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Yield of intersystem (singlet-triplet) crossing in phthalocyanines evaluated on the basis of a time in resolved photothermal method

The yields of triplet state formation for seven various phthalocyanines (Pc) unsubstituted by metal and substituted by Mg, Zn, Cu and Al in dimethyl-sulfoxide (DMSO) solutions were compared on the basis of time resolved photothermal measurements. The sample with atmospheric oxygen and bubbled by nitrogen were investigated. Absorption, fluorescence, and steady state photoacoustic spectra (PAS) of the same samples were also measured. The yields of triplet state formation for most of metal substituted dyes were higher than for investigated metal-free dyes. The triplets for various dyes were differently quenched by oxygen. The possibility of the application of investigated dyes in photodynamic therapy and diagnosis was discussed.

ROOM TEMPERATURE TIME‐RESOLVED IN μs RANGE DELAYED LUMINESCENCE OF CHLOROPHYLL a AND CHLOROPHYLL‐LUTEIN MIXTURE SOLUTIONS

Using time‐resolved in μS range luminescence spectroscopy, we observed at 20°C the emission of chlorophyll a, pheophytin a and chlorophyll a‐lutein mixture solutions. This delayed emission exhibits several maxima in the650–750 nm region. The positions and kinetics of decay of delayed emission bands depend on chlorophyll concentration, and vary as a result of pheophytinization and addition of lutein. Our results can be explained by supposition that upon excitation, charge transfer species are formed in various pigment complexes. The back electron transfer reactions yield chlorophyll excited singlet states contributing to observed delayed emission. Delay in emission seems to be due also to the trapping of excitation on the triplet states of various forms of pigment and its detrapping with the participation of thermal energy followed by energy transfer to the forms of pigment characterized by different decay times.

SPECTRAL PROPERTIES OF BCHL C IN NEMATIC LIQUID CRYSTALS. II : AGGREGATED FORM OF DYE

Abstract The orientation properties of aggregated bacteriochlorophyll c in a nematic liquid crystal matrix have been studied by means of polarized steady-state and time-resolved spectroscopy. The pigment introduced to the model system was predominantly in tetrameric (or hexameric) form with absorption maxima located at about 670 and 710 nm. Two tetramers with long-wavelength maxima at 704 and 722 nm could be distinguished. They exhibited different orientations in liquid crystal and different yields of fluorescence. Both tetramers exhibited delayed luminescence in the μs time region.

MELANIN-PORPHYRIN INTERACTION MONITORED BY DELAYED LUMINESCENCE AND PHOTOACOUSTICS

Even though melanin is commonly viewed as a photoprotective agent, the molecular mechanism of the melanin-related photoprotective action remains unclear. We studied the interaction of a synthetic dopa melanin with positively and negatively charged porphyrins using an array of spectroscopic techniques. Thus absorption, fluorescence, time-resolved delayed luminescence in the microsecond time range, and photoacoustic spectra at different modulation frequencies of both porphyrins and their mixtures with dopa melanin were measured in buffered aqueous solution at room temperature. It has been confirmed that dopa melanin forms a complex with the cationic porphyrin in its ground state, which significantly modifies the optical properties of the dye molecule. Although no such complex can be detected for the anionic porphyrin, some interaction between melanin and the porphyrin molecule can be detected in its excited state. Both porphyrins show delayed luminescence with a decay time of 35-38 microseconds, which is substantially reduced by melanin. Our photoacoustic measurements indicate that upon photoexcitation, melanin efficiently liberates heat. Our data also suggest that the thermal properties of melanin can be modified by porphyrins, particularly by the cationic form of the dye. The ionic complexes of dopa melanin with the positively charged porphyrin exhibit properties typical for a supermolecular system that liberates heat as a whole.

Fluorescence of aggregated forms of Chl a in various media

Abstract The fluorescence lifetimes in the picosecond time range of various aggregated forms of chlorophyll a (Chl a ) in different media (wet n -haxane, adsorbed layers and polymer films) were measured. Photographs of Chl a and Chl b poly(vinyl alcohol) (PVA) samples containing pigment oligomers are compared and discussed on the basis of previous and present spectral data concerning chlorophyll aggregation. In the investigated samples, the aggregated forms of Chl a can be divided into three groups: “dry monomer” (isolated from interaction with water), hydrated dimers and oligomers built from such dimers. These three forms have different emission lifetimes and, from photoacoustic spectra, yields of thermal deactivation. The excitation energy transfer from dry monomers to aggregated forms is rather ineffective.

Spectral properties of bacteriochlorophyll c in nematic liquid crystal. Part 1. Monomeric forms of dye

Abstract The spectroscopic features of bacteriochlorophyll c and bacteriopheophytin c in a nematic liquid crystal matrix have been investigated. Absorption, circular dichroism, fluorescence and time resolved delayed luminescence spectra have been measured. The pigment is introduced to the liquid crystal from a dry and from a hydrated chloroform solution. In both cases the pigment is in the monomeric form. Hydration of the solvent and the presence or absence of the central Mg atom affect the interaction of the pigment molecules with the liquid crystal matrix, changing the fluorescence anisotropy. A model for the bacteriochlorophyll c orientation in the liquid crystal is proposed and the averaged angles between the transition moments and the liquid crystal orientation axis are determined. A slow process (in the microsecond range) of radiative deactivation of energy absorbed by the pigments is observed. This delayed emission could be due to pigment ionization and delayed charge recombination and/or thermal activation from the triplet to the excited singlet state.

Action spectra of the photopotential generation for pigment and dye solutions in nematic liquid crystals located in the electrochemical cell

Abstract The method of measurements of the action spectra of photopotential generation using a modulated acting light beam was elaborated and applied to two systems: the solutions of stilbazolium merocyanines and to the mixture of chlorophyll a and luteine. Both types of samples were dissolved in nematic liquid crystal and located either between two semiconducting electrodes, or between a transparent gold electrode and a semiconducting electrode. By changing the frequency of light modulation or the phase shift between a modulated light beam and a measured photopotential signal, information about the kinetics of photopotential generation is obtained. The free base and protonated merocyanine forms exhibit different kinetics of potential generation. The amplitude of the photopotential of the chlorophyll and luteine mixture is higher than the sum of the amplitudes of the photopotential of these pigments located in separated cells. In a cell with two semiconducting electrodes, the light gradient (e.g., different illumination of front and back electrodes due to light absorption) plays an important role in photopotential generation.

Photoelectric properties of chlorophyll and carotene solutions in nematic liquid crystal located between semiconducting electrodes.

The photopotential and photocurrent generation for chlorophyll a, beta-carotene and a mixture of these pigments dissolved in nematic liquid crystal and located between transparent semiconducting electrodes were measured. Both pigments exhibit photopotential and photocurrent generation. From the photocurrent amplitudes it follows that the efficiency of electron transfer to a semiconducting electrode from beta-carotene is higher than from chlorophyll alpha. The photocurrent amplitude of the pigment mixture is slightly lower than that calculated as a sum of amplitudes of pigments located in separated cells. This difference can be explained by secondary effects, such as competition between carotene and chlorophyll molecules in a process of adsorption on a semiconducting electrode. Therefore it seems that no charge transfer complexes of chlorophyll and carotene are formed in the investigated model system.

Fast and slow processes of thermal deactivation of excited stilbazolium merocyanine dyes

Abstract The long living triplet states play important role in sensitizing action in all photochemical reactions. The yield of generation of triplet states of dyes can be evaluated on the basis of measurements of their slow (microsecond) thermal deactivation (TD). All experiments were carried out in the oxygen presence, it means under quenching dye triplets. The pulse dye laser generates in the investigated solution pressure signal. The high of the amplitudes of first maximum of this pressure wave-form signal in the solution of the investigated dye and in the reference sample were measured. Reference sample exhibits only fast processes of TD. The comparison of the first maximum of wave-form photothermal signal of sample and of reference enable to calculate part of energy exchanged into heat in time longer than time resolution of arrangement. The fluorescence yields of investigated dyes were also established. On the basis of such data, using procedure described in literature, the yield of singlet–triplet intersystem crossing (ISC) was evaluated. It was shown that this yield depends on the length of stilbazolium merocyanine chain. The product of triplet state yield and energy was lower for merocyanines with longer chains. At lower temperatures the yield of fluorescence increases and amount of excitation exchanged in short time into heat decreases. The slow TD process increases in low temperature because of the decrease in the quenching of the dyes triplet states by oxygen. The amount of energy exchanged into heat in a time longer than time resolution of apparatus is due predominantly through TD of the dye triplet states.

Aggregation of chlorophyll b in model systems

Abstract Chlorophyll b (Chl b ) in model systems occures in several forms. The relationship between these forms and the forms occurring in organisms is not yet clear. Chl b aggregation has been investigated less than that of Chl a . In this paper, several spectral features, e.g. fluorescence lifetimes in the picosecond time range, time-resolved delayed luminescence spectra, photoacoustic spectra and photopotential generation of Chl b embedded in model systems (poly(vinylalcohol) film or nematic liquid crystal), were measured. The decay of the fluorescence of Chl b in polymer films can be analyzed, to a good approximation, on the basis of the following three exponential components: 3300–4300 ps, 400–900 ps and 40–71 ps. These decays are tentatively related to the emission of “dry” monomers and dry and wet dimers and oligomers respectively. The delayed luminescence spectra of the same samples in the microsecond range (at 8–295 K) are located in a similar spectral range to the fluorescence spectra. The intensity ratio between the delayed luminescence and prompt fluorescence is higher in the long-wavelength region, in which the oligomer emission is observed, than in the short-wavelength region, in which the emission of monomers and small aggregates predominates. The yield of thermal deactivation of the aggregated forms is higher than that of the monomers. The differences between the lifetimes of the various forms can be explained by the competition between the emission of prompt fluorescence, thermal deactivation and energy trapping (which is, in part, later deactivated as delayed luminescence). The excitation energy transfer from “dry” monomers to aggregated forms is not very effective. The most effective process of excitation trapping followed by delayed luminescence emission occurs in oligomers of Chl b . On the basis of photopotential generation, the delayed luminescence is due, at least in part, to pigment ionization followed by slow charge recombination. The kinetics of photopotential generation and decay depend on the aggregation of the pigment.