# Experimental Study Redefines the Mechanism of Heptamethine Cyanine Phototruncation

## TL;DR

This study reveals how a specific cyanine dye breaks down under light, forming a shorter version of itself through a complex chemical process.

## Contribution

The paper experimentally identifies a detailed mechanism for phototruncation of heptamethine cyanine dyes involving electron transfer and oxygen.

## Key findings

- Phototruncation of Cy7 occurs efficiently in aqueous solutions with specific buffer compositions.
- The process involves ultrafast electron transfer from excited Cy7 to oxygen, forming a radical dication intermediate.
- Cy5 can also undergo phototruncation via the same mechanism but with lower efficiency.

## Abstract

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.