T. López Arbeloa

Correlations between photophysics and lasing properties of dipyrromethene–BF2 dyes in solution

Abstract UV–Vis absorption, fluorescence (steady-state and time-correlated) and lasing properties of three pyrromethene–BF 2 dyes (PM546, PM556 and PM567) have been determined in several solvents. Good correlations between photophysical properties and lasing characteristics of these dyes have been obtained not only for a given dye in several solvents but also for the three dyes in a common solvent. Both the observed spectral changes and the deactivation mechanism affecting the lasing gain are explained on the basis of the possible resonance structures of the chromophore.

On the mechanism of radiationless deactivation of rhodamines

Abstract Absorption and emission (fluorescence spectra and radiative decay curves) spectroscopies are used to determine the photophysical properties of rhodamines with non-alkylated amino groups (rhodamine 110 and 123) in water and in ethanol. The results are compared to those previously reported for rhodamines with alkylamino groups. Different mechanisms for internal conversion such as the amino group rotation hindered by solvent viscosity, amino NH vibrations, the formation of a twisted intramolecular charge transfer state (TICT) and the open-close umbrella-like motion model (ULM) are considered. Both the TICT and the ULM models can explain the experimental results if an intramolecular xanthene-COOR interaction and specific rhodamine-solvent interactions are taken into account.

Photophysical properties of rhodamines with monoethylamino groups R19 and R6G in water-ethanol mixtures

Abstract Absorption and emission (fluorescence spectra and radiative decay curves) spectroscopy techniques are applied to study the photophysical properties of rhodamine 6G and of the cationic and the zwitterion molecular forms of rhodamine 19 in different water—ethanol mixtures. The present results show the influence of the specific solute-solvent interactions on these properties, mainly on the rate constant of the non-radiative deactivation of rhodamines. The internal conversion mechanism of rhodamines with monoethylamino groups is discussed.

Spectroscopic study of the adsorption of rhodamine 6G on clay minerals in aqueous suspensions

Abstract Absorption and fluorescence spectroscopies were applied to study the adsorption of rhodamine 6G on several smectite-type clays in aqueous suspension. The dye can be adsorbed as the monomeric and the dimeric forms on both the external and the interlamellar surfaces of the clay. The presence of these species and their evolution with the stirring time of the sample and the relative dye/ clay concentration depend on the nature of the smectite, the clay concentration and the particle size, factors that also affect the dispersion degree of the clay particles in water.

Influence of fluorinated group on the photophysics of 7-aminocoumarins

Abstract The photophysical properties of fluorinated 7-aminocoumarin derivatives with different alkylation degree at the amino group are studied in water/ethanol mixtures. The effect of the molecular structure of the chromophore and solute-protic solvent interactions are considered to explain the observed spectral shifts and the internal conversion process. The radiationless deactivation process is analyzed on the basis of the formation of a twisted intramolecular charge transfer state and a planar-to-pyramidal structural change of the amino group.

Electronic spectroscopy of pyrromethene 546

Abstract UV–Vis absorption and fluorescence (steady state and time-correlated) spectroscopies are applied to study the photophysics of pyrromethene 546 in a multitude of apolar, polar, and protic solvents, including water/ethanol mixtures. The solvent effect on the absorption and fluorescence bands is analysed in terms of the stabilization of several resonance structures of the π-system of the chromophore by general and specific H-bond solute/solvent interactions. Highest and lowest values for the rate constant of radiative and radiationless deactivation of PM546 are observed in high polar and H-bond donor solvents, respectively, and they are recommended as the best media to obtain the highest laser efficiency of PM546 in liquid solutions.

Binary solvent effects on the absorption and emission of 7-aminocoumarins

Abstract The photophysical properties of di-, mono- and non-ethyl 7-aminocoumarins have been investigated in different water/ethanol mixtures. The effect of the solvent composition on the spectral shifts and on the radiationless deactivation process depends on the molecular structure of the dye. The solvent effect is analyzed in terms of hydrogen bondings between the dye and protic solvents. The mechanism for internal conversion is discussed by two models: via a twisted intramolecular charge transfer (TICT) state, and by an hybridational change of the amino group (open-close umbrella-like motion, ULM model). Both models can explain the experimental results if solute-solvent hydrogen bonds are considered.

Luminescence properties of rhodamines in water/ethanol mixtures

Abstract The absorption and the emission (fluorescence spectra and radiative decay curves) characteristics of rhodamine R3B and R6G and of the molecular forms of RB and R19 are determined in different water/ethanol mixtures. The influences of molecular structure (alkylation of the amino groups and photonation or esterification of the carboxyphenyl group) and the nature of the solvent (specific solute-solvent interactions) on these properties are investigated. The internal conversion mechanism of rhodamines is discussed.

Photophysical and lasing properties of a new ester derivative of rhodamine 6G

Abstract Absorption and fluorescence spectra, fluorescence decay curves and laser emission characteristics of a newly synthesized dye belonging to the rhodamine family, dissolved in several aliphatic monoalcohols and in water/ethanol mixtures, have been obtained. The new dye incorporates the chromophoric group of rhodamine 6G, but with the ethyl substituent in the ester group of this molecule replaced by the p-ethoxycarbonylbenzyl group, and can be considered as a molecular model for a previously described rhodamine dye with a polymerizable double bond that has demonstrated to lase efficiently with good photostability in solid polymeric matrices. The results obtained indicate that the modifications in the dye molecule scarcely affect the photophysics of the rhodamine chromophore. The new rhodamine dye has lasing properties that correlate well with its photophysical characteristics. Laser emission in water solution with efficiencies as high as 10.5% has been obtained.

Environmental effects on the photophysics of pyrromethene 556

Visible absorption and fluorescence (steady-state and time-correlated) spectroscopies were applied to study the photophysics of the dye pyrromethene 556 in liquid solutions. Environmental factors, such as dye concentration, pH and nature of the solvent, were considered. The shape of the absorption spectrum does not change with the dye concentration, suggesting that the dye does not aggregate up to the solubility limit. The changes in the fluorescence signal by increasing the dye concentration (shift to lower energies in the fluorescence band and decrease/increase in the fluorescence quantum yield/lifetime) are attributed to re-absorption and re-emission effects. The solvent effects are analysed in terms of multicomponent linear regressions in which several solvent parameters (dipolarity/polarizability, H-bond donor, electron donor capacities, etc.) are individually and simultaneously treated. The influence of the solvent on the maximum of the absorption and fluorescence bands and on the rate constant of the radiative deactivation is explained on the basis of the stabilization of several resonance structures of the chromophore by general solvent effects and by specific solute–solvent interactions.