Fluorescence excitation spectrum of triptycene in a supersonic jet

The laser-induced fluorescence excitation spectrum of triptycene in pulsed supersonic expansions of He reveals the electronic origin of the S ₀→S₁(1A′₁→¹E′) transition at 2750.85 Å (36 352 cm^-1), followed by 19 vibrational excitations in the energy range 350 cm^-1 above it. This vibrational structure corresponds to the excitations of four torsional modes of the phenyl rings. Mechanical models for these torsional modes indicate the existence of three doubly degenerate vibrations, exhibiting Jahn-Teller coupling with the ¹E′ state, together with double excitations of a nondegenerate vibration. At excess vibrational energies of 350-1000 cm ^-1 above the S₁ origin the laser-induced fluorescence spectrum is extremely weak and devoid of any structure, implying the onset of a new effective intramolecular decay channel, which may result in isomerization in the isolated molecule.