Marcelo G. Vivas

Molecular Structure – Optical Property Relationships for a Series of Non-Centrosymmetric Two-photon Absorbing Push-Pull Triarylamine Molecules

This article reports on a comprehensive study of the two-photon absorption (2PA) properties of six novel push-pull octupolar triarylamine compounds as a function of the nature of the electron-withdrawing groups. These compounds present an octupolar structure consisting of a triarylamine core bearing two 3,3′-bis(trifluoromethyl)phenyl arms and a third group with varying electron-withdrawing strength (H < CN < CHO < NO2 < Cyet < Vin). The 2PA cross-sections, measured by using the femtosecond open-aperture Z-scan technique, showed significant enhancement from 45 up to 125 GM for the lowest energy band and from 95 up to 270 GM for the highest energy band. The results were elucidated based on the large changes in the transition and permanent dipole moments and in terms of (i) EWG strength, (ii) degree of donor-acceptor charge transfer and (iii) electronic coupling between the arms. The 2PA results were eventually supported and confronted with theoretical DFT calculations of the two-photon transition oscillator strengths.

Nonlinear spectra of ZnO: reverse saturable, two- and three-photon absorption

We present a broadband (460 - 980 nm) analysis of the nonlinear absorption processes in bulk ZnO, a large-bandgap material with potential blue-to-UV photonic device applications. Using an optical parametric amplifier we generated tunable 1-kHz repetition rate laser pulses and employed the Z-scan technique to investigate the nonlinear absorption spectrum of ZnO. For excitation wavelengths below 500 nm, we observed reverse saturable absorption due to one-photon excitation of the sample, agreeing with rate-equation modeling. Two- and three-photon absorption were observed from 540 to 980 nm. We also determined the spectral regions exhibiting mixture of nonlinear absorption mechanisms, which were confirmed by photoluminescence measurements.

Broadband three-photon absorption spectra of platinum acetylide complexes

We investigate the three-photon absorption spectra of four platinum acetylides complexes employing femtosecond pulses. We observed strong three-photon absorption cross-section in the near-infrared region (from 850 nm to 1200 nm). The three-photon absorption (3PA) spectra present resonance enhancement effect as two photons of the excitation wavelength approach the lower two-photon allowed states of the molecules as well as a 3PA allowed band around 1180 nm. The 3PA cross-section spectra were interpreted using the sum-over-essential-states approach, considering a three-energy-level diagram.

Two-photon absorption spectra of carotenoids compounds

Carotenoids are biosynthetic organic pigments that constitute an important class of one-dimensional π-conjugated organic molecules with enormous potential for application in biophotonic devices. In this context, we studied the degenerate two-photon absorption (2PA) cross-section spectra of two carotenoid compounds (β-carotene and β-apo-8′-carotenal) employing the conventional and white-light-continuum Z-scan techniques and quantum chemistry calculations. Because carotenoids coexist at room temperature as a mixture of isomers, the 2PA spectra reported here are due to samples containing a distribution of isomers, presenting distinct conjugation length and conformation. We show that these compounds present a defined structure on the 2PA spectra, that peaks at 650 nm with an absorption cross-section of approximately 5000 GM, for both compounds. In addition, we observed a 2PA band at 990 nm for β-apo-8′-carotenal, which was attributed to a overlapping of 11Bu+-like and 21Ag–-like states, which are strongly one...

Experimental and Theoretical Study on the One- and Two-Photon Absorption Properties of Novel Organic Molecules Based on Phenylacetylene and Azoaromatic Moieties

This Article reports a combined experimental and theoretical analysis on the one and two-photon absorption properties of a novel class of organic molecules with a π-conjugated backbone based on phenylacetylene (JCM874, FD43, and FD48) and azoaromatic (YB3p25) moieties. Linear optical properties show that the phenylacetylene-based compounds exhibit strong molar absorptivity in the UV and high fluorescence quantum yield with lifetimes of approximately 2.0 ns, while the azoaromatic-compound has a strong absorption in the visible region with very low fluorescence quantum yield. The two-photon absorption was investigated employing nonlinear optical techniques and quantum chemical calculations based on the response functions formalism within the density functional theory framework. The experimental data revealed well-defined 2PA spectra with reasonable cross-section values in the visible and IR. Along the nonlinear spectra we observed two 2PA allowed bands, as well as the resonance enhancement effect due to the presence of one intermediate one-photon allowed state. Quantum chemical calculations revealed that the 2PA allowed bands correspond to transitions to states that are also one-photon allowed, indicating the relaxation of the electric-dipole selection rules. Moreover, using the theoretical results, we were able to interpret the experimental trends of the 2PA spectra. Finally, using a few-energy-level diagram, within the sum-over-essential states approach, we observed strong qualitative and quantitative correlation between experimental and theoretical results.

Femtosecond laser induced synthesis of Au nanoparticles mediated by chitosan.

This paper reports the synthesis of Au nanoparticles by 30-fs pulses irradiation of a sample containing HAuCl4 and chitosan, a biopolymer used as reducing agent and stabilizer. We observed that it is a multi-photon induced process, with a threshold irradiance of 3.8 × 10(11) W/cm2 at 790 nm. By transmission electron microscopy we observed nanoparticles from 8 to 50 nm with distinct shapes. Infrared spectroscopy indicated that the reduction of gold and consequent production of nanoparticles is related to the fs-pulse induced oxidation of hydroxyl to carbonyl groups in chitosan.

Two-Photon Circular−Linear Dichroism of Perylene in Solution: A Theoretical−Experimental Study

Herein, we report on the theoretical-experimental analysis of the two-photon absorption (TPA) and two-photon circular-linear dichroism (TPCLD) spectra of a highly conjugated, rigid, and centrosymmetric molecule in solution, that is, perylene/CH2Cl2. We show how a three-energy-level diagram, under the sum-over-essential states approach, assists in the determination of the magnitude of transition electric dipole moments and the angle between them for the main TPA transitions. We demonstrate the potential of TPCLD to reveal the symmetry of excited states and the angles between their transition electric dipole moments and that of the ground state. By means of TPCLD, we explain how the overwhelming contribution of certain TPA transitions can mask important spectral features in regions where the transition electric dipole moments are perpendicular. TPCLD is expected to enhance the understanding of the photophysical properties of materials that are not accessible using conventional linear and two-photon spectroscopy. TPA and TPCLD measurements were performed employing the open-aperture Z-scan technique using an amplified femtosecond system. Time-dependent density functional theory (TD-DFT) calculations were carried out using response theory at the B3LYP level with the aug-cc-pVDZ basis set. Solvent effects were included through the polarizable continuum model (PCM).

Experimental and theoretical investigation of the first-order hyperpolarizability of a class of triarylamine derivatives.

This paper reports on the static and dynamic first-order hyperpolarizabilities of a class of push-pull octupolar triarylamine derivatives dissolved in toluene. We have combined hyper-Rayleigh scattering experiment and the coupled perturbed Hartree-Fock method implemented at the Density Functional Theory (DFT) level of theory to determine the static and dynamic (at 1064 nm) first-order hyperpolarizability (βHRS) of nine triarylamine derivatives with distinct electron-withdrawing groups. In four of these derivatives, an azoaromatic unit is inserted and a pronounceable increase of the first-order hyperpolarizability is reported. Based on the theoretical results, the dipolar/octupolar character of the derivatives is determined. By using a polarizable continuum model in combination with the DFT calculations, it was found that although solvated in an aprotic and low dielectric constant solvent, due to solvent-induced polarization and the frequency dispersion effect, the environment substantially affects the first-order hyperpolarizability of all derivatives investigated. This statement is supported due to the solvent effects to be essential for the better agreement between theoretical results and experimental data concerning the dynamic first-order hyperpolarizability of the derivatives. The first-order hyperpolarizability of the derivatives was also modeled using the two- and three-level models, where the relationship between static and dynamic first hyperpolarizabilities is given by a frequency dispersion model. Using this approach, it was verified that the dynamic first hyperpolarizability of the derivatives is satisfactorily reproduced by the two-level model and that, in the case of the derivatives with an azoaromatic unit, the use of a damped few-level model is essential for, considering also the molecular size of such derivatives, a good quantitative agreement between theoretical results and experimental data to be observed.

Revealing the Electronic and Molecular Structure of Randomly Oriented Molecules by Polarized Two-Photon Spectroscopy.

In this Letter, we explored the use of polarized two-photon absorption (2PA) spectroscopy, which brings additional information when compared to methods that do not use polarization control, to investigate the electronic and molecular structure of two chromophores (FD43 and FD48) based on phenylacetylene moieties. The results were analyzed using quantum chemical calculations of the two-photon transition strengths for circularly and linearly polarized light, provided by the response function formalism. On the basis of these data, it was possible to distinguish and identify the excited electronic states responsible for the lowest-energy 2PA-allowed band in both chromophores. By modeling the 2PA circular-linear dichroism, within the sum-over-essential states approach, we obtained the relative orientation between the dipole moments that are associated with the molecular structure of the chromophores in solution. This result allowed to correlate the V-shape structure of the FD48 chromophore and the quantum-interference-modulated 2PA strength.

Polarization effect on the two-photon absorption of a chiral compound.

In this report, we investigate the polarization effect (linear, elliptical and circular) on the two-photon absorption (2PA) properties of a chiral compound based in azoaromatic moieties using the femtosecond Z-scan technique with low repetition rate and low pulse energy. We observed a strong 2PA modulation between 800 nm and 960 nm as a function the polarization changes from linear through elliptical to circular. Such results were interpreted employing the sum-over-essential states approach, which allowed us to model the 2PA circular-linear dichroism effect and to identifier the overlapping of the excited electronic states responsible by the 2PA allowed band.