Excited-State Symmetry Breaking Dynamics Driven by Mode-Resolved Jahn–Teller Distortion in Luminescent Tris(2,4,6-trichlorophenyl)methyl Radicals

Understanding the electronic excited-state dynamics of luminescent tris(2,4,6-trichlorophenyl)methyl (TTM) radicals is important for optimizing them as optical-spin interfaces for quantum information applications. Symmetric TTMs possess degenerate lowest-energy excited states for which the ultrafast photophysics remains unexplored. Here, we use broadband transient absorption (BBTA) spectroscopy on these systems at cryogenic temperatures to provide insight into their complex ultrafast electronic state relaxation via Jahn–Teller (JT) distortion and solvent stabilization. Global lifetime fitting of time-dependent BBTA spectra shows a 421 ± 15 fs JT distortion in TTM followed by a 13.9 ± 0.1 ps stabilization of the symmetry-broken relaxed excited state geometry of the radical by the solvent. Coherent wavepacket dynamics reveal fast dephasing of low frequency vibrations and a growth of coherent oscillations associated with an 807 cm–1 mode with a time constant of ∼485 fs, matching the JT distortion time scale. Electronic structure calculations reveal a conical intersection, while Huang–Rhys factor calculations exhibit significant vibronic coupling, thus corroborating the experimental results. Manifestation of similar vibronically coupled excited-state symmetry breaking was also found in a series of 4-aryl-substituted TTM derivatives and verified by complementary theoretical calculations.