Stanislav Stanimirov

Synthesis and Photophysical Properties of Yellow-Emitting Iridium Complexes. Effect of the Temperature on the Character of Triplet Emission

The parent study includes synthesis end photophysical characterization of four new phosphorescent cyclometalated iridium (III) complexes. The cyclometaled ligands used here are 2-(4-chlorophenyl)benzo[d]thiazole, 2(4isopropylphenyl)benzo[d] thiazole, 2-(4-methoxyphenyl)benzo[d]thiazole, 4-(benzo[d]thiazol-2-yl)-N,N-dimethylaniline. The auxiliary ligand is acetylacetone (Hacac). The photometric analyses show temperature dependence on phosphorescent spectra and the nature of the emitting state of the synthesized complexes. Using fluorescent and absorption spectroscopy we suggest an explanation of the observed effect points out the transition from ligand centered (LC) to MLCT main character of the emitting state. The quantitative photometric analysis states high phosphorescent quantum yields of the complexes in solution.

Photophysical properties of novel fluorescent poly(oxyethylene phosphate) tris(β-diketonate) europium (III) complexes

In the present study, photophysical properties of fluorescent poly(oxyethylene phosphate) tris(beta-diketonate) europium (III) complexes have been studied by means of stationary and time-resolved fluorescence spectroscopy (in ethanol at room temperature). The luminescent quantum yields and efficiency for the energy transfer from beta-diketonate ligands to Eu(III) ion have been determined for the studied complexes by using diffusion-enhanced fluorescence resonance energy transfer. Obtained results show effect of the polymer ligands upon photophysical properties of the complexes and a relation has been established with length of the oxyethylene spacer between two phosphonate groups. The Förster radiuses of the synthesized compounds with SulfoRhodamine 101 as acceptor have been calculated. Measured distances between molecules of the donors and acceptor at identical acceptor/donor molar ratios have been illustrated the difference in structure of the ternary and polymer complexes in solution even at low concentration.

Novel pH responsive luminescent poly(oxyethylene phosphate) tris(β-diketonate) europium (III) complexes

The paper reports on the dependence of the absorbance and luminescent intensity from pH of novel poly(oxyethylene phosphate) tris(β-diketonate) europium (III) complexes. The photophysical data obtained allow some preliminary assumptions about the nature of this phenomenon. Increase in luminescent efficiency is a consequence of enhanced efficiency of energy transfer caused by structural changes in complexes after water deprotonation. Remarkable change has been observed in photophysical properties of the polymer complexes by studying the fluorescent emission and excitation spectra and absorption recorded at various pH both in solution and in the solid state. Some of the complexes derivative of the dibenzoylmethane (DBM) show more than hundred times increase in the luminescence after alkalization. The pH value, at which the maximum luminescent efficiency appears, depends on the type of the fourth ligand. The difference between luminescent efficiency of the complexes in alkaline and neutral environment depends on the β-diketonate ligands and on polymer type as well.

The luminescence response of Eu(III)-thenoyltrifluoroacetonate complexes upon preresonant excitation with femtosecond laser pulses.

The luminescence of thenoyltrifluoroacetonate (TTA) coordination complexes of trivalent europium ion (Eu(III)) in aqueous solutions and in solid-state polymeric films is probed upon single- and two-photon preresonant excitation with Ti:sapphire femtosecond laser. Particularly, diamine-liganded Eu(III)(TTA)(3) and poly(oxyethylene phosphate)tris(beta-diketonate)Eu(III) complexes are examined aiming their possible applications as luminescent labels for sensing and imaging of biological molecules. Even at a pre-resonance, the excitation of these compounds with high-intensity, broadband light of frequency-doubled Ti:sapphire femtosecond laser centered around 400 nm results in a luminescence response suitable for fluorometric applications.

Femtosecond laser spectroscopy of europium complexes in solutions

The luminescence response of a new coordination complex of europium Eu(III) ion, namely tris[4,4,4-trifluoro-1-(2-thienyl)-1,3-butanediono]Eu(III) 1,4-diaza-bicyclo[2.2.2]octane, applicable in microbiology, biochemistry and molecular biology, is probed upon ultra-high intensity photo-excitation with high-power femtosecond laser pulses. The use of laser light source with ultrashort and high-power pulses should increase by many orders of magnitude the intensity of photoexcitation, and therefore the efficiency of the bioassays and fluorescent microscopy in the research on molecules and structures of biomedical and pharmacological interest, labeled with luminescent complexes. The results obtained show that the luminescence response of the examined compound, even in dilute aqueous solutions is sufficient for fluorometric applications, such as luminescent labels in biomedical diagnostics and bioanalyses, as well as in laser scanning fluorescent microscopy.