Unraveling the Mechanism of a Reversible Photoactivated Molecular Proton Crane

Structural dynamics of the photoactivated mol. proton crane 7-​hydroxy-​8-​(morpholinomethyl)​quinoline has been studied 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 arom. quinoline nitrogen by rotation around the C-​C bond. Time-​resolved vibrational spectroscopy allows us to study this process in unprecedented detail. 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 intramol. hydrogen bond that locks the crane arm breaks with a time const. of 36 ± 1 ps. Subsequently, the protonated crane arm rotates with a time const. 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 const. 700 ± 22 ps, the proton is transferred back from the quinoline nitrogen to the neg. charged O atom. The time const. of the back rotation is 39.8 ± 0.2 ns, about 200 times slower than the forward proton transfer.