S_2 State Optical Property Enhancement of Indocyanine Green Due to Optical Exposure

摘要

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 NIRoptical properties. in addition to the well-known NIR absorption, ICG also shows an absorptionband with a peak maximum at ~400 nm. However, insignificant information is available about itsoptical properties in the visible wavelength range. The absorption peak at ~400 nm could bereferred to as the transition of the electrons from the ground (S_0) state to second excited (S_2) state.After excitation to the S_2 state, these electrons could directly relax to S0 state, this transition causesan emission with a maximum at ~557 nm. ICG is an optically unstable molecule, its NIRabsorption and emission deteriorate upon optical exposure. However, optical exposure does notseem to affect the higher energy transitions of ICG adversely. Unlike the NIR emission, thefluorescence emission of ICG at ~557 nm increases upon continuous optical exposure. To the bestof our knowledge, this is the first report on the S_2 state mediated emission enhancement due to thelight exposure of ICG in the visible wavelength range. Improvement in the optical properties ofthe ICG due to optical exposure might open several exciting areas of research where its applicationcould be broadened.