Photophysical processes in dimers of thiacarbocyanine cations

The results of quantum-chemical calculations of absorption, fluorescence, and induced T-T absorption spectra and the rate constants of photophysical processes for the monomer and dimers of the 3,3′-diethylthiacarbocyanine cation are presented. It has been shown that the formation of the dimers leads to a substantial reduction in the efficiency of the radiative pathways and enhancement of the efficiency of the non-radiative pathways of deactivation of the fluorescent state, thereby resulting in a decrease in the quantum yield of fluorescence. The fluorescence quantum yield depends in this case on the relative orientation of the molecules in the dimer with respect to each other. The calculation results satisfactorily describe experimental data on the spectral and luminescent properties of the dimers of the 3,3′-diethylthiacarbocyanine cation and their photophysical processes.     

Specificity of the redox photoreaction of dimers of thiadicarbocyanine dyes

The primary steps of the redox reaction of dimers of the thiadicarbocyanine dye and its 5,5′-dichloro derivative in aqueous solutions were studied in the presence of 4-nitroacetophenone, ascorbic acid, or hydroquinone. In water the dye molecules (anion, M^?) mainly exist as dimers M₂ ^2?. The laser pulse irradiation (10 ns, 532 nm) results in the population of the lowest triplet level M₂ ^2?, whose depletion occurs due to both intersystem crossing to the ground state and photoinduced transition to the highest triplet state of the dimer followed by photoionization. Photoionization at low intensities of a laser pulse proceeds via the one-quantum mechanism going to the two-quantum mechanism with an increase in the laser pulse intensity. The photooxidation of the dimer in the lowest triplet state with 4-nitroacetophenone results in the formation of unstable radical anion M₂ ^?· that spontaneously dissociates to monomer M^? and radical M^· of the dye. In the presence of electron donors (ascorbic acid, hydroquinone), the dimers in the triplet state are not photoreduced, but the electron donors reduce M₂ ^?· and M^· to the dye dimer and monomer, respectively.     

Primary processes in the photosensitized polymerization of cationic monomers

A detailed time-resolved laser spectroscopy investigation has been carried out on the electron transfer reactions of substituted thioxanthone derivatives with diphenyliodonium (Ph-I⁺) salts having different metal halide counterions (MX^?_n). Quenching of thioxanthones' triplet state has been followed under various conditions, by changing the number and nature of substituents on the thioxanthone skeletone, using anion with different nucleophilicity and employing different solvents, namely methanol and acetonitrile. A Photosensitization mechanism is proposed involving an electron transfer from thioxanthone to diphenyliodonium salt. The absorption spectra of the thioxanthone's excited state and the formed new transient are recorded and the rate constants of the excited state processes are measured. The triplet state of thioxanthone derivatives has been quenched by cationically polymerizable monomers and the quantum yield of the major processes has been evaluated. Photolytic experiments have been performed to measure the extent of acid formation. Form photopolymerization experiments using different photoinitiating systems, the rate of polymerization and percentage of monomer conversion have been determined. Both the reactivity in the excited states and the nucleophilicity of the anions affect the efficiency of the photopolymerization reaction.     

Specifics of deactivation processes of excited states of thiacarbocyanine dyes adsorbed on cellulose

Spectral and kinetic study on deactivation processes of electronically excited states of molecules of cationic (Dye1, Dye2 and anionic (Dye3–Dye5) thiacarbocyanine dyes adsorbed on cellulose was performed. Adsorption of the dyes leads to an increase in the concentration of dimers. The dimers of adsorbates of Dye3–Dye5 exhibit only phosphorescence, whereas the monomers of the adsorbates are capable of both phosphorescence and delayed fluorescence. Adsorbates of Dye3–Dye5 prepared from water and methanol exhibit phosphorescence of cis-monomers. Adsorbates of Dye1 from water and ethanol as well as adsorbates of Dye2–Dye5 from chloroform exhibit delayed fluorescence of trans-monomers. Phosphorescence and delayed fluorescence are observed at room temperature in the presence of air oxygen.     

Investigation of the pathways of thermal fragmentation of thiacarbocyanine dyes

The pathways of the thermal transformations of thiacarbocyanines with various alkyl groups attached to the ring nitrogen atoms of the heteroresidues, of 3,3-diethylthiacarbocyanine with various anions, and its anhydro base were investigated. The results of the studies are compared with the results of quantumchemical calculations of the labilities of the bonds in these compounds by the Pariser-Parr-Pople method.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1626–1630, December, 1981.     

Effects of NaCl on the J-aggregation of two thiacarbocyanine dyes in aqueous solutions

The effects of NaCl on the aggregation of two typical thiacarbocyanine dyes (3,3'-di(3-sulfopropyl)-4,5,4',5'-dibenzo-9-phenyl-thiacarbocyanine triethyl ammonium salt (Dye 1) and 3,3'-di(3-sulfopropyl)-4,5,4',5'-dibenzo-9-methyl-thiacarbocyanine triethyl ammonium salt (Dye 2)) in aqueous solution have been studied by using absorption spectroscopy, fluorescence spectroscopy, and 1H- and 23Na-NMR measurements. It is found that the J-aggregation of two dyes can be promoted by the addition of NaCl and that the effective coherence length of the J-aggregate is shorter than that obtained without NaCl. Fluorescence spectra demonstrate that the fluorescence intensities of the J-aggregates of two dyes are quenched by addition of NaCl. This is consistent with the decrease of the effective coherence length of J-aggregates of the two dyes in the presence of NaCl. 1H-NMR spectra of two dyes show that the Na(+) ions penetrate into the J-aggregates and replace the counterion (triethylammonium ions) in two dyes. The measurements of the chemical shifts of 23Na nuclei provide further information about the interaction between the Na(+) ions and dye anions in the J-aggregates of the two dyes. Due to this interaction, the electrostatic repulsion between the dye anions in the J-aggregates can be reduced and thus accelerate the aggregation of the two dyes in the presence of NaCl. The apparent association constants between Na(+) ions and dye molecules in J-aggregates of Dye 1 and Dye 2 estimated from the measured chemical shifts of 23Na nuclei are about 2.38 M(-1) and 1.35 M(-1), respectively.     

Spectral study of the noncovalent interaction of thiacarbocyanine dyes with hyaluronic acid

The noncovalent interaction with hyaluronic acid (HA) of four cyanine dyes having similar structures, 3,3′,9-trimethylthiacarbocyanine (Cyan 2), 3,3′-dimethylthiacarbocyanine (Cyan 45), 3,3′-diethylthiacarbocyanine (DTC), and 3,3′-diethyl-9-methylthiacarbocyanine (DMTC), was studied using the absorption spectra. Dyes Cyan 2, Cyan 45, and DMTC were shown to form H-aggregates in the presence of HA, which is reflected in the appearance of a short-wavelength absorption band with a maximum at 440–450 nm. By the tendency to H-aggregation in the presence of HA, the dyes can be arranged in the series Cyan 2 > Cyan 45 > DMTC > DTC. The aggregation number for Cyan 2 was determined: n = 3. In buffer solutions pH 4.5, 7.0, and 9.1, the absorption spectra of H-aggregates of Cyan 2 have longer wavelengths (with maximums at 460–470 nm) than those in the absence of buffers. Cyan 2 is suggested as a spectral probe to detect HA in biological systems.     

Photochemical properties of meso-substituted thiacarbocyanine dyes in solutions and in complexes with DNA

The processes of cis-trans photoisomerization and thermal back isomerization as well as the effect of DNA on the spectral and kinetic characteristics of the triplet state of a number of meso-substituted thiacarbocyanine dyes: 3,3’-diethyl-9-methoxythiacarbocyanine iodide (K1), 3,3’,9-triethylthiacarbocyanine iodide (K2), 3,3’-diethyl-9-methylthiacarbocyanine iodide (K3), and 3,3’-diethyl-9-chlorothiacarbocyanine perchlorate (K4), were studied by the flash photolysis method. Upon flash photoexcitation, the processes of trans-cis and cis-trans photoisomerization were observed for dye K1; the data on the structure of the absorption bands of the photoisomers were obtained. Complexation with DNA leads to an increase in the quantum yield of the triplet state of the dyes, which is explained by growing rigidity of the bound molecules. In the presence of DNA, triplet state deactivation follows the two-exponential law, thus showing that the dyes form complexes of two different types. The processes of quenching of the dye triplet state by oxygen were studied in solutions and in complexes with DNA. The rate constants for oxygen quenching of the triplet state of the dyes in complexes with DNA were found to be much lower than the values expected for the diffusion-controlled reactions (with allowance for the spin statistical factor, $k_{qO_2 } < 1/9k_{dif} $ ), which is explained by the steric factor of the complexation.     

Modeling the structure,absorption spectra,and cis-trans isomerization of thiacarbocyanine dyes

The relative stability of the trans-and cis-isomers of 3,3′-diethylthiacarbocyanine (Dye1) and 3,3′-diethyl-9-methylthiacarbocyanine (Dye2)¹, as well as sections of the potential energy surfaces along the internal coordinate of the isomerization reaction, were studied using the density functional theory. Calculation of the minimum energy pathway for the isomerization reaction showed that the barrier for rotation about the C₈–C₉ bond is higher for Dye1 than for Dye2. Local minimums were found for the singlet excited state of the 8,9-cis-and trans-isomers of the dyes. In the case of the trans-isomers, substantial changes in the dye structure do not occur and the local minimum of the excited state corresponds to the geometry of the starting trans-isomers, which favors efficient fluorescence. A search for the nearest local minimum of the singlet excited state of the 8,9-cis-isomers leads to structures, which differ significantly from the starting structures, and the intensity of the S₁ → S₀ transition in those structures appears to be practically zero. The results are in agreement with experimental data on the absorption, fluorescence, and fluorescence excitation spectra of the dyes.     

Photonics of dimers of cyanine dyes

The results of study on the properties of dimers of thiamonomethine-and thiatrimethinecyanines (thiacarbocyanines) in the ground and electronically excited states in aqueous solutions are presented. Dimers of cyanine dyes have the sandwiched structure with near-parallel alignment of the polymethine chains of the monomers in the dimer. The formation of dimers is manifested by two absorption bands of different intensities due to splitting of the S* level of the monomers upon their resonance interaction. Dimers of thiacarbocyanines are characterized by a low fluorescence quantum yield φ_fl as compared to monomers; however, φ_fl of dimers of thiamonomethinecyanines are markedly higher than that of monomers. Dimers of cyanine dyes are also characterized by a relatively high quantum yield of intersystem crossing to the triplet state. In the triplet-triplet absorption spectra, two bands of different intensities are revealed, which are due to the splitting of the higher triplet level of the monomers that form the dimer. In the presence of electron donors (ascorbic acid, hydroquinone) and/or acceptors (p-benzoquinone, p-nitroacetophenone, methylviologen), the triplet state of dimers is quenched as a result of electron transfer yielding radical products. Dimers in the triplet state can serve as photosensitizers of redox reactions.     

Tuning photosensitized singlet oxygen generation efficiency of novel aza-BODIPY dyes

Novel aza-BODIPY derivatives substituted with heavy atoms such as bromine and iodine were synthesized, and their triplet and singlet oxygen generation efficiencies have been investigated. These derivatives showed absorption in the NIR region with high molar extinction coefficients. The dye substituted with four iodine atoms showed yields of Φ(T) = 0.78 and Φ((1)O(2)) = 0.70, which are the highest values so far obtained for the aza-BODIPY derivatives.     

Triphenylmethane dyes as redox indicators in dichrometry

Summary The use of some triphenylmethane dyes, Erioglaucine A, Eriogreen B, Xylenecyanol FF, Setoglaucine O, Setocyanine Supra, Fast Green FCF, and Night Blue as redox indicators in the dichrometric titration of iron(II), ferrocyanide, uranium(IV), molybdenum(V), and hydroquinone in hydrochloric acid, sulphuric acid and perchloric acid media has been studied. Conditions for the satisfactory titrations employing these indicators have been established.

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Triphenylmethanfarbstoffe als Redoxindicatoren bei Titrationen mit Dichromat

Zusammenfassung Bei der Titration von Eisen(II), Hexacyanoferrat(II), Uran(IV), Molybdän(V) und Hydrochinon mit Dichromat in salz-, schwefeloder perchlorsaurer Lösung wurden folgende Triphenylmethanfarbstoffe als Indicatoren verwendet: Erioglaucin A, Eriogrün B, Xylolcyanol FF, Setoglaucin O, Setocyanin Supra, Echtgrün FCF und Nachtblau. Die entsprechenden Titrationsbedingungen werden mitgeteilt.

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We are grateful to the Council of Scientific & Industrial Research, India, for the award of a Junior Research Fellowship to one of us (V. V. S. E. D.).     

Primary processes in plant photosynthesis: photosystem I reaction center

The photosystem I (PSI) pigment-protein complex of plants converts light energy into a transmembrane charge separation, which ultimately leads to the reduction of carbon dioxide. Recent studies on the dynamics of primary energy transfer, charge separation, and following electron transfer of the reaction center (RC) of the PSI prepared from spinach are reviewed. The main results of femtosecond transient absorption and fluorescence spectroscopies as applied to the P700-enchied PSI RC are summarized. This specially prepared material contains only 12–14 chlorophylls per P700, which is a special pair of chlorophyll a and has a significant role in primary charge separation. The P700-enriched particles are useful to study dynamics of cofactors, since about 100 light-harvesting chlorophylls are associated with wild PSI RC and prevent one from observing the elementary steps of the charge separation. In PSI RC energy and electron transfer were found to be strongly coupled and an ultrafast up-hill energy equilibration and charge separation were observed upon preferential excitation of P700. The secondary electron-transfer dynamics from the reduced primary electron acceptor chlorophyll a to quinone are described. With creating free energy differences (ΔG₀) for the reaction by reconstituting various artificial quinones and quinoids, the rate of electron transfer was measured. Analysis of rates versus ΔG₀ according to the quantum theory of electron transfer gave the reorganization energy, electronic coupling energy and other factors. It was shown that the natural quinones are optimized in the photosynthetic protein complexes. The above results were compared with those of photosynthetic purple bacteria, of which the structure and functions have been studied most.     

Cyanine dyes as protectors of K562 cells from photosensitized cell damage

Several cyanine dyes were found to protect K562 leukemia cells against toxicity mediated by cis-di(4-sulfonatophenyl)diphenylporphine (TPPS2) and light. Most cyanine dyes derived from dimethylindole were better photoprotectors than cyanine dyes with other structures. This correlated with the fact that cyanine dyes derived from dimethylindole were predominately monomeric at millimolar concentrations within K562 cells, while other cyanine dyes formed aggregates. For cyanine dyes that are derived from dimethylindole and have absorption band wavelengths greater than 700 nm, fluorescence-energy transfer from TPPS2 to the cyanine dye was the most important mechanism for photoprotection. There was no spectroscopic evidence for complex formation between the cyanine dyes and TPPS2. The dimethylindole derivative, 1,1',3,3,3',3'-hexamethylindodicarbocyanine, was an excellent photoprotector, but a poor quencher of TPPS2 fluorescence and a relatively poor singlet-oxygen quencher. This cyanine dye may act by quenching excited triplet TPPS2. Singlet-oxygen quenching may contribute to the photoprotection provided by cyanine dyes not derived from dimethylindole. Differences in the subcellular distribution of the various cyanine dyes studied may have contributed to the different apparent mechanisms of photoprotection.     

Photoprocesses in dimers and dimeric complexes of polymethine dyes with cucurbiturils

Russian Chemical Bulletin - The review is devoted to the structure and photoprocesses in dimers of polymethine dyes and their complexes as objects of supramolecular photochemistry. The results of...