Multiphoton processes in photoreactions of indocyanine green in ethanol

In cyanine dye, new photoreaction paths involving multiphoton processes were uncovered by ultrafast transient absorption spectroscopy. In contrast to the conventional reaction path that proceeded through one-photon resonance to the S1 state by near-infrared light irradiation, the new reaction paths were found to proceed through near-infrared multiphoton excitation to higher excited states and the optical order of the electronic excitation was dependent on the type of photoproduct: a four-photon process for cis isomers and three for photodegradation. Leuco forms with saturated hydrocarbon chain seem to be formed by photodegradation. The ultraviolet pump fluence dependence of the cis isomer production was consistent with the near-infrared pump fluence dependence. Changing the near-infrared pump fluence controlled the number ratio of cis isomers to degraded molecules.