Alfons Penzkofer

Fluorescence behaviour of highly concentrated rhodamine 6G solutions

The fluorescence quantum distributions E(X) and fluorescence quantum efficiencies qF of rhodamine 6G in methanol and in water are measured for various concentrations up to the solubility limit. The fluorescence spectra are separated in monomer and dimer (ground-state dimer and closely spaced pair) contributions. The stimulated emission cross sections for the monomers and the dimers are resolved.

Absorption and fluorescence spectroscopic investigation of indocyanine green

Abstract Absorption spectra, fluorescence quantum distributions, fluorescence quantum yields and degrees of fluorescence polarization vs. dye concentration were determined for indocyanine green in methanol, water and aqueous albumin solution. The monomer fluorescence quantum yield is limited to a few per cent by internal conversion. In water, dimerization starts at low concentrations (approximately 3 × 10 −8 m ol dm −3 ) and lowers the fluorescence quantum yield. The strong affinity of the dye to albumin shifts the onset of dimerization to higher concentrations. In methanol, dimerization is weak and closely spaced pair formation dominates at high concentrations. Dye adsorption to albumin and dye aggregation in the solvents were analysed experimentally and theoretically.

Fluorescence quenching of rhodamine 6G in methanol at high concentration

Abstract The fluorescence lifetime of rhodamine 6G dissolved in methanol is measured over a wide concentration region from 10 −5 to 0.6 mol/l. The rapid reduction of fluorescence lifetime above 10 −2 mol/l is found to be mainly due to energy transfer to quenching centers. The decrease of the fluorescence lifetime is limited by the finite fluorescence lifetime of the quenching centers of ≈ 1 ps.

PHOTOSTABILITY AND THERMAL STABILITY OF INDOCYANINE GREEN

The photo-fading of the S0-S1 absorption band of the infrared dye indocyanine green sodium iodide (ICG-NaI) has been studied by cw laser excitation to the S1 band. Monomeric solutions in water, heavy water, aqueous sodium azide, human plasma, methanol and dimethyl sulfoxide (DMSO) as well as J-aggregated solutions in H2O and D2O have been investigated. A leucoform of indocyanine green seems to be formed by photodegradation. The degradation slows down with exposure time. The initial degradation yield, phi D,0, is determined. In monomeric and dimeric water, heavy water and sodium azide solutions the initial photostability is of the order of phi D.0 approximately 10(-3), in the organic solvents methanol and DMSO it is of the order of phi D.0 approximately 10(-5), and in human plasma it is phi D.0 approximately 2 x 10(-6). J-aggregates at high concentration are very stable. The thermal stability of the ICG-NaI solutions at room temperature in the dark is compared with their photostability. The thermal degradation time of monomeric and dimeric ICG-NaI in water, heavy water and sodium azide solutions is t(th) approximately 10 days, while no thermal degradation is observed for ICG-NaI J-aggregates and ICG-NaI in methanol, DMSO and human plasma.

Nonresonant third order hyperpolarizability of rare gases and N2 determined by third harmonic generation

Abstract The third order hyperpolarizabikity γ (3) 1111 (-ω 3 ;ω 1 ,ω 1 ,ω 1 ) of the rare gases He, Ne, Ar, Kr, Xe and of N 2 are determined by third harmonic generation involving picosecond light pulses of a Nd-glass laser. The results are compared with reported experimental and theoretical values.

pH dependence of the absorption and emission behaviour of riboflavin in aqueous solution

The absorption and fluorescence behaviour of riboflavin is investigated in aqueous solution over a wide range of pH values from pH=−1.1 to pH=13.4. Absorption spectra, fluorescence quantum distributions, fluorescence quantum yields, and fluorescence signal decays are measured. Riboflavin exists in three different forms depending on the pH of the solutions. There is an equilibrium between the cationic form (RFloxH2+) and the neutral form (RFloxH) at low pH (pKc=0.4), and between the neutral form and the anionic form (RFlox−) at high pH (pKa=9.75). The cationic, neutral, and anionic species have different absorption spectra. The fluorescence quantum yields and fluorescence lifetimes of the various forms are separated. The cationic form is non-fluorescent (fluorescence quantum yield φF<5×10−5), and the anionic form has a low fluorescence quantum yield of φF≈1.2×10−3. The fluorescence quantum yield of the neutral form is around φF≈0.26 above pH=4 and decreases strongly below pH=4 because of [H+]-dependent excited-state reaction of RFloxH* to RFloxH2+*. At low pH the fluorescence signal decay is single-exponential since only the neutral form, RFloxH, is emitting. At high pH a bi-exponential fluorescence decay occurs because RFloxH and RFlox− emit with different time constants.

Absorption behaviour of methanolic rhodamine 6G solutions at high concentration

Abstract The optical constants of rhodamine 6G dissolved in methanol and of a solid rhodamine 6G film are determined. The concentration of the solutions is varied between 10 −5 and 0.66 mol/l. Deviations from Beers law at high concentrations are found. They are thought to be due to the mutual interaction of neighbouring molecules. The absorption behaviour of rhodamine 6G in methanol is compared with the absorption behaviour of rhodamine 6G in water.

Vibronic relaxation in the S1 state of rhodamine dye solutions

Abstract The relaxation of vibronic energy levels was investigated by two different techniques. (A) The transmission of pico-second pulses was measured as a function of the incident peak intensity and (B) the transmission of a second probe pulse was studied versus delay time. Population lifetimes of the excited levels between 0.5 ps and 1.0 ps were found for rhodamine 6G and rhodamine B in the solvents water, ethanol, ethylene glycol and glycerol. The lifetimes are independent of solvent viscosity.

Absorption and emission spectroscopic investigation of cyanovinyldiethylaniline dye vapors

Abstract The dyes 4-dicyanovinyl-N,N-diethylaniline (DCVA) and 4-tricyanovinyl-N,N-diethylaniline (TCVA) are investigated spectroscopically in the vapor phase. The absorption and emission spectra are compared with solution spectra. The thermal stability of the dye powder is determined. The saturated vapor density versus temperature is deduced from absorption measurements. The vapor absorption peaks at 382 nm (DCVA) and 437 nm (TCVA). The fluorescence quantum efficiency of both dyes in the vapor phase is approximately 1.5×10 −4 .

Fluorescence spectroscopic behaviour of neat and blended conjugated polymer thin films

The fluorescence quantum distributions, quantum yields, degrees of polarisation, and stimulated emission cross-section spectra of four para-phenylene vinylene and two para-phenylene ethynylene luminescent polymers were determined. Neat luminescent polymer films and polystyrene films doped with luminescent polymers were investigated. The fluorescence spectroscopic data analysis accounted for fluorescence absorption and reemission in thin films. The fluorescence spectroscopic behaviour of the solid thin films on fused silica substrates was compared with the corresponding behaviour of the luminescent polymers dissolved in liquid tetrahydrofuran.