Abstract
Pyrazine derivatives DIPY, TETPY and CNDIPY have been designed and synthesized which form fluorescent supramolecular assemblies in mixed aqueous media due to their AIEE and ICT characteristics. Among all the derivatives, the assemblies of TETPY and CNDIPY show strong absorption in the visible region with high absorption coefficients, low HOMO-LUMO gap, and high photostability. Further, the supramolecular nanoassemblies of TETPY and CNDIPY show excellent potential to generate reactive oxygen species (ROS) under the visible light irradiation. Owing to their strong absorption in the visible region and ROS generation ability, the supramolecular nanoassemblies of TETPY and CNDIPY act as efficient photoredox catalytic systems for carrying out (a) oxidative amidation of aromatic aldehydes (b) hydroxylation of boronic acid and (c) oxidative homocoupling of benzylamines under mild conditions such as aqueous media, aerial environment, and natural sunlight as a source of irradiation. All the mechanistic investigations suggest the participation of in-situ generated ROS in the organic transformations upon light irradiation.
Highlights
Very recently, supramolecular assemblies of perylene bisimide (PBI) derivatives having the potential to generate reactive oxygen species have been reported as efficient photosensitizers for degradation of dyes and photodynamic therapy under visible light irradiation[14]
Suzuki Miyaura coupling between dibromo pyrazine/tetrabromo pyrazine derivative (1a/1b)[16] and 4-(2,2′,6′,2′′-terpyridine-4′-yl) phenyl boronic acid (2)[17] in dioxane furnished DIPY/TETPY derivative in 80%/85% yield (Fig. 1)
The 1H NMR spectrum of DIPY/TETPY in CDCl3 shows a characteristic singlet at 8.80/8.80 ppm corresponding to Tpy aromatic protons and characteristic doublet/ multiplet at 8.23/8.36-8.33 ppm corresponding to Py aromatic protons
Summary
Pyrazine derivatives DIPY, TETPY and CNDIPY have been designed and synthesized which form fluorescent supramolecular assemblies in mixed aqueous media due to their AIEE and ICT characteristics. Over the past few years, the pursuit of researchers to make organic transformations more ‘green’ resulted in the development of new organic photosensitizers (such as BODIPY derivatives and carbon-based derivatives) having strong absorption in the visible region and long-lived triplet states[8,9,10,11,12,13] Though these photosensitizers could overcome some of the limitations associated with off-the-shelf complexes/dyes such as low photostability and requirement of high catalytic loading, presence of toxic heavy atoms and their multi-step synthesis (especially in carbon-based materials) which undermines the overall goal of an alternative ‘green’ catalytic system. To best of our knowledge, this is the first report which demonstrates the development of AIEE-ICT active ‘metal/heavy atom’ free supramolecular nanoassemblies of pyrazine based photosensitizers as efficient photoredox catalytic systems for carrying out oxidative organic transformations using natural sunlight as the source of irradiation
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