I. López Arbeloa

Dimeric states of rhodamine B

Abstract Aggregation of the coloured neutral and cationic forms of rhodamine β in aqueous solution has been studied. The formation constants and absorption of the dimers were determined. The geometric disposition of monomers in both aggregates and the nature of the association forces are discussed. Trimers are not formed.

Flourescence self-quenching of the molecular forms of Rhodamine B in aqueous and ethanolic solutions

Abstract The fluorescence quenching produced by the aggregates of the zwitter-ion and the cationic molecular forms of Rhodamine B in water and in ethanol is evaluated in this paper. Experimental results indicate that the quenching is higher in ethanol than in water. In ethanolic systems the trimer produces a higher fluorescence quenching than the dimer. The participation of different quenching processes is discussed on the basis of these results.

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.

Structural, photophysical and lasing properties of pyrromethene dyes

The molecular structure and the photophysical properties of pyrromethene-BF2 (PM) dyes are studied with the aim of finding the best structural and environmental conditions which optimize the laser performance of these dyes. To this end, UV–Vis absorption and fluorescence spectra and fluorescence decay curves of several PM dyes are registered in a multitude of solvents with different physicochemical properties and in polymeric solid matrices. Quantum mechanical calculations at different levels are also applied in order to explain the photophysical behaviour of these dyes. The studied pyrromethenes incorporate alkyl (methyl, ethyl and tert-butyl), sulfonate, cyano, and acetoxy- and methacryloyloxy-polymethylene groups in different positions of the chromophore. In the last case, the presence of the polymerizable acryloyl group facilitates the covalent linkage of the chromophore to a polymeric chain, of special technological interest in the development of tunable dye lasers in the solid state. From the experimental results and the theoretical calculations, we discuss different mechanisms of internal conversion for PM dyes, such as the loss in the planarity of the chromophore, the electron flow through the delocalized π-system, the vibrational coupling and the formation of an intramolecular charge transfer state. The present work demonstrates the good correlation between the photophysical and the lasing properties of PM dyes with different structural (substituents) and environmental (solvents and polymers) conditions. Contents PAGE 1. Introduction 340 2. Methodology 342 2.1. Dyes 342 2.2. Instrumental 344 2.3. Computational 345 3. Results and discussion 346 3.1. Preliminary considerations. Molecular structure 346 3.2. General photophysics 348 3.3. 2,6-Disubstituted-pyrromethenes 351 3.3.1. Solvent e.ect 355 3.3.2. Photophysics-lasing correlations 357 3.3.3. PM567 in polymeric matrices: cross-linking e.ect 357 3.4. Acetoxy-pyrromethenes 360 3.4.1. Linear 8-acetoxy analogues 361 3.4.2. 8-phenyl analogues 363 3.4.3. Solid polymeric matrices 365 3.5. 8-Cyano group: intramolecular charge transfer (ICT) state 367 4. Conclusions 371 Acknowledgements 371 References 372

Molecular forms of rhodamine B

Abstract Three molecular forms of rhodamine B (neutral, cationic and cationic-lactonic) in aqueous solutions are reported. The pka of the equilibria between the neutral and cationic forms as well as cationic and cationic-lactonic ones are evaluated. The dimerization constants of neutral and cationic forms (coloured) are also determined.

Solvent effect on photophysics of the molecular forms of rhodamine B. Solvation models and spectroscopic parameters

Abstract Photophysics and spectral characteristics of the zwitterionic and cationic forms of rhodamine B have been studied in a number of protic and aprotic solvents with different solvation properties. The results have been discussed on the basis of the molecular structures of the dye, role of carboxyphenyl group and specific solute-solvent interactions.

Solvent effects on the photophysics of the molecular forms of rhodamine B. Internal conversion mechanism

Abstract The fluorescence characteristics, φ f and τ f , for two molecular forms of rhodamine B, RB ± and RBH + , have been measured in a number of solvents selected for their hydrogen donor and/or acceptor properties, dielectric constant, viscosity, etc. The activation energies E a calculated from the temperature dependence of φ f are also solvent dependent. The results do not agree with the internal conversion mechanism suggested by Drexhage based on rotational motion of the diethylamino group. No single mechanism seems to be valid for both molecular forms. Two likely mechanisms are suggested.

Influence of the molecular structure and the nature of the solvent on the absorption and fluorescence characteristics of rhodamines

Abstract A study of the equilibrium of the molecular forms of rhodamine 19 in aqueous and ethanolic solution is carried out by determining the absorption and fluorescence characteristics of the zwitterionic and the cationic forms of the dye. The optical properties of rhodamine 19 are compared with those obtained for rhodamine 6G and also with those previously reported for rhodamine 3B and for the molecular forms of rhodamine B in water and ethanol. Different aspects of the molecular structure of the rhodamines and solvent effects are discussed, as well as their influence on the photophysical properties of the rhodamines. The aggregation of the molecular forms of rhodamine 19 is also studied in water and ethanol.

On the aggregation of rhodamine B in ethanol

Abstract The dimer and the trimer aggregation constants of the cationic form of rhodamine B are determined in very concentrated ethanolic solutions of the dye. The absorption spectrum, the geometrical structure and the association bonding of the aggregates are discussed and compared with those previously obtained in aqueous solution.

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.