Hyper-Rayleigh Scattering and Third-Harmonic Scattering in Chiral Liquids: Basic Evidences and Differences with Linear Chiroptical Techniques.

Nonlinear chiroptical methods like hyper-Rayleigh optical activity (HROA) and third-harmonic optical activity (THOA) have a great potential in characterizing structure-chiroptical properties of molecular systems. Here, with these methods, we bring for the first time experimental and theoretical evidence of nonlinear chiroptical differences between enantiomers of simple chiral molecules, using exclusively linearly polarized incident light. The origin of these unexpected nonlinear chiroptical contributions comes from a new nonlinear source term of the form ßOA(∇×µ(nω)), which is a bilinear coupling term including the linear chirality parameter, ßOA, and the curl of the nonlinear induced electric dipole moment, µ(nω), both involving dipolar magnetic interactions. Through a simple nonlinear chiroptical model that we propose, we show that specific nonlinear chiroptical parameters can be quantified, thus bringing new insights into stereochemical and electronic structural features of molecular and supramolecular systems.

Second-order nonlinear optical properties of X-shaped pyrazine derivatives.

This work reports an investigation of the second-order NLO properties of two isomer series of X-shaped pyrazine derivatives, by means of HRS measurements and DFT calculations. The systems differ in the relative position of the donor and acceptor substituents with respect to the axis formed by the nitrogen atoms of the central pyrazine ring. Although the magnitude of the second harmonic signal is similar, HRS measurements revealed that the anisotropy of the NLO response strongly differs in the two chromophore series, the one of the 2,3-isomers being strikingly dipolar, while the one of the 2,6-isomers is mostly octupolar. The experimental observations are well supported by DFT calculations. In particular, the sum-over-states approach allows us to rationalize the different NLO anisotropies observed in the two isomer series through a detailed analysis of the symmetry of the low-lying excited states.

First hyperpolarizability of the di-8-ANEPPS and DR1 nonlinear optical chromophores in solution. An experimental and multi-scale theoretical chemistry study.

The solvent effects on the linear and second-order nonlinear optical properties of an aminonaphtylethenylpyridinium (ANEP) dye are investigated by combining experimental and theoretical chemistry methods. On the one hand, deep near infrared (NIR) hyper-Rayleigh scattering (HRS) measurements (1840-1950 nm) are performed on solutions of di-8-ANEPPS in deuterated chloroform, dimethylformamide, and dimethylsulfoxide to determine their first hyperpolarizablity (ßHRS). For the first time, these HRS experiments are carried out in the picosecond regime in the deep NIR with very moderate (≤3 mW) average input power, providing a good signal-to-noise ratio and avoiding solvent thermal effects. Moreover, the frequency dispersion of ßHRS is investigated for Disperse Red 1 (DR1), a dye commonly used as HRS external reference. On the other hand, these are compared with computational chemistry results obtained by using a sequential molecular dynamics (MD) then quantum mechanics (QM) approach. The MD method allows accounting for the dynamical nature of the molecular structures. Then, the QM part is based on TDDFT/M06-2X/6-311+G* calculations using solvation models ranging from continuum to discrete ones. Measurements report a decrease of the ßHRS of di-8-ANEPPS in more polar solvents and these effects are reproduced by the different solvation models. For di-8-ANEPPS and DR1, comparisons show that the use of a hybrid solvation model, combining the description of the solvent molecules around the probe by point charges with a continuum model, already achieves quasi quantitative agreement with experiment. These results are further improved by using a polarizable embedding that includes the atomic polarizabilities in the solvent description.

Optical and structural characterization of femtosecond laser written micro-structures in germanate glass.

We report on direct femtosecond laser writing in zinc barium gallo-germanate glasses. A combination of spectroscopic techniques allows to progress in the understanding of the mechanisms taking place depending on the energy. In the first regime (type I, isotropic local index change) up to 0.5 µJ, the main occurrence is the generation of charge traps inspected by luminescence, together with separation of charges detected by polarized second harmonic generation measurements. At higher pulse energies notably at the threshold corresponding to 0.8 µJ or in the second regime (type II modifications corresponding to nanograting formation energy domain), the main occurrence is a chemical change and re-organization of the network evidenced by the appearance of molecular O2 seen in the Raman spectra. In addition, the polarization dependence of the second harmonic generation in type II indicates that the organization of nanogratings may be perturbed by the laser-imprinted electric field.

Nonlinear Optical Responses of Photoswitchable Donor-Acceptor Stenhouse Adducts.

This work combines hyper-Rayleigh scattering (HRS) experiments performed in the NIR range (1.30 and 1.60 µm) and quantum chemical calculations to provide a comprehensive description of the second harmonic generation (SHG) responses of donor-acceptor Stenhouse adducts (DASAs). Representative derivatives of the three generations of DASAs, which differ by the nature of their electron-donating and withdrawing moieties and also include clickable species, have been synthesized and their photoswitching behavior fully characterized. The HRS measurements allow us to establish relationships between the magnitude of the SHG response of open forms and the nature of the donor and acceptor groups. The largest SHG responses are obtained for derivatives incorporating either a barbituric acid or an indanedione acceptor unit, while N-methylaniline appears as the most efficient donor group. The calculations support well the experimental data and show that high hyperpolarizabilities are associated to low excitation energies and large extent of the photoinduced intramolecular charge transfer, which enhances the dipole moment variation between the ground and first dipole-allowed electronic excited state. In addition, a complete investigation of the photoswitching kinetics of DASAs in chloroform solution shows important differences, highlighting in particular the role of the donor group on the photoswitching efficiency.

Multi-State Second-Order Nonlinear Optical Switches Incorporating One to Three Benzazolo-Oxazolidine Units: A Quantum Chemistry Investigation.

This contribution employs quantum chemistry methods to describe the variations of the second nonlinear optical responses of molecular switches based on benzazolo-oxazolidine (BOX) units, connected by π-linkers, along their successive opening/closing. Under the fully closed forms, all of them display negligible first hyperpolarizability (ß) values. When one BOX is opened, which is sketched as C→O, a push-pull π-conjugated segment is formed, having the potential to enhance ß and to set the depolarization ratio (DR) to its one-dimensional-like value (DR = 5). This is observed when only one BOX is open, either for the monoBOX species (C→O) or for the diBOX (CC→CO) and triBOX (CCC→CCO) compounds, i.e., when the remaining BOXs stay closed. The next BOX openings have much different effects. For the diBOXs, the second opening (CO→OO) is associated with a decrease of ß, and this decrease is tuned by controlling the conformation of the π-linker, i.e., the centrosymmetry of the whole compound because ß vanishes in centrosymmetric compounds. For the triBOXs, the second opening gives rise to a Λ-shape compound, with a negligible change of ß, but a decrease of the DR whereas, along the third opening, ß remains similar and the DR decreases to the typical value of octupolar systems (DR = 1.5).

Self-assembling, structure and nonlinear optical properties of fluorescent organic nanoparticles in water.

Owing to their intense emission, low toxicity and solubility in aqueous medium, fluorescent organic nanoparticles (FONs) have emerged as promising alternatives to inorganic ones for the realization of exogenous probes for bioimaging applications. However, the intimate structure of FONs in solution, as well as the role played by intermolecular interactions on their optical properties, remains challenging to study. Following a recent Second-Harmonic Scattering (SHS) investigation led by two of us [Daniel et al., ACS Photonics, 2015, 2, 1209], we report herein a computational study of the structural organization and second-order nonlinear optical (NLO) properties of FONs based on dipolar chromophores incorporating a hydrophobic triphenylamine electron-donating unit and a slightly hydrophilic aldehyde electron-withdrawing unit at their extremities. Molecular dynamics simulations of the FON formation in water are associated with quantum chemical calculations, to provide insight into the molecular aggregation process, the molecular orientation of the dipolar dyes within the nanoparticles, and the dynamical behavior of their NLO properties. Moreover, the impact of intermolecular interactions on the NLO responses of the FONs is investigated by employing the tight-binding version of the recently developed simplified time-dependent density functional theory (sTD-DFT) approach, allowing the all-atom quantum mechanics treatment of nanoparticles.

Combining Benzazolo-Oxazolidine Twins toward Multi-state Nonlinear Optical Switches.

Molecular switches are chemical compounds exhibiting the possibility of reversible transformations between their different forms accompanied by a modification in their properties. Among these, switching of multi-addressable Benzazolo-OXazolidines (BOXs) from a closed form to an open form results in drastic modifications in their linear and nonlinear optical properties. Here, we target molecules containing two identical BOX units (DiBOX) connected by different π-conjugated linkers, and we combine synthesis, UV/visible absorption, and hyper-Rayleigh scattering (HRS) measurements, together with density functional theory (DFT) calculations. Three derivatives have been considered, which differ by the linker: (i) a bithiophene moiety (Bt), (ii) two 3,4-ethylenedioxythiopene (EDOT) units, and (iii) a triad composed of an EDOT-thiophene-EDOT sequence (TtO). As a matter of fact, these systems can adopt three states (CF-CF, POF-POF, and CF-POF) depending on the closed form (CF) or the protonated open form (POF) of each BOX unit. Despite chemical equivalence, stepwise switching of such systems under the addition of a chemical acid or an oxidant has been experimentally evidenced for two of them (DiBOX-Bt and DiBOX-TtO). Then, DFT calculations show that the first BOX opening leads to the formation of a push-pull π-conjugated segment, exhibiting a huge increase in the first hyperpolarizability (ß) and a bathochromic shift with respect to the fully closed form. On the contrary, the second BOX opening induces not only a slight bathochromic shift but also a reduction in their ß values conferring the great and uncommon abilities to modulate their linear and nonlinear properties over three discrete levels. Among these results, those on DiBOX-Bt agree with the experimental data obtained by HRS measurements and further shed light on their structure-property relationship.

Hyper-Rayleigh Scattering as a New Chiroptical Method: Uncovering the Nonlinear Optical Activity of Aromatic Oligoamide Foldamers.

Molecular helices based on self-organized aromatic oligoamide foldamers have been designed and prepared in their two enantiomeric forms in order to probe their second-order nonlinear chiroptical properties in solution. The quinoline oligoamides were rationally functionalized by electron-donating and electron-withdrawing groups to afford a gradual increase of the electronic polarization of the helical architectures. Their hyper-Rayleigh scattering (HRS) responses in solution were accordingly assessed, using either linearly polarized or circularly polarized incident light. Both methods allowed us to observe nonlinear optical activity that was quantified, for the first time for molecular systems, through circular differential scattering intensity ratios. The hyper-Rayleigh optical activity study reveals important charge-transfer differences within the aromatic oligomers, depending on the helix handedness and on the extent of electronic polarization induced by the appended substituents. The origin of the enantiomeric difference is discussed considering both achiral and chiral contributions. Overall, using aromatic oligoamide foldamers as a chiral model, we demonstrate the capabilities of HRS as a complementary chiroptical method, ideally suited for the analysis of various chiral molecular and supramolecular systems in solution. The reliability and chiral discrimination sensitivity of the method can be further improved through dynamic measurements using standard polarization modulation and heterodyning techniques.

Long-lived monolithic micro-optics for multispectral GRIN applications.

The potential for realizing robust, monolithic, near-surface refractive micro-optic elements with long-lived stability is demonstrated in visible and infrared transmitting glasses capable of use in dual band applications. Employing an enhanced understanding of glass chemistry and geometric control of mobile ion migration made possible with electrode patterning, flat, permanent, thermally-poled micro-optic structures have been produced and characterized. Sub-surface (t~5-10 µm) compositional and structural modification during the poling process results in formation of spatially-varying refractive index profiles, exhibiting induced Δn changes up to 5 × 10-2 which remain stable for >15 months. The universality of this approach applied to monolithic vis-near infrared [NIR] oxide and NIR-midwave infrared [MIR] chalcogenide glass materials is demonstrated for the first time. Element size, shape and gradient profile variation possible through pattern design and fabrication is shown to enable a variety of design options not possible using other GRIN process methodologies.

Diagnostic guide enabling distinction between taphonomic stains and enamel hypomineralisation in an archaeological context.

OBJECTIVE: Molar Incisor Hypomineralisation (MIH) is a structural anomaly that affects the quality of tooth enamel and has important consequences for oral health. The developmentally hypomineralised enamel has normal thickness and can range in colour from white to yellow or brown with or without surface breakdown. The possibility of finding MIH in 'ancient populations' could downplay several current aetiological hypotheses (e.g., dioxin derivatives, bisphenols, antibiotics) without excluding the possible multifactorial aspect of the anomaly. In an archaeological context, chemical elements contained in the burial ground can stain teeth yellow or brown and therefore might create a taphonomic bias. The purpose of the present study is to test a proposed diagnostic guide enabling determination of the pathological or taphonomic cause of enamel discolouration and defects that resemble MIH present on 'ancient teeth'. DESIGN: Two sample groups including MIH discoloration (n=12 teeth) from living patients, taphonomic discoloration (n=9 teeth) and unknown discoloration (n=2 teeth) from medieval specimens were tested. Three non-destructive methods-Raman spectroscopy, X-ray micro-computed tomography and X-ray fluorescence were utilised. RESULTS: Hypomineralised enamel has decreased mineral density (p<0.0001) and increased phosphate/ß-carbonate ratio (p<0.01) compared to normal enamel whereas relative concentrations of manganese, copper, iron and lead are similar. In taphonomic discoloration, relative concentrations of these elements are significantly different (p<0.05) to normal enamel whereas mineral density and Raman spectra profile are comparable. CONCLUSIONS: Enamel hypomineralisation can be distinguished from taphonomic staining in archaeological teeth.

Oxazines: A New Class of Second-Order Nonlinear Optical Switches.

A combined experimental-theoretical investigation has revealed that oxazine-based compounds are multiaddressable, multistate, and multifunctional molecular switches exhibiting contrasts of both linear and second-order nonlinear optical properties. The switching properties are particularly large when the substituent is a donor group. In this study, the cleavage of the C-O bond at the junction of the indole and oxazine cycles (of the closed a forms) is acido-triggered, leading to an open form (b(+)) characterized by larger first hyperpolarizabilities (ßHRS) and smaller excitation energies than in the closed form. These results are confirmed and interpreted utilizing ab initio calculations that have been carried out on a broad set of compounds to unravel the role of the substituent. With respect to acceptor groups, oxazines bearing donor groups are characterized not only by larger ßHRS and ßHRS contrast ratios but also by smaller excitation energies, larger opening-induced charge transfer, and reduction of the bond length alternation, as well as smaller Gibbs energies of the opening reaction. Compared to protonated open forms (b(+)), calculations on the zwitterionic open forms (b) have pointed out similarities in the long-wavelength UV/vis absorption spectra, whereas their ßHRS values might differ strongly as a function of the substituent. Indeed, the open forms present two NLOphores, the indoleninium-substituent entity and the nitrophenol (present in the protonated open form, b(+)) or nitrophenolate (present in the zwitterionic open form, b) moiety. Then, nitrophenolate displays a larger first hyperpolarizability than nitrophenol and the ß tensor of the two entities might reinforce or cancel each other.

Second-Order Nonlinear Optical Properties of a Dithienylethene-Indolinooxazolidine Hybrid: A Joint Experimental and Theoretical Investigation.

The nonlinear optical (NLO) properties of a double photochrome molecular switch are reported for the first time by considering the four trans forms of a dithienylethene-indolinooxazolidine hybrid. The four forms are characterized by means of hyper-Rayleigh scattering (HRS) experiments and quantum chemical calculations. Experimental measurements provide evidence that the pH- and light-triggered transformations between the different forms of the hybrid are accompanied by large variations of the first hyperpolarizability, which makes this compound an effective multistate NLO switch. Quantum chemical calculations conducted at the time-dependent Hartree-Fock and time-dependent DFT levels agree with the experimental data and allow a complete rationalization of the NLO responses of the different forms. The HRS signal of the forms with an open indolinooxazolidine moiety are more than one order of magnitude larger than that measured for the other forms, whereas the open/closed status of the dithienylethene subunit barely influences the dynamic NLO properties. However, extrapolation of the NLO responses to the static limit leads to univocally distinguishable intrinsic responses for three of the various forms. This hybrid system thus acts as a highly efficient multistate NLO switch for eventual exploitation in optical memory systems with multiple storage and nondestructive readout capacity.

Huge electro-/photo-/acidoinduced second-order nonlinear contrasts from multiaddressable indolinooxazolodine.

In this work, linear and nonlinear optical properties of electro-/acido-/photoswitchable indolino[2,1-b]oxazolidine derivatives were investigated. The linear optical properties of the closed and open forms have been characterized by UV-visible and IR spectroscopies associated with DFT calculations. Nonlinear optical properties of the compounds have been obtained by ex situ and in situ hyper-Rayleigh experiments in solution. We show that protonated, oxidized, and irradiated open forms exhibit the same visible absorption and NLO features. In particular, the closed and open forms exhibit a huge contrast of the first hyperpolarizability with an enhancement factor of 40-45. Additionally, we have designed an original electrochemical cell that allows to monitor in situ the hyper-Rayleigh response upon electrical stimulus. We report notably a partial but good and reversible NLO contrast in situ during oxidation/reduction cycles. Thereby, indolinooxazolidine moieties are versatile trimodal switchable units which are very promising for applications in devices.

Photorheological ionic liquids.

Two room temperature ionic liquids (ILs) bearing coumarin and p-hydroxycinnamic acid moieties are synthesized, and their photochemistry is studied in solution and neat conditions. Irradiation at absorption maxima leads to photochemical transformations and results in changes of their rheological properties which are evaluated by rotational rheometry. Samples of ionic liquids are also studied by Hyper-Rayleigh scattering, and the effect of their photochemistry on ionic nanoaggregation is discussed.

Experimental and theoretical investigation of the first-order hyperpolarizability of octupolar merocyanine dyes.

Hyper-Rayleigh scattering experiments and quantum chemical calculations are combined to investigate the second-order nonlinear optical responses of a series of three-arm merocyanine derivatives. They exhibit an octupolar hyperpolarizability response with lower amplitude than crystal violet due to a lower extent of the photoinduced charge transfer and reduced bond length alternation. Strong effects on the second-order optical response measured close to the two-photon absorption level are clearly evidenced; for example, the effective measured polarization ratio deviates below the ideal octupolar value of 3/2 even at very low excitation power. These effects are attributed to two-photon absorption resonance, which we believe modifies dynamically the population of the ground state versus that of the excited state.

A multistate pH-triggered nonlinear optical switch.

By using hyper-Rayleigh scattering experiments and quantum-chemical calculations, we demonstrate that nonlinear optics can be used to probe unequivocally, within a non-destructive process, the multiple electronic states that are activated upon pH- and light-triggered transformations of the 4'-hydroxyflavylium ion. These results open new perspectives in the design of molecular-scale high-density optical memory.

Design and characterization of molecular nonlinear optical switches.

Nanoscale structures, including molecules, supramolecules, polymers, functionalized surfaces, and crystalline/amorphous solids, can commute between two or more forms, displaying contrasts in their nonlinear optical (NLO) properties. Because of this property, they have high potential for applications in data storage, signal processing, and sensing. As potential candidates for integration into responsive materials, scientists have been intensely studying organic and organometallic molecules with switchable first hyperpolarizability over the past two decades. As a result of this, researchers have been able to synthesize and characterize several families of molecular NLO switches that differ by the stimulus used to trigger the commutation. These stimuli can include light irradiation, pH variation, redox reaction, and ion recognition, among others. The design of multistate (including several switchable units) and multifunctional (triggered with different stimuli) systems has also motivated a large amount of work, aiming at the improvement of the storage capacity of optical memories or the diversification of the addressability of the devices. In complement to the synthesis of the compounds and the characterization of their NLO responses by means of hyper-Rayleigh scattering, quantum chemical calculations play a key role in the design of molecular switches with high first hyperpolarizability contrasts. Through the latter, we can gain a fundamental understanding of the various factors governing the efficiency of the switches. These are not easily accessible experimentally, and include donor/acceptor contributions, frequency dispersion, and solvent effects. In this Account, we illustrate the similarities of the experimental and theoretical tools to design and characterize highly efficient NLO switches but also the difficulties in comparing them. After providing a critical overview of the different theoretical approaches used for evaluating the first hyperpolarizabilities, we report two case studies in which theoretical simulations have provided guidelines to design NLO switches with improved efficiencies. The first example presents the joint theoretical/experimental characterization of a new family of multi-addressable NLO switches based on benzazolo-oxazolidine derivatives. The second focuses on the photoinduced commutation in merocyanine-spiropyran systems, where the significant NLO contrast could be exploited for metal cation identification in a new generation of multiusage sensing devices. Finally, we illustrate the impact of environment on the NLO switching properties, with examples based on the keto-enol equilibrium in anil derivatives. Through these representative examples, we demonstrate that the rational design of molecular NLO switches, which combines experimental and theoretical approaches, has reached maturity. Future challenges consist in extending the investigated objects to supramolecular architectures involving several NLO-responsive units, in order to exploit their cooperative effects for enhancing the NLO responses and contrasts.

Nonlinear optical molecular switches as selective cation sensors.

This work demonstrates that the recognition of cations by molecular switches can give rise to large contrasts of the second-order nonlinear optical (NLO) properties, which can therefore be used as a powerful and multi-usage detection tool. The proof of concept is given by evidencing, by means of ab initio calculations, the ability of spiropyran/merocyanine systems to selectively detect alkali, alkaline earth, and transition-metal cations.

Reference molecules for nonlinear optics: a joint experimental and theoretical investigation.

Hyper-Rayleigh scattering (HRS) experiments and quantum chemical calculations are combined to investigate the second-order nonlinear optical responses of a series of reference molecules, namely, carbon tetrachloride, chloroform, trichloroacetonitrile, acetonitrile, and dichloromethane. The multipolar decomposition of the first hyperpolarizability tensor through the use of the spherical harmonics formalism is employed to highlight the impact of the symmetry of the molecular scatterers on their nonlinear optical responses. It is demonstrated that HRS is a technique of choice to probe the molecular symmetry of the compounds. Coupled-cluster calculations performed at the coupled-cluster level with singles, doubles, and perturbative triples in combination with highly extended basis sets and including environment effects by using the polarizable continuum model qualitatively reproduce the molecular first hyperpolarizabilities and depolarization ratios of the molecular scatterers.