THE JAHN-TELLER EFFECT IN 9-FLUOROTRIPTYCENE

The vibronic structure in the S1(E) <-- S0(A1) resonant two-photon ion ization (R2PI) spectrum of supersonically cooled 9-fluorotriptycene is assigned using three different Jahn-Teller (JT) model Hamiltonians fo r the excited 1E state-E x e, (A + E) x e, and (A + E) x (a2 + e). The basic E x e interpretation is satisfactory. However, the fitted vibro nic band frequencies and intensities are improved by including couplin g to a second excited state 1A1 in an exciton model. Some further obse rved absorption bands are only assignable by invoking a molecular Barn ett effect (momentum coupling to an a2 vibration). The measured fluore scence emission spectra from different S1 vibronic levels are quantita tively reproduced within all three coupling schemes by the parameters fitted to the R2PI spectrum. Results are compared to previous calculat ions on unsubstituted triptycene. The JT stabilization energy is decre ased by approximately 10% upon fluoro bridgehead substitution, which i s rationalized by the electron-withdrawing effect of the F atom. For t he same reason, the exciton splitting between the S1 and the S2 states , as calculated in the (A + E) x e model, is reduced relative to tript ycene. The ground state vibrational frequencies in the range 0-700 cm- 1 are calculated using the semiempirical MOPAC 6.0/AM1 method and comp ared with the measured S0 frequencies, as well as those of triptycene. A 273 cm-1 degenerate C-F bending mode predicted by the AM1 calculati on may explain several unassigned features in the higher-energy (200-3 60 cm-1) part of the R2PI spectrum, and may represent an example of e x e multimode coupling within a degenerate electronic state.