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

Abstract

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-perylenetetracarboxylicdianhydride (PTCDA) molecule in real space by imaging the spatial distribution of single-molecule electroluminescence 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-supported ultrathin 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°, which reflects the change in the transition dipole orientation from along the molecular short axis to the long axis. Such a change directly reveals the occurrence of strong vibronic coupling associated with a large Herzberg-Teller (HT) contribution, which goes beyond the conventional 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 (A_g) 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-dominated nature. Notably, the vibration-induced emission associated with HT-dominated contributions (such as the v₂ (B_3g) 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 B_3g 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>i.e.</i>, induce efficient mixing of the electronic excited state S₁ with the electronic excited state S₂), so that the v₂-vibration induced 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.