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

Abstract 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.