C. Bojarski

FLUORESCENT DIMERS OF RHODAMINE 6G IN CONCENTRATED ETHYLENE GLYCOL SOLUTION

Abstract Aggregation of rhodamine 6G in concentrated ethylene glycol solutions is studied as a function of temperature. The measurements of absorption and fluorescence spectra evidence that the system of interest consists of two fluorescent species: monomers and dimers. Strong overlaps between all absorption and fluorescence bands in this system enable forward and reverse energy transport between monomers and dimers. The quantitative analysis of the effect of nonradiative energy transport (NET) on the fluorescence spectra as well as on the fluorescence quantum yields of monomers and dimers explains the temperature and concentration regularities observed. In particular, it is shown that in the concentrated solution the calculated monomer quantum yield is underrated compared to that measured, if the reverse NET is neglected. The lack of information on the value of the dimer quantum yield does not allow for full analysis of the forward and reverse NET in the system. However, it is shown that if that quantum yield is determined as the best fit parameter, an excellent agreement between the experimental and calculated values is obtained.

Nonradiative excitation energy transport in one-component disordered systems.

AbstractHigh-accuracy Monte Carlo simulations of the time-dependent excitation probabilityGs(t) and steady-state emission anisotropyrM/r0M for one-component three-dimensional systems were performed. It was found that the values ofrM/r0M obtained for the averaged orientation factor

The structure of the flavomononucleotide dimer

\overline {\kappa ^2 }

Spectroscopic manifestations of flavomononucleotide dimers in polyvinyl alcohol films.

only slightly overrate those obtained for the real values of the orientation factor κik2. This result is essentially different from that previously reported. Simulation results were compared with the probability coursesGs(t) andR(t) obtained within the frameworks of diagrammatic and two-particle Huber models, respectively. The results turned out to be in good agreement withR(t) but deviated visibly fromGs(t) at long times and/or high concentrations. Emission anisotropy measurements on glycerolic solutions of Na-fluorescein and rhodamine 6G were carried out at different excitation wavelengths. Very good agreement between the experimental data and the theory was found, with λex≈λ0-0 for concentrations not exceeding 3.5·10−2 and 7.5·10−3M in the case of Na-fluorescein and rhodamine 6G, respectively. Up to these concentrations, the solutions investigated can be treated as one-component systems. The discrepancies observed at higher concentrations are caused by the presence of dimers. It was found that forλex λ0-0), they lie higher than the respective theoretical values. Such a dispersive character of the energy migration can be explained qualitatively by the presence of fluorescent centers with 0-0 transitions differing from the “mean” at λ0-0.

On the mechanism of nonradiative electronic excitation energy transport in two-component systems: Concentration depolarization investigations

Abstract The electronic absorption spectra of the flavomononucleotide (FMN) in aqueous solution and in glycerine-water solution with change of the dye concentration have been measured. The FMN dimer absorption spectrum, monomer absorption spectrum, dimerization constant K and molar fraction of the monomer were calculated. It was found that FMN dimerization constants in aqueous solution were Ka = 118.0 l/mol and in glycerine Kg = 20.5 l/mol. In the region of the monomer absorption band two dimer absorption bands appear, in accordance with the Kasha molecular exciton theory.

INVESTIGATIONS ON THE FLUORESCENCE CONCENTRATION QUENCHING OF FLAVOMONONUCLEOTIDE IN GLYCERINE‐WATER SOLUTIONS

Abstract Structural parameters of the flavomononucleotide (FMN) dimer in water and glycerin-water solution have been established on the basis of spectroscopic studies and the Kasha theory. A parallel-plane dimer model is proposed, in which the monomer units are arranged face to face. For the FMN dimer in water, the distance between the planes of monomer units is equal to R = 3.5 ± 0.3 A , and the angle α1 ( ∢ ( M I M I ′ )) between the moments of transition S1 → S0 (absorption band I) and the angle αII ( ∢ ( M II M II ′) ) between the moments of transition S2 → S0 (band II) of monomer units are equal to α1 = 71 ± 4° and αII = 0 ± 4°, respectively. From the mentioned orientation of absorption transition moments of FMN monomer units in the dimer, a conclusion may be drawn that the angel, φ, between the transition moments S1 → S0 and S2 → S0 is equal to 36 ± 4°. This orientation is consistent with the results of Johansson obtained for FMN molecules in lamellar liquid crystals. Similar R, αI and αII values were obtained for the FMN dimer in the water-glycerin solution.

Direct and reverse energy transport in systems of monomers and imperfect traps: Monte Carlo simulations.

Excitation energy transport and trapping is studied for monomer-fluorescent dimer system of flavomononucleotide (FMN) in polyvinyl alcohol films (PVA). It is shown that the theory neglecting reverse energy transfer (RET) from dimers to monomers does not allow for the explanation of concentration quenching and concentration depolarization results presented herein. Much better agreement has been obtained using generalized energy transport theory in which fluorescent dimers are treated as imperfect traps for excitation energy. Such parameters like the dimer quantum yield and its emission anisotropy are estimated.

The influence of fluorescence concentration quenching on the emission anisotropy of flavins in glycerine-water solutions.

Absorption and fluorescence spectra of flavomononucleotide (FMN) in polyvinyl alcohol films (PVA) over a very wide concentration range are investigated. The dimerization constant as well as the pure monomer and dimer spectra are calculated and the structural parameters of FMN dimer are established. Excitation wavelength and temperature dependencies of FMN/PVA fluorescence spectra for different FMN concentrations were carried out. These measurements together with those of absorption reveal that dimers are imperfect traps for excitation energy and that the energy transfer can occur both in forward and in reverse direction. Moreover, it was shown that the observed temperature changes in fluorescence spectra may be qualitatively explained by the effect of inhomogeneous broadening of FMN energy levels and by the presence of fluorescent dimers.