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 beta-gamma cis isomer. Exchange rates extrapolated from 2D EXSY spectra under slow exchange conditions are in agreement with that estimated from methine H-1 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 gamma position.