Accessing Energetically Restricted Optical Transitions in a Single Free-Base Porphyrin Molecule

TL;DR

This work demonstrates a general strategy to access optical transitions in single-molecule STML by engineering energy-level alignment through substrate work function control and tip-gating in a double-barrier junction. By decoupling porphyrin H$_{2}$TBP with NaCl and tuning $D_{0}^{+}$ relative to $S_{1}$, the otherwise forbidden $D_{0}^{+} \rightarrow S_{1}$ transition becomes radiatively active, with quantitative support from a polaron-based model that accounts for NaCl relaxation energy. Static work-function tuning (Ag(111) vs Ag(110)) further confirms energy-level realignment and reveals a potential $S_{2}$ emission at ~2.15 eV, highlighting the role of vibronic coupling in enhancing or shaping emission. TD-DFT analyses of Franck–Condon and Herzberg–Teller contributions, together with spatial emission maps, illuminate how local photophysics and plasmon coupling influence the observed spectra, underpinning a framework to predict and control electroluminescence in single molecules and related devices.

Abstract

Characterizing the electronic properties of single atoms, molecules, and nanostructures is the hallmark of scanning tunneling microscopy (STM). Recently, exploration of a complex manifold of nonequilibrium many-body electron configurations has been enabled by the development of STM electroluminescence methods (STML). STML provides access to optical properties of individual molecules through a cascade of relaxation processes between many-body states that obey energy conservation. Insufficient charge attachment energies quench the relaxation cascade via optically excited states, causing even intrinsically bright molecules to remain dark in STML. Here, we leverage substrate work function control and tip-induced gating of the double barrier tunnel junction to induce an energy shift of the ionic transition state of a single free-base tetrabenzoporphyrin (H2TBP) to gain access to optically excited states and bright exciton emission. The experimental observations are validated by a rate equation and polaron model considering the relaxation energy of the NaCl decoupling layer upon charging of the molecule.

Figures (4)

• Figure 1: Electronic and optical fingerprints of H$_{2}$TBP (a) Comparison of fluorescence intensity from a single H$_{2}$TBP and H$_{2}$Pc molecule decoupled from Ag substrate by a thin-film of NaCl (b) STM luminescence of H$_{2}$TBP measured at -2.3 V and H$_{2}$Pc measured at -2.5 V. (c) Scanning tunneling spectroscopy of H$_{2}$TBP shown with comparison to H$_{2}$Pc to emphasize the upward shift of the molecular transport resonances. Constant-height STM images show the spatial distribution of the PIR and NIR recorded at -2.4 V and 1.1 V, respectively. The STS and STML data in panel (b) and (c) were recorded from molecules adsorbed on the same NaCl island with the same tip (plasmonic cavity) to ensure fair comparison of optical intensity. The inset in (c) shows a polaron model fit to the PIR which calculates a D$_{0}^{+}$ state energy of 1.812 eV.
• Figure 2: Evidence of local dielectric gating in single molecule fluorescence (a) Many-body diagram of the H$_{2}$TBP molecule. The grey region illustrates the effect of $\Delta\,E$ in (b). Black arrows indicate charge transfer with the STM tip while non-radiative relaxation via charge transfer with the substrate are shown in grey. Radiative relaxation follows the colored arrows from D$_{0}^{+}$$\rightarrow$ S$_{1}$$\rightarrow$ S$_{0}$ (b) [left axis] Bias dependent STML intensity of the 0-0 emission from H$_{2}$TBP . Spectral intensity is integrated between 652 - 642 nm and normalized to the excitation rate of the measurement (t$_{acq}$ x $|$I$_{avg}$$|$). [right axis] Corresponding STS spectra of an H$_{2}$TBP PIR for reference to the molecule electronic response. A rate equation model is used to simulate the voltage dependent emission rate (dashed blue line).
• Figure 3: Engineered energy level alignment for enhanced optical characterization (a) Comparison of STS measurements of H$_{2}$TBP when the Ag(111) substrate is replaced with Ag(110). (b) STM luminescence of H$_{2}$TBP adsorbed on NaCl/Ag(110). Measurement performed with V = -3.0 V, I = -314 pA, t_acq = 15 s and g = 150 l/mm. (c) [left axis] Bias dependent STML intensity of the 0-0 emission from H$_{2}$TBP. Spectral region is indicated by the grey shaded region in b and normalized to the excitation rate of the measurement (t$_{acq}$ x $|$I$_{avg}$$|$). [right axis] Reference STS spectra. Each of the measurements were recorded at the position indicated in the inset of b.
• Figure 4: Local probing the optical emission of a single free-base porphyrin (a) [top] Vibronic emission from the S$_{1}$ zero-phonon-line at 1.915 eV. [bottom] TD-DFT calulcation of FC (dashed) and FC-HT (solid) assisted emission from gas phase geometry optimized for the S1 exicted state. (b) [top] Vibronic emission from the S$_{2}$ zero-phonon-line at 2.15 eV. [bottom] TD-DFT calulcation of FC (dashed) and FC-HT (solid) assisted emission from gas phase geometry optimized for the S2 excited state. The experimental spectra were measured at -3.0 V with a current of -340 pA (red) and -280 pA (grey) and are vertically offset for clarity. (c)[top] STML intensity map of the S$_{1}$ peak area. [bottom] Simulated STML map for the S$_{1}$ state. (d)[top] STML intensity map of the S$_{2}$ peak area. [bottom] Simulated STML map for the S$_{1}$ state.