Visible Light-Induced Decomposition of Acyl Peroxides Using Isocyanides: Synthesis of Heteroarenes by Radical Cascade Cyclization.

Visible light-mediated facile synthesis of heteroarenes, namely, isoquinolines, benzothiazoles, and quinazolines, is demonstrated by employing isocyanides and inexpensive acyl peroxides. It is shown for the first time that singlet-excited isocyanides decompose acyl peroxides into aryl/alkyl radicals. The latter attack isocyanides, yielding imidoyl radicals that subsequently cyclize to afford heteroarene products. The protocol involving radical cascade reactions obviates the requirement of any external photocatalyst, oxidant, additive, and base.

Oxidation-Induced Differentially Selective Turn-On Fluorescence via Photoinduced Electron Transfer Based on a Ferrocene-Appended Coumarin-Quinoline Platform: Application in Cascaded Molecular Logic.

Differentially selective molecular sensors that exhibit differential response toward multiple analytes are cost-effective and in high demand for various practical applications. A novel, highly differentially selective electrochemical and fluorescent chemosensor, 5, based on a ferrocene-appended coumarin-quinoline platform has been designed and synthesized. Our designed probe is very specific toward Fe3+ via a reversible redox process, whereas it detects Cu2+ via irreversible oxidation. Interestingly, it exhibits differential affinity toward the Cu+ ion via complexation. High-resolution mass spectrometry, 1H NMR titration, and IR spectral studies revealed the formation of a bidentate Cu+ complex involving an O atom of the amide group attached to the quinoline ring and a N atom of imine unit, and this observation was further supported by quantum-chemical calculations. The metal binding responses were further investigated by UV-vis, fluorescence spectroscopy, and electrochemical analysis. Upon the addition of Fe3+ and Cu2+ ions, the fluorescence emission of probe 5 shows a "turn-on" signal due to inhibition of the photoinduced electron transfer (PET) process from a donor ferrocene unit to an excited-state fluorophore. The addition of sodium l-ascorbate (LAS) as a reducing agent causes fluorescence "turn off" for the Fe3+ ion because of reemergence of the PET process but not for the Cu2+ ion because it oxidizes the ferrocene unit to a ferrocenium ion with its concomitant reduction to Cu+, which further complexes with 5. Thermodynamic calculations using the Weller equation along with density functional theory calculations validate the feasibility of the PET process. A unique combination of Fe3+, LAS, and Cu2+ ions has been used to produce a molecular system demonstrating combinational "AND-OR" logic operation.

Metal-coordination driven intramolecular twisting: a turn-on fluorescent-redox probe for Hg2+ ions through the interaction of ferrocene nonbonding orbitals and dibenzylidenehydrazine.

A unique C2 symmetric azine bridged bi-ferrocenyl receptor (4) has been modelled and synthesized. In this work, we are able to synthetically regulate conjugation of the dibenzylidenehydrazine fluorophore unit to unexpectedly reveal metal-coordination driven intramolecular twisting. The present probe shows a dramatic turn-on fluorescence response with 91 fold increment of quantum yield along with 17 nm blue shift upon binding with Hg2+ ions selectively with a limit of detection as low as 15 nM. Upon Hg2+ recognition, the ferrocene/ferrocinium redox peak was anodically shifted by ΔE1/2 = 78 mV, indicating the formation of a new complex species. A plausible binding mode of Hg2+ ions with compound 4 has been proposed based on 1H NMR titration, a high-resolution mass spectrometry (HRMS) study and a density functional theory (DFT) study along with the Job's plot analysis. Interestingly, DFT calculations have revealed that the reason for fluorescence enhancement after coordination to Hg2+ ions is not due to conventional restricted C[double bond, length as m-dash]N isomerization or interrupted N-N single bond rotation rather it is due to the increase of the π ← π* transition at the expense of the n → π* (aromatic) transition of the free ligand. Furthermore, TD-DFT calculations of the first excited singlet state of 4 and [4·Hg2+] revealed the involvement of the aromatic π electrons with the vacant site of Hg2+ ions which may be further attributed to the fluorescence enhancement phenomenon. In addition, receptor 4 was successfully applied for the detection of Hg2+ ions in real samples.

ICT-Isomerization-Induced Turn-On Fluorescence Probe with a Large Emission Shift for Mercury Ion: Application in Combinational Molecular Logic.

A unique turn-on fluorescent device based on a ferrocene-aminonaphtholate derivative specific for Hg2+ cation was developed. Upon binding with Hg2+ ion, the probe shows a dramatic fluorescence enhancement (the fluorescence quantum yield increases 58-fold) along with a large red shift of 68 nm in the emission spectrum. The fluorescence enhancement with a red shift may be ascribed to the combinational effect of C═N isomerization and an extended intramolecular charge transfer (ICT) mechanism. The response was instantaneous with a detection limit of 2.7 × 10-9 M. Upon Hg2+ recognition, the ferrocene/ferrocenium redox peak was anodically shifted by ΔE1/2 = 72 mV along with a "naked eye" color change from faint yellow to pale orange for this metal cation. Further, upon protonation of the imine nitrogen, the present probe displays a high fluorescence output due to suppression of the C═N isomerization process. Upon deprotonation using strong base, the fluorescence steadily decreases, which indicates that H+ and OH- can be used to regulate the off-on-off fluorescence switching of the present probe. Density functional theory studies revealed that the addition of acid leads to protonation of the imine N (according to natural bond orbital analysis), and the resulting iminium proton forms a strong H-bond (2.307 Å) with one of the triazole N atoms to form a five-membered ring, which makes the molecule rigid; hence, enhancement of the ICT process takes place, thereby leading to a fluorescence enhancement with a red shift. The unprecedented combination of H+, OH-, and Hg2+ ions has been used to generate a molecular system exhibiting the INHIBIT-OR combinational logic operation.