The photophysical property of the tricarbocyanine dye IR144 has been extensively studied in non-aqueous solvents. However, as a potential near-infrared biomedical imaging probe, the photophysical property of IR144 in water is still little known. So, the aggregation behaviors of IR144 in water with steady-state absorption spectroscopy and integrated polarization dependent femtosecond pump-probe spectroscopy were investigated. Through comparing the absorption spectral bandshape of IR144 in water and in water pool of AOT reverse micelles, It is found that IR144 form dimer aggregates in water even at very low concentration （〈 1.0× 10^- 7 moloL 1）. And the absorption spectrum of the IR144 aggregates always displays a bimodal feature, which is independent of the dye concentration ranging from 1.0 × 10^-7 to 1.0 × 10^-4 mol·L^-1. For better understanding the aggregation behaviors of IR144 in water, we measured the ground state recovery kinetics and the reorientation kinetics of IR144 in water and in water pool of AOT reverse micelles （W0= [H2O]/[AOT], W0=40）. It is found that the fluorescence quantum yield of 1R144 in water is lower than that in water pool of AOT reverse micelles, and the reorientation time of IR144 in water is slower than that in water pool of AOT reverse micelles. Those kinetic measurements also verify that IR144 exists as dimer aggregates in water.
