title: Molecular Seesaw: How Increased Hydrogen Bonding Can Hinder Excited-State Proton Transfer.
abstract: A previously unexplained effect in the relative rate of excited-state intramolecular proton transfer (ESIPT) in related indole derivatives is investigated using both theory and experiment. Ultrafast spectroscopy [ J. Phys. Chem. A, 2015, 119, 5618-5625 ] found that although the diol 1,3-bis(2-pyridylimino)-4,7-dihydroxyisoindole exhibits two equivalent intramolecular hydrogen bonds, the ESIPT rate associated with tautomerization of either hydrogen bond is a factor of 2 slower than that of the single intramolecular hydrogen bond in the ethoxy-ol 1,3-bis(2-pyridylimino)-4-ethoxy-7-hydroxyisoindole. Excited-state electronic structure calculations suggest a resolution to this puzzle by revealing a seesaw effect in which the two hydrogen bonds of the diol are both longer than the single hydrogen bond in the ethoxy-ol. Semiclassical rate theory recovers the previously unexplained trends and leads to clear predictions regarding the relative H/D kinetic isotope effect (KIE) for ESIPT in the two systems. The theoretical KIE predictions are tested using ultrafast spectroscopy, confirming the seesaw effect.
authors.affiliation: Division of Chemistry and Chemical Engineering, California Institute of Technology , 1200 East California Blvd., Pasadena, California 91125, United States.
authors.forename: Ralph
authors.initials: R
authors.lastname: Welsch
authors.affiliation: Department of Chemistry, University of Southern California , Los Angeles, California 90089-1062, United States.
authors.forename: Eric
authors.initials: E
authors.lastname: Driscoll
authors.affiliation: Department of Chemistry, University of Southern California , Los Angeles, California 90089-1062, United States.
authors.forename: Jahan M
authors.initials: JM
authors.lastname: Dawlaty
authors.affiliation: Division of Chemistry and Chemical Engineering, California Institute of Technology , 1200 East California Blvd., Pasadena, California 91125, United States.
authors.forename: Thomas F
authors.initials: TF
authors.lastname: Miller