Dipole-Bound-State Spectroscopy Reveals the Vibrational Structure of a Strongly Reorganized Radical and an Unusual Regime of Anharmonic Autodetachment.

Dipole-bound states (DBSs) are typically viewed as weakly bound spectroscopic intermediates near the electron detachment threshold. Here we demonstrate that a DBS can serve as both a high-resolution spectroscopic probe of an otherwise inaccessible neutral vibrational structure and a platform for fundamentally altered autodetachment dynamics. Using cryogenic photoelectron spectroscopy, nanosecond VIS + IR spectroscopy, and picosecond time-resolved photoelectron imaging, we investigate the DBS of the deprotonated tropolone anion (Tp-). Direct photodetachment is dominated by severe Franck-Condon congestion arising from aromaticity loss and pronounced structural distortion upon electron detachment, obscuring the vibrational structure of the neutral radical. In contrast, DBS-mediated spectroscopy resolves extensive progressions of torsional, out-of-plane, and ring-deformation modes, enabling detailed vibrational characterization of the nonaromatic neutral species. The DBS exhibits an unusually large binding energy of 652 ± 63 cm-1, exceeding the energies of the dominant low-frequency vibrational modes. As a result, efficient propensity-rule autodetachment through these modes becomes energetically inaccessible, substantially suppressing conventional autodetachment pathways. Time-resolved measurements reveal markedly prolonged relaxation dynamics and direct competition between autodetachment and internal conversion, leading to thermionic electron emission from vibrationally hot ground-state anions. These findings establish DBS spectroscopy as a powerful route to vibrational structure obscured by Franck-Condon limitations and suggest that sufficiently large DBS binding energies can fundamentally alter excess-electron relaxation dynamics.