N. I. Makarova

Synthesis, Photophysical and Redox Properties of the D–π–A Type Pyrimidine Dyes Bearing the 9-Phenyl-9H-Carbazole Moiety

Novel donor-π-acceptor dyes bearing the pyrimidine unit as an electron-withdrawing group have been synthesized by using combination of two processes, based on the microwave-assisted Suzuki cross-coupling reaction and nucleophilic aromatic substitution of hydrogen. Spectral properties of the obtained dyes in six aprotic solvents of various polarities have been studied by ultraviolet–visible and fluorescence spectroscopy. In contrast to the absorption spectra, fluorescence emission spectra displayed a strong dependence from their solvent polarities. The nature of the observed long wavelength maxima has been elucidated by means of quantum chemical calculations. The electrochemical properties of these dyes have been investigated by using cyclic voltammetry, while their photovoltaic performance was evaluated by a device fabrication study. The experimental and calculation data show that all of the dyes can be regarded as potentially good photosensitizers for dye-sensitized solar cells.

2-Hetaryl-1,3-tropolones based on five-membered nitrogen heterocycles: synthesis, structure and properties

Summary A series of derivatives of 2-hetaryl-1,3-tropolone (β-tropolone) was prepared by the acid-catalyzed reaction of 2-methylbenzoxazoles, 2-methylbenzothiazoles and 2,3,3-trimethylindoline with 3,4,5,6-tetrachloro-1,2-benzoquinone. The molecular structures of the three representative compounds were determined by X-ray crystallography. In crystal and (as shown by the DFT PBE0/6-311+G** calculations) in solution, 2-hetaryl-4,5,6,7-tetrachloro- and 2-hetaryl-5,6,7-trichloro-1,3-tropolones exist in the NH-tautomeric form with a strong resonance-assisted intramolecular N–H···O hydrogen bond. The mechanism of the formation of 1,3-tropolones in the reaction of methylene-active five-membered heterocycles with o-chloranil in acetic acid solution has been studied using density functional theory (DFT) methods. The reaction of 2-(2-benzoxa(thia)zolyl)-5,6,7-trichloro(4,5,6,7-tetrachloro)-1,3-tropolones with alcohols leads to the contraction of the seven-membered tropone ring with the formation of 2-(2-benzoxa(thia)zolyl)-6-alkoxycarbonylphenols. The molecular structure of 2-(2-ethoxycarbonyl-6-hydroxy-3,4,5-trichlorophenyl)benzoxazole has been determined by X-ray diffraction. 2-(2-Benzoxa(thia)zolyl)-6-alkoxycarbonylphenols display intense green fluorescence with anomalous Stokes shifts caused by the excited state intramolecular proton transfer (ESIPT) effects.

Synthesis and Photochromic Properties of Asymmetric Dihetarylethenes Based on 5-methoxy-1,2-dimethylindole and 5-(4-bromophenyl)-2-methylthiophene

We report the synthesis of new asymmetric dihetarylethenes – 4-[5-(4-bromophenyl)-2-methylthiophen-3-yl]-3-(5-methoxy-1,2-dimethyl-1H-indol-3-yl)furan-2,5-dione and 1-alkyl(phenyl)-3-(5-methoxy-1,2-dimethyl-1H-indol-3-yl)-4-[5-(4-bromophenyl)-2-methylthiophen-3-yl]-1H-pyrrole-2,5-diones, which exhibit photo-chromic and fluorescent properties in solution. Thermally stable photoinduced cyclic isomers were observed for dihetarylethenes derived from 5-methoxy-1,2-dimethylindole and 5-(4-bromophenyl)-2-methylthiophene, unlike for their structural analogs based on 5-methoxy-1,2-dimethylindole and unsubstituted thiophene. Changing the thiophene fragment of dihetarylethenes to a 5-(4-bromophenyl)-2-methylthiophene fragment gave rise to photochromic properties in furan-2,5-dione derivatives.

Benzoid-Quinoid Tautomerism of Schiff Bases and Their Structural Analogs: LII.* Schiff Bases Derived from 6-tert-Butyl-5-hydroxy- and 5-Hydroxy-6-iodo-2,3-tetra-methylenebenzo[b]furan-4-carbaldehydes

Two series of Schiff bases, 4-aryl(alkyl)iminomethyl-6-tert-butyl-5-hydroxy- and 4-aryl(alkyl)iminomethyl-5-hydroxy-6-iodo-2,3-tetramethylenebenzo[b]furans were synthesized. These compounds in solution give rise to tautomeric benzoid-quinoid equilibrium; the fraction of the quinoid form in the equilibrium mixture increases with rise in solvent polarity and in going from N-aryl to N-alkyl derivatives. The benzoid tautomer shows fluorescence with an anomalous Stokes shift. The absorption and luminescence spectral properties of the examined Schiff bases make them promising as signal fragments of chemosensors for metal cations.

The peculiarities of the spectral luminescence properties of N-anthryl-substituted pyridinium cations

Abstract A structureless fluorescence band λ max f ≈ 530 nm) in liquid solution and a fluorescence band with vibrational structure (λ = 376–480 nm) in rigid media (ethanol, 77 K and poly(methyl methacrylate) (PMMA), 296 K) were observed for N -anthryl-substituted pyridinium cations. In liquid solution the long-wave fluorescence is attributed to the biradical structure resulting from the photoinduced electron transfer (PET) from anthracene to the pyridinium ring, and the long-wave, low intensity absorption (and fluorescence excitation) band (λ=465 nm) is attributed to this interfragment PET which is weakly allowed by the torsional vibrations. In rigid media the PET is strongly forbidden (in the strict orthogonal pyridinium-anthracene structure) and radiative deactivation of the anthracene fragment is observed.

Proton-transfer-induced radiationless deactivation in 2,4,6-triarylpyridines

Abstract Excited state proton transfer (ESPT) of 2,4,6-triarylpyridines (TAPs) with carboxylic acid in dioxane, tetrahydrofuran, ethylacetate, acetonitrile and ethanol has been studied by absorption and fluorescence spectroscopy and kinetic fluorometry. The electronic structures of the TAPs and their cations have been calculated by the CNDO/S method. The ESPT proceeds via a dynamic mechanism. The fluorescence-quenching rate constant is lower than the limiting diffusion rate constant of bimolecular reaction by more than an order of magnitude. This proves the existence of a potential barrier for the ESPT reaction of the TAPs. The excited product quantum yield in the ESPT reaction (ϕ r ) is less than unity and depends on the concentration of acetic acid (AcOH). This dependence can be explained neither by a static ESPT reaction nor by different reaction efficiencies for acid solvates of different structure. The limiting ([AcOH] = 0) value of ϕ r increases as the TAP basicity and acid strength increase. This value does not depend on the medium polarity but increases as the “concentration” of hydroxyl groups in the TAP solvate shell grows. The efficiency of ESPT correlates with the electron-accepting ability of the excited pyridinium ion.

Photochemistry of pyridinium salts 2: Photobiscyclization of N-azomethine pyridinium derivatives☆

Abstract The photobiscyclization reaction of N -azomethine pyridinium derivatives was investigated and the final and intermediate photoproducts were isolated. The first step of the reaction involves dihydro intermediate self-trapping by a 1,5-sigmatropic proton shift.

Spiropyrans and spirooxazines: 11. Complexation of photochromic 5-(1,3-benzothiazol-2-yl)-substituted 1´,3´-dihydrospiro[benzo[f]chromene-3,2´-indole] with metal ions

Abstract5-(1,3-Benzothiazol-2-yl)substituted 1´,3´-dihydrospiro[benzo[f]chromene-3,2´-indole] forms intensely colored complexes with metal ions in acetone solution in the absence of irradiation. The composition and stability of the complexes were shown to depend on the nature of the metal ion. The molecular structure of the zinc complex was determined by X-ray diffraction. The complexes with diamagnetic ions display negative photochromism associated with their reversible dissociation induced by visible light.

N,N′-Bis(9-anthrylmethyl)diamines as fluorescent chemosensors for transition metal cations

N,N′-Bis(9-anthryl)-substituted diamines, imidazolidines, and hexahydropyrimidines were synthesized, and their luminescence properties and complexing ability were studied. N,N′-Bis(9-anthrylmethyl)ethane-1,2-diamine and N,N′-bis(9-anthrylmethyl)butane-1,4-diamine were found to be effective and selective PET chemosensors for Zn2+ and H+, respectively. 2-[1,3-Bis(9-anthrylmethyl)hexahydropyrimidin-2-yl]-6-methoxyphenol can be used as fluorescent chemosensor for mercury(II) cations.

The Dynamics of Intramolecular Excited State Relaxation of N-Anthryl Substituted Pyridinium Cations

N-(1-Anthryl)-2,4,6-trimethyl-pyridinium (I), N-(2-anthryl)-2,4,6-trimethyl-pyridinium (II) and 10-(1-anthryl)-1,2,3,4,5,6,7,8-octahydro-acridinium cations (III) with anomalously high fluorescence Stokes’ shift have been investigated. Fluorescence kinetics analysis at various temperatures showed that in the range 293–77 K, the radiative deactivation rate constants (kf) increase by 5.5 to 30 times. The low-temperature time-resolved emission spectra of I–III were found to be consistent with the model: A→ A*⇚ B* where A* is the local excited twisted form and B* is the relaxed more planar, bent conformer of the molecule. The rate constants of the excited relaxed state formation (k1) and back reaction (k−1) of compounds studied were estimated.