Experimental Study Redefines the Mechanism of Heptamethine Cyanine Phototruncation

Cyanine dyes are popular chromophores used in contemporary biomedical fields due to their tunable near-infrared absorption and fluorescence properties. One such application of cyanines involves the photochemical shortening of the polymethine chain by a two-carbon fragment. This process is referred to as phototruncation or photoblueing because the resulting cyanine's absorption band shifts hypsochromically. A recent quantum-chemical study has proposed a mechanism for this process; however, it cannot explain the substantial enhancement of the reaction under specific conditions. Here, we present the results of an extensive investigation of phototruncation of the prototypical heptamethine cyanine dye (Cy7). To elucidate the underlying mechanism, a comprehensive analytical approach was employed, encompassing kinetic studies, isotopic labeling, transient spectroscopy, femtosecond stimulated Raman spectroscopy, collision-induced dissociation, and infrared photodissociation spectroscopy. Our findings demonstrate that phototruncation occurs efficiently in aqueous solutions of a specific organic buffer composition. It is sensitive to reactant concentrations and pH, and its efficiency increases with the addition of electron acceptors. The reaction involves ultrafast electron transfer from a singlet-excited cyanine dye to oxygen, forming a radical dication intermediate. This intermediate reacts with another oxygen molecule and subsequently with a buffer constituent featuring an ethanolamine scaffold. The reaction continues with oxidation, cyclization, and elimination steps to form pentamethine cyanine (Cy5) in yields up to 33%. We also demonstrate that Cy5 undergoes phototruncation via the same mechanism but with lower efficiency. The triplet-excited Cy7 also undergoes phototruncation. The findings of this study lay the foundation for the further exploitation of this unique process.