Gating upconversion electroluminescence in a single molecule via adsorption-induced interaction of unpaired spin

TL;DR

The paper shows that upconversion electroluminescence (UCEL) from a single radical molecule, vanadyl phthalocyanine (VOPc), can be gated by the adsorption geometry on NaCl/Au(111). By combining STML and TD-DFT, the authors demonstrate that the O-down geometry enables a CI-mediated UCEL pathway via cationic states, $D_{0} \rightarrow T_{0}^{+} \rightarrow D_{1,2} \rightarrow T_{1-12}^{+} \rightarrow D_{3,4} \rightarrow D_{0}$, while the O-up geometry blocks this route due to higher-lying triplet states. They identify two emission lines, Q ($\hbar \omega \approx 1.80$ eV) from the neutral molecule and $X^+$ ($\hbar \omega \approx 1.45$ eV) from the cation, with UCEL visible for O-down but quenched for O-up. The results reveal that the unpaired electron, via its interaction with the substrate, can reorder excited states and alter transition probabilities, offering a route to tune spin-dependent molecular luminescence without altering the molecule itself.

Abstract

Molecules with unpaired spins (radicals) offer promising alternatives to closed-shell molecules as they are less limited regarding the spin statistics in their electroluminescence. Here, we combine scanning tunneling microscopy induced luminescence and density functional theory to study single vanadyl phthalocyanine molecules, which are stable neutral radicals. Two distinct adsorption geometries of the molecule on NaCl/Au(111) lead to a difference in the interaction of the unpaired electron with the substrate, which in turn allows us to investigate its effects on the light emission process. Remarkably, we observe that up-conversion electroluminescence is gated by the adsorption geometry of the molecule, an effect we attribute to a reordering of excited states and enhanced excited state transition probabilities. The profound influence of the unpaired electron via state reordering opens new possibilities for tuning not only molecular electroluminescence but also many other spin dependent phenomena.

Figures (16)

• Figure 1: STML of VOPc in different adsorption configurations.. (a) Schematic of the experimental setup (tip and molecule are not to scale) showing a single VOPc molecule decoupled from the Au(111) surface by 3ML of NaCl. (b) Topographic overview showing two distinct configurations of VOPc identified as O-up (blue) and O-down (red) ($V_\mathrm{b}$ = -2.0 V, and $I$ = 1 pA). (c) STML spectrum on an isolated O-up recorded with $V_\mathrm{b}$ = -2.20 V, and $I$ = 65 pA, $t_\mathrm{exp}$ = 100 s. Emission lines at two distinct energies are labeled as $Q$ and $X^+$. Corresponding topography is shown in the inset ($V_\mathrm{b}$ = -2.0 V, and $I$ = 1 pA). (d) STML spectrum on an isolated O-down recorded with $V_\mathrm{b}$ = -2.05 V, $I$ = 45.5 pA, $t_\mathrm{exp}$ = 10 s. Emission lines at two distinct energies are labeled as $Q$ and $X^+$. Corresponding topography is shown in the inset ($V_\mathrm{b}$ = -2.35 V, and $I$ = 4 pA).
• Figure 1: STML spectra recorded on VOPc in (a)-(b) O-up and (c)-(d) O-down configurations. Experimental data are shown in black, whereas the colored solid curves show the Lorentzian fit of the charged (blue and red) and the neutral (dark green) emission lines. The orange shaded region in all the spectra represents the 95% confidence interval of the fit. (e) and (f) show the topographies recorded on a single VOPc (O-up) at $V_\mathrm{b}$ = -2.0 V, $I$ = 1 pA and $V_\mathrm{b}$ = 1.9 V, $I$ = 1 pA. (g) and (h) show the topographies of VPc at the same $V_\mathrm{b}$ and current as for e and f, respectively. (i) Topography of VOPc in O-down configuration ($V_\mathrm{b}$ = -2.35 V, $I$ = 4 pA). (j) STML spectrum recorded at VPc ($V_\mathrm{b}$ = -2.7 V, and $I$ = 30.5 pA, $t_{exp}$ = 5 s). Topographies (e-h) are 4.5 nm$\times$ 4.5 nm and 2.5 nm$\times$ 2.5 nm in i.
• Figure 2: Electronic structure of VOPc and energetics of the emission bands. (a) and (b) $dI/dV_\mathrm{b}$ spectrum recorded on O-up and O-down, respectively (inset: tip positions are marked with a blue/red cross). Dotted lines indicate the onset of the PIR at -1.35 V. (c) and (d) Integrated photon counts of $Q$ (black) and $X^+$ (brown) bands as a function of $V_\mathrm{b}$ recorded on O-up and O-down, respectively. Parameters for recording the spectra on O-up $I$ = 40 pA, $t_\mathrm{exp}$ = 30 s and on O-down $I$ = 45 pA, $t_\mathrm{exp}$ = 10 s. (e) and (f), Dependency of integrated photon counts of $Q_x$ (black) and $X^+$ (brown) emission bands on tunneling current at fixed bias voltages $V_\mathrm{b}$ = -2.20 V (O-up) and $V_\mathrm{b}$ = -2.40 V (O-down). Shaded areas show the 95% confidence band for the fits. Parameters for recording the inset topographies in (a) and (b) are $V_\mathrm{b}$ = -2.0 V, $I$ = 1 pA and $V_\mathrm{b}$ = -2.35 V, $I$ = 4 pA, respectively.
• Figure 2: (a) d$I$/d$V_\mathrm{b}$ for O-up (blue) and O-down (red) plotted in log scale to highlight the onset of the PIR. The vertical dashed line is at -1.35 V. (b) STML spectra recorded as a function of applied bias voltages for VOPc in O-up configuration ($I$ = 40 pA, $t$ = 30 s) (c) VOPc in O-down configuration ($I$ = 45 pA, $t$ = 10 s).
• Figure 3: Calculated TD-PBE0 excitation spectra and NTOs (a) and (b) Oscillator strength in the velocity representation of the lowest excitations of the ground state in the duplet spin configuration of neutral molecules in O-up, O-down, respectively. (c) and (d) Oscillator strength of the lowest excitations of the positively charged molecule in the triplet spin configuration of O-up, O-down, respectively. Note that for the O-down configuration, there are additional low-energy states (yellow band). (e) and (f) Oscillator strength of the lowest second excitations starting from $D_{1,2}$ in the Duplet spin configuration of O-up, O-down, respectively, as obtained from quadratic response TD-DFT. Colored excitation bands in (a)-(f) that are referred in the text are indicated as a guide to the eye and labeled according to the spin configuration of the excited state. (g) and (h) Side view of particle NTO of the $D_{1,2}\rightarrow D_0$ transition of O-up, O-down orientations. (i) and (j) Calculated spin density of O-up and O-down in the neutral molecule. (k) and (l) Calculated spin density of O-up and O-down in the cation.