The Jahn–Teller effect in 9-fluorotriptycene.

The vibronic structure in the S1(E)←S0(A1) resonant two-photon ionization (R2PI) spectrum of supersonically cooled 9-fluorotriptycene is assigned using three different Jahn–Teller (JT) model Hamiltonians for the excited 1E state—Exe, (A+E)xe, and (A+E)x(a2+e). The basic Exe interpretation is satisfactory. However, the fitted vibronic band frequencies and intensities are improved by including coupling to a second excited state 1A1 in an exciton model. Some further observed absorption bands are only assignable by invoking a molecular Barnett effect (momentum coupling to an a2 vibration). The measured fluorescence emission spectra from different S1 vibronic levels are quantitatively reproduced within all three coupling schemes by the parameters fitted to the R2PI spectrum. Results are compared to previous calculations on unsubstituted triptycene. The JT stabilization energy is decreased by ∼10% upon fluoro bridgehead substitution, which is rationalized by the electron-withdrawing effect of the F atom. For the same reason, the exciton splitting between the S1 and the S2 states, as calculated in the (A+E)xe model, is reduced relative to triptycene. The ground state vibrational frequencies in the range 0–700 cm-1 are calculated using the semiempirical MOPAC 6.0/AM1 method and compared with the measured S0 frequencies, as well as those of triptycene. A 273 cm-1 degenerate C–F bending mode predicted by the AM1 calculation may explain several unassigned features in the higher-energy (200–360 cm-1) part of the R2PI spectrum, and may represent an example of e+e multimode coupling within a degenerate electronic state. [ABSTRACT FROM AUTHOR]