Lorenzo Echevarria

Two-photon absorption and two-photon circular dichroism of hexahelicene derivatives: a study of the effect of the nature of intramolecular charge transfer

Herein we report on the theoretical–experimental analysis of the one- and two-photon absorption and circular dichroism spectra of two intrinsically chiral aromatic molecules – hexahelicene derivatives – with helical chirality and intramolecular charge transfer (ICT). The primary outcomes of our investigation demonstrate that the TPA cross-section and the amplitude of the TPCD signal of this type of helicenes are strongly affected by the strength of the ICT and the nature of the extension of the electronic delocalization, i.e. beyond (EXO-ICT) or within (ENDO-ICT) the helicene core. These results were corroborated through the comparative theoretical analysis of the corresponding contributions of the magnetic dipole transition moment and the electric quadrupole transition moment to the TPA rotatory strength on a series of five similar helicene derivatives with different molecular electron delocalization disposition. Two-photon absorption (TPA) and two-photon circular dichroism (TPCD) spectra were obtained using the double L-scan technique over a broad spectral range (400–900 nm) using 90 fs pulses at a low repetition rate (2–50 Hz) produced by an amplified femtosecond system. The theoretical simulations were performed using modern analytical response theory within the Time-Dependent Density Functional Theory (TD-DFT) approach using B3LYP and CAM-B3LYP, and the aug-cc-pVDZ and 6-311++G(d,p) basis sets.

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).

Thermo-optical properties of gold nanoparticles in colloidal systems

In this work, we report the thermo-optical properties of nanoparticles in colloidal suspensions. Spherical gold nanoparticles obtained by laser ablation in condensed media were characterized using thermal lens spectroscopy pumping at 532 nm with a 10 ns pulse laser-Nd-YAG system. The obtained nanoparticles were stabilized in the time by surfactants (sodium dodecyl sulfate or SDS) in water with different molar concentrations. The morphology and size of the gold nanoparticles were determined by transmission electron microscopy (TEM) and UV–visible techniques. The plasmonic resonance bands in gold nanoparticles are responsible for the light optical absorption, and the positions of the absorption maximum and bandwidth in the UV–visible spectra are given by the morphological characteristics of these systems. The thermo-optical constants such as thermal diffusion, thermal diffusivity, and (dn/dT) are functions of the nanoparticle sizes and the dielectric function of the media. For these reasons, the thermal lens (TL) signal is also dependent on nanoparticle sizes. An analysis of the TL signal of the nanoparticles reveals the existence of an inverse dependence between the thermo-optical functions and the size. This methodology can be used in order to evaluate these systems and characterize nanoparticles in different media. These results are expected to have an impact in bioimaging, biosensors, and other technological applications such as cooling systems.

A fluorescent benzothiazole probe with efficient two-photon absorption.

In this work, we report the two-photon absorption of 2-[4-(dimethylamino)phenyl]-1,3-benzothiazole-6-carbonitrile (DBC) in DMSO solution pumping at 779 nm with a 10 ns pulse laser-Nd:YAG system. The obtained two-photon absorption cross-section in DBC (407 ± 18 GM) is considerably high. Because DBC is a novel compound and have high values of fluorescence quantum yield, this result is expected to have an impact in biomolecules detection, diagnosis and treatment of cancer. Similar structures have previously been reported to show remarkable antitumour effects.

Evaluation of photosensitizers singlet oxygen production for photodynamic therapy applications using time resolved thermal lens

We report on the optical characterization and measurement of oxygen singlet quantum yield of Chlorophyll and Chlorin e6 in water-ethanol mixtures by direct observation of thermal relaxation using time resolved thermal lens method. The analysis of the time-resolved curve allows the determination of the quantum yield of singlet oxygen formation. The quantum yield is deduced from the relative magnitudes of the fast and slow components.

Stimulated Raman red-light generation by acetone as a perspective source for photodynamic therapy applications

We report on a new source of coherent red-light with perspective applications in laser photodynamic therapy. The red light is generated through stimulated Raman scattering (SRS) process by acetone of a frequency-doubled Nd:YAG nanosecond laser radiation (532 nm). The Stokes transition is found at 630 nm and with spectral line narrowing similar to the pump one. When pumping using the radiation from a Rhodamine-6G Dye laser (560 - 575 nm), tuning between 640 and 670 nm is demonstrated. Conversion efficiencies up to 50% are reported.

Assessment of the tautomeric population of benzimidazole derivatives in solution: a simple and versatile theoretical–experimental approach

Herein, we present a simple and versatile theoretical–experimental approach to assess the tautomeric distribution on 5(6)-aminobenzimidazole (5(6)-ABZ) derivatives in solution via one-photon absorption. The method is based on the optimized weighted sum of the theoretical spectra of the corresponding tautomers. In this article we show how the choice of exchange-correlation functional (XCF) employed in the calculations becomes crucial for the success of the approach. After the systematic analysis of XCFs with different amounts of exact-exchange we found a better performance for B3LYP and PBE0. The direct test of the proposed method on omeprazole, a well-known 5(6)-benzimidazole based pharmacotherapeutic, demonstrate its broader applicability. The proposed approach is expected to find direct applications on the tautomeric analysis of other molecular systems exhibiting similar tautomeric equilibria.Graphical abstractUsing a weighted sum of the corresponding individual tautomer theoretical spectra, the tautomeric population of benzimidazole derivatives in solution and at room temperature is directly determined through the theoretical–experimental fitting of the UV–Vis spectra of the tautomeric mixture at equilibrium. The reliability of the proposed method is based on the existent spectral difference between the two species.

Development of a Thermal Lens Microscope for Asphaltene Analysis

Thermal lens microscopy is a very sensitive photothermal technique with many applications in environment, organic chemistry, biochemistry and biomedicine. This paper shows the development of a thermal lens microscope for asphaltene analysis. Two coaxial laser beams were pumped through the microscope objective to the sample. The wavelength of the excitation beam was selected to coincide with an absorption band of the analyte, and the wavelength of the probe beam was chosen so that the sample solution has no absorption. When the analyte molecules absorb the excitation beam, they emitted heat increasing the temperature in the confocal region that decreases the refractive index generating a lens-like object. This instrument was used to measure the response of thermal lens in different Venezuelan asphaltene solutions, finding variations in the signal with the concentration of asphaltene and the degree of aggregation or flocculation of the sample in the solvent.

Photophysical study of the Dibenzylideneacetones and 3-Benzylidenethiochroman-4-ones

In this work, a study of the optical properties of Dibenzylideneacetones a-c and 3-Benzylidenethiochroman-4-ones {d-e} is presented. Optical measurements and theoretical approaches are used to establish the contribution of electron donor-acceptor groups in the apparition of fluorescence emission.

Concentration dependence of surface nonlinear susceptibility of vanadyl-porphyrin at silica surfaces

Resonant molecular optical second harmonic generation (SHG) has been obtained from [2, 3, 7, 8, 12, 13, 17, 18- Octaethyl-porphinato] vanadyl, (OEP)V=O, adsorbed on the hypotenuse face of a glass prism. Rotational intensity patterns are showed by plotting the second harmonic intensity as a function of the incident polarization angle of the fundamental wave, at a fixed angle of incidence. Evaluation of these patterns by Fourier analysis allowed the determination of the second order susceptibilities components of the adsorbed layers. Assuming a delta distribution function, the mean orientation angles from normal to the surface plane, were obtained for different solution concentrations. Our results indicate an abrupt change in the angle for two concentration ranges.