2-Aminopyridine derivative as fluorescence ‘On–Off’ molecular switch for selective detection of Fe3+/Hg2+

2-Amino-6-methyl-4-phenyl-nicotinonitrile 1, a 2-aminopyridine-based fluorescent compound, was found to be a fluorescent chemosensor for the detection of Fe³⁺ and Hg²⁺ ions over a number of other metal ions. Compound 1 was synthesized in one step using a multicomponent reaction, and characterized using common spectroscopic tools. During Fe³⁺/Hg²⁺ sensing the compound 1 followed a ‘switch-off’ mechanism. Further, compound 1 could sense Fe³⁺ over Hg²⁺ by its distinct absorption and fluorescence quenching behaviors. 1:1 complex formation of 1 with Fe³⁺ and Hg²⁺ was clearly understood from Job’s plot. The present work brings additional evidence on the importance of multicomponent reactions which could lead to the development of fluorescence chemosensor in one step for the selective detection of biologically important metal ions.     

Chromophore Multiplication To Enable Exciton Delocalization and Triplet Diffusion Following Singlet Fission in Tetrameric Pentacene

A tetrameric pentacene, PT , has been used to explore the effects of exciton delocalization on singlet fission (SF). For the first time, triplet decorrelation through intramolecular triplet diffusion was observed following SF. Transient absorption spectroscopy was used to examine different decorrelation mechanisms (triplet diffusion versus structural changes) for PT and its dimeric equivalent PD on the basis of the rate and activation barrier of the decorrelation step. Charge‐separation experiments using tetracyano‐p‐quinodimethane ( TCNQ ) to quench triplet excitons formed through SF demonstrate that enhanced intersystem crossing, that is, spin catalysis, is a widely underestimated obstacle to quantitative harvesting of the SF products. The importance of spatial separation of the decorrelated triplet states is emphasized, and independent proof that the decorrelated triplet pair state consists of two (T₁) states per molecule is provided.     

Understanding stress-induced disorder and breakage in organic crystals: beyond crystal structure anisotropy

Crystal engineering has advanced the strategies for design and synthesis of organic solids with the main focus being on customising the properties of the materials. Research in this area has a significant impact on large-scale manufacturing, as industrial processes may lead to the deterioration of such properties due to stress-induced transformations and breakage. In this work, we investigate the mechanical properties of structurally related labile multicomponent solids of carbamazepine (CBZ), namely the dihydrate (CBZ·2H₂O), a cocrystal of CBZ with 1,4-benzoquinone (2CBZ·BZQ) and the solvates with formamide and 1,4-dioxane (CBZ·FORM and 2CBZ·DIOX, respectively). The effect of factors that are external (e.g. impact stressing) and/or internal (e.g. phase transformations and thermal motion) to the crystals are evaluated. In comparison to the other CBZ multicomponent crystal forms, CBZ·2H₂O crystals tolerate less stress and are more susceptible to breakage. It is shown that this poor resistance to fracture may be a consequence of the packing of CBZ molecules and the orientation of the principal molecular axes in the structure relative to the cleavage plane. It is concluded, however, that the CBZ lattice alone is not accountable for the formation of cracks in the crystals of CBZ·2H₂O. The strength and the temperature-dependence of electrostatic interactions, such as hydrogen bonds between CBZ and coformer, appear to influence the levels of stress to which the crystals are subjected that lead to fracture. Our findings show that the appropriate selection of coformer in multicomponent crystal forms, targetting superior mechanical properties, needs to account for the intrinsic stress generated by molecular vibrations and not solely by crystal anisotropy. Structural defects within the crystal lattice, although highly influenced by the crystallisation conditions and which are especially difficult to control in organic solids, may also affect breakage.

Crystal engineering has advanced the strategies of design and synthesis of organic solids with the main focus being on customising the properties of the materials.     

Deformation-induced phosphorescence shift in a 2D elastically flexible organic single crystal: role of chalcogen-centered weak interactions

Mechanically responsive organic luminescent crystals are one of the promising choices of materials for flexible photonic devices. However, the change in phosphorescence emission as a function of the flexibility of a crystal has never been reported. Our current findings demonstrate two-dimensional (2D) and one-dimensional (1D) macroscopic elastic deformability, under mechanical stress, in elastically flexible single crystals of dibenzothiophene, and its brominated derivative, respectively. Unlike the presence of dual fluorescence (FL) and room temperature phosphorescence (RTP) in dibenzothiophene single crystals, the derivative was found to show only RTP. Interestingly, upon elastic deformation, single crystals of the dual emissive dibenzothiophene show a noticeable blue shift (∼20 nm) of RTP emission when compared to their pristine crystals (straight and naturally bent). However, their FL peaks remain nearly unchanged irrespective of the crystal deformation. A hierarchy of structure-elastic functionality to RTP modulation has been quantitatively mapped by rationalizing the role of chalcogen-involved weak interactions.

In response to macroscopic elastical bending, single crystals of dual emissive dibenzothiophene depict a significant blue shift (∼20 nm) of RTP emission when compared to their pristine crystals (straight and naturally bent).     

Photoionization mass spectrometry of aliphatic monoterpene alcohols

Summary 1. The dissociative processes observed in the photoionization of monoterpene alcohols have been investigated. It has been established that the relative intensities of the peaks of the heavy ions, including the molecular ions, are higher on photoionization than on electron impact.2. It has been found that the mass spectra of the isomeric alcohols obtained on photoionization differ from one another to a greater degree than the electronic mass spectra.Agrophysical Scientific-Research Institute. Translated from Khimiya Prirodnykh Soedinenii, No. 6, pp. 723–726, November–December, 1974.     

Donor/Acceptor-Substituted Tetraethynylethenes: Systematic Assembly of Molecules for Use as Advanced Materials

A comprehensive series of tetraethynylethenes (= 3,4-diethynylhex-3-ene-1,5-diynes, TEEs) bearing electrondonating (p-methoxyphenyl or p-aminophenyl) and/or electron-accepting (p-nitrophenyl) groups was prepared via [Pd]-catalyzed Sonogashira cross-coupling reactions. The electronic and photonic properties of these molecules were investigated with a special emphasis on the effects caused by degree and pattern of donor/acceptor substitution around the central TEE core. This analysis showed that intramolecular donor-acceptor interactions, as evidenced by a long-wavelength charge-transfer band, are considerably more effective in TEEs 44 and 46 , with trans and cis, linearly-conjugated electronic pathways between donor and acceptor, than in 11 , with a geminal, cross-conjugated electronic pathway. UV/VIS Spectroscopy revealed a steady bathochromic shift of the longest-wavelength absorption band (λ_max) as the number of donor-acceptor conjugation paths increased upon changing from bis-arylated ( 11, 44 , and 46 ) to tetrakis-arylated ( 14, 31 , and 35 ) TEEs. The position of the longest-wavelength absorption was also found to be strongly dependent on the nature of the N-substituents in the R₂NC₆H₄-donor groups. Electronic emission spectroscopic investigations demonstrated a considerable solvent dependency of the fluorescence of donor-acceptor-substituted TEEs such as 11 or 44 , in agreement with the presence of highly polarized excited states in these molecules. Correspondingly, fluorescence spectra of TEEs bearing only donor or acceptor substituents showed little solvent dependency. The large majority of the donor/acceptor-substituted TEEs are thermally and environmentally stable molecules. They can be stored for months as solids in the air at room temperature, and many decompose only upon heating to temperatures above 200°. X-Ray analysis showed the conjugated C-atom scaffolds of 44, 46 , and 67 to be essentially planar, whereas the aryl substituents in 28 and 30 are rotated out of the plane of the TEE core by varying degrees.     

Esters of 2-iodoxybenzoic acid: hypervalent iodine oxidizing reagents with a pseudobenziodoxole structure

Esters of 2-iodoxybenzoic acid (IBX-esters) were prepared by the hypochlorite oxidation of the corresponding 2-iodobenzoate esters and isolated as chemically stable, microcrystalline products. These hypervalent iodine compounds are potentially valuable oxidizing reagents belonging to a new class of pentavalent iodine compounds with a pseudobenziodoxole structure. Methyl 2-iodoxybenzoate can be further converted to the diacetate or a bis(trifluoroacetate) derivative by treatment with acetic anhydride or trifluoroacetic anhydride, respectively. Single-crystal X-ray diffraction analysis of methyl 2-[(diacetoxy)iodosyl]benzoate 8a reveals a pseudobenziodoxole structure with three relatively weak intramolecular I...O interactions. The dimethyl and diisopropyl esters of 2-iodoxyisophthalic acid were prepared by oxidation of the respective iodoarenes with dimethyldioxirane. Single-crystal X-ray diffraction analysis of diisopropyl 2-iodoxyisophthalate 6b showed intramolecular I...O interaction with the carbonyl oxygen of only one of the two carboxylic groups, while NMR spectra in solution indicated equivalency of both ester groups. IBX-esters, methyl 2-[(diacetoxy)iodosyl]benzoate, and 2-iodoxyisophthalate esters can oxidize alcohols to the respective aldehydes or ketones in the presence of trifluoroacetic acid or boron trifluoride etherate. The bis(trifluoroacetate) derivative can oxidize alcohols to carbonyl compounds without acid catalyst.