Isomerization and Dimerization of Indocyanine Green and a Related Heptamethine Dye

Indocyanine green (ICG) and a second heptamethine cyanine dye were studied in detail using 2D NMR spectroscopy at multiple temperatures. In addition to the all‐trans conformation, we found in both cases a small percentage of a specific β‐γ cis isomer. Exchange rates extrapolated from 2D EXSY spectra under slow exchange conditions are in agreement with that estimated from methine 1H linewidths near coalescence and with previous photo‐isomerization studies. We could also confirm our previous hypothesis that, under aerobic conditions in combination with exposure to daylight, ICG undergoes radical dimerization specifically at the γ position. Introduction