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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]

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]

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]

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]

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]