ChemInform Abstract: The Jahn-Teller Effect in Triptycene.

Abstract

The irregular vibronic structure resolved in the S1←S0 resonant two‐photon ionization (R2PI) spectrum of supersonically cooled triptycene (9,10‐dihydro‐9,10[1’2’]benzenoanthracene) is assigned in terms of a single‐mode E’⊗e’ Jahn–Teller vibronic Hamiltonian for the excited state, with linear and quadratic coupling terms. The Jahn–Teller active vibrational mode is a benzene wagging framework mode. To fit to the observed vibronic levels yields a very low frequency νe’ =47.83 cm−1 and linear and quadratic terms are k=1.65 and g=0.426. This fit accounts for ≊98% of the observed absorption band intensities over the observable range 0–350 cm−1. The quadratic term is unusually large, leading to localization of the lowest vibronic levels in the three symmetry‐equivalent minima. Emission spectra from 13 vibronic levels in the excited E’ state show extended vibrational progressions with up to 25 members in the analogous e’ ground state vibration, which is highly harmonic in the electronic ground state. The Franck–C...