Proton tunneling of tropolone in durene single crystal as studied by time-resolved EPR detected excitation spectroscopy

We have measured the excitation spectra for tropolone–OH in durene single crystal and tropolone–OD in deuterated durene using a time-resolved electron paramagnetic resonance (TREPR) detection method that makes possible to separate the signals due to magnetically different sites. The tunneling doublet with 3 cm⁻¹ was observed in the sharp zero-phonon line. The small splitting indicates that the crystal field increases the barrier of double-minimum potential for the proton tunneling in the $S1$ state. Moderately asymmetric potentials of the $S0$ and $S1$ states, where the energetic imbalance between two wells in the $S1$ state potential is opposite the $S0$ state potential, reasonably explained the observed unusual intensity ratio of the tunneling doublet $(01+<01−).$ A well-resolved progression of a phonon band with a 15 cm⁻¹ separation was also obtained in durene crystal at very low temperature. From a Franck–Condon analysis of the relative intensity of the phonon band, it was clarified that the stable configuration of the excited state tropolone in durene differed from that of the ground state.