Probing vibronic coupling of a transiently charged state of a single molecule through subnanometer resolved electroluminescence imaging

The intramolecular vibronic coupling has a great effect on molecular electronic transitions and associated spectral characteristics, which is a central topic in the study of molecular spectroscopy. In this paper, we investigate the vibronic coupling of a transiently charged state within a single 3,4,9,10-perylenetetracarboxy-licdianhydride (PTCDA) molecule in real space by imaging the spatial distribution of single-moleculeelectroluminescence via highly localized excitation of tunneling electrons in a plasmonic nanocavity. The electron injections from a scanning tunneling microscope tip into a PTCDA molecule on a silver-supportedultrathin salt layer produce a transient doubly charged molecular anion that emits vibrationally resolved fluorescence. The sub-molecular resolved spectroscopic imaging for the -2 valence transiently charged state shows a two-spot pattern along the molecular short axis for the purely electronic 0-0 transition. However, the observed two-spot orientation for certain anti-symmetric vibronic-state imaging is found to be evidently different from the purely electronic 0-0 transition, rotating 90 degrees, which reflects the change in the transition dipoleorientation from along the molecular short axis to the long axis. Such a change directly reveals the occurrenceof strong vibronic coupling associated with a large Herzberg-Teller (HT) contribution, which goes beyond theconventional Franck-Condon (FC) picture. Combined with theoretical calculations, the anti-symmetric vibration is found to have a strong dynamic disturbance to the transition density of purely electronic transitions, especially those atoms with large transition densities, which induces a strong transition charge oscillation along the long axis of the molecule and thus leads to a transition dipole along the long axis of the molecule. On the other hand, for vibronic emissions associated with the totally symmetric molecular vibration(such as the v1 (Ag) mode described above), the observed two-spot orientation in the vibronic-state imaging pattern is found to be the same as the purely electronic 0-0 transition, which directly reveals its FC-dominatednature. Notably, the vibration-induced emission associated with HT-dominated contributions (such as the v2(B3g) mode) is often discussed in the literature by using an intensity borrowing mechanism via the state mixing with other high-lying eigenstates. In the present work, the v2-vibration with B3g symmetry is likely to modulate the zero-order electronic wavefunction of the S1 state in a way to best resemble that of the S2 state (i.e., induce efficient mixing of the electronic excited state S1 with the electronic excited state S2), so that the v2-vibrationinduced emission seems to borrow intensities from neighboring S2 -> S0 transitions. Our results provide a new route for the real-space understanding of the microscopic picture for the vibronic coupling within a single molecule in a transiently charged state