András Grofcsik

Quantum chemical studies on excited state intermolecular proton transfer of oxazine dyes

Abstract The excited state lifetimes of oxazine dyes nile blue and oxazine 720 show a significant solvent dependence. As an aid to the interpretation of the decay in acidic environments the ground electronic states and the first singlet excited states of oxazine monocations and dications have been studied by methods of quantum chemistry. Ab initio calculations carried out on the lower members on this series show a significant increase of the electron densities of the ring nitrogen on excitation. The calculated electron energies also suggest that the second proton attaches to the ring nitrogen in the excited states, in contrast to the ground states where (in the case of oxazine 720) the second proton attaches to the terminal nitrogen. A kinetic scheme is proposed for the decay process in which in acidic environments the main deactivation path is the reaction of excited state monocations with protons, followed by internal conversion and subsequent deprotonation of the ground state dications.

Synthesis of chiral 1,3-calix[4](crown-6) ethers as potential mediators for asymmetric recognition processes

Abstract Novel chromogenic 1,3-calix[4](crown-6) derivatives comprised of 1,1′-binaphthyl-, methyl-α- d -glucoside-and d -mannitol moieties in the crown ether ring have been synthesized. UV–vis spectroscopic measurements of the 2,4-dinitrophenylazo chromogenic molecules indicated noticeable chiral discrimination associated with coloration towards primary amine enantiomers.

Rotational reorientation dynamics of nile blue A and oxazine 720 in protic solvents

Abstract The temperature dependence of the orientational relaxation times of the cationic oxazine dyes nile blue A and oxazine 720 have been determined in water, methanol, ethanol and ethyleneglycol, by measuring the picosecond decays of the polarization dichroism of transient absorption signals. It has been shown that the observed orientational relaxation times are attributed to the ground state solutes. The equilibrium geometries of the two ground state cations, their dipole moments and their atomic charge distributions have been computed in accurate density functional calculations. The hydrodynamic and dielectric contributions to the orientational relaxation times have been estimated on the bases of generalized Stokes–Einstein–Debye theory and of point charge dipole approach, using the molecular parameters obtained in the quantum chemical calculations. The results indicate that the solvent shells around the solutes significantly enhance the orientational relaxation times. The temperature dependence of this effect is negligible in water and methanol, small, but observable in ethanol, and strong in ethyleneglycol.

Triplet state spectroscopic studies on some 5,10,15,20-tetrakis(methoxyphenyl)porphyrins

Abstract Extensive triplet state spectroscopic investigations were carried out with a series of 5,10,15,20-tetrakis(methoxyphenyl)porphyrins. Triplet absorption spectra, triplet lifetime, triplet quantum yield and quantum yield for singlet oxygen production were determined with different absorption and emission techniques, using the frequency-doubled beam of a Nd:YAG laser. It has been found that these synthetic porphyrins are effective photosensitizers which can be used as model compounds to investigate the theoretical and instrumental aspects of PDT.

Intermolecular photoinduced proton transfer in nile blue and oxazine 720

Abstract The S1 excited state lifetimes of laser dyes nile blue (NB) and oxazine 720 (OX) have been determined in acidic aqueous solutions as functions of pH. The decay constant show a linear dependence on [H+], with an identical slope for both molecules. The reason for the faster decay in acidic solutions results from the formation of dications by reaction of excited state monocations with hydrogen ions. These results suggest that in the excited state reaction the additional proton locates on the ring nitrogen atom rather than on the terminal amino groups.

Photochromism of a spiropyran derivative of 1,3-calix[4]crown-5

A new spiropyran derivative containing a calixcrown ring has been synthesised. Thermal conversion from merocyanine to the spiropyran form was studied in ethanol and in acetonitrile, and equilibrium and kinetic data were compared with those of the parent 6-nitrospiropyran. The kinetics of thermal decay of the merocyanine form of the calixcrown-spiropyran molecule shows a biexponential character in both solvents, while the decay is much slower in ethanol than in acetonitrile.

Fluorescence decay dynamics of organic dye molecules in solution

Abstract A picosecond laser system consisting of two tuneable dye lasers pumped synchronously by a mode locked argon-ion laser has been used for studying the relaxation of the S 1 excited states of dye molecules Nile Blue and Oxazine 720. The dependence of the excited state lifetimes on the solvent and on the temperature are discussed. Both intramolecular and intermolecular decay channels contribute to the radiationaless de-excitation. The mechanism of intramolecular decay is different, while that of intermolecular decay is very similar for the two molecules.

Spectroscopic study on the complex formation of chromogenic bridged calixarenes with aliphatic amines

Abstract The complex formation between chromogenic capped calix 4 arene derivatives comprising indophenol indicator group(s), and aliphatic amines has been studied by UV/Vis spectroscopy. The equilibrium constants have been determined in ethanol, and—for one ligand—also in dimethylsulfoxide. The results have been interpreted in terms of various types of host–guest interactions and of steric effects.

Stimulated emission cross-sections in fluorescent dye solutions: gain spectra and excited-state lifetimes of Nile blue A and oxazine 720

Gain spectra of two laser dyes, Nile blue A and oxazine 720, have been recorded with the pump–probe spectroscopic method using a modulated-pump–unmodulated-probe method of high sensitivity. By locating the pump laser within the region of sample absorption with the probe set to shorter wavelengths, where fluorescence is negligible, it is shown how it is possible to determine the absorption cross-section free of the influence of stimulated emission. By contrast, by locating the pump laser away from the region of fluorescence emission but locating the probe laser in those regions where absorption and fluorescence can both occur, it is shown that the gain signal contains contributions from both of these processes. By combining the two sets of measurements, it is shown how both absorption and stimulated emission cross-sections can be separately determined. Measurement of the integrated areas shows that within the experimental error the areas under the spectral profiles of the absorption and stimulated emission cross-sections are identical.Temporal scans of the gain have been performed for both samples in order to determine excited-state lifetimes and orientational relaxation times. These lifetimes have been shown to vary significantly in the different solvents investigated and explanations for this behaviour have been proposed.

NONLINEAR RAMAN SPECTROSCOPY OF LIQUID CRYSTALS : ORIENTATIONAL ALIGNMENT AND SWITCHING BEHAVIOUR IN A FERROELECTRIC LIQUID CRYSTAL MIXTURE

Picosecond inverse Raman spectroscopy has been employed to probe the alignment behaviour and switching characteristics of a 6 μm thick ferroelectric liquid crystal based on a host mixture of fluorinated phenyl biphenyl-carboxylates and a chiral dopant. Optical bistability is observed in the Raman signal on application of dc electric fields of opposite polarity. For particular polarities of the applied field, the Raman signals display a cos4 θ dependence on the angle of rotation around the beam direction. Reorientational rate constants of 300 μs and 590 μs are observed for the aromatic core at the high-voltage limit for the rise and decay of the 1600 cm−1 Raman signal on application of a switching ac electric field.