Exploring organic photosensitizers based on hemicyanine derivatives: a sustainable approach for preparation of amide linkages.

Hemicyanine derivatives C1-C4 have been synthesized and show strong absorption in the visible region, good water solubility, efficient intersystem crossing, a high singlet oxygen quantum yield and high ability to transport electrons from the donor to acceptor. These hemicyanine derivatives were utilized as photocatalysts in additive/base free oxidative amidation of aromatic aldehydes in mixed aqueous media under visible light irradiation at low catalytic loading. The hemicyanine derivative C4 exhibited recyclability upto four cycles and reusability upto five cycles in oxidative amidation of aromatic aldehydes. Among all the hemicyanine derivatives, C4 shows a high photocatalytic efficiency due to a high singlet oxygen quantum yield. All the mechanism investigations showed involvement of reactive oxygen species generated by the organic triplet photosensitizer based on hemicyanine derivative for carrying out oxidative amidation of aldehyde. Our results will encourage the design of new "metal free" organic photosensitizers and their application in photocatalysis.