W. Holzer

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.

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.

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.

Spectroscopic Characterization of Flavin Mononucleotide Bound to the LOV1 Domain of Phot1 from Chlamydomonas reinhardtii

The absorption and emission behavior of flavin mononucleotide (FMN) in the light‐, oxygen‐ and voltage‐sensitive (LOV) domain LOV1 of the photoreceptor Phot1 from the green alga Chlamydomonas reinhardtii was studied. The results from the wild‐type (LOV1‐WT) were compared with those from a mutant in which cysteine 57 was replaced by serine (LOV1‐C57S), and with free FMN in aqueous solution. A fluorescence quantum yield of φF= 0.30 and a fluorescence lifetime of τF= 4.6 ns were determined for FMN in the mutant LOV1‐C57S, whereas these quantities are reduced to about φF= 0.17 and τF= 2.9 ns for LOV1‐WT, indicating an enhanced intersystem crossing in LOV1‐WT because of the adjacent sulfur of C57. A single‐exponential fluorescence decay was observed in picosecond laser time‐resolved fluorescence measurements for both LOV1‐WT and LOV1‐C57S as expected for excited singlet state relaxation by intersystem crossing and internal conversion. An excitation intensity dependent fluorescence signal saturation was observed in steady‐state fluorescence measurements for LOV1‐WT, which is thought to be because of the formation of a long‐lived intermediate flavin‐C(4a)–cysteinyl adduct in the triplet state (few microseconds triplet lifetime, adduct lifetime around 150 s). No photobleaching was observed for LOV1‐C57S, because no thiol group is present in the vicinity of FMN for an adduct formation.

Fluoresence quenching of riboflavin in aqueous solution by methionin and cystein

Abstract The fluorescence quantum distributions, fluorescence quantum yields, and fluorescence lifetimes of riboflavin in methanol, DMSO, water, and aqueous solutions of the sulphur atom containing amino acids methionin and cystein have been determined. In methanol, DMSO, and water (pH=4–8) only dynamic fluorescence reduction due to intersystem crossing and internal conversion is observed. In aqueous methionin solutions of pH=5.25–9 a pH independent static and dynamic fluorescence quenching occurs probably due to riboflavin anion–methionin cation pair formation. In aqueous cystein solutions (pH range from 4.15 to 9) the fluorescence quenching increases with rising pH due to cystein thiolate formation. The cystein thiol form present at low pH does not react with neutral riboflavin. Cystein thiolate present at high pH seems to react with neutral riboflavin causing riboflavin deprotonation (anion formation) by cystein thiolate reduction to the cystein thiol form.

Optical constants measurement of single-layer thin films on transparent substrates

A reflection and transmission technique is described, which allows the determination of the absorption coefficient spectrum, the refractive index dispersion, and the thickness of thin films on transparent substrates. The method may be used at any angle of incidence for s-polarized or p-polarized light. The absorption spectrum and the refractive index spectrum of a neat rhodamine 6G dye film on a fused silica substrate are determined. The absorption cross-section spectrum of the film is compared with the monomeric dye absorption cross-section spectrum in highly diluted solution.

Photo-physical characterization of rhodamine 6G in a 2-hydroxyethyl-methacrylate methyl-methacrylate copolymer

Abstract Some photo-physical parameters of 1×10−4 and 5.9×10−4 M samples of rhodamine 6G in a 2-hydroxyethyl-methacrylate methyl-methacrylate copolymer (volume mixture 1:1) are determined and compared with rhodamine 6G in methanol. They include the absorption cross-section and stimulated emission cross-section spectra, the fluorescence quantum distributions, quantum yields and lifetimes. The photo-degradation under steady-state and pulsed excitation is investigated. The saturable absorption behaviour under picosecond laser excitation is studied.

Travelling-wave lasing of TPD solutions and neat films

Abstract The travelling-wave lasing action (amplified spontaneous emission (ASE)) of the electrically active triphenylamine dimer N , N ′-diphenyl-bis(3-methylphenyl)-biphenyl-4,4′-diamine (TPD) in 1,4-dioxane solution and as neat film is studied. The samples are transversally pumped with picosecond excitation pulses (wavelength, 347.15 nm; duration, 35 ps). Lasing occurs at 401 nm for TPD in 1,4-dioxane and at 418 nm for TPD neat films. For the photo-physical characterization, the optical constants of neat films are determined by reflection and transmission measurement, the absorption cross-sections are extracted, and fluorescence spectroscopic parameters are determined. The fluorescence quantum yield of TPD in 1,4-dioxane is 0.69 and for neat TPD films, the fluorescence quantum yield is 0.31.

Spectroscopic and travelling-wave lasing characterisation of Gilch-type and Horner-type MEH-PPV

Abstract Two different types of poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene-vinylene) (MEH-PPV) (i) a strictly linear, well-defined material of moderate molar mass prepared through polycondensation (Horner-type) and (ii) a commercially available MEH-PPV-sample (Gilch-type, high-molecular and defect-rich) resulting from a dehydrohalogenation process, are carefully analysed by GPC and 13 C / 1 H -NMR spectroscopy. These studies show that for the Gilch-type polymer there is a marked deficiency of regular vinylene groups (≈30%), and therefore, a lack of long-range polyconjugation. The samples are characterized by thin-film optical constants measurements, absorption and fluorescence spectroscopic studies, and thin-film wave-guided travelling-wave laser action investigations. The influence of film-forming solvents (chlorobenzene (CB), tetrahydrofuran (THF)) and of film heating on the absorption, emission and laser behaviour is investigated. The fluorescence quantum yields of neat films (≈25% for Horner-type MEH-PPV and ≈36% for Gilch-type MEH-PPV) and toluene solutions (≈38% for Horner-type MEH-PPV and ≈32% for Gilch-type MEH-PPV) are similar. The exciton delocalisation (chromophore size) is found to be about four repeat units for both types of MEH-PPV. Remarkably good laser performance with low pump laser threshold ( 2 ) was achieved for both polymers independent of the synthesis pathway, the deviations in the molecular fine structure and the film-forming solvents. However, the preparation of high quality optical films is much easier for the Horner-type MEH-PPV than for the Gilch-type MEH-PPV making the Horner-type MEH-PPV easier to use for thin-film lasers and photonic devices.

Amplified spontaneous emission in neat films of arylene-vinylene polymers

Amplified spontaneous emission (travelling-wave lasing) was achieved for a set of 18 poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene-vinylene) (MEH-PPV) related arylene-vinylene copolymers, a part of which substituted with additional phenyl groups at the vinyl double bond. Wave-guiding neat thin films on glass substrates were used. The samples were transversally pumped with single second harmonic pulses of a mode-locked ruby laser (wavelength 347.15 nm). Travelling-wave emission occurred in the wavelength region between 485 and 650 nm. Repeat unit based absorption cross-section spectra, normalized fluorescence quantum distributions, and amplified spontaneous emission spectra are presented. Ground-state absorption cross-sections at the wavelengths of peak amplified emission are extracted from effective gain length measurements. Effective stimulated emission cross-sections are derived from pump pulse energy densities necessary for optical narrowing.