Alina Dudkowiak

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.

Electrochemical cell with bacteriochlorophyll c and chlorophylls a and b in nematic liquid crystal

Abstract The photocurrent generation and current-voltage (I(V)) characteristics of the electrochemical cells consisting of a layer of chlorophyll a, chlorophyll b or bacteriochlorophyll c solution in a nematic liquid crystal sandwiched between two semiconducting (In2O3) electrodes covered by an orienting layer SiOx were investigated. Bacteriochlorophyll c was introduced into the nematic liquid crystal in monomeric and in aggregated predominantly tetrameric forms. The photoelectrical properties of bacteriochlorophyll c in monomeric and aggregated forms are different. All investigated samples exhibit cathodic photocurrent related to the extraction of electrons from the conduction band of the illuminated semiconducting electrode. Amplitudes of generated photocurrents and the shapes of the current-voltage voltammogramic loops, at the same conditions of illumination, are for various chlorophylls different.

THE ORIENTATION OF BACTERIOCHLOROPHYLL C IN GREEN BACTERIA CELLS AND CELL FRAGMENTS

Abstract Green bacteria cells, their fragments and isolated pigment molecules were oriented by embedding them in anisotropic polymer film. Two methods of film deformation were used: uniaxial stretching, and skew stretching in which a previously uniaxially elongated film was again stretched along the axis, forming a 40 ° angle with the previous direction of stretching. Photographs of samples in transmitted light and under a polarized fluorescence microscope were taken. Polarized absorption and fluorescence spectra were measured, and order parameters were calculated and discussed. In the 670–680 nm region absorption and emission of disaggregated bacteriochlorophyll c from chlorosomes and bacteriopheophytin c is observed. The decrease of mutual pigment interactions caused the spectral shift of the absorption band from 750 to 670 nm; however, disaggregated pigment is still located in chlorosomes and preserves the orientation. The film stretching diminished the absorption of bacteriochlorophyll c in chlorosomes (at about 750 nm) but only slightly increased the absorption of disaggregated bacteriochlorophyll c (670 nm). This suggests that as a result of stretching, part of the transition moments of the chlorosome bacteriochlorophyll c red band became located outside of the plane of the electric vector of the spectrophotometer light beam. In the case of a large sample such as whole bacteria or their fragments, the axis of orientation is closer to the direction of the first stretching axis than to the second skew axis. Separated pigment molecules are oriented along the second axis of stretching. The dimensions of objects seen under the fluorescence microscope are larger than those of bacteria, therefore they are clusters of bacteria or bacteria fragments. Results obtained from microscopic investigations and from polarized spectroscopy suggest that two pools of clusters of bacteria or fragment clusters exist with different angles of orientation (one under about 15 °, the second almost parallel to the film axis), and different average length. It was shown also that the pigment orientation depends strongly on the conditions of bacteria culture and sample preparation.

Photoelectrical properties of green bacteria cells and cell fragments located in electrochemical cell

Abstract Photopotential action spectra as well kinetics of their generation and decay for green bacteria Prosthecochloris aestuarii cells and cell fragments were measured. The sample was located between two transparent electrodes, both semiconducting or one semiconducting and second one metallic. The electric properties of the cells in darkness and under illumination were also established. In the photopotential action spectrum the maximum about 750 nm, responsible for the bacteriochlorophyll c oligomers absorption, is absent. It shows that bacteriochlorophyll c molecules located in chlorosome oligomers are not taking part in the photopotential generation. From the action spectra it follows that predominantly desaggregated bacteriochlorophyll c and bacteriopheophytin, and may be also some chlorophyll a -like pigments are responsible for a photopotential generation. In the 670 nm region the desaggregated bacteriochlorophyll c as well the bacteriopheophytin absorption are located, whereas the Soret band is characteristic rather for the bacteriopheophytin, but some contribution from the desaggregated bacteriochlorophyll c cannot be excluded. With the sample pheophytinization the photopotential amplitudes increase. The complex kinetic of the photopotential generation suggests the superposition of at least two processes: the fast one, occurring just after light absorption, which probably is due to charge separation in the reaction centers and the second one—slower—may be related to some diffusion of the charge generated in the antenna pigments through the cell membrane. Due to slower photopotential decay and higher concentrations of the antenna pigments than that of the reaction centers, the contributions from the desaggregated antenna pigments, at used in the experiments frequencies of light modulation, predominant in the photopotential action spectra. For each sample the sign of the photopotential signal changes with the change of the side of illumination of the electrochemical cell.

Spectral properties of stilbazolium merocyanines – potential sensitizers in photodynamic therapy and diagnosis: Part I. Merocyanines in model systems

Abstract Spectral and photochemical properties of seven stilbazolium merocyanines of different chain length and side groups attached, dissolved in ethanol or methanol solutions as well as in polyvinyl alcohol water solutions and films, were investigated. The absorption, fluorescence emission and fluorescence excitation spectra as well as steady state photoacoustic and laser induced optoacoustic spectroscopy signals were measured. Triplet states are, because of their slow decay time, usually very effective in photochemical reaction; therefore dyes with efficient triplet states generation are suitable for destruction of illuminated diseased cells. The laser induced optoacoustic measurements were used to evaluate the efficiency of generation and decay times of dye triplet states. The quenching of the dye triplet state by oxygen was also studied. The results show that properties of merocyanines and their interactions with macromolecules depend strongly on the type of side groups attached. The dyes with NO 2 groups attached exhibit low yield of fluorescence therefore they could be useful rather in photodynamic therapy (PDT) than for diagnostic purposes. Some of the merocyanines investigated exhibit high yields of triplet state generation and, therefore, should be effective in photodynamic reactions. Further investigations will be carried out with the same dyes on healthy and diseased cells in order to show whether it is possible, on the grounds of spectral and photochemical properties of dye molecules in simple model systems, to predict the efficiencies of the dye incorporation into cells and the yield of photodynamic reactions.

Molecular symmetry determines the mechanism of a very efficient ultrafast excitation-to-heat conversion in Ni-substituted chlorophylls.

In the Ni-substituted chlorophylls, an ultrafast (<60 fs) deactivation channel is created, which is not present in Ni-porphyrins. This observation prompted us to investigate in detail the mechanism of excitation-to-heat conversion in Ni-substituted chlorophylls, experimentally, using time-resolved laser-induced optoacoustic spectroscopy, and theoretically, using group theory approach. The Ni-substituted chlorophylls show exceptional photostability and the optoacoustic measurements confirm the prompt and very efficient (100%) excitation-into-heat conversion in these complexes. Considering their excellent spectral properties and the loss-free excitation-into-heat conversion they are likely to become a new class of versatile photocalorimetric references. The curious features of the Ni-substituted chlorophylls originate from the symmetry of a ligand field created in the central cavity. The central N-Ni(2+) bonds, formed via the donation of two electrons from each of the sp(2) orbitals of two central nitrogens to an empty [Formula: see text] hybrid centered on Ni(2+), have a considerable covalent character. The extreme rate of excited state relaxation is then not due to a ladder of the metal centered d-states, often invoked in metalloporphyrins, but seems to result from a peculiar topology of the potential energy surface (a saddle-shaped crossing) due to the covalent character of the N-Ni(2+) bonds. This is confirmed by a strong 0→0 character of electronic transitions in these complexes indicating a similarity of their equilibrium geometries in the ground (S(0)) and the excited states (both Q(X) and Q(Y)). The excitation energy is very efficiently converted into molecular vibrations and dissipated as heat, involving the central Ni(2+). These Ni-substituted pigments pose a fine exemplification of symmetry control over properties of excited states of transition metal complexes.

FLUORESCENCE LIFETIMES OF ORIENTED GREEN BACTERIA CELLS, CELL FRAGMENTS AND ORIENTED BACTERIOCHLOROPHYLL C MOLECULES

Abstract The decays of the polarized components of fluorescence (excited at 410 nm by a 70 fs laser flash) of whole bacteria Prosthecochloris aestuarii cells, their fragments and isolated bacteriochlorophyll c molecules located in isotropic and stretched polymer films have been measured and analysed using exponential components which in the literature data are assigned to monomers, small aggregates and oligomers of bacteriochlorophyll c. The amplitudes of the lifetime components depend only slightly on the light polarization but they change strongly with a change in the spectral region of fluorescence observation and the type of sample. Two spectral regions of fluorescence emission have been observed: at 680 nm (bacteriochlorophyll c and bacteriopheophytin monomers) and at 750 nm (bacteriochlorophyll c oligomers). The longest decay component (from 5.6 to 7.4 ns for various samples and light polarizations) is similar to that observed for monomer bacteriochlorophyll c in various solutions. The middle component (1.1–2.7 ns) is probably due to small bacteriochlorophyll c aggregates, whereas the shortest one (ranging from 0.001 to 0.37 ns depending on the type of sample, polarization of light, etc.) is related to bacteriochlorophyll c oligomers. Some components exhibit negative amplitudes, showing that the emission is generated by excitation transfer from other pigment forms. The amplitude of oligomer component emission is low in bacteria and bacteria fragments in stretched film, whereas it is high for artificial oligomers. This is because in organisms the excitation is efficiently transferred to bacteriochlorophyll a complexes emitting in a longer-wavelength range, whereas in the model system oligomers of bacteriochlorophyll c are the final acceptors of excitation. In bacteria the stretching of a polymer matrix not only orients the whole object but also, to some extent, perturbs the natural arrangement of pigment and the disaggregated part of oligomers to monomers and smaller aggregates. Therefore in stretched polymer we observe the middle component of decay. The intensities of various polarized components of emission measured for the same sample are different. The coefficients of the anisotropy of emission a short time (about 100 fs) after excitation and averaged over the whole decay time for anisotropic and isotropic samples have been calculated. In some cases a strong depolarization of fluorescence during the decay time is observed; in others the anisotropy only changes slowly in this time. The results are compared with literature data for lifetimes and anisotropy of fluorescence of bacteriochlorophyll c in organisms and in artificial aggregates.

The influence of the presence of lipid on the aggregation of 8,12-diethyl farnesyl bacteriochlorophyll c located in adsorbed layers and monolayers

The photoacoustic spectra and time-resolved delayed luminescence spectra in the microsecond time range were measured for layers of 8,12-diethyl farnesyl bacteriochlorophyll c adsorbed on quartz supports by solvent evaporation and as Langmuir-Blodgett monolayers. Both types of model system were also investigated with the addition of lipid. The data showed a very strong influence of lipid addition on pigment aggregation. In samples with synthetic and natural lipid addition, the pigments were found to be predominantly in the monomeric and dimeric states, whereas in the same type of sample without lipid, the pigments were aggregated to a higher degree. The influence of the presence of lipid on the aggregation of bacteriochlorophyll c in monolayers and adsorbed layers may also suggest that the contact of various pigment molecules with the lipids surrounding the chlorosome may influence the formation of various pigment aggregates in vivo. The synthetic lipid L-alpha-phosphatidylcholine dipalmitoyl and the natural lipid L-alpha-phosphatidylcholine type IVS from soy beans were used. In the latter case, only adsorbed layers were investigated. Our interpretation is preliminary as only one 8,12-diethyl farnesyl bacteriochlorophyll c homologue was present in our systems.

DELAYED MICROSECOND LUMINESCENCE OF PHYCOBILISOMES

The time‐resolved luminescence spectra (in the microsecond range) of phycobilisomes and biliproteins in buffer and polymer matrix were measured in the temperature range from 8 K. to 293 K. Delayed luminescence located in the same spectral region as prompt fluorescence of investigated samples (DLF) and the long‐wavelength delayed emission in the720–760 nm range (DL1) was observed. The temperature and viscosity dependencies of DLF and DL1 luminescences were different, but both do not have uniexponential decays and are not quenched by oxygen. This means that delayed luminescence could be generated without the participation of the triplet states, or the chromophores could be shielded by protein against interaction with oxygen. The linear dependence of delayed luminescence on exciting light intensity shows that delayed luminescence is monophotonically induced.

Energy deactivation pathways of porphyrins and dopa melanin in polyvinyl alcohol systems

Abstract Absorption, fluorescence, delayed luminescence and photoacoustic spectra have been measured to learn about the radiative and non-radiative processes in two porphyrins: (4- N,N,N,N -trimethylanilinium) porphyrin (TAP) and (4-sulphonatophenyl) porphyrin (TPPS 4 ) and their mixtures with a synthetic dihydroxyphenylalanine (dopa) melanin in polyvinyl alcohol (PVA) solution and in PVA film. Due to ionic interactions, the positively charged TAP in its ground state forms a complex with dopa melanin. Absorption and emission spectra show that the TAP—dopa melanin complex is preserved in PVA solution, whereas when the complex is embedded in PVA film a competitive interaction between the PVA and dopa melanin in the dye—polymer complex is observed. In the case of TPPS 4 , no strong interaction between dye and polymer has been observed either in PVA solution or in PVA film. Our photoacoustic spectra indicate that dopa melanin in both PVA solution and in buffer solution liberates heat promptly. Evidently, the PVA matrix does not affect, to any significant extent, the thermal properties of dopa melanin. The casting of PVA film changes the interaction of dopa melanin with porphyrins and in consequence their thermal properties. Under such conditions, porphyrin and dopa melanin behave like individual species. The new results are compared with spectroscopic data previously obtained for TAP and TPPS 4 in phosphate buffer solution, and show that phosphate buffer solution and PVA solution are suitable media for porphyrin—dopa melanin complex electron exchange study, whereas the PVA film can be an appropriate system for investigation of the energy transfer process between porphyrin and dopa melanin.