Unraveling the Mechanism of a Reversible Photoactivated Molecular Proton Crane

We study the structural dynamics of the photoactivated molecular proton crane 7-hydroxy-8-(morpholinomethyl)quinoline using femtosecond UV-pump IR-probe spectroscopy. Upon electronic excitation, a proton is transferred from the hydroxy to the amine group located on the rotatable morpholino side group. This morpholino group subsequently delivers the proton to the aromatic quinoline nitrogen by rotation around the CC bond. Time-resolved vibrational spectroscopy allows us to study this process in unprecedented detail. We find that the transport of the proton involves multiple time scales. Upon photoexcitation, the OH proton is transferred within <300 fs to the morpholino side group. After this, the intramolecular hydrogen bond that locks the crane arm breaks with a time constant of 36 +/- 1 ps. Subsequently, the protonated crane arm rotates with a time constant of 334 +/- 12 ps to deliver the proton at the quinoline moiety. After the proton crane has returned to its electronic ground state with a time constant 700 +/- 22 ps, the proton is transferred back from the quinoline nitrogen to the negatively charged O atom. The time constant of the back rotation is 39.8 +/- 0.2 ns, about 200 times slower than the forward proton transfer.