Indocyanine Green Fluorescence: Quenching by a Twisted Charge-Transfer State Violates the Kasha-Vavilov Rule

The near-infrared dye indocyanine green (ICG) is widely used for biomedical imaging, yet its photophysical behavior remains incompletely understood. We have investigated how viscosity affects ICG fluorescence in methanol:glycerol mixtures. Increasing the glycerol content raised the viscosity and increased the fluorescence quantum yield (FQY) and lifetime in a way that could not be explained by changes in the solvent's polarity. ICG also exhibited an unusual violation of the Kasha-Vavilov rule that FQYs are generally independent of the excitation wavelength. Calculations using density functional theory revealed that the changes in FQY reflect formation of a nonfluorescent twisted conformation. Twisting around one of the bonds in the central polymethine chain has a calculated energy barrier of about 0.3 eV that can be surmounted at the expense of excess vibrational energy when the molecule is excited on the blue side of the absorption band but not with excitation at the absorption maximum.