S2 state optical property enhancement of indocyanine green due to optical exposure

Indocyanine green (ICG) is a United States Food and Drug Administration approved near-infrared (NIR) chromophore used in clinics for the last six decades. It has been used for cardiac output, hepatic function measurements, NIR imaging, and anticancer therapy. ICG is known for its NIR optical properties. in addition to the well-known NIR absorption, ICG also shows an absorption band with a peak maximum at ~400 nm. However, insignificant information is available about its optical properties in the visible wavelength range. The absorption peak at ~400 nm could be referred to as the transition of the electrons from the ground (S0) state to second excited (S2) state. After excitation to the S2 state, these electrons could directly relax to S0 state, this transition causes an emission with a maximum at ~557 nm. ICG is an optically unstable molecule, its NIR absorption and emission deteriorate upon optical exposure. However, optical exposure does not seem to affect the higher energy transitions of ICG adversely. Unlike the NIR emission, the fluorescence emission of ICG at ~557 nm increases upon continuous optical exposure. To the best of our knowledge, this is the first report on the S2 state mediated emission enhancement due to the light exposure of ICG in the visible wavelength range. Improvement in the optical properties of the ICG due to optical exposure might open several exciting areas of research where its application could be broadened.