Bipolar single-molecule electroluminescence and electrofluorochromism

Activating the Fluorescence of a Ni(II) Complex by Energy Transfer

“…This excites the molecule into a higher doublet state (D x ) and permits access to S 1 upon discharging. 32 We have confirmed this by dI/dV and STML spectroscopy down to voltages of V s = −3 V (see Figure S1), and there is still no detectable NiPc emission despite fulfilling E(D x ) > E(S 1 ) (see also Section 2 of the Supporting Information). However, there is usually an energy mismatch, and therefore, the discharging will end in a vibrationally hot S 1 state.…”

“…We note that also homodimers of two NiPc molecules showed no fluorescence in STML (see Figure S6a). In comparison, homodimers of ZnPc and PdPc (Figure S6b) showed a redshift and sharpening of the Q-band emission, which is an expected effect of coherent excitonic coupling. , Overall, our observations are in line with previous submolecularly resolved STML studies of Pc-based donor–acceptor dimers, confirming our interpretation that the fluorescence of NiPc is enabled by RET from one of the MPc donor molecules (M = Zn, Pd, or Pt).…”

“…27,32 In cases where the doublet ground state D 0 has a lower energy than the S 1 state, the molecule can only go back to S 0 by a nonradiative discharging process. 32 In NiPc, both the NIR and PIR threshold magnitudes are accessed at voltages where eV s < E(Q NiPc ); hence, both anionic and cationic D 0 levels are lower than S 1 . A similar situation was previously found for other nonluminescent Pc molecules.…”

“…In STML experiments of isolated NiPc molecules, the excitation is mostly preceded by an electron tunneling into (or out of) the molecule, leading to an anionic (or cationic) doublet state, which is drawn schematically in Figure a (red arrows). , In cases where the doublet ground state D 0 has a lower energy than the S 1 state, the molecule can only go back to S 0 by a nonradiative discharging process . In NiPc, both the NIR and PIR threshold magnitudes are accessed at voltages where eV s < E (Q NiPc ); hence, both anionic and cationic D 0 levels are lower than S 1 .…”

“…While the established strategy is to tweak the intramolecular structure chemically toward optoelectronic applicability, an interesting alternative approach may be the physical design of intermolecular interactions, by controlling the local environment around the luminophore. In this context, recent studies combining scanning tunneling microscopy (STM) with light detection, referred to as STM-induced luminescence (STML) spectroscopy, not only enabled to fundamentally understand single-molecule fluorescence with submolecular resolution, − but especially the influence of the local environment and neighboring chromophores could be investigated using STM-based atomic manipulation techniques. For example, the impacts of adsorption at defects and step edges of an insulating surface on the fluorescence spectrum were studied, exciton delocalization and superradiance were observed in J-aggregated ZnPc dimers and chains, ,, and resonant energy transfer (RET) in molecular donor–acceptor dimers and trimers was investigated with atomic-scale resolution. − The latter is particularly interesting, as it may offer the opportunity to activate luminescence in otherwise dark open-shell 3d metal complexes by physical design of the local environment rather than intramolecular chemical design.…”