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Lipid molecules are involved in a wide range of fundamental cell functions, explaining the enormous structural diversity of lipids, partly due to the large number of different fatty acids, which can vary in length and in the number and position of unsaturated bond(s). Interestingly, imbalances between (un)saturated species have been reported in various pathologies. Hence, it is of particular importance to study in detail the structural consequences associated with variations in the nature of the lipid hydrocarbon tails. To this end, 17 single‐component lipid bilayers, varying only in the nature of the hydrocarbon core are studied by molecular dynamics (MD) simulations. The structural analysis reveals that the effect of the nature of the two fatty acids on the overall membrane structure is additive. In addition, TDDFT quantum mechanical (QM) calculations are performed to study the modulations of the nonlinear optical (NLO) properties of an embedded probe, serving as local structural microscopes. However, these calculations highlight the difficulty of rationalizing changes in the NLO responses, in relation with the nature of the hydrocarbon region of the host environment. Nevertheless, less dense and packed membranes, which allow the probe to adopt more planar conformations, are associated with larger NLO responses. [ABSTRACT FROM AUTHOR]

Recent measurements of the third harmonic scattering responses of molecules have given a new impetus for computing molecular second hyperpolarizabilities (γ) and for deducing structure–property relationships. This paper has employed a variety of wavefunction and density functional theory methods to evaluate the second hyperpolarizability of the p-nitroaniline prototypical push-pull π-conjugated molecule, addressing also numerical aspects, such as the selection of an integration grid and the impact of the order of differentiation vs the achievable accuracy by using the Romberg quadrature. The reliability of the different methods has been assessed by comparison to reference Coupled-Cluster Singles and Doubles with perturbative treatment of the Triples results. On the one hand, among wavefunction methods, the MP2 scheme offers the best accuracy/cost ratio for computing the static γ. On the other hand, using density functional theory, γ remains a challenging property to compute because all conventional, global hybrid or range-separated hybrid, exchange–correlation functionals underestimate static γ values by at least 15%. Even tuning the range-separating parameter to minimize the delocalization errors does not enable to improve the γ values. Nevertheless, the original double-hybrid B2-PLYP functional, which benefits from 27% of PT2 correlation and 53% Hartree–Fock exchange, provides accurate estimates of static γ values. Unfortunately, the best performing exchange–correlation functionals for γ are not necessarily reliable for the first hyperpolarizability, β, and vice versa. In fact, the β of p-nitroaniline (pNA) could be predicted, with a good accuracy, with several hybrid exchange–correlation functionals (including by tuning the range-separating parameter), but these systematically underestimate γ. As for γ, the MP2 wavefunction method remains the best compromise to evaluate the first hyperpolarizability of pNA at low computational cost. [ABSTRACT FROM AUTHOR]

3,4‐Dimethylenecyclobutene (DMCB) is an unusual isomer of benzene. Motivated by recent synthetic progress to substituted derivatives of this scaffold, we carried out a theoretical and computational analysis with a particular focus on the extent of (anti)aromatic character in the lowest excited states of different multiplicities. We found that the parent DMCB is non‐aromatic in its singlet ground state (S0), lowest triplet state (T1), and lowest singlet excited state (S1), while it is aromatic in its lowest quintet state (Q1) as this state is represented by a triplet multiplicity cyclobutadiene (CBD) ring and two uncoupled same‐spin methylene radicals. Interestingly, the Q1 state, despite having four unpaired electrons, is placed merely 4.8 eV above S0, and there is a corresponding singlet tetraradical 0.16 eV above. The DMCB is potentially a highly useful structural motif for the design of larger molecular entities with interesting optoelectronic properties. Here, we designed macrocycles composed of fused DMCB units, and according to our computations, two of these have low‐lying nonet states (i. e., octaradical states) at energies merely 2.40 and 0.37 eV above their S0 states as a result of local Hückel‐ and Baird‐aromatic character of individual 6π‐ and 4π‐electron monocycles. [ABSTRACT FROM AUTHOR]

The frequency dispersion of the hyper-Rayleigh scattering first hyperpolarizabilities (ßH R S) of five reference molecules for nonlinear optics, namely, carbon tetrachloride, chloroform, dichloromethane, acetonitrile, and trichloroacetonitrile, is described using the coupled-cluster singles and doubles quadratic response function (CCSD-QRF) as well as approximate schemes. Comparisons to approximate schemes in which the frequency dispersion is evaluated as either a multiplicative or an additive correction to the static hyperpolarizability yield the following observations: (i) errors of the order of 10% or less are usually encountered when using the multiplicative scheme for photon energies far from the lowest dipole-allowed excitation energies, (ii) spurious cases cannot be excluded as evidenced by carbon tetrachloride where the multiplicative scheme predicts a decrease of ßH R S in contradiction to the increase obtained using the CCSD-QRF method, and (iii) the additive scheme is at best as reliable as the multiplicative approximation. The two-state approximation presents the advantage of correcting the wrong behavior of the additive and multiplicative schemes for carbon tetrachloride, but it is not an improved solution for the other compounds, while the question of selecting the appropriate dominant excited state remains unanswered. Finally, a new ßßxyz value of 18.9 a.u. is proposed for carbon tetrachloride in gas phase at λ = 1064 nm, to be compared with the measured 16.9 ± 1.4 a.u. value due to Shelton. [ABSTRACT FROM AUTHOR]

The second-order nonlinear optical properties of 1,3-indandione-derived spirocyclohexadine compounds, a new class of photochromes showing sensitivity to both UV and visible lights, are investigated by means of density functional theory. The calculations predict that these compounds should display a large first hyperpolarizability variation upon commutation between their cyclic spiropyran and their merocyanine form. We also show that significant enhancement of the NLO response of the merocyanine form, as well as of the NLO contrast, can be achieved by means of appropriate chemical substitution, which makes these derivatives highly appealing for potential use in photo-responsive NLO devices. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 (βHRS). 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 βHRS 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 βHRS 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. [ABSTRACT FROM AUTHOR]

Novel magenta dyes were obtained via an unprecedented base-promoted intramolecular cyclization of strongly dipolar elongated merocyanine dyes bearing a phthalimide substituted tricyanopropylidene terminal segment. These shorter diarylpolyene derivatives retain a dipolar character and show a typical intramolecular charge transfer (ICT) transition. The cyclization noticeably modifies the electronic structure inducing both onset of bond length alternation and reduction of dipole moment as well as a marked hypsochromic and hypochromic shift of the ICT absorption band compared to their merocyanine precursors. Yet the new dyes show improved quadratic hyperpolarizability and thermal stability as compared to the merocyanine dyes having the same number of conjugated double bonds in the polyenic linker. As such these derivatives hold promise as new class of dyes for NLO materials and SHG probes. [ABSTRACT FROM AUTHOR]

The linear and nonlinear optical properties of a series of arylvinyldiazine derivatives are investigated by means of quantum chemical first principles calculations. The UV/vis absorption spectra of the various derivatives, as well as the NLO properties previously characterized by means of EFISHG measurements, are well reproduced by the theoretical computations. The reported results provide complementary insights on the structure–properties relationships linking the nature of the chemical substituents and of the conjugated π-electron system that constitute the push–pull structure to the amplitude of the NLO responses. [ABSTRACT FROM AUTHOR]

In this computational experiment, students use advanced quantum chemistry tools to simulate the photochromic reaction mechanism in naphthopyran derivatives. The first part aims to make students familiar with excited-state reaction mechanisms and addresses the photoisomerization of the benzopyran molecule by means of semiempirical quantum chemical calculations. This part introduces in particular the concept of conical intersection between potential energy surfaces and can be conducted under instructor guidance during a 4 h laboratory session. The second part investigates the photophysical and photochromic properties of two diphenyl-naphthopyran isomers and can be carried out independently by the students. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Highlights: [•] The first hyperpolarizability of a redox switching RuII/III complex has been investigated. [•] Both TDDFT and SOS methods confirm and explain experimental β contrast. [•] MLCT state dominates RuII β whereas ILCT contribution is negligible. [•] MLCT state disappears and β drops substantially upon oxidation. [•] The depolarization ratio changes little upon oxidation. [Copyright &y& Elsevier]

Abstract: A joint experimental and theoretical study is carried out to investigate the structural and absorption properties of the C212 organic dye adsorbed onto the TiO2 anatase surface. UV–Vis measurements evidence a significant blue shift of the main absorption band of the dye upon absorption, which is well reproduced by theoretical calculations combining tight-binding and time-dependent DFT calculations. The blue shift is substantiated by a loss in the efficiency of the light-induced charge transfer along the anchored dye, which partly originates from the deprotonation of the carboxylic acid function. [Copyright &y& Elsevier]

Abstract: A new soluble n-type material based on the 2,9-bis(triisopropylsilylethynyl)triphenodioxazine (TIPS-triphenodioxazine) has been designed as a quinonoid equivalent of TIPS-pentacene and synthesized using an innovative synthetic pathway. Both materials showed identical electronic affinities, intermolecular arrangements and thin film topographies. However, the TIPS-triphenodioxazine led to better performances than TIPS-pentacene in n-channel organic field-effect transistors (OFETs) evidencing the potentialities of this π-conjugated core in the field of organic electronics. [Copyright &y& Elsevier]

A semi-empirical valence bond/Hartree–Fock (VB/HF) method is developed to calculate one- and two-electron interactions between molecular fragments in conducting supramolecular stacks. This fragment orbital-based formalism allows for consistent extraction of an effective Hamiltonian defined as a “frontier orbital” model. This Hamiltonian quantitatively describes transfer and electrostatic interactions between conducting electrons, while reducing the active space so dramatically that the electronic eigenstates of very large systems may be investigated. The capabilities of the VB/HF method are illustrated on two different supramolecular stacks involving a π–π interacting fragment. In the first part of this study, the framework of the VB/HF method is used to evaluate the relative magnitude of the electronic interactions between conduction electrons in organic conductors and superconductors derived from Bechgaard salts. In the second part of this study, the VB/HF formalism is extended to derive an effective model for conduction holes along doped DNA double strands. Transferable intra- and intersite parameters were first evaluated from VB/HF calculations carried out on nucleoside pairs. From this interaction databank, the effective Hamiltonian of any type of nucleoside sequence can be defined. The thermalized charge distribution for a single hole delocalized along a DNA sequence containing 240 Watson–Crick pairs is then calculated and compared with the experimental yields of damage revealed by photocleavage experiments. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [ABSTRACT FROM AUTHOR]

The linear and nonlinear optical (NLO) properties of a series of Λ -shaped derivatives containing a 4,5-dicyanopyrazine acceptor unit, N,N -dimethylamino donor groups and systematically enlarged π-conjugated linkers are investigated by means of UV/Visible and Hyper-Rayleigh scattering spectroscopies. Density functional theory calculations are also carried out to rationalize the magnitude and symmetry of the NLO responses. The results demonstrate that these compounds possess two low-lying excited electronic states close to each other in energy, which are accessible through one-photon optical transitions respectively polarized perpendicular and parallel to the two-fold molecular axis. These two states contribute additively to the first hyperpolarizability, which exhibits a dominant dipolar character. We also show that the NLO responses significantly deviate from Kleinman's index permutation symmetry. • Λ -shaped pyrazine derivatives deliver strong second-order NLO responses. • They result from the contribution of low-lying perpendicular electronic transitions. • The first hyperpolarizabilities are not Kleinman-symmetric. • These second-order NLO responses have a dominant dipolar character. • Their amplitudes depend on the nature of the π-conjugated linkers. [ABSTRACT FROM AUTHOR]

Molecular-based Fluorescent Organic Nanoparticles (FONs) are versatile light-emitting nano-tools whose properties can be rationally addressed by bottom-up molecular engineering. A challenging property to gain control over is the interaction of the FONs' surface with biological systems. Indeed, most types of nanoparticles tend to interact with biological membranes. To address this limitation, we recently reported on two-photon (2P) absorbing, red to near infrared (NIR) emitting quadrupolar extended dyes built from a benzothiadiazole core and diphenylamino endgroups that yield spontaneously stealth FONs. In this paper, we expand our understanding of the structure-property relationship between the dye structure and the FONs 2P absorption response, fluorescence and stealthiness by characterizing a dye-related series of FONs. We observe that increasing the strength of the donor end-groups or of the core acceptor in the quadrupolar (D-π-A-π-D) dye structure allows for the tuning of optical properties, notably red-shifting both the emission (from red to NIR) and 2P absorption spectra while inducing a decrease in their fluorescence quantum yield. Thanks to their strong 1P and 2P absorption, all FONs whose median size varies between 11 and 28 nm exhibit giant 1P (106 M−1.cm−1) and 2P (104 GM) brightness values. Interestingly, all FONs were found to be non-toxic, exhibit stealth behaviour, and show vanishing non-specific interactions with cell membranes. We postulate that the strong hydrophobic character and the rigidity of the FONs building blocks are crucial to controlling the stealth nano-bio interface. [ABSTRACT FROM AUTHOR]

Due to their versatility and easy processing, Surface-Anchored Metal–Organic Frameworks (SurMOFs) have gained interest in recent times as promising electrochromic thin films. Herein a step forward in their use and characterization was achieved thanks to the integration of {Zn2(PDICl4)2} SurMOFs in a multi-layer electrochromic device (ECD), based on a membrane-like electrolyte. The optical and electrochemical properties of the ECD were fully characterized, revealing a two-step reduction process localized on the organic ligand and involving subsequent near infra-red (NIR) and cyan absorbing states, leading to optical modulation of the films. The species responsible for this absorption were isolated and identified in the reduced states. In parallel to experimental characterization, quantum chemistry was successfully used to investigate the structure–property relationship of the SurMOF, revealing additional information regarding the structure and the local environment of the electrochromic ligand. [ABSTRACT FROM AUTHOR]

The reactivity of novel chiral lactamide‐substituted diselenide‐based reagents under oxidative conditions was exploited to develop a metal‐free method for the preparation of enantioenriched allenylamides from simple alkynes in good yields, and with enantiomeric excesses up to 99 %. The key of the success in this method is attributed to the hydrogen‐bonded lactamide appendages that ensure configurational stability of chiral vinyl selenoxide intermediates for an optimal enantiotopic β‐syn‐elimination step. [ABSTRACT FROM AUTHOR]

The reactivity of novel chiral lactamide‐substituted diselenide‐based reagents under oxidative conditions was exploited to develop a metal‐free method for the preparation of enantioenriched allenylamides from simple alkynes in good yields, and with enantiomeric excesses up to 99 %. The key of the success in this method is attributed to the hydrogen‐bonded lactamide appendages that ensure configurational stability of chiral vinyl selenoxide intermediates for an optimal enantiotopic β‐syn‐elimination step. [ABSTRACT FROM AUTHOR]

[Display omitted] • Uncaging efficiency is retained in dimeric DEAC conjugated structures. • Two-photon absorption properties (2PA) are enhanced by up to two order of magnitude. • The aromatic cores influence both the uncaging efficiency and the 2PA properties. • Both electron-releasing and electron-withdrawing cores lead to unprecedented 2P uncaging sensitivities (550 GM). A series of symmetrical conjugated dimeric coumarinyl cages has been synthesized by grafting 7-diethlyaminocoumarin (DEAC) cages via π-linkers onto different aromatic cores displaying various electronic character (electron-withdrawing: benzobisthiazole BBTz, electron-donating: carbazole or ambivalent: fluorenyl). Due to their symmetrical design, all conjugated dimeric derivatives showed enhanced two-photon absorption at 730 nm, the electron-withdrawing core leading to the highest response (over 3000 GM). All dimeric derivatives retained the uncaging ability of the DEAC cage, with a strong influence of the nature of the core on the uncaging quantum yield. While the fluorenyl core led to an uncaging quantum yield similar to DEAC, the BBTz core led to a twice larger uncaging quantum yield and the carbazole core to even larger uncaging quantum yield (60 %). As a result, both electron-donating and electron-withdrawing cores lead to symmetrical dimeric DEAC derivatives showing unprecedented two-photon uncaging photo-sensitivity (up to 550 GM). [ABSTRACT FROM AUTHOR]

Model solutions of white wines containing phenolic acids have been investigated by means of UV–vis, laser induced fluorescence and Raman spectroscopic techniques. In order to interpret the spectra, density functional theory calculations of phenolic acids have been performed. This work demonstrates that only hydroxynamic acids are in resonance with a laser excitation line with 325 nm wavelength and are therefore at the origin of the strong enhancement of the Raman light scattering. Real white wines also display such resonance Raman scattering so that their content in hydroxycinnamic acids may be quite precisely determined. The analysis of the Raman spectrum of a real dry white wine reveals qualitatively the preponderance in its composition of p-coumaric and caftaric acids. [ABSTRACT FROM AUTHOR]

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 (β) 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 β 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 β, and this decrease is tuned by controlling the conformation of the π-linker, i.e., the centrosymmetry of the whole compound because β vanishes in centrosymmetric compounds. For the triBOXs, the second opening gives rise to a Λ-shape compound, with a negligible change of β, but a decrease of the DR whereas, along the third opening, β remains similar and the DR decreases to the typical value of octupolar systems (DR = 1.5). [ABSTRACT FROM AUTHOR]

The first hyperpolarizability ofthe four trans forms of a dithienyletheneindolinooxazolidine biphotochrome in acetonitrile solution has beenevaluated by using two solvation models, an explicit and an implicitone. The implicit solvation model is the integral equation formalismof the polarizable continuum model (IEF-PCM), whereas in the explicitone, the solvent molecules are represented by point charges, of whichthe positions have been generated by Monte Carlo simulations whereasthe solute is treated quantum mechanically. At optical frequencies,first hyperpolarizabilities calculated with the implicit solvationmodel are usually larger than those obtained with the multiscale approach.However, both approaches predict similar contrasts, indicating thatimplicit solvation models such as IEF-PCM are well-suited to describethe variations in the NLO responses of molecular switches. In addition,the analysis of the contrasts of first hyperpolarizabilities showsthat the biphotochrome can act as a three-state NLO switch. [ABSTRACT FROM AUTHOR]

This work demonstrates by means of DFT and ab initio calculations that recognition of alkali cations can be achieved by probing the variations of the second-order nonlinear optical properties along the commutation process in spiropyran/merocyanine systems. Due to the ability of the merocyanine isomer to complex metal cations, the switching between the two forms is accompanied by large contrasts in the quadratic hyperpolarizability that strongly depend on the size of the cation in presence. Exploiting the nonlinear optical responses of molecular switches should therefore provide powerful analytical tools for detecting and identifying metal cations in solution. [ABSTRACT FROM AUTHOR]

[Display omitted] • Synthesis of molecular photoswitches based on 2-substituted N H and N CH 3 imidazole. • Tuning of switching properties by preventing imidazole tautomerism and benzene substitution. • Pentafluorosulfanyl derivative showed the highest stability of the (Z)–isomer. • Insertion of SF 5 group proved to be useful strategy towards a bistable photoswitch. Azo–imidazole photoswitches based on N H– and N –methylimidazole were designed and prepared via a convenient synthetic route. These azo-compounds were investigated by means of XRD analysis, cyclic voltammetry, thermal analysis, UV–Vis absorption spectroscopy and DFT calculations. The photoswitching was mostly affected by imidazole N -substitution with the half-life of (Z)–isomers found within the range of tens of seconds (N H-imidazoles) vs. tens of hours (N -methylimidazoles). N -Methylimidazole switches can be further tuned by altering electronic effects of the appended substituents (acyl, bromine, trifluoromethyl and pentafluorosulfanyl). The latter group proved to be particularly interesting in stabilizing (Z)–isomer. Its large half-life (∼24 h), high photoconversion (89 %) and diminished nonlinear optical response in contrast to the (E)-isomer (β E / Z = 3.82) point to a very useful strategy in introducing SF 5 group in the structure of molecular switches. In summary, this detailed study demonstrates that photoswitching properties of azo-imidazoles can be significantly tuned in a broad extent by (preventing) imidazole tautomerism as well as by altering electronic effects of appended substituents. [ABSTRACT FROM AUTHOR]

The contrast of second-order nonlinear optical response in the dihydroazulene (DHA)-vinylheptafulvene (VHF) equilibrium has been investigated as a function of the nature of the substituent (R) on the phenyl ring by means of quantum chemistry calculations including electron correlation, frequency dispersion, and solvent effects. By considering the hyper-Rayleigh scattering (HRS) response, the contrast for R = H and R = CH3between the DHA and VHF forms is larger than 5 while the contrast between the cisand transVHF forms is close to 1. Adding the NH2donor group in paraposition of the phenyl leads to a substantial increase of the HRS first hyperpolarizability of the three forms, which is detrimental to the contrast. Then, in the case of the NO2acceptor group, a contrast is recovered because the HRS first hyperpolarizability of the DHA form is about 2–3 times larger than for both VHF forms. These variations of first hyperpolarizability as a function of the substituents as well as the associated contrasts have been explained in terms of donor/acceptor strengths and geometrical parameters. [ABSTRACT FROM AUTHOR]

Many natural products of plant or microbial origins are derived from enzymatic dearomative oxygenation of 2‐alkylphenolic precursors into 6‐alkyl‐6‐hydroxycyclohexa‐2,4‐dienones. These so‐called ortho‐quinols cyclodimerize via a remarkably selective bispericyclic Diels–Alder reaction. Whether or not the intervention of catalytic or dirigent proteins is involved during this final step of the biosynthesis of these natural products, this cyclodimerization of ortho‐quinols can be chemically reproduced in the laboratory with the same strict level of site‐specific regioselectivity and stereoselectivity. This unique yet unified process, which finds its rationale in the inherent chemical reactivity of those ortho‐quinols, is illustrated herein by an efficient and bioinspired first chemical synthesis of one of the most structurally complex and synthetically challenging examples of such natural cyclodimers, the bisditerpenoid (+)‐maytenone. [ABSTRACT FROM AUTHOR]

Many natural products of plant or microbial origins are derived from enzymatic dearomative oxygenation of 2‐alkylphenolic precursors into 6‐alkyl‐6‐hydroxycyclohexa‐2,4‐dienones. These so‐called ortho‐quinols cyclodimerize via a remarkably selective bispericyclic Diels–Alder reaction. Whether or not the intervention of catalytic or dirigent proteins is involved during this final step of the biosynthesis of these natural products, this cyclodimerization of ortho‐quinols can be chemically reproduced in the laboratory with the same strict level of site‐specific regioselectivity and stereoselectivity. This unique yet unified process, which finds its rationale in the inherent chemical reactivity of those ortho‐quinols, is illustrated herein by an efficient and bioinspired first chemical synthesis of one of the most structurally complex and synthetically challenging examples of such natural cyclodimers, the bisditerpenoid (+)‐maytenone. [ABSTRACT FROM AUTHOR]

The linear and second-order nonlinear optical (NLO) properties of clickable azobenzene derivatives are investigated in chloroform solution by means of UV/Visible and Hyper-Rayleigh scattering (HRS) spectroscopies. Their absorption properties are also monitored along time in order to characterize the rate and kinetics of the E-Z photoswitching reaction. The magnitude and symmetry of the first hyperpolarizability of the compounds are rationalized with the help of DFT calculations, which support well the experimental data. We demonstrate that the second harmonic generation (SHG) intensity of the E form, as well as its variation upon photoisomerization, can be tuned by varying the nature of the peripheral substituents. A fully characterized library of ready-to-click switchable compounds displaying large hyperpolarizability contrasts, and differing in the amplitude and orientation in their dipole moments is provided. This library expands the database of photoswitching compounds for further anchorage into polymer matrices or nano- or micro-sized solid substrates. [Display omitted] • A series of new clickable azobenzene derivatives have been synthesized. • Photoswitching rates and speed strongly depend on the chemical functionalization. • The substitution pattern largely impacts the magnitude of the second-harmonic signal. • Clicking a triazole with a long alkyl chain slightly enhances the NLO responses. [ABSTRACT FROM AUTHOR]

We report the first study dealing with the self-assembly of an α-amino acid derivative in perfluorocarbons. Rheology, microscopy, and spectroscopy studies reveal that the fluorous sodium L -prolinate derivative 1 self-assembles in perfluorocarbons to form a three-dimensional network of left-handed nano-helices resulting in solvent gelation. Singlet oxygen lifetime measured in a gel of perfluorodecalin is about 1000 times longer than in pure water. [ABSTRACT FROM AUTHOR]

We synthesized and characterized a series of dyes built from a spirofluorene or truxene core. The quadrupolar spirofluorene system is the initial building unit for the design and preparation of more complex star‐shaped dyes consisting of a truxene core bearing three di‐ or triphenylamine moieties with or without a thiophene connector. Their photophysical, electrochemical, and electrochemiluminescence (ECL) properties were first investigated in solution. Structure/activity relationships were derived and rationalized by comparing the quadrupolar system and trigonal truxene‐core derivatives using computational studies. The photophysical and redox characteristics are drastically tuned by the introduction of a thiophene bridge and electron‐donor substituents at their terminal branches. These comparative studies show the essential role of the stability of both radical cations and anions to obtain efficient ECL dyes. The stabilization of the radicals is directly related to the charge delocalization due to the π‐conjugation by the thiophene bridge. The brightest ECL is achieved by annihilation and coreactant (benzoyl peroxide) pathways with the blue‐emitting truxene dye, which is 2‐ and 4.5‐times greater than that of the quadrupolar compound and reference [Ru(bpy)3]2+ emitter, respectively. Such an extensive study on these extended π‐conjugated molecules presenting different core structures may guide the design and synthesis of new ECL dyes with a strong efficiency. [ABSTRACT FROM AUTHOR]

We describe the synthesis of a series of covalently linked dimers of quadrupolar curcuminoid‐BF2 dyes and the detailed investigation of their solvent‐dependent spectroscopic and photophysical properties. In solvents of low polarity, intramolecular folding induces the formation of aggregated chromophores, the UV/Vis absorption spectra of which display the optical signature characteristic of weakly‐coupled H‐aggregates. The extent of folding and, in turn, of ground‐state aggregation is strongly dependent on the nature of the flexible linker. Steady‐state and time‐resolved fluorescence emission spectroscopies show that the Frenkel exciton relaxes into a fluorescent symmetrical excimer state with a long lifetime. Furthermore, our in‐depth studies show that a weakly emitting excimer lies on the pathway toward a photocyclomer. Two‐dimensional 1H NMR spectroscopy and density functional theory (DFT) allowed the structure of the photoproduct to be established. To our knowledge, this represents the first example of a [2π+2π] photodimerization of the curcuminoid chromophore. [ABSTRACT FROM AUTHOR]

The linear and nonlinear optical properties of the 3-methyl-4-nitropyridine-1-oxyde (POM) crystal have been evaluated using semiempirical quantum chemistry techniques. The scheme includes (i) the evaluation of the polarizability and first hyperpolarizability of increasingly large one-dimensional, two-dimensional, and three-dimensional clusters of POM, (ii) the use of the time-dependent Hartree–Fock approach to determine the static and dynamic responses in combination with semiempirical Austin model 1 Hamiltonian, (iii) the assessment, for the POM monomer and dimer, of the electron correlation effects using second-order Møller–Plesset perturbation theory with several basis sets containing polarization and diffuse functions, (iv) the assessment of the validity of the multiplicative scheme and its application to get effective polarizability and first hyperpolarizability of the POM unit cell in the crystal, (v) the use of a sum-over-states approach to attribute the first hyperpolarizability to a dominant charge-transfer excited state, and (vi) comparison with experimental data as well as with calculated values obtained using the oriented gas approximation. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract The synthesis and the characterization of the linear and second-order nonlinear optical properties are reported for a new derivative of the indolino-oxazolidine family, which exploits the strong donor capacity of the ferrocenyl unit. Hyper-Rayleigh scattering experiments show that the pH-triggered switching between the closed and protonated open form of this compound enhances the first hyperpolarizability by roughly a factor 5, as a result of the increase of the push-pull π -electron conjugation within the molecule, between the donor ferrocenyl unit and the acceptor indoleninium moiety. Although derivatives exhibiting larger β responses in their nonlinear optical active form and larger β P O F / β C F contrasts were previously reported within this class of materials, the figure of merit of this ferrocenyl-based compound is high enough for device application. Quantum chemical calculations of the linear and nonlinear optical properties are at least in qualitative agreement with the experimental trends. Calculations also show that β of the protonated open form originates from two low-lying excited states, of which the contributions partly cancel each other due to the opposite directions of the photo-excited charge transfer. Highlights • pH-triggered switching between the closed and protonated open form of an indolino-oxazolidine compound bearing a donor ferrocenyl unit enhances the first hyperpolarizability by roughly a factor 5. • This enhancement results from the increase of the electron conjugation within the molecule. • Quantum chemical (QC) calculations of the (non)linear optical properties are at least in qualitative agreement with experiment. • QC calculations show that β of the protonated open form originates from two excited states, of which the contributions partly cancel each other due to the opposite directions of the photo-excited charge transfer. [ABSTRACT FROM AUTHOR]

The electronic and optical properties of a large series of symmetrical D– π –A– π –D chromophores are investigated. Vertical transition energies and related wavelengths, as well as absorption strengths, are computed by means of density functional theory and analyzed in the light of structural and electronic parameters such as torsional angles, bond length alternation, ground-state charge transfer and photo-induced charge displacement. This computational design allows establishing structure-property relationships linking the chemical structure of the chromophores to their absorption capability in the near-infrared (NIR) region, providing comparative guidelines for eventual syntheses. [ABSTRACT FROM AUTHOR]

TDDFT calculations are associated to an extrapolation scheme to predict the optical properties of twelve donor-acceptor copolymers, in which carbazole, benzodithiophene or cyclopentadithiophene donor subunits are combined to benzofurazane, benzothiadiazole or diketopyrrolopyrrole acceptor moieties. Relative excitation energies and absorption strengths are rationalized in terms of geometrical, electronic and optical descriptors. It is shown that the combination of a cyclopentadithiophene with a diketopyrrolopyrrole leads to the lowest excitation energy and highest light absorption efficiency within the series. However, combining cyclopentadithiophene and benzodithiophene subunits appears as more efficient for solar cell applications due to better energy level alignment with respect to PCBM. [ABSTRACT FROM AUTHOR]

Iodoperfluooralkylation of terminal alkenes and alkynes is effectively photo-promoted by benzophenone 2 (BP) or the photoreducible copper(II) complex 1. In particular, BP at 1 mol% in methanol upon 365 nm irradiation with a low-pressure mercury lamp (type TLC=thin layer chromatography, 6 W) results in a fast reaction with excellent reaction yields. Complex 1 and BP 2 exhibited very similar reactivity, suggesting that the reactions involving 1 are likely to be governed by the benzophenone photoactivation processes, rather than copper(I)/(II) redox processes. Mechanistic investigations using transient absorption spectroscopy revealed that a deactivation pathway of the benzophenone triplet (3BP*) is via its reaction with the methanol solvent. We propose that the generated radicals, in particular .CH2OH, play a key role in the initiation step forming Rf. by reacting with RfI, Rf. then entering a radical chain cycle. 1H NMR studies provided evidence that a substantial amount (∼7% NMR yield) of the hemiacetal CH3OCH2OH is formed, i.e., the possible by-product of the reaction between .CH2OH and RfI. Finally, DFT calculations indicate that a triplet-triplet energy transfer (TTET) process from 3BP* to perfluorooctyl iodide (C8F17I) is unlikely or should be rather slow under the reaction conditions, consistent with the transient absorption studies. [ABSTRACT FROM AUTHOR]

We describe the synthesis and spectroscopic study of the bichromophoric molecule DA-meso consisting of two boron difluoride curcuminoid (BF 2 -curcuminoid) units tethered by the rigid diacetylenic bridge. This structural feature imparts the quadrupolar-like DA-meso with a strong ability to self-assemble in solution, which induces a profound change in the optical properties. The UV–vis absorption spectrum of the aggregate is characterized by the appearance of a redshifted band at low energy and its fluorescence emission occurs in the near infrared (λ max = 700 nm) with a low quantum yield (5%). This is in contradiction with the strong emission expected for J-aggregates considering the Frenkel exciton model. From concentration-dependent UV–vis absorption spectroscopy we demonstrate the formation of a dimer composed of four BF 2 -curcuminoid units. Single-crystal X-ray diffraction structure analysis and in-depth quantum chemical calculations based on density functional theory (DFT) and subsequent decomposition of the electronic excited states on a diabatic basis enable to discuss the geometry of the DA-meso dimer, and to establish the presence of excited states close in energy to that of the unperturbed monochromophore, which are not predicted by the classical Frenkel exciton model. Different dimer structures characterized by π-stacks of two and up to four chromophores are investigated, in which low-lying excited states with vanishing oscillator strength are found to be redshifted with respect to the lowest-energy allowed state of the aggregate. From these experimental and theoretical data, the spectral features of the DA-meso dimer are rationalized by considering the formation of a "non-fluorescent J-aggregate", illustrating the trend in non-Kasha behavior observed for quadrupolar-like dyes. • Two boron difluoride curcuminoid dye units are linked by a diacetylenic bridge. • The bichromophoric system self-assembles into a dimer in solution. • Absorption and fluorescence spectra are red shifted. • Quantum chemical calculations give energies of adiabatic excited states of aggregates. • A non-fluorescent J-aggregate results from the interaction between quadrupolar dyes. [ABSTRACT FROM AUTHOR]

In the design of non-linear optical chromophores, the conjugation pathways in multi-branched dyes are known to influence heavily their optical properties. Herein, we investigate this strategy for the design of two-photon (2P) responsive photolabile protecting groups (PPG) by assembling via a triphenylamine core extended coumarinylmethyl derivatives. The experimental study reveals an enhancement of the 2P absorption in the tri-branched compound, but a strikingly different photophysics behaviour resulting in a decrease in bond cleavage efficiency, and aggregation in aqueous acetonitrile. These combined effects results in much poorer 2P uncaging efficiency of the three-branched derivatives. In contrast, the corresponding mono-branched coumarin exhibit very high 2P photochemical efficiency (450 GM). [Display omitted] • Efficient synthesis of trimeric coumarinylmethyl cages via covalent assembling on a triphenylamine core. • Enhancement and broadening of the two-photon absorption response of the trimer with respect to its monomeric analogue. • The trimeric compound release three equivalents of caged molecules, yet with reduced overall uncaging efficiency. [ABSTRACT FROM AUTHOR]

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 (βHRS) 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 βHRS and βHRS 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 βHRS 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 β tensor of the two entities might reinforce or cancel each other. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Merocyanine dyes that exhibit antithetic cyanine-like behaviour and giant first-order hyperpolarisability ( β) values have been designed. These cyanine-type dyes open up an intriguing route towards molecular-based electro-optic materials as well as new second-harmonic generation dyes for imaging. [ABSTRACT FROM AUTHOR]