N. A. Dudina

Regularities of the formation of binuclear homo- and heteroleptic complexes of d metals with 3,3′-bis(dipyrrolylmethenes) in DMF

The reactions of seven symmetrically alkylated tetradentate ligands 3,3′-bis(dipyrrolylmethenes) (H2L) with d-metal acetates (M(AcO)2) in DMF solutions at 298.15 K were studied by spectrophotometry. Helicands H2L were found to be structurally preorganized to form stable binuclear homoleptic two-helix helicates [M2L2] with Co(II), Ni(II), Cu(II), Zn(II), Cd(II), and Hg(II) acetates. The coordination of the ligands by the metal ions included consecutive stages of formation of the heteroleptic [M2L(AcO)2] and homoleptic [M2L2] complexes. The [M2L(AcO)2] complexes were spectrally revealed in solutions containing a ligand excess (cH2L / cM(AcO)2 > 1). An increase in the salt concentration shifted the system of equilibria to the homoligand product [M2L2]. The thermodynamic constants of the reactions increased in the series of complexing agents: Cu(II) < Cd(II) < Hg(II) < Ni(II) < Co(II) < Zn(II). An analysis of the data on the thermodynamic constants of [M2L2] helicate formation in solutions and the earlier obtained results of the IR and 1H NMR studies of the hydrobromic salts of the ligands (H2L · 2HBr) showed that the key regularities of the influence of the structural factors on the coordination properties of the ligands were in an increase in the stability of the [M2L2] complexes with an increase in the basicity of the ligands.

Characteristic features of formation, synthesis, and properties of binuclear zinc(II) helicates with alkyl-substituted 3,3′-bis(dipyrrolylmethenes)

On the basis of spectrophotometric studies, it was demonstrated that complexation of hydrobromides of seven alkylated 3,3′-bis(dipyrrolylmethenes) (H2L · 2HBr) with zinc(II) acetate dihydrate in DMF (298.15 K) includes successive steps of formation of binuclear helicates of two types: [Zn2(AcO)2L] (heteroleptic single-helix complex) and [Zn2L2] (homoleptic double-helix complex). An increase in the degree of alkylation of the pyrrole rings of the ligands from 4 to 10 methyl groups induces an increase of overall complexation constants from 7.60 to 13.73. The synthesis of helicates [Zn2L2] was described. The products were studied by elemental analysis and IR, 1H NMR, U/Vis, and fluorescence spectroscopy. The effect of the helicand structure and the solvent nature on the chromophore and fluorescence properties of [Zn2L2] was discussed. The kinetic stability of zinc(II) with decamethyl-substituted 3,3′-bis(dipyrrolylmethene) in HCl-DMF medium was estimated.

The high sensitive and selective "off-on" fluorescent Zn2+ sensor based on the Bis(2,4,7,8,9-pentamethyldipyrrolylmethene-3-yl)methane.

A fluorescent chemosensor based on the 3,3′-bis(dipyrrin) bearing two chromophoric dipyrrin units was synthesized, which showed a strongly enhanced fluorescent intensity in the presence of Zn2+ ions and a high selectivity toward Zn2+ ions over a wide range of tested metal ions in organic solvents.

Synthesis and spectral analysis of alkyl-substituted 3,3′-bis(dipyrrolylmethenes)

Four new alkyl-substituted 3,3′-bis(dipyrrolylmethene) dihydrobromides containing from 4 to 10 alkyl substituents were synthesized. In a highly alkylated ligand of these substances one of the hydrogen atoms of the 3,3′-methylene spacer was substituted with a phenyl group. The compounds obtained were studied by IR, 1H NMR, electron absorption, and fluorescent spectroscopy. The increased alkylation degree of pyrroles and the introduction of an aryl substituent in the 3,3′-spacer causes a significant high-frequency shift of the N-H stretching vibrations in the IR spectra, an upfield shift of the NH-proton signals in 1H NMR spectra, a decrease in the auxochromic effects of protons on the aromatic system of chromophore in the composition of salts. The red shift of maximum of the strong band in electron absorption spectra and the emission spectra of compounds in DMF, DMSO, C5H5N, C6H6, and CHCl3 was established. The salts obtained are stable in benzene and chloroform, while in electron-donor solvents the irreversible processes of solvolytic dissociation of salts to free organic base and HBr take place.

3,3′-bis(dipyrrolylmethenes) as new chelating ligands: Synthesis and spectral properties

Hydrobromides of three new alkyl-substituted tetrapyrrole ligands with an open chain, in which the dipyrrolylmethene fragments are linked by a CH2-spacer at the 3,3′-pyrrole carbon atoms, were synthesized and studied by IR, 1H NMR, and electronic absorption spectroscopy. As compared to the 2,2′ isomers (alkyl derivatives of biladiene-a,c) and monomers (2,2′-, 2,3′-, and 3,3′-dipyrrolylmethenes, the effect of structural factors is manifested in a considerable (up to 19–31 nm) bathochromic shift of the strong band in the electronic spectrum, an increase in the N-H stretching vibration frequency in the IR spectra (by more than 30 cm−1), and a decrease in the stability of 3,3′-bis(dipyrrolylmethene) salts. The solvent effect is manifested in small changes in the quantitative characteristics of the electronic absorption spectra of 3,3′-tetrapyrrole hydrobromides in C6H6, CCl4, CH2Cl2, CHCl3, and alcohols. In DMF, DMSO, and C5H5N, the salts undergo solvolytic dissociation to the free ligands and HBr, which accelerates in dilute solutions (<10−4 M) and with an increase in the electron-donor power of the solvent. The auxochromic effects of protons in the electronic absorption spectra of the salts, compared to ligands, were estimated quantitatively.

Thermal properties of ligands, salts and metal complexes of linear oligopyrroles

The effect of structure on the thermal properties of dipyrrolylmethanes, dipyrrolylmethenes, bis (dipyrrolylmethenes), their salts, and chelates is analyzed proceeding from the results of thermogravimetric analysis. The common tendency is a decrease in the thermal stability of compounds with a disturbed symmetry of alkyl substitution of ligands and with a higher degree of their alkylation, with the increased size of alkyl substituents, with the nitrogen atom in the five-member cycles replaced by S or O. The opposite effect occurs at the change in the mode of attaching methylene spacer in going from 2,2′- to 3,3′-bis(dipyrrolylmethenes). The effects of the themostabilization of the ligands of linear oligopyrroles in the composition of chelates and HBr salts are evaluated quantitatively.

Photonics of zinc(II) and boron(III) chelates with methyl- and phenyl-substituted dipyrromethenes and azadipyrromethenes

A comparative study of spectral, luminescent, and photophysical parameters has been carried out for a series of zinc(II) and boron(III) chelates with dipyrromethene ligands of a similar structure: tetramethyl- and tetraphenylpyrrodimethenes bearing the substituents on the pyrrole rings and tetraphenylazapyromethene. In some cases, the luminescence efficiency of the chelates depends on the excitation wavelength. The replacement of the central atom boron(III) by zinc(II) results in not only a twofold increase in the number of coordinated chromophoric ligands and the absorption coefficient, but also an increase in the contribution of nonradiative processes to excitation energy deactivation and, hence, a decrease in the yield of fluorescence. The efficiency of intersystem crossing is higher in the zinc(II) dipyrromethene complexes than in the corresponding boron-chelated dipyrromethenes (BODIPYs), resulting in the appearance of phosphorescence in frozen solutions. It has been found that the replacement of the meso-spacer in the dipyrromethene ligand by the nitrogen atom not only shifts the absorption and fluorescence band maximums to longer wavelengths, but also decreases the fluorescence efficiency and leads to the appearance of long-lived luminescence even in aza-BODIPY. An analysis of the results has shown that these behavior can be due to involvement of the nπ* (πσ*, σπ*) states localized on the meso-nitrogen in the deactivation of excitation energy.

Synthesis and photophysical properties of Cd(II) and Cu(II) complexes with decamethylated bis(dipyrrolylmethene)

Copper(II) and cadmium(II) complexes with bis(2,4,7,8,9-pentamethyldipyrrolylmethene-3-yl)-methane were synthesized. Influence of the complex-forming ion nature on the optical properties of [Cd2L2] and [Cu2L2] helicates was studied.

Synthesis and spectral properties of helicate of cobalt(II) with Bis(1,2,3,7,9-pentamethyldipyrrolylmethen-3-yl)methane

Reaction of cobalt(II) acetate tetrahydrate with compound I in a chloroform–methanol mixture in the presence of triethylamine afforded a binuclear homoleptic double-helical compound [Co2L2] II (Scheme 2). The electron absorption spectra of [Co2L2] solutions in organic solvents (С6Н6, DMF, CHCl3) contain two bands. Depending on the nature of the solvent, the most intensive band has a maximum in the range of 524–528 nm with a shoulder on the left slope at 490– 493 nm; a broadened less intense band of the charge transfer is in the range of 373–374 nm. Type of the electron absorption spectrum of helicate [Co2L2] in the organic solvents is consistent with the published data for the corresponding by composition Co(II) helicate with the structurally related bis(dipyrrolylmethenes) [4, 5]. In a non-polar inert benzene instead of the polar solvents, the electron-donating DMF and protonDOI: 10.1134/S1070363211010324

Optical Properties of Zinc(II) and Boron(III) Dipyrrinates with Different Structures

The spectral luminescent, photophysical, photochemical, lasing, nonlinear optical, and sensor characteristics of a series of new synthesized complexes of zinc and difluoroborate with dipyrrines of different structure have been studied. It is found that many of these compounds exhibit stimulated emission in different solvents when excited by the second (532 nm) and third (355 nm) harmonics of a Nd:YAG laser in the range of 548–692 nm. It is shown that not only efficient fluorophores belonging to dipyrrine difluoroborates (with a quantum fluorescence yield close to 1), but also compounds with a fluorescence yield equal to 0.3, generate laser radiation with a high resource; phosphorescence is also observed along with fluorescence. Transmission of UV radiation (355 nm) is shown to decrease with an increase in the pulsed excitation power density; this is a manifestation of nonlinear optical properties. The change of phosphorescence signal in dependence of the composition of the gas mixture around a solid-state sample colored by dipyrrinate complexes indicates that a number of the compounds under consideration exhibit sensor abilities.