M. Gabriela Lagorio

Modeling of fluorescence quantum yields of supported dyes

Aggregation equilibrium and the fluorescence properties of hydroxoaluminium tricarboxymonoamidephthalocyanine adsorbed on microgranular cellulose have been studied at different dye loadings. Up to a concentration of nearly 3 × 10−6 mol phthalocyanine (g cellulose)−1 diffuse reflectance spectra may be interpreted on the basis of a simple monomer–dimer equilibrium. Monomer and dimer spectra are similar to the spectra of the monomeric dye in solution. The solid-state dimer spectrum is red-shifted with respect to that of the monomer and this is attributed to the coplanarity of dimers. Fluorescence spectra and quantum yields show typical effects of re-absorption and re-emission of light. In particular, the observed fluorescence quantum yields depend on concentration and span the range 0.29–0.07. To account for these effects as well as the effect of aggregation on fluorescence quantum yields and to obtain corrected fluorescence spectra a model based on the Kubelka–Munk theory of diffuse reflectance is developed. The application of this model to the case under study yields a true fluorescence quantum yield ϕ = 0.46 ± 0.02 in the whole range of concentrations, which is slightly higher than the value found for the same dye in dimethyl sulfoxide solution.

Quantum yield of singlet molecular oxygen sensitization by copper(II) tetracarboxyphthalocy anine

Abstract The quantum yield φ Δ for sensitization of singlet molecular oxygen, O 2 ( 1 Δ g ), by copper(II) tetracarboxyphthalocyanine (CuTCPc) in dimethyl sulphoxide was determined under stationary illumination, using diphenyl-isobenzofurane as chemical quencher. A constant value of φ Δ = 0.16 ± 0.03 was obtained, independent of sensitizer concentration in the range 2–26 μM even though dimers and possibly oligomers are present at the highest concentrations. The monomer and the aggregates of CuTCPc physically quench O 2 ( 1 Δ g ) with a nearly diffusional rate constant.

Visible and near-IR spectroscopic and photochemical characterization of substituted metallophthalocyanines

Abstract The UV—visible spectra of copper(II) tetracarboxyphthalocyanine (CuTCPc), copper(II) tetrapentyloxycarbonyl- phthalocyanine (CuTPOCPc) and hydroxyaluminium tricarboxymonoamidephthalocyanine (AlTCPc) were studied as a function of concentration in various solvents. Dye aggregation equilibria were investigated and pure monomet and dimer spectra were obtained. Near-IR absorption maxima for the above compounds and for cobalt(II) dicarboxydiamidephthalocyanine (Co(II)TCPc) and dicyanocobalt(III) dicarboxydiamidephthalocyanine (Co(III)TCPc) were also obtained and their assignment to triplet—multiplet transitions is discussed. The analysis of the monomer and dimer spectra on the basis of exciton theory and results obtained from molecular mechanics computational calculations allows conclusions to be drawn on the monomer and dimer structures and observed differences in aggregation ability to be interpreted. Quantum yields of singlet molecular oxygen photoproduction by monomers and dimers were determined for aluminium and copper dyes by steady state irradiation using diphenylisobenzofuran as singlet oxygen quencher. The lack of activity in dimers, in accordance with the literature available, is interpreted in terms of the vibronic coupling between monomeric units in the dimer (exciton theory).

Rose Bengal adsorbed on microgranular cellulose: evidence on fluorescent dimers

Rose Bengal adsorbed on microgranular cellulose was studied in the solid phase by total and diffuse reflectance and steady-state emission spectroscopy. A simple monomer-dimer equilibrium fitted reflectance data up to dye loadings of 4 x 10(-7) mol (g cellulose)(-1) and allowed calculation of monomer and dimer spectra. Further increase of dye loading resulted in the formation of higher aggregates. Observed emission and excitation spectra and quantum yields were corrected for reabsorption and reemission of luminescence, using a previously developed model, within the assumption that only monomers are luminescent [M. G. Lagorio, L. E. Dicelio, M. I. Litter and E. San Roman, J. Chem. Soc., Faraday Trans., 1998, 94, 419]. An apparent increase of fluorescence quantum yield with dye loading was found, which was attributed to the occurrence of dimer fluorescence. Extension of the model to two luminescent species (i.e. monomer and dimer) yielded constant fluorescence quantum yields for the monomer, phiM= 0.120 +/- 0.004, and for the dimer, phiD= 0.070 +/- 0.006. The monomer quantum yield is close to the value found for the same dye in basic ethanol. The presence of fluorescent dimers and calculated quantum yields are supported by analysis of the excitation spectra and other experimental evidence. The possible occurrence of non-radiative energy transfer and the effect of surface charge on the properties of the dimer are analyzed.

Meso-substituted cationic porphyrins of biological interest. Photophysical and physicochemical properties in solution and bound to liposomes.

Abstract A series of cationic porphyrins with 1–4 positive charges are studied: mono(N-methyl-4-pyridyl)triphenylporphine chloride [Mono], cis(N-methyl-4-pyridyl)diphenylporphine chloride [Cis], tri(N-methyl-4-pyridyl)monophenylporphine chloride [Tri] and tetra(N-methyl-4-pyridyl)porphine chloride [Tetra]. Their photophysical properties are measured in small unilamellar vesicles and compared with those in homogeneous solution. Liposomes of l-α-dimyristoyl-phosphatidylcholine (100 nm diameter) and l-α-dipalmitoyl-phosphatidylcholine (50 nm diameter) in phosphate-buffered saline (pH = 7.4) or D2O 0.15 M NaCl were used. The effect of the medium microheterogeinity is discussed. The triplet quantum yields in liposomes for all the porphyrins are about 0.7, similar to the value obtained for Tetra in aqueous media. The singlet molecular oxygen quantum yields for the hydrophilic compounds Tri and Tetra are greater than those of the hydrophobic ones, Mono and Cis. Also, association constants (KL) of the dyes to liposomes and their localization within the membranes are determined from fluorescence and fluorescence polarization measurements, respectively. KL values are in the range of 104–105 M−1 for all the compounds, indicating that hydrophobic and coulombic interactions between porphyrins and liposomes are responsible for the dye association. Fluorescence polarization experiments indicate that Mono and Cis can penetrate into the lipidic phase, and that Tri and Tetra are located near the polar heads of the lipidic molecules.

Photophysics of supported dyes: phthalocyanine on silanized silica

Aluminum tetracarboxyphthalocyanine is adsorbed onto silanized silica particles at different dye to particle ratios. Diffuse reflectance spectra are measured and the equilibrium between dye monomers and aggregates is studied. Between 4.0 × 10−8 and 2.0 × 10−6 mol of dye g−1 of support, a simple dimerization equilibrium explains experimental results. Fluorescence spectra and quantum yields are calculated and corrected for light absorption by non-fluorescent dimers and reabsorption of emitted radiation. Once corrected, fluorescence quantum yields are of the same order of magnitude than in solution. Absorption spectra are also obtained for suspensions of the same materials in water. The hydrophilic environment enhances aggregation dramatically. Singlet oxygen quantum yields measured in suspension are lower than those found for the same phthalocyanine in solution of DMSO. Results are compared with those previously obtained for similar materials.

PHOTOPHYSICAL PROPERTIES OF SUPPORTED DYES. QUANTUM YIELD CALCULATIONS IN SCATTERING MEDIA

Aluminum tricarboximonoamidephthalocyanine and Rose Bengal were immobilized by adsorption and covalent linkage to different solid supports: silica, silanized silica, silanized silica modified with polylysine chains and NovaSyn® TG amino resin (low cross-linked polystyrene with polyethyleneglycol chains). Specially designed tools were used to evaluate absolute quantum yields in scattering media. A previously developed model was used to calculate fluorescence quantum yields in the solid state and a method is presented for the evaluation of singlet molecular oxygen quantum yields in suspensions. The method yields reliable results for dilute suspensions of particles with diameters much greater than the irradiation wavelength. Diffuse reflectance and, in the case of suspensions, total transmittance spectra are the basic inputs required. In this way the efficiencies of suspended solid photosensitizers were obtained for all samples and correlated with the dye concentration and the way of linking to the surface.

Photophysics on surfaces: Absorption and luminescence properties of Pheophorbide-a on cellulose

Solid samples of Pheophorbide-a adsorbed on microcrystalline cellulose were prepared with a wide range of dye concentrations (7.1 × 10−9 to 8.9 × 10−6 mol (g cellulose−1) and studied by diffuse reflectance and steady-state and time-resolved fluorescence spectroscopy in the solid state. Absorption spectra obtained by means of reflectance spectroscopy were compared with those found in different environments and carrier systems. No aggregation may be inferred from the spectroscopic data. Changes in the fluorescence spectra with increasing dye concentration are attributed to: (a) reabsorption of luminescence and re-emission and (b) interaction between the excited dye molecule and ground state neighboring molecules. The first effect accounts for the observed differences in emission spectra for thin and thick layers of a given sample. Emission decays are non-exponential even at low dye concentrations. A Forster energy transfer mechanism leading to energy trapping by molecules of the same type as the excited dye is proposed.

Microcrystalline cellulose as a carrier for hydrophobic photosensitizers in water

Samples of pheophorbide-a adsorbed on microcrystalline cellulose, which have been previously characterized in the solid state (M.G. Lagorio, E. San Roman, A. Zeug, J. Zimmermann and B. Röder, Phys. Chem. Chem. Phys., 2001, 3. 1524-1529), were washed with water, leading to stable suspensions of ultrafine particles (d < 2 microm) carrying photoactive, monomeric dye molecules. Detachment can be controlled through the particle size distribution. Suspensions are fluorescent and generate singlet molecular oxygen efficiently. A similar effect has been observed on washing samples containing hematoporphyrin IX adsorbed on the same support. Thus, using cellulose as a heterogeneous carrier, it is possible to introduce hydrophobic photosensitizers into the aqueous medium while avoiding aggregation, thus preserving their photophysical properties. At the same time, the spectroscopic properties of dyes attached to cellulose particles are compared with those in ethanol and ethanol-water mixtures and their differences are explained in terms of medium polarity and dye aggregation.

Biosorption of arsenic from groundwater using Vallisneria gigantea plants. Kinetics, equilibrium and photophysical considerations.

Arsenic (V) uptake from groundwater by using Vallisneria gigantea plants was studied using batch experiments. Reflectance and fluorescence of intact plants were investigated and changes in photophysical properties following arsenic absorption were reported. Good correlations have been found between arsenic concentration in groundwater and parameters derived from reflectance and fluorescence measurements. This system reached its equilibrium after seven days when the removal quantities were strongly dependent on the initial arsenic concentration. Interestingly, Vallisneria plants were able to accumulate from 100 to 600 mg As kg(-1) in roots and fronds although the translocation factors were low (0.6-1.6). Kinetic data for biosorption process followed a first-order law. At low arsenic concentrations the uptake in plants was governed by diffusion aspects. Langmuir, Freundlich and Dubinin-Radushkevich models were applied and results demonstrated that arsenic uptake was better described by the Langmuir model. As a final remark we concluded that a plant of this species should be able to remove 1mg As per week.