Stereospecific Formation of the rctt Isomer of Bis-crown-Containing Cyclobutane upon [2 + 2] Photocycloaddition of an (18-Crown-6)stilbene Induced by Self-Assembly via Hydrogen Bonding

The formation and the spectroscopic and structural properties of 1:1 and 2:1 (ligand-to-dication) complexes of an (18-crown-6)stilbene with ethane-1,2-diammonium diperchlorate in MeCN were studied by UV-vis and NMR spectroscopy and by density functional theory calculations. Prolonged UV irradiation of 2:1 mixtures of the crown stilbene and the diammonium salt led to the formation of two main photoproducts, namely, the single syn-"head-to-head" photodimer of the crown stilbene (rctt cyclobutane) due to supramolecular-assisted [2 + 2] photocycloaddition and a crown ether derivative of phenanthrene due to a photoinduced electrocyclization reaction. The rctt cyclobutane was isolated by preparative photolysis, followed by chromatography. The selectivity of the [2 + 2] photocycloaddition is explained by supramolecular pre-organization of crown stilbene molecules into the 2:1 complexes that have a pseudo-sandwich structure with stacking interactions between the stilbene moieties.

[2+2] Photocycloaddition in a bichromophoric dyad: photochemical concerted forward reaction following Woodward-Hoffmann rules and photoinduced stepwise reverse reaction of the ring opening via predissociation

A novel biphotochromic dyad with styrylbenzo[f]quinoline photochromes was designed and synthesized to study the [2+2] photocycloaddition (PCA) reaction leading to cyclobutane with two benzo[f]quinoline (BQ) substituents and the reverse four-membered ring opening reaction. In the dyad, PCA occurs in a concerted manner according to the Woodward-Hoffmann rules in the ππ* excited state after excitation of the whole conjugated π-system comprising the ethylene group. Nanosecond time-resolved emission spectroscopy indicated formation of an excimer as a possible intermediate of the PCA reaction. The reverse reaction of photoinduced cyclobutane ring opening is assumed to proceed stepwise according to the predissociation mechanism: after excitation of the BQ substituent, energy transfer (ET) occurs from the bonding ππ* term localized on the BQ substituent to the dissociative πσ* term localized on cyclobutane; the efficiency of such a process was measured for the first time. For the first time, it is suggested that the predissociation mechanism is common to the ring-opening reaction of any cyclobutane with an unsaturated substituent where the π-system of the substituent rather than the σ-system of cyclobutane is excited under light irradiation.

Emissive and reactive excimers in a covalently-linked supramolecular multi-chromophoric system with a balanced rigid-flexible structure

A novel multi-chromophoric system, triad, in which two styrylbenzoquinoline (SBQ) photochromes are connected by a balanced rigid-flexible linker comprising 2,3-naphthylene framework (a residue of 3-oxy-2-naphthoic acid) and tetramethylene groups, was designed and synthesized to study an excimer formation in the excited state. The ¹H NMR data testified that triad exists in solution as folded conformers with asymmetric parallel-displaced SBQ units. Under light irradiation, in the triad, competitive photoisomerization and [2 + 2] photocycloaddition reactions were observed, both reactions being reversible. The photocycloaddition resulted in a tetrasubstituted cyclobutane. The red-shifted fluorescence spectrum and the appearance of a long-lived component in the triad fluorescence decay indicated formation of an 'emissive' excimer. The photocycloaddition is assumed to occur in a 'reactive' excimer, in which the ethylene groups of the SBQ photochromes are located at a distance sufficient for the formation of the σ-bonds between them. Quantum-chemical density functional theory (DFT) calculations at M06-2X/6-31G* level predicted the existence of the triad conformers with π-stacking interaction of SBQ photochromes, the structure of which is pre-organized for the excimer formation and photocycloaddition. For the first time, both emissive and reactive excimers were experimentally observed in the multi-chromophoric system with two diarylethylene photochromes undergoing [2 + 2] photocycloaddition.

Development of Photoactive Supramolecular Devices and Machines

Abstract

Among the various directions of the science of photoactive organic compounds, the design, creation, and study of photoactive supramolecular devices and machines are attracting special attention, since in this way it is possible to discover new approaches to construct complex hierarchical “smart” materials.

Pseudodimeric Complexes of an (18-Crown-6)stilbene with Styryl Dyes Containing an Ammonioalkyl Group: Synthesis, Structure, and Stereospecific [2 + 2] Cross-Photocycloaddition

A new efficient method was proposed for the synthesis of (18-crown-6)stilbene; the structure of the product was confirmed by X-ray diffraction analysis. In MeCN, this compound forms pseudodimeric complexes with N-(2-ammonioethyl)-4-styrylpyridinium and N-(3-ammoniopropyl)-4-styrylpyridinium diperchlorates via hydrogen bonding between the ammonium group and the crown ether oxygen atoms. The ammonioethyl derivative was synthesized for the first time. The stability constants and spectral characteristics of the complexes were measured by spectrophotometric and fluorescence titration. Photoirradiation of the pseudodimeric complex of (18-crown-6)stilbene with the ammoniopropyl dye resulted in the stereospecific [2 + 2] cross-photocycloaddition reaction. The replacement of the stilbene moiety in the crown compound by a styrylpyridine moiety led to a 5-fold increase in the quantum yield of the photoprocess. The most probable cause for this effect is the presence of photoinduced electron transfer in (18-crown-6)stilbene complexes. This assumption is confirmed by fluorescence lifetime spectroscopy and density functional theory calculations.

The regioselective [2 + 2] photocycloaddition reaction of 2-(3,4-dimethoxystyryl)quinoxaline in solution

The [2+2] photocycloaddition between two molecules of (E)-2-(3,4-dimethoxystyryl)-quinoxaline (1) in an acetonitrile solution is the first example of the regioselective and stereospecific formation of a cyclobutane derivative without applying any method for the preorganization of molecules in a dilute solution.

Self-assembly of cucurbiturils and cyclodextrins to supramolecular millstones with naphthalene derivatives capable of translocations in the host cavities

Naphthylpyridine derivatives and cavitands form inclusive complexes of different structure and stoichiometry; the guests are capable of translocations within cucurbit[7,8]uril cavities.

Synthesis, structure, and stereospecific cross-[2+2] photocycloaddition of pseudodimeric complexes based on ammonioalkyl derivatives of styryl dyes

Two different styryl dyes form pseudodimeric complexes via hydrogen bonding and stacking interactions; irradiation of these complexes gives rctt-cyclobutane derivatives.

Supramolecular Dimerization and [2 + 2] Photocycloaddition Reactions of Crown Ether Styryl Dyes Containing a Tethered Ammonium Group: Structure-Property Relationships

Molecular self-assembly is an effective strategy for controlling the [2 + 2] photocycloaddition reaction of olefins. The geometrical properties of supramolecular assemblies are proven to have a critical effect on the efficiency and selectivity of this photoreaction both in the solid state and in solution, but the role of other factors remains poorly understood. Convenient supramolecular systems to study the structure-property relationships are pseudocyclic dimers spontaneously formed by styryl dyes containing a crown ether moiety and a remote ammonium group. New dyes of this type were synthesized to investigate the effects of structural and electronic factors on the quantitative characteristics of supramolecular dimerization and [2 + 2] photocycloaddition in solution. Variable structural parameters for the styryl dyes were the size and structure of macrocyclic moiety, the nature of heteroaromatic residue, and the length of the ammonioalkyl group attached to this residue. Quantum chemical calculations of the pseudocyclic dimers were performed in order to interpret the relationships between the structure of the ammonium dyes and the efficiency of the supramolecular photoreaction. One of the dimeric complexes was obtained in the crystalline state and studied by X-ray diffraction. The results obtained demonstrate that the photocycloaddition in the pseudocyclic dimers can be dramatically affected by the electronic structure of the styryl moieties, as dependent on the electron-donating ability of the substituents on the benzene ring, and by the conformational flexibility of the pseudocycle, which determines the mobility of the olefinic bonds. The significance of electronic factors is highlighted by the fact that the photocycloaddition quantum yield in geometrically similar dimeric structures varies from ≤10(-4) to 0.38. The latter value is unusually high for olefins in solution.