A. V. Kulinich

Synthesis, structure, and solvatochromism of merocyanine dyes based on barbituric acid

Di-, tetra-, and hexamethinemerocyanines were synthesized on the basis of barbituric and 1,3-dimethylbarbituric acids and heterocycles characterized by a strong (1-benzyl-1,2-dihydrobenzo[cd]indole), medium (1,3-dihydro-1,3,3-trimethyl-2H-indole and 3-ethyl-2,3-dihydro-1,3-benzothiazole), and strong electron-donor power (1,3-diphenyl-2,3-dihydro-1H-benzimidazole). Their solvatochromic properties were analyzed using both conventional parameters (absorption maxima and molar absorption coefficients) and centroids, oscillator strengths, widths, asymmetry and excess coefficients, and fine structure of the long-wave absorption bands, which were determined by mathematical processing (method of moments). A combination of these parameters unambiguously characterizes limiting electronic states of merocyanines (neutral polyene, polymethine, or charged polyene) responsible for the type of solvatochromism. Relations were found between solvatochromic properties, on the one hand, and donor-acceptor power of the terminal groups, length of the polymethine chain, and solvent nature, on the other. The difference in solvatochromism (up to reversal of sign) of merocyanines derived from barbituric acid and their analogs based on 1,3-dimethylbarbituric acid originates from both the difference in their electronic asymmetry and the ability of the NH groups in the former to participate in hydrogen bonding with proton-acceptor solvents.

Electronic structure and fluorescent properties of malononitrile-based merocyanines with positive and negative solvatochromism

The behavior of the positions and shapes of the fluorescence bands of di-, tetra-, and hexamethine merocyanine dyes with 3H-indolyliden (dyes 1–3) and benzoimidazolyliden (dyes 4–6) as electron-donating substituents and malononitrile as an electron-accepting substituent is studied by the method of moments in solvents of different polarity. The solvatofluorochromic shifts have been found to be smaller than the solvatochromic shifts not only for negatively solvatochromic merocyanines 4–6, but also for dyes 1–3 whose solvatochromism is positive. For dyes 4–6, cases of a change of the sign of solvatofluorochromism with respect to the sign of solvatochromism are revealed. These nontrivial effects are accounted for by transitions between the polyene and polymethine electronic structures of merocyanines in the fluorescence state S1 that occur with increasing medium polarity. In contrast to the absorption spectra of merocyanines 1–3, an increase in the chain length results in an increase in the vinylene shifts in the fluorescence spectra of these dyes, as well as in a decrease in the deviations and in the narrowing of the bands. This is explained by the fact that the electronic structure of these merocyanines in the S1 state is closer to that of the ideal polymethine (the cyanine limit) than in the S0 state. The fluorescence bands of merocyanines 4–6 are observed to be broader compared to the absorption bands. This broadening is caused by a change in the relation between intermolecular and vibronic interactions during absorption and emission of light. The interactions of these types have a decisive effect on the behavior of the Stokes shifts and fluorescence quantum yields of merocyanines 1–6.

Thermochromism and thermofluorochromism of merocyanines with a positive solvatochromism

The thermochromism and thermofluorochromism of ethanol solutions of di-, tetra-, and hexamethine merocyanines based on 1,3-dihydro-1,3,3-trimethyl-(2H)-indol-2-yliden and malonodinitrile, having a positive solvatochromism as compared to the corresponding cationic and anionic symmetric dyes, are studied at T = 348, 293, and 4.2 K. It is found that, upon a decrease in the temperature, the absorption spectra exhibit bathochromic shifts of bands, an increase in the vinylene shifts, and a decrease in deviations, whereas the fluorescence spectra exhibit opposite changes. In this case, the differences in the shapes of the absorption and luminescence bands decrease so considerably that the bands become virtually mirror-symmetric. It is shown that the spectral and luminescent effects observed arise because, at high temperatures, the electronic structure of the ground state of merocyanines approaches that of neutral polyenes, whereas, at low temperatures, this structure is similar to the structure of ideal polymethine. Such a transformation of the structure is caused by an increase in specific electrostatic interactions (nucleophilic and electrophilic solvations) with a solvent, as a result of which the alternation of the charges of a chromophore increases, while its bond orders become equalized.

Croconium dye as a sensitizer of photothermoplastic holographic media for the near-IR region

This paper discusses the spectral and photoconductive properties of films based on a co-oligomer of glycidylcarbazole with additives of croconium dye in comparison with similar films with squarylium, merocyanine, and cationic polymethine dyes. It is concluded that croconium dye, like merocyanine dye, belongs to the electron-nonsymmetric organic dyes. The possibility of using croconines as sensitizers of photothermoplastic holographic recording media for the near-IR region is demonstrated for the first time. The lower holographic sensitivity of recording media with croconium dye by comparison with squarylium and merocyanine dyes is associated with the presence of deep traps for charge carriers, which are formed by the molecules of an intraionic dye and are liberated when heated in the process of development of the holograms.

Effect of concentration of anionic polymethine dye in poly-N-epoxypropylcarbazole polymer film composite on the spectral-luminescent properties and photoconductivity.

The absorption, fluorescence and fluorescence excitation spectra of polymer thin film composites based on poly-N-epoxypropylcarbazole (PEPC) doped with different content of anionic polymethine dye testify to the formation of contact ion-pair associates at high concentration of the latter. The photoconductivity of such composites in the visible spectrum region is observed, despite the absence in the polymer of any fragment capable of accepting a photoexcited electron from anionic dye chromophore. Analysis of the spectral data together with DFT/B3LYP quantum-chemical calculation of the HOMO and LUMO energies of the monomer N-methylcarbazole unit and the radical of the polymethine dye allowed us to suggest the possible mechanism of internal photoeffect in the investigated composites which is based on the formation of the contact ion-pair associates.

Near-infrared squarate and croconate dianions derived from tetranitrofluorene

Squarylium and croconium dianionic dyes with tetranitrofluorene terminal groups have been synthesized. A distinctive feature of their absorption is multiple fairly intense bands in the VIS and NIR regions (up to 1060 nm). DFT quantum chemical calculation (including polarizing continuum simulation of solvent) was used to interpret their optical behaviour.

Near-infrared photosensitive composites with electron conductivity

Polymer composite films were prepared on the basis of poly(vinyl ethylal) doped with a low concentration of an anionic polymethine dye that absorbs light in the near IR region and a higher concentration of 2,4,5,7-tetranitro-9-fluorenone or tetrakis(benzylamino)octahydrofullerene organic acceptor. The photoconductivity of the polymer composite films increases on passing to fullerene derivatives. The main reason for this effect is a decrease in the influence of steric factors on intermolecular electronic transitions in the case of the near-spherical structure of acceptor molecules as compared with the planar structure.

Influence of the Polarity of a Medium on the Photonics of a Merocyanine Dye with a High Quadratic Polarizability

The influence of the polarity of a medium on the spectral and luminescent properties of 2-[(2E,4E)-6-(1,3,3-trimethyl-2,3-dihydro-1H-2-indolyliden)-2,4-hexadienyliden]malononitrile (THDM) in solutions and polymer matrices is studied at room temperature under conditions of steady-state and pulsed laser excitation. A large bathochromic shift of the absorption spectra observed upon an increase in the polarity of a solvent is caused by a strong increase in the molecular dipole moment μ due to a transition of molecules from the ground state (μg = 7.6 D) to an excited Franck-Condon state (μFC = 33.5 D). Based on the solvatochromic data, the quadratic polarizability was calculated to be β = (3.2 ± 0.6) × 10−28 esu, which is close to the experimentally determined value βex = (3.9 ± 0.2) × 10−28 esu. A strong narrowing of the fluorescence spectra in comparison with the absorption spectra is observed upon an increase in the solvent polarity. This narrowing is explained by a decrease in the bond length alternation parameter and by weakening of vibronic interactions in the singlet excited state. The dynamic solvatofluorochromism of THDM in the picosecond range is caused by reorientations of molecules of the polar environment occurring during a time period consistent with the dielectric relaxation time of these molecules.

Fluorescent properties of merocyanines based on 1,3-indandione

We have studied the fluorescent properties of vinylogous series of merocyanine dyes based on 1,3-indandione and heterocycles of different electron-donating powers. Using a set of solvents with polarities lying in a wide range, we have analyzed the dependence of their solvatofluorochromism on the key structural parameters—the donor heterocyclic group and the length of the polymethine chain. It has been found that the signs of solvatochromism and solvatofluorochromism of merocyanines under study coincide. However, the solvent more weakly affects the position and the shape of their fluorescence bands than in the case of absorption spectra, especially, for negatively solvatochromic derivatives of 1,3-diphenylbenzimidazole. We have found that, upon passage from polar aprotic solvents to alcohols, the fluorescence quantum yields of dyes under investigation (irrespective of the sign of their solvatochromism) decrease. We have performed quantum-chemical calculations of merocyanine molecules by the DFT/B3LYP/6-31G(d,p) and TDDFT methods, including calculations taking into account the polarity of the medium by the PCM method. Based on the analysis of electronic transitions, we have explained the fluorescence quenching of indandione merocyanines in alcohol, which is unusual for carbonyl-containing intraionic dyes.

Quantum-chemical study of relation of spectral and luminescent properties of positively solvatochromic malononitrile-based merocyanine dyes with their structure

The electronic structure, the spectra, and the efficiency of photophysical processes of energy deactivation are calculated by a semiempirical method for three positively solvatochromic merocyanine dyes with different polymethine chain lengths. The electronic structure and spectra calculated by different modifications of the INDO method at different molecular geometries are compared, and the optimum geometry of the isolated molecule is chosen. The absorption spectra of dyes are interpreted, including the short-wave-length bands related to transitions to highest excited states. The possibility of a specific electrophilic solvation of these compounds in the ground and fluorescent states, as well as the contribution of specific intermolecular interactions to the total interaction with the medium, is estimated. The role played by the trans-cis isomerization of isolated merocyanine molecules in the deactivation of their excited states is considered.