Excited state energy relaxation dynamics in near-infrared dye via transient absorption spectroscopy

Understanding the transient absorption dynamics of IR-780 infrared dye is important since it is extensively used in in vivo imaging and phototherapy treatments. It is widely used in biomedical imaging and cancer therapy. Linear absorption spectra of this dye show a strong absorption around 780 nm and vibronic sidebands in higher energy side. For nondegenerate pump–probe studies, the IR-780 dye is excited with a 780-nm pump pulse and probed with a white light continuum to obtain a two-dimensional transient spectrum. We observe different types of signals, such as ground state bleach, stimulated emission, and excited state absorption (S₁–S_n states), corresponding to different energy transitions and relaxation processes in the dye. Also, we perform open aperture Z-scan measurements to confirm that single-photon absorption is the dominant process of pump excitation. We observed the overall transient spectrum dynamics fitted for a double exponential decay process with time constants of 449 ps and 162 ps, associated with the excited state decay and bleach decay, respectively.