Photoredox catalytic organic reactions promoted with broadband visible light-absorbing Bodipy-iodo-aza-Bodipy triad photocatalyst

Abstract

The 4-bora-3a,4a-diaza-s-indacene scaffold is known as Bodipy. 2,6-Diiodo-aza-Bodipy (B-1) and the Bodipy-2,6-diiodo-aza-Bodipy triad (B-2) have been used as novel photocatalysts for photoredox catalytic organic reactions with tetrahydroisoquiniline as substrate to prepare highly functionalized organic compounds via a [3 + 2] cycloaddition–aromatization tandem reaction and Cu(I)-catalyzed alkynylation reactions. In distinction to conventional photocatalysts such as Ru(bpy)₃Cl₂, Eosin Y or Rose Bengal, which are based on a mono-visible light-harvesting chromophore profile and show weak absorption in the visible region, the new photocatalysts are strong visible absorbers (B-1, ε = 73 000 M⁻¹ cm⁻¹ at 683 nm). More importantly, resonance energy transfer (RET) has been used to increase the absorption of photocatalyst B-2 in the visible region, in which two Bodipy units were used as energy donor and diiodo-aza-Bodipy as energy acceptor. B-2 shows broadband absorption in the range 400–750 nm (ε = 165 000 M⁻¹ cm⁻¹ at 504 nm, and 71 000 M⁻¹ cm⁻¹ at 683 nm). Iodo-aza-Bodipy is more efficient than conventional photocatalysts such as [Ru(bpy)₃]Cl₂. Furthermore, the broadband visible light-absorbing B-2 is more efficient as a photocatalyst than previously reported monochromophore photocatalyst B-4 (diiodo-Bodipy). Our results will be useful for the design of efficient organic triplet photosensitizers as photocatalysts for photoredox catalytic organic reactions.