Two-photon-induced intramolecular excited-state proton transfer process and nonlinear optical properties of HBI in ethanol solution

► The optical nonlinearity of HBI was investigated for the first time. ► The nonlinear absorption was confirmed to originate from TPA. ► By establishing ESIPT kinetic model, the TPA cross section was obtained. ► The HBI molecules exhibit much larger TPA cross section than other similar structural ones. ► The high nonlinearities were interpreted on the basis of the nonlinear optical molecular design and their origin.