Fernando López Arbeloa

8-PropargylaminoBODIPY: unprecedented blue-emitting pyrromethene dye. Synthesis, photophysics and laser properties

Highly emitting 8-propargylaminoBODIPY (8-PAB) 2 was prepared in 94% yield. Unlike any other BODIPY structure hitherto described in the literature, 2 displays efficient emission in the blue region of the visible spectrum with a fluorescence quantum yield up to 0.94 and high laser efficiency (35%) at 483 nm.

Aggregate formation of rhodamine 6G in aqueous solution

The formation constant and absorption spectrum of the dimer of Rhodamine 6G have been evaluated: the trimer is not formed. The geometric structure of the aggregate is determined using exciton theory. The thermodynamics of the dimerization process permit the study of association forces, justifying the structural differences from those exhibited by Rhodamine B dimers.

Orientation and aggregation of cationic laser dyes in a fluoromica: polarized spectrometry studies

Abstract Polarized infrared (IR) and polarized ultraviolet–visible light (UV–VIS) absorption spectra were measured for cationic laser dyes, rhodamine 6G (R6G) and oxazine 4 (Ox4), incorporated in an oriented film of synthetic fluoromica (Li taeniolite, LiTN) to determine the orientation of dye molecules in the interlayer space. Dependence of absorbance on the incident angle of polarized light was observed for characteristic vibrational modes of R6G and Ox4 in the infrared region. It indicated qualitatively that dye molecules were tilted with the longest axis of the condensed ring nearly perpendicular to the host layer (vertical orientation). Polarized UV–VIS spectra showed the absorption of monomer, dimer and high-order aggregates of dyes in the interlayer space of TN. The monomer and dimer absorption bands of the dyes did not show any dependence on the polarized incident light. The dichroic ratio dependence on the incident angle of polarized light was observed for H-aggregates, from which the H-aggregates were calculated to be oriented at 71° for Ox4/TN and 73° for R6G/TN against the layer. The obtained molecular orientation in the interlayer space is consistent with the basal spacings derived from powder X-ray diffraction (PXRD) data—22.4 A for Ox4/TN and 23.7 A for R6G/TN. Other states of dyes showed only negligible dependence, implying that interlayer distance is determined by the high-order H-aggregate in the interlayer space. Such vertical orientation was ascribed to the high cation exchange capacity (CEC) of LiTN and the dye aggregation in the interlayer space.

Dimerization and trimerization of rhodamine 6G in aqueous solution. Effect on the fluorescence quantum yield

The trimerization equilibrium constant of rhodamine 6 G in aqueous solution as determined by absorption spectroscopy is higher than that of the dimer previously reported. Similarities observed between the absorption spectra of both aggregates are explained by exciton theory considering a sandwich structure for the aggregates. The lack of emission from the aggregates implies an important decrease in the fluorescence quantum yield of the dye, and the rate constant of the quenching produced by the dimer and trimer are calculated separately. The dominant mechanism for the quenching seems to be long-range energy transfer from the monomer to the aggregates, being affected by molecular diffusion as well as excitation energy migration between monomers.

Structural and spectroscopic characteristics of Pyrromethene 567 laser dye. A theoretical approach

Quantum mechanic calculations at the DFT (B3LYP) and semiempirical PM5 levels were performed to study the structural and electronic properties of Pyrromethene 567 laser dye. TD-DFT and semiempirical ZINDO and CISD methods were carried out to predict the photophysical characteristics of the dye. The effect of the solvent was evaluated by means of SCRF (PCM) and the semiempirical COSMO models in c-hexane, acetone and methanol, and the results are compared to the experimental data. Both algorithms indicate an increase of the charge separation through the chromophore π-system in the polar solvents. This result explains the increase in the dipole moment and the hypsochromic shift of the absorption and fluorescence spectral bands in polar solvents, which is also experimentally observed, inducing a diminution in the dipole transition moment.

Application of fluorescence with polarized light to evaluate the orientation of dyes adsorbed in layered materials.

A new method of fluorescence polarization is applied to evaluate the angle of the preferential orientation of Rhodamine 6G (R6G) dye adsorbed in supported thin films of Laponite (Lap) clay. The method is based in the determination of the fluorescence dichroic ratio, obtained from the recorded fluorescence spectra with the detection polarizer horizontally and vertically oriented, as a function of the twisted angle of the film around its vertical axis, keeping the excitation polarizer in a fixed direction. The validity of the method is checked by comparing the experimental results obtained with both vertically and horizontally polarized excitations to that previously provided by absorption spectroscopy with linearly polarized light. A preferential orientation angle with respect to the normal to the clay layer of 62° is derived for R6G monomers adsorbed in Lap films.

On the arrangements of R6G molecules in organophilic C12TMA/lap clay films for low dye loadings.

Absorption and fluorescence spectroscopies with linearly polarized light are applied to characterize the adsorbed species of rhodamine 6G (R6G) laser dye in ordered organophilic laponite (Lap) clay films for low dye loadings. The organophilic character of the clay was controlled by the number of organic surfactant dodecyl-trimethylammonium (C12TMA) cations intercalated into the interlayer space of the clay. Experimental results suggest that for moderate to high surfactant contents (>70% of the total cation exchange capacity, CEC, of the clay) the accessibility of the interlayer space for R6G molecules is reduced. In these cases, the first stage in the adsorption of R6G molecules is at the external surface, where dye molecules can self-associate even for very low dye loading (around 0.1%CEC), probably for the limitation of the external surface area. The presence of a nonfluorescent H-type aggregate with a short-displaced coplanar structure and a fluorescent oblique head-to-tail J aggregate is reported. The inclined structure of this J aggregate is stabilized by the surfactant molecules at the external surface.

Two-step resonance energy transfer between dyes in layered silicate films.

Single- and two-step fluorescence resonance energy transfer (FRET) was investigated between laser dyes rhodamine 123 (R123), rhodamine 610 (R610), and oxazine 4 (Ox4). The dye molecules played the role of molecular antennas and energy donors (ED, R123), energy acceptors (EA, Ox4), or both (R610). The dye cations were embedded in the films based on layered silicate laponite (Lap) with the thickness of several μm. Optically homogeneous films were prepared directly from dye/Lap colloids. Dye concentration in the films was high enough for FRET to occur but sufficiently low to prevent the formation of large amounts of molecular aggregates. The films were characterized by absorption and fluorescence spectroscopies, and their optical properties were compared with colloid precursors and dye aqueous solutions. The phenomenon of FRET was confirmed by means of steady-state and time-resolved fluorescence spectroscopies. Significant quenching of ED emission in favor of the luminescence from EA molecules was observed. FRET led to the decrease in the lifetimes of excited states of ED molecules. Molecular orientation of dye molecules was determined by polarized absorption and fluorescence spectroscopies. Almost parallel orientation with respect to silicate surface (∼30°) was determined for all fluorescent species of the dyes. Theoretical model on relationship between anisotropy and molecular orientation of the fluorophores fits well with measured data. The analysis of anisotropy measurements confirmed the significant role of FRET in the phenomenon of light depolarization.