Transition-Metal-Free C-S Bond Forming Strategy towards Synthesis of Highly Diverse Pyrazole Tethered Benzothiazoles: Investigation of their Photophysical Properties

A metal-free domino approach has been devised towards construction of two C-S bonds for the synthesis of bioactive N, S-heterocycles. The developed method was extended for the preparation of structurally diverse pyrazole tethered benzothiazole frameworks by using one-pot operation of pyrazole C-3/4/5 carbaldehydes, electron rich aromatic amines and elemental sulfur. This protocol provides excellent fluorophores with several additional advantages such as transition metal-free approach, inexpensive and odorless sulfur source, superior atom economy, and broad substrate scope including gram scale synthesis. The synthesized pyrazole tethered benzothiazole fluorophores were evaluated for their luminescent properties including absorbance, excitation, emission, molar extinction coefficient, brightness, and Stokes shift. The photophysical studies revealed that these pyrazole and benzothiazole hybrids emerged as excellent fluorophores and could exhibit fluorescence quantum yield up to 66%.