Excited State Proton Transfer of Super Photo Acid Tethered to Lipids Membranes Reveals the Presence of Contact Ion Pair

Photoacid molecules experience a significant decrease in pKa upon electronic excitation, facilitating excited state proton transfer (ESPT) to nearby proton acceptors. Super photoacids are characterized by pKa* < 0, hence, they are ‘super’ proton donors to even acidic acceptors. Here, we synthesized a new super photoacid molecule – C12-Hydroxyquinoline (C12-HQ). According to the Eigen–Weller model, an ion pair forms during the dissociation process, where the proton transfers to the acceptor while still bound to the photoacid. However, to date, the experimental evidence of this ion pair formation was elusive. In our study, we use C12-HQ tethered to a lipid membrane composed of either phosphatidylcholine (POPC) or phosphatidic acid (POPA). Using ultrafast spectroscopy, we were able to observe the transient formation on an ion pair between C12-HQ and the lipid membrane. By comparing POPC membranes to POPA membranes, we concluded that it is the phosphate group in the POPA membrane that acts as the proton acceptor and forms the ion pair with the dissociated proton. This capability of the phosphate group to form an ion pair with the proton is only possible while using a super photoacid, while a regular photoacid is too ‘weak’ to serve as the proton donor to the acidic phosphate group.