L.F. Vieira Ferreira

Fluorescence quantum yield evaluation of strongly absorbing dye solutions as a function of the excitation wavelength

Abstract A simple method to evaluate fluorescence quantum yields based on corrected fluorescence emission spectra of dyes in dilute and strongly absorbing solutions using different excitation wavelengths is presented. The method is supported by a detailed knowledge of the apparatus geometry and energy profile of excitation. Several recommended quantum counters were used (9,10-diphenylanthracene, rhodamines 101 and B, cresyl, violet, oxazine 1 and 1-ethyl-4-(4-p-dimethylaminophenyl)-1,3 butadienyl)-quinolinium perchlorate, (LDS 798) to cover the emission range from the UV to the visible and near-IR A curve of correction factor f vs. the optical density of the samples was obtained enabling an accurate determination to be made of the fluorescence quantum yield as a function of concentration and excitation wavelength. A case study of two squaraines (bis[4-(dimethylamino)phenyl]squaraine (HSQ) and bis[4-(dimethylamino)-2-methylphenyl]squaraine (MeSQ)) is presented. The quantum yields of these squaraines were obtained in saturated solutions in dichloromethane and in dilute samples at room temperature. The quantum yields determined are the same for high and low concentrations of the compounds indicating that no aggregation effects occur.

Photophysics of cyanine dyes on surfaces. A new emission from aggregates of 2,2′-cyanines adsorbed onto microcrystalline cellulose

Strong aggregation has been detected using diffuse reflectance ground-state absorption techniques to study both 1,1′-diethyl-2,2′-cyanine iodide and 1,1′-diethyl-2,2′-carbocyanine iodide adsorbed onto microcrystalline cellulose. For the former dye, H and J aggregates were formed, while for the latter only H aggregates were observed. The fluorescence quantum yields determined for both dyes adsorbed onto microcrystalline cellulose are about three orders of magnitude greater than those observed in ethanolic solutions being 0.30 for 2,2′-cyanine and 0.33 for the 2,2′-carbocyanine. Laser-induced time-resolved absorption and emission spectra of these samples were obtained as a function of the laser fluence using the technique of diffuse-reflectance laser flash photolysis using a photomultiplier for detection for the absorption measurements and a diode array spectrometer with nanosecond time resolution for the emission experiments. A new fluorescence emission from molecular aggregates of the 1,1′-diethyl-2,2′-carbocyanine iodide was detected. This new emission was only observed when excitation was with high laser fluences and from concentrated samples (0.50 µmol of the dye per gram of microcrystalline cellulose) where monomers and H aggregates coexist in the ground state. The new emission was absent in the case of 1,1′-diethyl-2,2′-cyanine iodide adsorbed onto microcrystalline cellulose, where the use of low or high fluences of the pulsed laser with low or high concentrations of this dye produced the same fluorescence spectrum as obtained with steady-state excitation.

Controlled growth of Cu2O nanoparticles bound to cotton fibres

A green, safe and fast procedure is presented for in situ generation of nanoparticles (NPs) of cuprous oxide (Cu2O) onto cotton fibres at room temperature using water as a solvent. The method is based on a mild surface oxidation of cellulose fibres to generate in a controlled way carboxylic groups acting as a binding site for the adsorption of Cu(2+) via electrostatic coordination. Then, the adsorbed Cu(2+) ions were readly converted into Cu2O by dipping the treated cotton fibres into a aqueous solution of a reducing agent. Field-emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), as well as UV-vis absorption and emission spectroscopic methods were used to analyse the size, morphology, chemical composition and the crystalline structure of the generated nanoparticles on the fabrics. The morphology of the ensuing Cu2O nanoparticles was shown to be dependent on the reduycing agent used. Antibacterial properties of the modified fibres were also investigated.

Aqueous photochemistry of pesticides triadimefon and triadimenol

The photochemistry of pesticides triadimefon and triadimenol was studied in aqueous solution and in methanol/water mixtures, in controlled and natural conditions. The photodegradation kinetics and product distribution are strongly dependent on the solvent and on the irradiation wavelength. The degradation rates are faster at 254 nm than at 313 nm. The kinetics is faster in water than methanol. Direct photoreaction is an important dissipation pathway of triadimefon in natural water systems while triadimenol is stable in these conditions. 1,2,4-Triazole and 4-chlorophenol are two of the major photodegradation products. The formation of the 4-chlorophenoxyl radical was detected for both pesticides in methanol and methanol/water mixtures. In methanol/water mixtures the reaction of both pesticides also occurs with 4-chlorophenolate formation, which increases with the water content. The photochemical studies of pesticides and other pollutants should be made in conditions as similar as possible to those observed in environmental systems.

Diffuse reflectance studies of β-phenylpropiophenone and benzophenone inclusion complexes with calix[4], [6] and [8]arenesDedicated to Professor Frank Wilkinson on the occasion of his retirement.

The formation of inclusion complexes of β-phenylpropiophenone and benzophenone with p-tert-butylcalix[4], [6] and [8]arenes (powdered solid samples) was studied with the use of diffuse reflectance techniques. Experimental evidence was obtained for inclusion in all cases. The benzophenone n → π* absorption band exhibits hypsochromic shifts with an increase of the calixarene ring aperture, providing evidence for an increase in cavity polarity. β-Phenylpropiophenone exhibits strong room temperature phosphorescence in contrast to its solution behaviour, and this is a clear evidence for the inclusion of this probe within the calixarene molecules. Calix[4]arene and calix[8]arene provide a more rigid environment to β-phenylpropiophenone whereas calix[6]arene appears to be more flexible. Transient absorption spectra of inclusion complexes with benzophenone (1 : 1 mol : mol, excited at 355 nm) show major triplet formation in the calix[4]arene case, while the ketyl radical of benzophenone is formed in the calix[6]arene case and is predominant in the calix[8]arene case. Phenoxyl radical transient absorption also becomes evident in the last two cases. For β-phenylpropiophenone (1 : 1 mol : mol) phenoxyl radicals are formed but no evidence was found for triplet or ketyl radical formation of this ketone in the calix[4]arene case. However, strong support for the ketyl radical of the aryl alkyl ketone was found for calix[6]arene and calix[8]arene inclusion complexes.

Potentialities of diffuse reflectance laser-induced techniques in solid phase: A comparative study of benzophenone inclusion within p-tert-butylcalixarenes, silicalite and microcrystalline cellulose

Abstract Diffuse reflectance and laser-induced techniques were used to access photochemical and photophysical processes in solid phases, namely calix[ n ]arenes ( n is the number of phenolic units in the ring), organic substances which are capable of forming inclusion complexes with several neutral organic substances and metal ions. We have used p - tert -butylcalix[4], p - tert -butylcalix[6] and p - tert -butylcalix[8]arenes as solid matrixes and benzophenone as probe. A comparative study is presented here, mainly using the results obtained with the calix[6]arene as host and those obtained with two other electronically inert supports: microcrystalline cellulose and silicalite, a hydrophobic zeolite. In all substrates, room temperature phosphorescence was obtained in air equilibrated samples. The decay times vary greatly and the largest lifetime was obtained for silicalite, where benzophenone is included into hydrophobic channels. Calix[6]arene and cellulose provide full protection against oxygen quenching while silicalite only protects the guest molecule partially. Calixarene molecules provide larger hydrophobic cavities than silicalite and also a better selectivity towards the guest size. This selectivity does not exist in the microcrystalline cellulose case. FTIR absorption spectra show that the distortion from planarity in the benzophenone molecule is larger in silicalite than in calix[6]arene, while in cellulose the distortion is slight. In spite of this, benzophenone exhibits the highest phosphorescence emission quantum yield in the case of silicalite. Benzophenone ketyl radical formation occurs with entrapment in cellulose and also with inclusion in calix[6]arene and calix[8]arene while in silicalite only triplet–triplet absorption is detected.

A comparative study of the photophysics and photochemistry of 4-chlorophenol adsorbed on silicalite and β-cyclodextrin

Abstract The photochemistry and photophysics of 4-chlorophenol (4-CP) were studied onto two model solid supports, silicalite and β-cyclodextrin (β-CD), using time resolved diffuse reflectance techniques and product degradation analysis. The results have shown that the photochemistry and photophysics of 4-CP are different from solution and depend on the solid. Ground state diffuse reflectance and time resolved luminescence demonstrated the inclusion of the probe in both substrates. 4-CP exhibits room temperature luminescence in both hosts, being structured and much more intense in β-CD. The emission was assigned to phosphorescence of the inclusion complex. Transient absorption demonstrated the formation of the unsubstituted phenoxyl radical and of 4-chlorophenoxyl radical in β-CD. In silicalite only the later was detected. The studies of the photodegradation products indicate that phenol is the main photoproduct in β-CD. In silicalite the chromatographic analysis indicates the presence of products that involve the ring cleavage.

Fluorescence quantum yield evaluation of strongly absorbing dye solutions as a function of the dye concentration

Abstract A simple method to evaluate fluorescence quantum yields using front surface geometry and based on corrected fluorescence emission spectra of dyes on diluted and strongly absorbing solutions was developed. The method is supported by a detailed knowledge of the apparatus geometry and energy profile of excitation, and uses an experimentally determined correction factor f . This factor relates the intensity of fluorescence obtained in the general case with the maximum value for the emission intensity obtained for very high optical densities. The method was applied to rhodamine 700 (R700) in ethanol and bis[4-(dimethylamino)-2-methylphenyllsquaraine (MeSQ) in dichloromethane in concentrations ranging from ∼ 10 −6 to ∼ 10 −3 M in both cases. The data shows a substantial quenching effect in the former case for very concentrated samples of the dye whereas no such effect was found in the methylsquaraine within experimental error.

Photochemistry on surfaces: Fluorescence emission of monomers and dimers and triplet state absorption of acridine orange adsorbed on microcrystalline cellulose

Abstract Prompt fluorescence as well as delayed fluorescence emission of acridine orange was detected at room temperature from samples where this dye is adsorbed on microcrystalline cellulose. Ground state absorption studies provided evidence for dimer formation of the dye when adsorbed on cellulose, and the equilibrium constant for dimerisation was determined as 1.6±0.1 × 106mol−1g. At low loadings of acridine orange on cellulose ( 20 μmol g−1), the fluorescence intensity decreases, and the emission is broad with its maximum at 620 nm, and is mainly due to the dimer. By assuming that the excited monomer and dimer of acridine orange are the only emitting species, it was possible to determine the fluorescence quantum yields for these two species when adsorbed on microcrystalline cellulose as 0.95±0.05 and 0.40±0.10, respectively. Pulsed emission studies at room temperature in the millisecond time-range also revealed monomer and dimer emissions on this longer time-scale. These are shown to be due to thermally activated delayed fluorescence arising from the triplet states of monomer and dimer acridine orange as confirmed by diffuse reflectance transient absorption studies.

Surface photochemistry: Photodegradation study of pyrene adsorbed onto microcrystalline cellulose and silica

Ground-state diffuse reflectance, time resolved laser-induced luminescence, diffuse reflectance laser flash-photolysis transient absorption and chromatographic techniques were used to elucidate the pho- todegradation processes of pyrene adsorbed onto microcrystalline cellulose and silica. Ground-state diffuse reflectance showed that on both substrates low concentrations display absorption of pyrene monomers. At high concentrations spectral changes attributed to aggregate formation were observed. Laser induced fluo- rescence showed that pyrene onto microcrystalline cellulose mainly presents fluorescence from monomers, while for silica, excimer-like emission was observed from low surface loadings (≥ 0.5 µmol g −1 ). Transient absorption and photodegradation studies were performed at concentrations where mainly monomers exist. On silica, pyrene presents transient absorption from its radical cation. On microcrystalline cellulose both radical cation, radical anion and pyrene triplet-triplet absorption were detected. Irradiation followed by chro- matographic analysis showed that pyrene decomposes on both substrates. For pyrene on microcrystalline cellulose 1-hydroxypyrene was the main identified photoproduct since in the absence of oxygen further ox- idation of 1-hydroxypyrene was very slow. For pyrene on silica photodegradation was very efficient. Almost no 1-hydroxypyrene was detected since in the presence of oxygen it is quickly oxidized to other photooxida- tion products. On both substrates, pyrene radical cation is the intermediate leading to photoproducts and oxygen it is not involved in its formation.