Coupling of a Jahn-Teller pseudorotation with a hindered internal rotationin an isolated molecule: 9-hydroxytriptycene

The irregular vibronic structure in the S-1<--S-0 resonant two-photon ioniz ation (R2PI) spectrum of supersonically cooled triptycene is a result of a classic Exe Jahn-Teller effect [A. Furlan et al., J. Chem. Phys. 96, 7306 ( 1992)]. This is well characterized and can be used as an effective probe of intramolecular perturbations. Here we examine the S-1<--S-0 R2PI spectrum of 9-hydroxytriptycene and the fluorescence from various excited state vibr onic levels. In this system the pseudorotation of the Jahn-Teller vibration is strongly coupled to the torsional motion of the bridgehead hydroxy grou p. This torsional motion results in a tunneling splitting in both the groun d and excited states. The population of the upper level in the ground elect ronic state results in additional vibronic transitions becoming symmetry al lowed in the R2PI spectrum that are forbidden in the bare triptycene molecu le. The assignment of the R2PI and fluorescence spectra allows the potentia l energy surfaces of these vibrational modes to be accurately quantified. T he full C-3v vibronic point group must be used to interpret the spectra. Th e time scale of the internal rotation of the-OH group and the butterfly fla pping of the Jahn-Teller pseudorotation are of similar magnitude. The tunne ling between the nine minima on the three dimensional potential energy surf ace is such that the Jahn-Teller pseudorotation occurs in concert with the- OH internal rotation. The Berry phase that is acquired during this motion i s discussed. The simple physical picture emerges of the angle between two o f the three benzene moieties opening in three equivalent ways in the S-1 el ectronic state. This geometry follows the position of the hydroxy group, wh ich preferentially orients itself to point between these two rings. (C) 199 8 American Institute of Physics. [S0021-9606(98)02348-4].