In this study, we have chosen the common 2-naphthol-6-sulfonate (2N6S) as the PA and 6aminoquinoline (6AQ) as the PB (Figure ). The hallmark of the ESPT process is the deprotonation of the PA (ROH) to RO -and the protonation of the PB (RN) to RNH + only upon light excitation, which is due to the different pKa values between the ground and excited states (Figure ). The ESPT process can be observed spectroscopically since the protonated and deprotonated states of both the PA and the PB absorb and emit at different wavelengths. Indeed, while the UV-Vis ground state absorption spectra of 2N6S and 6AQ show that 2N6S is in its ROH state and 6AQ is in its RN state (Figure , black curves), the emission spectra, which probe the excited state, show that 2N6S is in its RO -* state and 6AQ is in its RNH +* state (Figure , red curves). Importantly, the emergence of the ionic forms following PA and PB excitation is only in the excited state. Thus, upon returning to the ground state, the ESPT will be followed by a recombination process that will eventually result in the molecules returning to their initial ROH/RN states.