Carbazole-driven ratiometric fluorescence turn on for dual ion recognition of Zn2+ and Hg2+ by thiophene-pyridyl conjugate in HEPES buffer medium: spectroscopy, computational, microscopy and cellular studies

A Comprehensive Review on Thiophene Based Chemosensors

The recognition and sensing of various analytes in aqueous and biological systems by using fluorometric or colorimetric chemosensors possessing high selectivity and sensitivity, low cost has gained enormous attention. Furthermore, thiophene derivatives possess exceptional photophysical properties compared to other heterocycles, and therefore they can be employed in chemosensors for analyte detection. In this review, we have tried to explore the design and detection mechanism of various thiophene-based probes, practical applicability, and their advanced models (design guides), which could be thoughtful for the synthesis of new thiophene-based probes. This review provides an insight into the reported chemosensors (2008-2020) for thiophene scaffold as effective emission and absorption-based chemosensors.

Improved Access to 1,8-Diformyl-carbazoles Leads to Metal-Free Carbazole-Based [2 + 2] Schiff Base Macrocycles with Strong Turn-On Fluorescence Sensing of Zinc(II) Ions

Development of a new and high yielding synthetic route to 1,8-diformyl-carbazoles 3 (3a 3,6-di- tert-butyl substituted; 3b 3,6-unsubstituted) is reported. Use of a Heck coupling reaction, followed by ozonolysis, has greatly facilitated the preparation of these interesting head units in useful quantities. An initial foray into the new generations of Schiff base macrocycles that ready access to these head units (3) opens up, has led to the direct (i.e., metal-free) synthesis of two [2 + 2] macrocycles from 3a or 3b with 1,2-diaminoethane, H₂L^tBu (4a) and H₂L^H (4b), respectively, obtained as yellow powders in high yields (87-88%). The dizinc complex [Zn₂L^H(OAc)₂] (5b) was isolated as a bright yellow solid in 83% yield, by 1:2:2 reaction of H₂L^H with zinc(II) acetate and triethylamine. Aldehydes 3a and 3b, macrocycle H₂L^H, and complex [Zn₂L^H(OAc)₂] (5b) have been structurally characterized. The carbazole NH makes bifurcated hydrogen bonds with the pair of flanking 1,8-diformyl-moieties in 3, or 1,8-diimine-moieties in H₂L^H, leading to a flat, all- cis conformation. The stepped conformation of the metal-free macrocycle H₂L^H is retained in [Zn₂L^H(OAc)₂], despite deprotonation and binding of two zinc(II) centers within the two tridentate pockets. The N₃O₂ coordination of the zinc ions is completed by one μ_1,1- and one μ_1,3- bridging acetate anion. Excitation of nanomolar [Zn₂L^H(OAc)₂] in DMF at 335 nm results in clearly visible blue fluorescence (λ_max = 460 nm). Further studies on the H₂L^H macrocycle revealed turn-on fluorescence, with selectivity (over Ca²⁺, Mg²⁺ and a range of 3d dications) and nanomolar sensitivity for zinc(II) ions, highlighting one of the many potential applications for these new carbazole-based Schiff base macrocycles.