Danuta Wróbel

Study of the influence of substituents on spectroscopic and photoelectric properties of zinc phthalocyanines

Abstract In this paper we describe conversion of light energy into electric energy in a photoelectrochemical cell containing zinc phthalocyanine (ZnPc) dyes. For all dyes investigated in liquid polyvinyl alcohol with dimethyl sulfoxide solution and located in the photoelectrochemical cell the following measurements have been done: absorption, fluorescence, photoacoustic spectra, photovoltaic spectra, kinetics of photocurrent and current–voltage characteristics. It has been shown that all dyes located in the photoelectrochemical cell are able to convert light into electric energy but with different effectiveness. The influence of substituted different peripheral groups to ZnPc core and the correlation between the molecular structure and effectiveness of solar to electric energy conversion were observed and described. The unique behavior of ZnPc substituted with fluorines was indicated.

Mixtures of synthetic organic dyes in a photoelectrochemical cell

Abstract Spectroscopic and photoelectric properties of phthalocyanine (Pc), tetranaphtyloporphyrin (TNP) and merocyanine (Me) and their mixtures have been studied. Dyes dissolved in polyvinyl alcohol in dimethylsulfoxide were immersed in a semiconducting/metallic photoelectrochemical cell. Absorption, fluorescence and photoacoustic spectroscopies have been used as methods to follow deactivation pathways of radiative and non-radiative processes of excited states as a competitive processes to charge separation process. The dependence of photovoltage signal on excitation wavelength (photovoltage action spectrum), kinetics of photocurrent rise and decay in second time scale have been presented. Electric parameters of the photoelectrochemical cell with dyes (conductivity, capacitance) and their mixtures in the dark and upon illumination have been estimated. It was shown that all investigated dyes are able to generate photosignal but the values of photocurrent generated were dependent on the dye used in the experiment. The complexity of photoconversion process in the electrochemical cell fulfilled with the dye mixtures has been discussed. The presence of Me dye was found to improve TNP photovoltaic effect, whereas weak enhancement of photoeffects were observed in photoelectrochemical cell based on Me and Pc. This different tetraphenyloporphyrin and Pc conversion effectiveness in the presence of Me was discussed in aspects of various mechanisms of interaction between dyes. It has been indicated that dye aggregation, energy transfer, charge recombination and other processes can lead to improvement or inhibition of the photovoltaic effects and of the conversion effectiveness depending on the composition of dyes embedded in an electrochemical cell.

Photovoltaic and spectral properties of tetraphenyloporphyrin and metallotetraphenyloporphyrin dyes

Abstract Spectroscopical properties and photocurrent (or photovoltage) of tetraphenyloporphyrins and metallotetraphenyloporphyrins in nematic liquid crystal have been studied. Photoelectric response has been measured in an electrochemical cell made of the semitransparent semiconducting and golden electrodes with porphyrin dyes embedded in liquid crystal. Fluorescence, time-resolved luminescence in microsecond time scale and photoacoustic spectra have also been measured. The competition between radiative, non-radiative processes and charge transfer is discussed. It has been shown that effectivity of porphyrins for photocurrent generation depends on the presence/absence of central metal in the macrocycle of porphyrin skeleton and the kind of metal. The schematic model of the contributions of the dye molecule and semiconducting electrode in the electron transfer process is shown.

Dichroism and polarized fluorescence of chlorophyll a, chlorophyll c and bacteriochlorophyll a dissolved in liquid crystals

Three photosynthetic pigments were studied: chlorophyll a, chlorophyll c and bacteriochlorcphyll a in nematic liquid crystal matrixes. The polarized absorption and fluorescence spectra as a function of the electric field have been measured. From the polarized components of the absorption A( parallel) and A( perpendicular) of the pigments in liquid crystals two reduced components A(x) and A(y) are calculated (x and y are the direction of the axis which is going through the second, fourth pyrrol rings, and the first, third rings, respectively). From these results the orientation of chlorophylls in liquid crystals and the configuration of the transition moments in the skeleton of the pigment molecules were determined.

Organic photovoltaic solar cells: spectroscopic and photoelectric properties of photoactive dyes

Abstract This paper presents the results of the study of correlation between the molecular structure of photoactive dyes and their spectroscopic properties with their ability for solar energy in electric energy conversion when immersed in a sandwich-like photoelectrochemical cell. Two families of dyes were investigated: porphyrins and phthalocyanines. Spectroscopic studies were accompanied with photoelectric examinations. The influence of metal incorporated into the dye molecular core and of the peripheral groups on the photoelectric response is described. Improvement of the photocurrent intensity is discussed in terms of increasing a number of π electrons in the molecular skeleton, their delocalization and weakening of their bonding in the conjugated system and energy transfer between dyes. To cite this article: D. Wrobel, C. R. Chimie 6 (2003).

Photocurrent generation in an electrochemical cell with substituted metalloporphyrins

Abstract Photocurrent generation in a semiconducting–metallic electrochemical cell filled with metallotetraphenyloporphyrins dissolved in polyvinylalcohol and metallotetranaphthyloporphyrins in nematic liquid crystal was investigated. The photopotential action spectra showed that the dye molecules are responsible for photoresponse. It has been shown that the intensity of photocurrent was dependent on the type of central metal complexed with the porphyrin ring of the dye molecule and also on the substituents attached to the methine bridge. The dependencies of the photopotential signal and of incident light–photosignal phase shift as a function of modulation light frequency indicate that the observed effect is due to more than one mechanism of electron transport. The results for metalloporphyrin dyes in fluid polyvinylalcohol and in nematic liquid crystal (D. Wrobel, J. Goc, R.M. Ion, J. Mol. Struct. 450 (1998) 239) were compared. The reasons of the different effectiveness of photoresponse in the electrochemical cell with different porphyrins complexed with various metals and substituted by different functional groups are discussed.

Porphyrins and Phthalocyanines – Functional Molecular Materials for Optoelectronics and Medicine

ABSTRACT This paper deals with photophysical characterization of some synthetic porphyrins and phthalocyanines as well as natural chlorophylls and bacteriochlorophylls as attractive multifunctional organic materials for novel molecular photovoltaics and for photodynamic therapy or diagnosis of cancerous tissues. The following spectral methods were used in our investigations: electronic absorption, fluorescence, steady-state photoacoustic spectroscopy and time-resolved optoacoustic spectroscopy to get insight into the mechanisms responsible for molecular processes occurring in dyes upon light illumination. Spectral study was supported by photoelectric examination. The spectral parameters of dyes which are essential for photovoltaics and/or photodynamic therapy/diagnosis were estimated: absorption molar coefficients, fluorescence quantum yields, natural life-times, thermal deactivation parameters, quantum yields of triplet-state population, triplet-state thermal relaxation decay. It was shown that some synthetic porphyrins, phthalocyanines and natural (bacterio)chlorophylls can be potentially used as functional materials for both optoelectronics and/or medicine.

Fluorescence and Time-Resolved Delayed Luminescence of Porphyrins in Organic Solvents and Polymer Matrices

Porphyrin dyes fulfill an essential function in photosynthesis and are important in photodynamic therapy and in a range of electronic devices. Their spectroscopic characteristics may play a crucial role in these processes. The spectral properties of two porphyrin dyes: tetraphenylporphyrin and tetraphenysulfporphyrin in organic solvents (acetone, chloroform, methyl alcohol, and dimethyl-sulfoxide) and in polyvinyl alcohol and poly(methylmethacrylate) films have been investigated. Absorption, fluorescence, and microsecond time-resolved delayed luminescence spectra have been measured at room temperature. The existence of different aggregated dye forms in the ground and excited states has been demonstrated. The manifold of dye species depends on the solvent/polymer. In the case of the polymers, it also depends on the solvent used to coat the polymer film. Delayed luminescence spectra and decay times of the two porphyrins in the different solutions and in polymeric matrices suggest that different mechanisms of deexcitation of the singlet excited states may be responsible for their generation in these and other porphyrin dyes.

Fluorescence quenching and ESR spectroscopy of metallic porphyrins in the presence of an electron acceptor

Abstract In this paper we determined the optical and ESR properties of metal-free and metal complex tetraphenylporphyrins in the presence of anthraquinone in an isotropic solution of dimethylsulfoxide. Such a system can mimic the electron transfer process between the excited porphyrin dyes and a semiconducting electrode in a photoelectrochemical cell. Dye fluorescence quenching were followed by ESR to determine the origin of an unpaired electron in porphyrins. The Stern–Volmer quenching constants ( K Q ), bimolecular quenching rates ( k q ) and electron transfer rates ( k ET ) were determined. The K Q values changed in the range of 0.5×10 4 to 2.3×10 4 M −1 depending on the metal ion incorporated in the porphyrin, k q changed from 7.0×10 13 to 1.3×10 14 M −1 s −1 . The g -values and Δ B were estimated, which showed that the conjugated porphyrin was the origin of the unpaired electron. Both Mg–tetraphenylporphyrin, and Zn–tetraphenylporphyrin showed different photophysical behaviour compared to their Pb–tetraphenylporphyrin and Pt–tetraphenylporphyrin counterparts. The relationship between dye fluorescence quenching and ESR results is discussed.

Spectroscopic and photoelectric studies of phthalocyanines in polyvinyl alcohol for application in solar energy conversion

Phthalocyanines are synthetic dyes which are shown to be good photoconverters which can be applied in a photoelectrochemical cell constructed of semiconducting and metallic electrodes. By means of spectroscopic investigation (absorption,fluorescence and photoacoustics) we have followed spectral properties of phthalocyanines complexed with metals dissolved in polyvinyl alcohol solution in order to determine the pathways of deactivation of their excited states. Spectroscopic investigations were accompanied with photovoltaic/photocurrent and current-voltage characteristics of the photoelectrochemical cell with dyes. It has been shown that there is a correlation between dye fluorescence feature and dye ability to electric photoconversion. The comparison of new data for metallic phthalocyanines with those obtained for porphyrins is discussed.