A. K. Chibisov

Complexes of spiropyran-derived merocyanines with metal ions Thermally activated and light-induced processes

The properties of complexes of five spiropyran-derived merocyanines with MgII, PbII, transition and rare-earth metal ions (Mn+, n = 2, 3) have been studied in acetone by steady-state and time-resolved techniques. Within nanoseconds, excitation of the spiropyran (Sp) form of 1′,3′,3′-trimethyl-6-nitro-8-methoxy-spiro[2H-1-benzopyran-2,2′-indoline] (3) and derivatives leads to atriplet state of the zwitterionic merocyanine form, which converts into a mixture of trans- and cis-isomers. Eventually, bidentatemetal complexes (trans-Mn+) were observed, with rate constants for 3 from 3 × 107 (with NiII) to 1.2 × 109 d mol−1 s−1 (with PbII). This reaction also occurs with similar rate constants for 1 (where only unidentate complexes are formed) and 2 (derivatives without the methoxy group). For 5, where the methyl group on N(1′) is replaced by a phenyl group, the yield of complex is small. The kinetics were studied for equilibria of the Sp form with both trans-merocyanine and trans-Mn+, being perturbed either thermally or photochemically. Upon excitation of complexes with 3 and 4 [where the methyl group on N(1′) is replaced by C2H4OH], triplet formation, trans → cis photoisomerization (into cis-Mn+) and photosubstitution (into the Sp form) were observed. The yields depend on the nature of the metal ion; they are negligible for CoII, NiII or CuII, substantial for CdII or ZnII and largest for PbII. The lifetime of the observed cis-Mn+ complex is limited by interaction with the metal ion and water.

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.

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 φfl as compared to monomers; however, φfl 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.

Photoprocesses in aqueous solutions of 9-ethylthiacarbocyanine dyes in the presence of surfactants

The effect of cationic (CTAB), anionic (SDS) and neutral (Triton X-100) surfactants on the spectral and luminescence properties and photoreactions of three sulfopropyl-9-ethylthiacarbocyanine dyes (Dyes 1–3) was studied in aqueous solutions. In the absence of the surfactants, Dyes 1–3 occur in the form of an equilibrium mixture of cis-monomers and dimers. Neither monomers nor dimers are capable of fluorescence and cis-trans photoisomerization, and only the dimers experience intersystem crossing into the triplet state. In the presence of any of the surfactants at a concentration below the critical micelle concentration, the dimers undergo disaggregation; it is only in the presence of CTAB that the formation of a J-aggregate—which dissociates into monomers with an increase in the CTAB concentration—takes place. As the surfactant concentration increases, dye fluorescence appears, which is accompanied by a decrease in the yield of the dimer triplet state, and the amenability of the thiacarbocyanine monomers to cis-trans photoisomerization and transition to the triplet state is observed. The spectral effects observed are related to the conversion of the cis-form of the monomer to the trans-form upon solubilization of the dye molecules.

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.