The concept of structural helicity to enhance the triplet conversion of BODIPY dyes is discussed with reported helical BODIPYs. These dyes have high triplet yields, long triplet lifetimes and are capable of generating high singlet oxygens even in hypoxia condition. Potential applications of these helical BODIPYs as heavy-atom-free triplet photosensitizers for photodynamic therapy of cancer is also reviewed.

Abstract

Despite of having many advantages over the popular cancer therapies, photodynamic therapy still is not widely acceptable for clinical applications. Scarcity of efficient triplet photosensitizers (PSs) is one of the main bottlenecks for this. Although numerous heavy atom attached PSs are developed as PDT agents, but these are not suitable for clinical applications due to several reasons. Thus, development of heavy-atom-free organic PSs is urgently required. In this context, development of new type of helical BODIPYs as PSs for PDT is discussed. BODIPYs have rich photophysical properties and structural helicity further enhances their triplet conversion rates. This new concept of structural helicity to enhance the triplet conversion of BODIPYs is discussed with reported helical BODIPYs. Helical geometries of these dyes are checked by X-ray crystallography studies and their high triplet conversions as compared to planar BODIPYs are also confirmed. Importantly, these dyes have high triplet lifetimes and are capable of generating high singlet oxygens even in hypoxia condition as compared to conventional heavy atom attached BODIPYs. All these make the helical BODIPYs excellent candidates as PDT agents. Finally, their successful applications as PDT agents in killing of various types of cancer cells are also discussed. The results are encouraging which indicate that helical BODIPYs could be next generation heavy-atom-free PSs for PDT applications.