The First Microseconds of the Life of Excited Heptamethine Cyanine Revealed by Femtosecond Stimulated Raman Spectroscopy

Heptamethine cyanines are a well-known class of organic near-infrared (NIR) fluorophores that play an indispensable role in chemistry and biology. Despite their ubiquity, the underlying photophysical and photochemical dynamics triggered by excitation remain surprisingly elusive. In this study, we investigated a prototypical heptamethine cyanine (Cy7) using femtosecond stimulated Raman spectroscopy. Combining transient Raman spectra with quantum chemical calculations allowed us to develop a comprehensive picture of the species produced during Cy7 excitation and their behavior on the fs to sub-ms time scale. We have unambiguously identified the excited singlet and triplet states and the resulting configurational photoisomers using clearly distinguishable Raman shifts. We also reveal solvent-mediated relaxation channels, in particular ultrafast photoinduced electron transfer to dissolved dioxygen, generating cyanine radical dication in addition to superoxide. Together, these insights provide a coherent mechanistic framework for Cy7 photodynamics and provide design guidance for next-generation NIR probes.