Square network based upon the molecular salt of the tetraprotonated photoproduct rtct-tetrakis(pyridin-4-yl)cyclobutane and the sulfate anion

The formation and crystal structure of a hydrated molecular salt that results in a square network is reported. The crystalline solid is based upon the tetraprotonated photoproduct rtct-tetrakis(pyridin-4-yl)cyclobutane (4H-rtct-TPCB)⁴⁺ along with two sulfate anions (SO₄^2-) and eight waters of hydration, namely, 4,4',4'',4'''-(cyclobutane-1,2,3,4-tetrayl)tetrapyridinium bis(sulfate) octahydrate, C₂₄H₂₄N₄⁴⁺·2SO₄^2-·8H₂O. The fully protonated photoproduct acts as a four-connecting node within the square network by engaging in four charge-assisted N⁺-H...O hydrogen bonds to the sulfate anion. The observed hydrogen-bonding pattern in this square network is akin to T-silica, which is a metastable form of SiO₂. The included water molecules and sulfate anions engage in numerous O-H...O hydrogen bonds to form various hydrogen-bonded ring structures.

Design of Green-Emitting Salts from Substituted Pyridines: Understanding the Solid-State Photodimerization of trans-1,2-bis(4-pyridyl)ethylene

Polymorphic salts of trans-1,2-bis(4-pyridyl)ethylene (bpe), 2[bpeH₂ ] ⋅ (SO₄ )(2HSO₄ ) (1) and [bpeH₂ ] ⋅ 2HSO₄ (2) have been synthesized and their structures determined by X-ray crystallography. The Schmidt postulate predicts that neither of the salts will give rise to photodimerization so they can both potentially be applied as green light emitters. Despite the predictions, 1 undergoes a stereospecific solid-state photodimerization reaction with 100 % yield. This is due to UV induced combination of sliding and pedal-like movement of the pyridyl ring system that influences the alignment of C=C bonds. The sliding motion is restricted in 2. Consequently, the green emission from 1 is completely quenched after photodimerization. It is evident that counter ions play a dominant role in dis- and enabling photodimerization; their degree of protonization and lattice placement are important solvent controlled design parameters towards crystal structures that can act as future light emitters.

Reversible photoswitching of the DNA-binding properties of styrylquinolizinium derivatives through photochromic [2 + 2] cycloaddition and cycloreversion

It was demonstrated that styrylquinolizinium derivatives may be applied as photoswitchable DNA ligands. At lower ligand:DNA ratios (≤1.5), these compounds bind to duplex DNA by intercalation, with binding constants ranging from K b = 4.1 × 104 M to 2.6 × 105 M (four examples), as shown by photometric and fluorimetric titrations as well as by CD and LD spectroscopic analyses. Upon irradiation at 450 nm, the methoxy-substituted styrylquinolizinium derivatives form the corresponding syn head-to-tail cyclobutanes in a selective [2 + 2] photocycloaddition, as revealed by X-ray diffraction analysis of the reaction products. These photodimers bind to DNA only weakly by outside-edge association, but they release the intercalating monomers upon irradiation at 315 nm in the presence of DNA. As a result, it is possible to switch between these two ligands and likewise between two different binding modes by irradiation with different excitation wavelengths.

Template-stereocontrolled [2 + 2] photoreactions directed by surface recognition on hydrophilic functionalized carbon materials

Supramolecular assistance either in solution or in the solid state to the regioselective synthesis of single photodimers from [2 + 2] photoreactions surface-directed by multivalent H-bonding exo-templates based on hydrophilic carbon nanomaterials is shown.

A novel example of double reactivity by either photochemical [2+2] or thermal additions of an ionic organic supramolecular assembly

An example of double reactivity in the solid state was achieved from an ionic array directed by charge-assisted hydrogen bonds.

A Zn(ii) coordination polymer and its photocycloaddition product: syntheses, structures, selective luminescence sensing of iron(iii) ions and selective absorption of dyes

Nanospheres of a Zn(ii) complex are used to selectively sense iron(iii) ions while its photocycloaddition product can selectively absorb dyes.

Study of temperature and ligand flexibility effects on coordination polymer formation from cyclobutanetetracarboxylic acid

Three novel coordination polymers (CPs) from rctt-H4Cbtc and 4,4’-bpe with Zn and Ni metal nodes have been synthesized. The CP derivates of Zn are an unusual example for the preparation of metastable phases involving temperature dependent and concomitant transformation.

Oxygen-initiated stereoselective thermal isomerisation of a cyclobutane derivative in the solid state

Solid-state [2+2] photochemical cycloaddition reactions have been extensively studied after the classical work of Schmidt in the 1960s. Of these, trans-1,2-bis(4'-pyridyl)ethylene (bpe) is one of the well-studied alkenes to synthesize tetrakis(4-pyridyl)cyclobutane (tpcb). However, almost all the solid-state [2+2] cycloaddition reactions of bpe yielded, almost exclusively, one of the four possible isomers, namely, the rctt-tpcb (r=regio c=cis and t=trans). Here we describe a stereoselective synthesis of the tetrahedrally disposed rtct-tpcb by the solid-state thermal isomerization of the rctt-isomer in atmospheric air. We propose that this isomerization occurs through a topochemical unimolecular mechanism by a radical chain pathway, initiated by molecular oxygen. This is supported by the nature of products formed in air and nitrogen, detection of a radical in ESR spectral studies, ESI-MS crossover experiments, VT PXRD studies along with QM, MD and docking calculations. The formation of a unique isomer by thermal isomerization may be a general phenomenon to quantitatively synthesize other useful stereoisomers from the existing isomers of cyclobutane derivatives.

1-[5-(4,5-Dimethyl-1,3,2-dioxaborolan-2-yl)thiophen-2-yl]ethanone and 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzaldehyde

In both title compounds, C10H13BO3S, (I), and C13H17BO3, (II), the molecules adopt nearly planar conformations. The crystal packing of (I) consists of a supramolecular two-dimensional network with a herringbone-like topology formed by self assembly of centrosymmetric pairs of molecules linkedviadipole–dipole interactions. The crystal structure of (II) consists of a supramolecular two-dimensional network built up from centrosymmetric pairs of moleculesviaπ–π interactions. These pairs of molecules are self-organized in an offset fashion related by a symmetry centre, generating supramolecular ribbons running along the [101] direction. Neighbouring ribbons are stackedviacomplementary van der Waals and hydrophobic methyl–methyl interactions.