G. V. Zakharova

Photoprocesses in dimers of thiacarbocyanines

The properties of monomeric and dimeric thiacarbocyanine (1) and three derivatives with methyl (2) and ethyl (3 and 4) meso-substituents were studied in aqueous solution by time-resolved and steady-state spectroscopic methods. Dimerization is characterized by a blue-shifted band at 512–536 nm, a high association constant, e.g. KD=3.8×104 and 2.8×105 d mol-1 for 1 and 3, respectively and a negative enthalpy and enthropy, e.g. ΔHD=-72 kJ mol-1 and ΔSD=-142 J mol-1 K-1 for 3. Dimers are composed from the all-trans isomers for 1 and from mono-cis isomers for 2–4 with face-to-face alignment. They are non-fluorescent and the observed weak fluorescence originates essentially from the all-trans monomers. The triplet state of the monomer/dimer composition was observed upon naphthalene-sensitized and direct excitation. The triplet yield is largest for 3 and is due only to the dimer, whereas for 2 both monomer and dimer contribute to the overall T-T absorption spectrum. The amounts of T-T absorption and bleaching as a function of temperature resemble that of the ratio of monomer to dimer concentrations. Reasons for the lack of trans→cis photoisomerization in the cases of 2–4 are discussed.

Photoprocesses of thiamonomethinecyanine monomers and dimers

The photophysical properties and the monomer–dimer equilibrium of four thiamonomethinecyanine dyes (1–4) were studied by time-resolved and steady-state spectroscopic methods. In ethanol and aqueous solution at room temperature the behaviour is mainly due to a monomer and dimer, respectively. The dimerization constant for 2, the triethylammonium salt of 3,3′-di-(γ-sulfopropyl)-4,5-benzo-5′-chlorothiamonomethinecyaninebetaine, in aqueous solution is KD = 7 × 105 M−1 and lower in the presence of ethanol. Fluorescence occurs from both monomer and dimer; the quantum yield is Φf⩽0.006 for 1–4 in ethanol and 10–20 times higher in aqueous solution. The triplet state of the dimers was characterized; the lifetime is 0.1–0.9 ms and the yield ⩽0.2. With increasing temperature the amount of dimer is reduced, resulting in a correspondingly smaller value for T–T absorption. In microheterogeneous media, e.g. cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS) or Triton X-100, the equilibrium is shifted at 25°C towards the solubilized monomer above the critical micellization concentration (c.m.c). The CTAB concentrations for dimer deaggregation of 1, 3 and 4 are much lower (10–30 μM) than the c.m.c. The amount of T–T absorption decreases with the CTAB concentration and resembles that of the dimer ground state. For 2 the dimers are converted into J-aggregates at [CTAB] in the 10–20 μM range and split into monomers around the c.m.c.

Effects of substituents in the polymethine chain on the photoprocesses in indodicarbocyanine dyes

1,1′,3,3,3′,3′-Hexamethylindodicarbocyanine perchlorate (1) and iodide (1′) and five indodicarbocyanine derivatives (with substituents in the meso-position of the polymethine chain; R = CH32, Cl 3, Br 4, CN 5 and 1,3,3-trimethylindoline-2-ethenyl 6) were studied by time-resolved spectroscopy in slightly and strongly polar solvents, where ion pairs with ClO4– are present and absent, respectively. The quantum yield of fluorescence at 24 °C is Φf= 0.012 for 6 in ethanol and larger for 1–5.Φf increases markedly with decreasing temperature, approaching 0.7–0.9 at –196 °C. The fluorescence lifetime, e.g. typically τf= 3 ns in glassy ethanol, is correlated. trans→cis Photoisomerization occurs in virtually all cases. The quantum yield depends significantly on the medium and meso substituent; Φt → c is substantial for 2 and 3 in ethanol at 24 °C and decreases with decreasing temperature. The initial rotation around one of the central double bonds in the excited singlet state is activated. At room temperature the lifetime of the observed cis form is in the ms range. No discernible effect of ion-pairing on the kinetic parameters of the thermal cis → trans reversion could be found in toluene. The triplet state of the six dyes was characterized by sensitization; it plays a (minor) role on direct excitation only for 4 or in the presence of a heavy-atom solvent.

Reversible aggregation, precipitation and re-dissolution of rhodamine 6G in aqueous sodium dodecyl sulfate

Study of the interactions between a cationic laser-dye (rhodamine 6G or R6G) and an anionic surfactant (sodium dodecyl sulfate or SDS) in aqueous solution has demonstrated that the so-called premicellar effect on absorbance and fluorescence occurring in oppositely charged dye/surfactant systems is associated with a slow ion-pair aggregation/precipitation process. The red-shift and UV/visible band broadening which occurs during the precipitation process has been attributed to the formation of nanoparticles with size-dependent spectral properties. A slight increase of the SDS concentration (less than four-fold) gives rise to a slow re-dissolution of the previously precipitated particles. From the absorbance vs. time kinetic curves recorded during the aggregation/precipitation processes it can be deduced that a reaction-limited aggregation process dominates the early stages of the growth, while the end of the precipitation is diffusion-limited. Re-dissolution is also diffusion-limited. These findings have been interpreted by the formation and the re-dissolution of nanoparticles with possible fractal geometry. The whole process is likely to include several reversible steps. Among them, dimerization of the dye, ion-pairs and larger aggregates formation, precipitation of optically visible crystals, surfactant micellisation and re-dissolution of previously formed aggregates are the most prominent.

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.

Effect of cucurbituril on the primary photoprocesses in indocarbocyanine dyes in water

The effect of cucurbit[7,8]urils (CB7, CB8) on photoprocesses in indocarbocyanine dyes has been studied. Complexation with CB7 and CB8 reveals bathochromic shifts of the maxima in the absorption and fluorescence spectra. Fluorescence enhancement and decreasing have been observed for the complexes with CB7 and CB8, respectively. The dyes and their complexes are capable of trans–cis photoisomerization and hardly undergo intersystem crossing to the triplet state. The rate of the thermal cis–trans isomerization reaction is decreased due to complexation.

Specificity of photonics of 3,3′-diethyl-5,5′-dichloro-9-ethylthiacarbocyanine dimers in the presence of cucurbit[7]uril

Molecules of 3,3′-diethyl-5,5′-dichloro-9-ethylthiacarbocyanine form dimers in aqueous solutions, which are capable of fluorescence and intersystem crossing to the triplet state. In the presence of cucurbit[7]uril and alkali metals or ammonium cations, dye dimer complexes are formed, which exhibit phosphorescence and thermally activated delayed (E-type) fluorescence in air-saturated solutions at room temperature. With the use of quantum-chemical calculations, the structure of dimeric dye complexes with cucurbit[7]uril is suggested.

Trans-cis photoisomerization and photoinduced electron transfer as competitive reactions of 9-etylthiacarbocyanine in binary mixtures

The effect of the composition of the dioxane-water mixture on the ability of 9-ethylthiacarbocyanine to participate in competitive reactions of trans-cis photoisomerization and photoinduced electron transfer was studied. An increase in the dioxane content in the range 0–50 vol % leads to a shift of the equilibrium between the dye dimers and monomers toward the monomers (cis-monomers), which is accompanied by a drop in the yield of the triplet dimer molecules, which appear under the action of a laser flash, and electron-transfer products that are formed via the triplet state of dimers in the presence of methylviologen. With growing the dioxane content in the range 50–80 vol %, a shift of the equilibrium between the cis-and trans-monomers toward the trans-monomers occurs, which is accompanied by an increase in the fluorescence intensity. At the dioxane content above 80 vol %, a further shift of the equilibrium toward the trans-monomers occurs, which is accompanied by a substantial increase in the fluorescence intensity and an appearance under the action of a laser flash of the band of the triplet-triplet absorption of the trans-monomer and the absorption band of the cis-monomer as a result of trans-cis photoisomerization. The trans-monomers in the triplet state participate in the electron-transfer reaction with methylviologen. The intersystem crossing process competes with fluorescence and the trans-cis isomerization reaction, which occurs via the excited singlet state of the trans-monomers.

Primary processes in the photosensitized redox reaction of dimers of thiacarbocyanine dyes

The kinetics and the mechanism of the reaction of donor (ascorbic acid) oxidation by electron acceptors (methylviologen and p-nitroacetophenone) photosensitized by dimers of sulfoalkyl-9-ethylthiacarbocyanine dyes (Dye1, Dye2, and Dye3) were studied in aqueous solutions. Dimers of the dyes (dianions) are capable of transition to the triplet state that is mainly quenched by acceptors to form radical anions of dimers, which are unstable and dissociate within 10–12 μs into the monomer (anion) and its radical (the limiting reaction stage). The presence of a donor in the dye-acceptor mixture leads to one-electron reduction of the monomer radical to its anion followed by the dimerization reaction. The results of the analysis of the experimental data obtained by the laser photolysis technique are in good agreement with the calculated kinetic curves for the formation and the decay of the dimer radical anions.

Effect of Poly(methacrylic acid) on Photoinduced Processes with Cyanine Dyes in Aqueous Solutions

The cyanine dyes thiamonomethinecyanine (I), thiacarbocyanine (II), and thiadicarbocyanine (III) in an aqueous poly(methacrylic acid) (PMA) solution form different molecular species: monomers anchored to PMA, dimers, and H aggregates, in proportions determined by the [dye]/[PMA] concentration ratio. Each molecular species is characterized by its own probabilities of radiative and nonradiative transitions: the absence of photoisomerization for I–IIIdimers and H aggregates, the absence of fluorescence for H aggregates of IIand III, the capability of transition to a triplet state for PMA-anchored monomers I–III, and considerable enhancement of intersystem crossing to the triplet state for the dimers of II.