical character is globally verified. Then, upon introducing donor groups, the zwitterionic character increases, leading to closed-shell species and small second hyperpolarizabilities. In the case of substitution by acceptor groups, the charge transfer is reduced but the diradical character and the second hyperpolarizability hardly changes.