Laser-induced fluorescence spectroscopy of intramolecular hydrogen bonding in tropolone derivatives

Laser-induced fluorescence spectra of tropolone derivatives were recorded in a free jet expansion to investigate electronic and vibrational effects on intramolecular H- atom tunneling. The results highlight the sensitivity of tunneling to both the vibrational and electronic states. Electron donation stiffens the pseudo-aromatic ring, so that even though 5-amino and 5-hydroxytropolone are substituted far from the tunneling coordinate, large effects are still observed. The derivatives form a series that ranges from symmetric to strongly asymmetric. For example, though 5-NH{dollar}sb2{dollar} is symmetric, 5-NHD has a slight asymmetry in the double minimum potential well due entirely to zero point differences. In the ground state, where the tunneling barrier is greater and NHD is non-planar, tunneling is quenched by this asymmetry and conformations are localized. In the excited state, where the tunneling barrier is lower and NHD is planar, tunneling is restored. 5-OH has a more pronounced asymmetric electronic structure, and the two inplane orientations of the 5-OH with respect to the 2-OH produce two ground state conformers, syn and anti. In the excited state, some vibrational modes display localized behavior, while a "promoter" mode couples certain other syn and anti levels to varying degrees. Photoisomerization can be driven reversibly and with vibronic state-selectivity. In this sense the molecule acts as an optical switch. Finally, 2,7-dihydroxytropone (C{dollar}sb{lcub}rm 2v{rcub}{dollar}) (or 2,3-dihydroxytropone if C{dollar}sb{lcub}rm s{rcub}{dollar}) has a strongly asymmetric potential with respect to single hydrogen transfer (2,7 {dollar}leftrightarrow{dollar} 2,3 isomerization), and tunneling splitting between the mirror image wells of 2,3 (indicating a concerted double hydrogen transfer) is not observed.