Photochemical behaviour of Schiff base liquid crystals based on isoxazole and isoxazoline ring. A kinetic approach

<p>The kinetic study on the hydrolysis of Schiff bases (SBs) <b>1a-c</b> and <b>2a-c</b> induced by UV-vis was undertaken as a complementary study of the stability in solution and in bulk of the SBs. Solutions in chloroform were exposed to UV-vis and acquisition data occurred for <b>1a-c</b> and for <b>2a-c</b> in 21°C, 30 °C, 35°C and 40°C. Kinetic profile for <b>1a-c</b> and <b>2a-c</b> displayed the similar photochemical behaviour in that four temperature values. At 21°C, two kinetic regimes were observed where the decomposition of SBs is faster at the initial stage, with no linear plot of absorbance vs time, and after the kinetic profile obeyed a linear behaviour. A mathematical treatment of the experimental data was applied, which allowed associating the initial stage data with a second-order reaction, and the final stage of the hydrolysis with a first-order reaction. The mechanism of photochemical hydrolysis of SBs <b>1a</b> and <b>2a</b> was addressed. It was composed of three parts, the excitation, then the isomerisation and activation processes of tetrahedral intermediate and, the last process, a collapse of the intermediate to the yielded products of the hydrolysis aldehydes and amines which were detected by their UV-vis spectrum.</p

Liquid-crystalline coumarin derivatives: contribution to the tailoring of metal-free sensitizers for solar cells

<p>Coumarins have been used in a wide range of applications, such as dye-sensitised solar cells, laser dyes and optical sensors. In order to further explore the properties of these materials, three new coumarin derivatives were obtained with different terminal arylalkyne linkages to the 6-position of the coumarin core. The synthesised materials were characterised by NMR, absorption and emission spectroscopy, and the liquid crystal properties were investigated through differential scanning calorimetry and polarised optical microscopy. In addition, dye-sensitised solar cells were assembled to evaluate the photoelectrochemical properties of the materials. Only the coumarin with a naphthyl group exhibited stable smectic A and nematic mesophases. All the coumarins were photoemissive in the range 420–461 nm. The adsorption of these dyes on TiO₂ was observed by UV–vis spectroscopy; in addition, by incident photon-to-electron conversion efficiency and <i>I</i>–<i>V</i> curves, photocurrent generation was observed.</p

Quinolines by Three-Component Reaction: Synthesis and Photophysical Studies

<div><p>The synthesis of five quinolines 8-octyloxy-4-[4-(octyloxy)phenyl]quinoline and 6-alkoxy- 2-(4-alkoxyphenyl)-4-[(4-octyloxy)aryl]quinolines are described by three-component coupling reaction mediated by Lewis acid FeCl3 and Yb(OTf)3. 4-n-octyloxybenzaldehyde, anisaldehyde, 4-n-octyloxyaniline p-anisidine, and 1-ethynyl-4-heptyloxybenzene, 1-ethynyl-4-octyloxybenzene and 2-ethynyl-6-heptyloxynaphthalene are the reagents in this protocol. A Yb3+ catalyst resulted in higher yields of quinolines than Fe3+. Polarizing optical microscopy (POM) revealed that none of the quinolines were liquid crystals, even the more anisotropic. UV-Vis measurements of one of the quinolines in polar solvent show two absorption bands at 280 and 350 nm related to π,π* and n,π* transitions. No changes were observed to lower-energy absorption band (ε < 104 mol L-1 cm-1) related to n,π* transition. A laser flash photolysis study for one of the quinolines relates a main transient band at 450 nm with a lifetime of 2.6 µs in ethanol, which is completely quenched in the presence of oxygen. This transient band was assigned to triplet-triplet absorption of one of the quinolines, which is semi-oxidised in the presence of phenol. Radiative rate constants have been determined along singlet and triplet excited state energies (3.39 and 3.10 eV, respectively). The chemical structure of one of the quinolines was also unequivocally confirmed by single-crystal X-ray analysis.</p></div