Fluorescent cyclophanes and their applications

With the serendipitous discovery of crown ethers by Pedersen more than half a century ago and the subsequent introduction of host-guest chemistry and supramolecular chemistry by Cram and Lehn, respectively, followed by the design and synthesis of wholly synthetic cyclophanes-in particular, fluorescent cyclophanes, having rich structural characteristics and functions-have been the focus of considerable research activity during the past few decades. Cyclophanes with remarkable emissive properties have been investigated continuously over the years and employed in numerous applications across the field of science and technology. In this Review, we feature the recent developments in the chemistry of fluorescent cyclophanes, along with their design and synthesis. Their host-guest chemistry and applications related to their structure and properties are highlighted.

Diversity-Oriented Synthesis and Optical Properties of Bichromophoric Pyrrole-Fluorophore Conjugates

The mild reaction conditions of the palladium-copper coupling-isomerization reaction open a highly convergent, chromogenic route to blue emissive pyrroles in the sense of a consecutive four-component reaction. By virtue of this strategy a phenol derivative can be readily accessed, which can be transformed in a level-2 transformation to a library of bichromophoric pyrrol-fluorophore conjugates by facile alkylation with fluorophore halides. The photophysics of the underlying blue emitter derivative and the conjugates is studied by absorption and emission spectroscopy, furnishing intramolecular energy transfer at short distances as well as competing fluorescence quenching. In some cases partial energy transfer results in the occurrence of dual emission, for instance seen as magenta-rose emission arising from blue and red orange luminescence. The experimental photophysical studies are rationalized by DFT and TD-DFT calculations.

Crystal structure of phenyl 2,4,5-tri-chloro-benzene-sulfonate

The title compound, C12H7Cl3O3S, was synthesized via a nucleophilic substitution reaction between phenol and 2,4,5-tri-chloro-benzene-sulfonyl chloride. The two aryl rings are oriented gauche to one another around the sulfonate S-O bond, with a C-S-O-C torsion angle of -70.68 (16)°, and the two rings are inclined to one another by 72.40 (7)°. In the crystal, mol-ecules are linked via various C-Cl⋯π inter-actions, forming ribbons propagating along [100]. Neighboring ribbons are linked by a weak C-Cl⋯π inter-action, forming layers parallel to (010).

Deprotonation of resorcinarenes by mono- and diamine bases: complexation and intermolecular interactions in the solid state

The deprotonation of a resorcinarene by small organic bases results in 1 : 1 inclusion and 1 : 2 pseudo-capsular ion-pair complexes.

Fluorescent probe for detection of bacteria: conformational trigger upon bacterial reduction of an azo bridge

Using biomimetic chemical reduction or Clostridium perfringens cell extract containing azoreductase, the dimer-fluorescent probe 2,4-O-bisdansyl-6,7-diazabicyclooct-6-ene, which possesses a conformationally constrained cis-azo bridge, is reduced to the tetra-equatorial 2,4-O-bisdansyl-cyclohexyl-3,5-bisammonium salt which exhibits fluorescence indicative of a dansyl monomer.

Evaluation of the Radical Scavenging Activity of Resorcinarenes by DPPH• Free Radical Assay

A series of methyl-pendanted resorcinarene derivatives have been synthesised and evaluated for radical scavenging potential by using a colorimetric DPPH• assay to reveal their utility as potent radical scavengers. An evaluation of the efficiencies in relation to the number of DPPH• molecules reduced by one molecule of antioxidant establishes that resorcinarene can reduce approximately eight DPPH• molecules at a fairly lower stoichiometric concentration. This behaviour is compared to that of well known radical scavengers (α-tocopherol, BHT and resorcinol) frequently used as reference compounds in scientific evaluations.

Photochemical cleavage reactions of 8-quinolinyl sulfonates in aqueous solution

Photochemical cleavage reactions of 8-quinolinyl benzenesulfonate derivatives and related sulfonates in aqueous solutions are reported. The 8-quinolinyl benzenesulfonates undergo photolysis upon photoirradiation at 300-330 nm to give the corresponding 8-quinolinols and benzenesulfonic acids with the production of only negligible amounts of byproducts. The effects of substituent groups of the 8-quinolinyl moiety and the benzene ring on the photolysis reactions were examined. Based on steady-state mechanistic studies using a triplet sensitizer, a triplet quencher, and electron donors, it was suggested that the photolysis proceeds mainly via the homolytic cleavage of S-O bonds in the excited triplet state.

Novel dansyl-appended calix[4]arene frameworks: fluorescence properties and mercury sensing

Covalently-attached fluorophores may impart enhanced chemosensing capabilities to calixarene frameworks. Synthesis and characterization of six novel dansyl-appended calix[4]arenes, namely, H/Dan4, NO2/Dan4, H/(OH)2Dan2, H/(Ester)2(Dan)2, t-Bu/(OH)2Dan2, and t-Bu/(Ester)2Dan2, containing two or four dansyl moieties are reported. Among these, fluorescence intensity of NO2/Dan4 is observed to decrease significantly in the presence Hg2+ in the solution. Based on the decrease in fluorescence, a limit of detection for Hg2+ of 20 ppb is obtained. NO2/Dan4 as a chemosensing agent for Hg2+ shows excellent selectivity and adequate reversibility. Complexation of NO2/Dan4 with Hg2+ is investigated using fluorescence spectroscopy and is observed to be 2:1. The formation constant of (NO2/Dan4)2Hg2+ is estimated to be 5.2(+/- 0.8) x 10(10) M(-2) at ambient conditions. These observations are traced to the fact that while all other dansyl-appended calix[4]arenes show cone conformation in the solution, NO2/Dan4 is in the 1,3-alternate conformation. Stokes shift versus solvent orientational polarizability for NO2/Dan4 also indicates the difference in the ground- to excited-state dipole moment of this compound to be the maximum among all six, rendering it most sensitive to its environment. Fluorescence emission of NO2/Dan4 in nonpolar chloroform, polar-aprotic acetonitrile, and polar-protic ethanol is observed to be different than that of the rest of the dansyl-appended compounds as well.

The complex formation of tetracyclohexylammonium C1-resorcinarene with various guests - an electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry study

The complex formation of a tetraammonium C1-resorcinarene (R+4HCl) was studied using electrospray ionization Fourier transform ion cyclotron resonance (ESI-FTICR) mass spectrometry. Although R+4HCl easily loses its counter ions in the ESI process, a neutral self-assembled structure with an intramolecular circular hydrogen-bonded 16-membered -N(+)-H ... X(-) ... H-N(+)- array with ammonium ion as the charge-giving species was observed in the gas phase. In addition to chloride, several other counter ions were also studied. The size and basicity of the counter ion as well as the size of the charge-giving cation strongly affected the gas-phase stability of the self-assembled system. H/D exchange experiments showed that the ammonium substituents in the apical position of R affect the hydrogen-bonding system in the resorcinarene. The complexation of the saturated dicarboxylic acids was found to depend on the length of the carbon chain. The rigidity of the molecular skeleton of the acid improved the complexation considerably. The orientation and position of the carboxylic groups also had an effect on the complexation and consequently enabled stereochemical differentiation of the acids. Mass spectrometric observations were supported by theoretical calculations.