Michael V. Alfimov

Photoswitchable molecular receptors

The review describes photoswitchable molecular receptors based on crown-ether systems. The important feature of the systems is reversible change in the capacity of a photochromic host molecule for association with guests upon irradiation. From the other side, the exploration of the complex formation process in crown-ether systems leads to a novel approach to the modification of photochromic behavior. The results obtained show that the introduction of crown-ether moieties into dye molecules affords compounds that can change their spectral and photochromic properties upon complex formation. The researchers believe that the novel photochromic systems can now be regarded as being promising for traditional applications (photochromic ophthalmic lenses or camera filters, reversible holographic systems and cosmetics) and molecular electronics, biomimetic chemistry, and optical information storage.

Theoretical Study of Structure and Electronic Absorption Spectra of Some Schiff Bases and Their Zinc Complexes

Tetradentate Schiff bases (H(2)L(i)), derivatives of salicylic aldehyde (H(2)L(1), H(2)L(2)) or o-vanillin (H(2)L(3), H(2)L(4)) with ethylenediamine or o-phenylenediamine as a bridge, and their zinc complexes were studied experimentally and theoretically in view of their possible application as emitters in organic light emitting diodes (OLEDs). The composition of thin films of the complexes was analyzed using a combination of different experimental and molecular modeling techniques taking into account changes in the Gibbs free energy of dehydration and dimerization reactions. The absorption spectra of the initial Schiff bases were investigated in methanol solutions, while the absorption spectra of their zinc complexes were investigated in thin films. Experimental results of elemental analysis, IR spectroscopy, laser desorption/ionization mass spectrometry (LDI MS), and X-ray diffraction as well as theoretical analysis of electronic absorption spectra by the quantum-chemical TD DFT method demonstrate that thin films of the zinc complexes contain binuclear anhydrous molecules. This conclusion should be taken into account when considering both transport and luminescence properties of these complexes in OLED heterostructures. A comparison of the results of CIS, TD DFT/PBE, and TD DFT/PBE0 calculations reveals the crucial importance of the inclusion of the exact exchange in the E(XC) functional for the further correct description of potential energy surfaces of excited states for the systems studied.

Ditopic complex formation of the crown-containing 2-styrylbenzothiazole

The complex formation of 2-styrylbenzothiazole containing a 15-crown-5 ether fragment with alkaline earth metal cations, proton, Ag+ and Hg2+ was studied by optical and X-ray diffraction methods. The compound is able to bind the metal cations through the participation of two centers: the crown ether moiety and the heterocyclic part. The alkaline earth metal cations form complexes with the macrocyclic part of the molecule. The formation of a strong sandwich complex was found in the case of Ba2+. The proton coordinates with the N atom of the heterocyclic fragment of the molecule. The Ag+ and Hg2+ cations bind with both centers of the molecule: the crown ether fragment and the heterocyclic residue.

Synthesis, Structure, Spectroscopic Studies, and Complexation of Novel Crown Ether Butadienyl Dyes

Butadienyl dyes of the benzothiazole series with various fragments of benzocrown ethers 1a-c were synthesized for the first time. The structures and spectral properties of crown-containing butadien ...

DFT calculations on the electronic and geometrical structure of 18-crown-6 complexes with Ag+, Hg2+, Ag0, Hg+, Hg0, AgNO3, and HgX2 (X=Cl, Br, and I)

Abstract Density functional theory is used for molecular simulation of the electronic and geometrical structure of 18-crown-6, its complexes with Ag + , Hg 2+ , Ag 0 , Hg + , Hg 0 , AgNO 3 , and HgX 2 (X=Cl, Br, and I). Ab initio MP2/6-31G ∗ calculations are performed for the two main conformations of the free crown ether and for anion-free complexes. The complex formation energies are analysed in terms of various contributions including Pauli and electrostatic repulsion and orbital interactions according to the Morokuma scheme. It is found that the Hg 2+ ion is most strongly bound to the crown ether; silver and mercury ions in the 18-crown-6 cavity can capture an electron, and neutral 18-crown-6 complex of silver can be bound through van der Waals-type interactions. The stability of metal complexes with supporting anionic ligands is determined not only by the cation charge, but also by the type of the ligand.

Supramolecular assemblies of photochromic benzodithia-18-crown-6 ethers in crystals, solutions, and monolayersElectronic supplementary information (ESI) available: crystal data, data collection, and structure solution and refinement parameters. See http://www.rsc.org/suppdata/nj/b1/b110630a/

We studied the assembly of dithiacrown ether styryl dye (CSD) molecules in crystals, solutions, and films in the presence of metal cations. X-Ray diffraction data allowed us to conclude that the anion affects the supramolecular architecture of CSDs in the crystal, specifically, the type of stacking of the dye molecules. In solution, in the presence of Pb2+, CSD molecules with the betaine structure spontaneously form dimeric complexes consisting of two dye molecules and two metal cations, with a fixed mutual arrangement of the double bonds. The dimer complex is stable due to coordination between the anion substituent of one molecule and the metal cation located in the crown ether cavity of the other molecule. Irradiation of the dimer complexes leads to regio- and stereoselective [2 + 2]-cycloaddition, giving only one cyclobutane derivative of the eleven theoretically possible products. The other photoreaction studied for CSDs is reversible Z–E isomerization. Due to its specific structure, the betaine-type CSD is able to form the ‘anion-capped’ Z-isomer. Intramolecular coordination in the ‘anion-capped’ isomer enhances its stability and causes a sharp deceleration of its dark Z–E isomerization. The amphiphilic CSD forms relatively stable monolayers on distilled water and various aqueous salt subphases. The results obtained indicate that it is possible to distinguish between two types of the dye monolayer structures based on the presence of alkali or heavy metal cations in the aqueous subphase.

4-Styrylquinolines: synthesis and study of [2 + 2]-photocycloaddition reactions in thin films and single crystals

Four new (E)-4-styrylquinoline compounds containing two methoxy substituents or an 18-crown-6-ether fragment were synthesized in order to investigate the [2 + 2]-photocycloaddition (PCA) reaction in the solid state. These compounds reveal different abilities to undergo the photoreaction depending on the packing of the quinoline molecules in thin polycrystalline films and single crystals. The only products from the irradiation of (E)-4-styrylquinolines were an rctt isomer of 1,2,3,4-tetrasubstituted cyclobutane and, rarely, a Z isomer of the styrylquinoline. The rctt cyclobutane derivative was formed as a result of the PCA of centrosymmetric syn-‘head-to-tail’ dimeric pairs of the reactant species that are preorganized in such a way as to promote this reaction. Peculiarities in the crystal packing motifs that are typical for single crystals of 4-styrylquinoline compounds are discussed. The solvate molecules can affect significantly the packing of styrylquinolines. Benzene solvate molecules create a soft, flexible, shell about the dimeric pairs that facilitates the PCA in the single crystal without causing degradation of the crystal. In crystal packing that contains CH2Cl2 and H2O solvate molecules, they are prone to form complicated systems of hydrogen bonds with crown-ether oxygen atoms and each other. This results in distorting the organic molecule geometry toward non-planarity and accomplishing a new type of crystal packing uncommon for styrylheterocycles, in which the PCA is impossible. The topochemical single-crystal-to-single-crystal PCA reaction was monitored for one of the species. Two cyclobutane derivatives obtained in single crystals were subjected to recrystallization from a solution. The new single crystals formed belong to different crystal systems, with different unit cell parameters as compared with initial single crystals; significant differences in the geometry of the cyclobutane molecules were also found. This implies that the cyclobutane derivatives obtained in the solid phase have a molecular geometry that is somewhat stressed.

The 1:1 host-guest complexation between cucurbit[7]uril and styryl dye.

The photophysical properties of aqueous solution of styryl dye, 4-[(E)-2-(3,4-dimethoxyphenyl)ethenyl]-1-ethylpyridinium perchlorate (dye 1), in the presence of cucurbit[7]uril (CB[7]) was studied by means of fluorescence spectroscopy methods. The production of 1:1 host-guest complexes in the range of CB[7] concentrations up to 16 μM with K = 1.0 × 10(6) M(-1) has been observed, which corresponds to appearance of the isosbestic point at 396 nm in the absorption spectra and a 5-fold increase in fluorescence intensity. The decay of fluorescence was found to fit to double-exponential functions in all cases; the calculated average fluorescence lifetime increases from 145 to 352 ps upon the addition of CB[7]. Rotational relaxation times of dye 1 solutions 119 ± 14 ps without CB[7] and 277 ± 35 ps in the presence of CB[7] have been determined by anisotropy fluorescence method. The comparison of the results of quantum-chemical calculations and experimental data confirms that in the host cavity dye 1 rotates as a whole with CB[7].

Ab initio study of phosphorescent emitters based on rare-earth complexes with organic ligands for organic electroluminescent devices.

An ab initio approach is developed for calculation of low-lying excited states in Ln(3+) complexes with organic ligands. The energies of the ground and excited states are calculated using the XMCQDPT2/CASSCF approximation; the 4f electrons of the Ln(3+) ion are included in the core, and the effects of the core electrons are described by scalar quasirelativistic 4f-in-core pseudopotentials. The geometries of the complexes in the ground and triplet excited states are fully optimized at the CASSCF level, and the resulting excited states have been found to be localized on one of the ligands. The efficiency of ligand-to-lanthanide energy transfer is assessed based on the relative energies of the triplet excited states localized on the organic ligands with respect to the receiving and emitting levels of the Ln(3+) ion. It is shown that ligand relaxation in the excited state should be properly taken into account in order to adequately describe energy transfer in the complexes. It is demonstrated that the efficiency of antenna ligands for lanthanide complexes used as phosphorescent emitters in organic light-emitting devices can be reasonably predicted using the procedure suggested in this work. Hence, the best antenna ligands can be selected in silico based on theoretical calculations of ligand-localized excited energy levels.

Photoinduced and dark complexation of unsaturated viologen analogues containing two ammonium tails with cucurbit[8]uril

Complex formation between cucurbit[8]uril (CB[8]) and unsaturated viologen analogues 1a,b bearing two ammoniopropyl substituents was studied using 1H NMR spectroscopy and X-ray diffraction. The complex stability constants were measured by 1H NMR titration. CB[8] was found to encapsulate 1,2-di(4-pyridyl)ethylene derivative (E)-1a in water/acetonitrile solution to form only inclusion complex of 1 : 1 composition. The high stability of CB[8]·1a (lg K1:1 ≥ 5) is apparently due to the possibility of hydrogen bond formation between two NH3+ groups of 1a and the OC fragments of both portals of CB[8]. Contrarily, the derivative of 1,2-di(4-quinolyl)ethylene (E)-1b gave three types of inclusion complexes upon mixing with CB[8]. The stability of the 1 : 1 complex (lg K1:1 = 4.6) is decreased compared to (E)-1a, suggesting a poorer fit with respect to the bulky (E)-1b in the cavity of CB[8] for the simultaneous hydrogen bonding of the both ammonium groups. This is the driving force for the formation of an unusual 2 : 1 complex, {CB[8]}2·(E)-1b. The expanded π-system of (E)-1b allows also two tetracationic molecules of the acceptor to form a 1 : 2 complex owing to stacking interactions. Crystallization of a 1b/CB[8] mixture under irradiation resulted in an inclusion complex of the Z-isomer of 1b. The supramolecular complex is formed because the shape of (Z)-1b fits well to the host cavity and the ammonium groups form a system of hydrogen bonds with oxygen atoms of both portals of CB[8]. The features of crystalline complexes CB[8]·(Z)-1b and CB[8]·HClO4 are discussed.