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arxiv: 2509.08496 · v2 · pith:OMO66PVXnew · submitted 2025-09-10 · ❄️ cond-mat.mes-hall

Probing up-conversion electroluminescence of decoupled porphyrin molecules in a plasmonic nanocavity

Li-Qing Zheng , F\'abio J.R. Costa , Abhishek Grewal , Ruonan Wang , Fengmin Wang , Wei Li , Anna Ros{\l}awska , Klaus Kuhnke
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Klaus Kern
This is my paper

Pith reviewed 2026-05-18 18:06 UTC · model grok-4.3

classification ❄️ cond-mat.mes-hall
keywords up-conversion electroluminescenceporphyrin moleculestriplet statessingle-molecule luminescencescanning tunneling microscopyplasmonic nanocavityPdOEPNaCl decoupling layer
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The pith

PdOEP molecules demonstrate triplet-mediated up-conversion electroluminescence where singlet photons exceed the energy of a single tunneling electron.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper examines individual Pd-octaethylporphyrin molecules on silver surfaces separated by an ultrathin NaCl layer in a scanning tunneling microscope setup. It records both singlet S1 and triplet T1 emission lines at visible wavelengths roughly 100 nm apart. The singlet emission persists even when the emitted photon energy surpasses the energy supplied by one tunneling electron, indicating an up-conversion process. Energy level diagrams for S1, D0, and T1 states combined with fits to the current dependence of the emission confirm that the triplet state stores energy between successive electrons and relays it to produce the higher-energy output.

Core claim

The triplet T1 state functions as a relay or shelving state that accumulates energy from multiple tunneling electrons, enabling the S1 singlet state to emit photons with energy higher than that delivered by any single electron in an up-conversion electroluminescence process; this is verified by comparing the energies of S1, D0, and T1 states and by fitting the observed current dependencies of S1 and T1 emission under up-conversion conditions.

What carries the argument

The triplet-mediated up-conversion model in which the T1 state serves as a stable shelving relay that stores energy between tunneling electrons to allow subsequent excitation and radiative decay from the S1 state.

If this is right

  • Singlet and triplet emission lines appear at visible wavelengths only about 100 nm apart, unlike typical phthalocyanines whose triplets lie in the infrared.
  • Current-dependence fits under up-conversion conditions match the two-electron process expected when the triplet acts as an intermediate.
  • The mechanism applies to molecules decoupled from the metal substrate, allowing high-resolution STM studies of triplet luminescence.
  • The approach provides a platform for examining triplet states in systems relevant to organic light-emitting devices at the single-molecule scale.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same triplet-relay approach could be tested on other porphyrin derivatives to map how molecular structure controls the up-conversion efficiency.
  • If the shelving lifetime proves tunable by the nanocavity, the setup might allow control over the minimum current needed for observable up-conversion.

Load-bearing premise

The triplet state remains a stable energy-storage relay without significant competing decay channels or direct excitation routes that would bypass the need for sequential electron tunneling.

What would settle it

A measurement showing that up-conversion S1 emission continues at the same rate when the tunneling current is lowered such that the average time between electrons exceeds the triplet lifetime, or direct spectroscopic evidence of a bypassing excitation path to S1.

Figures

Figures reproduced from arXiv: 2509.08496 by Abhishek Grewal, Anna Ros{\l}awska, F\'abio J.R. Costa, Fengmin Wang, Klaus Kern, Klaus Kuhnke, Li-Qing Zheng, Ruonan Wang, Wei Li.

Figure 1
Figure 1. Figure 1: a. Schematic of the excitonic emission (S1 and T1 emissions) of a PdOEP molecule decoupled from an Ag substrate by 3ML NaCl under the excitation of tunneling electrons. b. Typical differential conductance (dI/dV) spectrum of a PdOEP molecule on 3ML NaCl / Ag (111). Insets are the HOMO and LUMO images of the PdOEP molecule acquired at -2.5 V and +1.7 V, respectively, on 2ML NaCl / Ag (111) (I = 1 pA). c. Ty… view at source ↗
Figure 2
Figure 2. Figure 2: a. STM luminescence spectra (gray dots) of a PdOEP molecule (S1 emission) as a function of voltage together with the Savitzky-Golay-smoothed spectra (solid lines), (I = 35 pA, acquisition time for each spectrum: 120 s). The spectra are vertically shifted for clarity. The dark blue spectra correspond to the range in which single electron excitation is sufficient to induce luminescence while the cyan spectra… view at source ↗
Figure 3
Figure 3. Figure 3: a. Two sections of STM luminescence spectra of a PdOEP molecule as a function of voltage (grey dots) together with Gaussian fits to the three main peaks, representing the S1 emission (solid blue line) and the T1 emission (red solid line) with a vibrational satellite. For all spectra the current is I = 42.5 pA, acquisition time per spectrum from top to bottom: 450s, 600s, 600s, 750s. The different acquisiti… view at source ↗
Figure 4
Figure 4. Figure 4: a. Current-dependent STM luminescence spectra of a PdOEP molecule (singlet emission) with Gaussian fit to the emission peak. (V = -2.25 V, acquisition time per spectrum: 120 s, current 20 pA to 60 pA as indicated at each curve). The spectra are vertically offset for clarity. b. Fluorescence intensity of the PdOEP molecule as a function of current with power law fits. Blue squares: peak height from the Gaus… view at source ↗
Figure 5
Figure 5. Figure 5: a. Current-dependent STM luminescence spectra of a PdOEP molecule (triplet emission) with Gaussian fit to the emission peak. (V = -2.5 V, acquisition time per spectrum: 120 s, I = 10 pA to 40 pA as indicated). The spectra are vertically offset for clarity. b. Phosphorescence intensity of the PdOEP molecule as a function of current with power law fits. Red squares: peak height from the Gaussian fit; black s… view at source ↗
Figure 6
Figure 6. Figure 6: Proposed many-body diagram describing the molecular states and transitions relevant for the excitation of singlet and triplet states (V < -2.2 V). The transitions drawn in black are tip￾mediated, in gray (dashed) substrate-mediated. The blue and red arrows represent the singlet and triplet radiative decays, respectively. energy of this state is too low. When the molecule is in the triplet state, a handful … view at source ↗
Figure 7
Figure 7. Figure 7: a Basic model for the up-conversion electroluminescence dynamics. The constants a1, a2, a3, and a4 indicate transition rates of the various processes. b/c. Data sets of T1 and S1 (circles) as a function of current from Figs. 5 and 4, respectively, and best fits (solid curves) to the model. For details of the model, see also Figure S9 and for details on the fit, see Figures S10, S11 and Table S1. The releva… view at source ↗
Figure 8
Figure 8. Figure 8: a. Current-dependent STM luminescence spectra of a PdOEP dimer (V = -2.25 V, acquisition time per spectrum: 120 s) and a plasmon spectrum at the same voltage (black curve). b. Fluorescence intensity as a function of current with power law fit. The inset is the STM image of the PdOEP dimer with tip position indicated (black dot). The relative distance scale at the top of the graph indicates the tip height v… view at source ↗
read the original abstract

Molecular triplet states can produce significant phosphorescence and act as a relay state for luminescence, such as in up-conversion processes. While this property makes triplet emitters interesting for organic light-emitting diodes (OLEDs), the study of their luminescence at the single molecule level in high resolution scanning tunneling microscopy (STM) is challenging. We investigate individual Pd-octaethylporphyrin (PdOEP) molecules decoupled from Ag(100) and Ag(111) by an ultrathin NaCl layer and observe singlet and triplet emission lines at visible wavelengths, only about 100 nm apart from each other. This is in stark contrast to the metal or free-base phthalocyanines, for which typically the lowest triplet transitions lie in the far red or infrared where the sensitivity of charge coupled device (CCD) detectors decrease significantly. The singlet S1 state of PdOEP emits photons even when the photon energy is higher than the energy provided by a tunneling electron, in an energy up-conversion process. This mechanism requires a relay (or shelving) state in which energy is stored in the molecule for the interval between tunneling electrons. Analyzing the energy levels of different molecular states (S1, D0, and T1 states) and fitting the current dependencies of S1 under up-conversion electroluminescence (UCEL) condition for S1 and T1 emission, we verify the validity of a triplet-mediated up-conversion model.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 2 minor

Summary. The manuscript reports STM-based observations of visible-wavelength singlet S1 and triplet T1 emission lines from individual PdOEP molecules decoupled from Ag(100)/Ag(111) by an ultrathin NaCl layer. It demonstrates up-conversion electroluminescence in which S1 photons are emitted at energies exceeding the tunneling-electron energy and supports a triplet-mediated relay model by analyzing the S1, D0, and T1 energy levels together with fits to the current dependence of the S1 UCEL intensity and T1 emission.

Significance. If the triplet-relay interpretation is robust, the result supplies a concrete single-molecule platform for studying triplet shelving and up-conversion in a porphyrin system whose S1–T1 separation lies in the visible, offering direct experimental input to models of molecular-scale OLED processes and plasmonic nanocavity luminescence.

major comments (1)
  1. [Current-dependence analysis] Results section on current-dependence fits: the reported power-law fits for S1 UCEL intensity versus tunneling current are presented under the assumption that population proceeds exclusively via sequential tunneling through T1; no quantitative comparison or goodness-of-fit metrics are given for alternative models (direct two-electron promotion to S1 or D0-mediated channels) that can produce indistinguishable I-dependence, leaving the exclusivity of the triplet-relay claim unverified.
minor comments (2)
  1. [Abstract and experimental results] The abstract states the S1 and T1 lines are 'only about 100 nm apart'; the main text should tabulate the precise peak wavelengths and linewidths for both transitions on each substrate to permit direct comparison with literature values.
  2. [Figures and energy diagram] Figure captions and energy-level diagrams should explicitly label the bias-voltage thresholds used for the UCEL regime and indicate which molecular states are accessed at each bias.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the thorough review and constructive feedback on our manuscript. We appreciate the positive assessment of the significance of our single-molecule up-conversion electroluminescence results. We address the major comment below in detail.

read point-by-point responses

  1. Referee: [Current-dependence analysis] Results section on current-dependence fits: the reported power-law fits for S1 UCEL intensity versus tunneling current are presented under the assumption that population proceeds exclusively via sequential tunneling through T1; no quantitative comparison or goodness-of-fit metrics are given for alternative models (direct two-electron promotion to S1 or D0-mediated channels) that can produce indistinguishable I-dependence, leaving the exclusivity of the triplet-relay claim unverified.

    Authors: We thank the referee for this observation. The triplet-relay interpretation rests primarily on the energy-level analysis (S1, D0, and T1 positions relative to the Fermi level and the applied bias), which shows that direct population of S1 lies outside the accessible energy window at the biases used, while sequential population via T1 is allowed. The current dependence is then fitted to the expected functional form for a shelving-state relay. We acknowledge that the I-dependence alone can be similar for certain alternative channels and that explicit goodness-of-fit metrics for those alternatives were not provided. In the revised manuscript we will add a quantitative comparison (including R^2 and residual analysis) of the triplet-relay model against direct two-electron and D0-mediated scenarios, together with a short discussion of why the alternatives remain inconsistent with the combined bias-threshold and energy-level data. This material will appear in the main text and as a supplementary note. revision: yes

Circularity Check

0 steps flagged

Current-dependence fits and energy-level analysis support triplet-mediated UCEL model without reducing to tautology

full rationale

The paper's central verification rests on experimental STM spectra, observed emission lines ~100 nm apart, and fits of S1 UCEL intensity versus tunneling current (compared to T1), using measured energy levels of S1, D0, and T1 states in the NaCl-decoupled PdOEP system. These steps draw on independent data rather than re-deriving a result from its own fitted parameters or a self-citation chain. No equation or claim reduces by construction to an input (e.g., no fitted exponent renamed as prediction of the same functional form). The assumption that T1 acts as the dominant relay is tested against the observed power-law dependence and threshold, leaving the claim self-contained against external benchmarks even if alternative channels remain possible.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The paper draws on standard molecular photophysics (energy level ordering of S1, T1, D0 states) and experimental assumptions about tunneling electron statistics; no new free parameters or invented entities are introduced beyond conventional fitting of current dependencies.

free parameters (1)
  • current-dependence fitting parameters
    Parameters used to match observed S1 and T1 emission intensities under UCEL conditions; their specific values are not stated in the abstract.
axioms (1)
  • domain assumption The triplet state acts as a long-lived relay that stores energy between successive tunneling electrons without rapid non-radiative decay.
    Invoked when linking the observed up-conversion threshold to the T1 lifetime and current statistics.

pith-pipeline@v0.9.0 · 5825 in / 1476 out tokens · 57967 ms · 2026-05-18T18:06:33.343690+00:00 · methodology

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Reference graph

Works this paper leans on

8 extracted references · 8 canonical work pages

  1. [1]

    Max-Planck-Institut für Festkörperforschung, 70569 Stuttgart, Germany

    work page

  2. [2]

    Institut de Physique, École Polytechnique Fédéral Lausanne, 1015 Lausanne, Switzerland

    work page

  3. [3]

    Gleb Wataghin Institute of Physics - University of Campinas – UNICAMP, Campinas 13083-859, Brazil

    work page

  4. [4]

    State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, People’s Republic of China

    work page

  5. [5]

    Key Laboratory of Mesoscopic Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, People’s Republic of China Present Addresses †: State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, People’s Republic of China. 30

    work page

  6. [6]

    NaCl was thermally sublimed from a Knudsen cell held at 900 K, with the Ag (100) and Ag (111) held at 300 K, to obtain a partial coverage of (100)-terminated NaCl islands

    Experimental section 1.1 Sample preparation All experiments were performed with a home-built low temperature ultrahigh-vacuum STM operated at 4.2 K (<10−11 mbar).1 Prior to use, Ag (100) and Ag (111) single crystals were cleaned by argon ion sputtering and subsequent annealing to 660 K and 670 K, respectively, for several cycles. NaCl was thermally sublim...

    work page

  7. [7]

    Processing steps of the exciton spectrum shown in Fig

    Supplementary figures Figure S1. Processing steps of the exciton spectrum shown in Fig. 1c. Points: raw data of the exciton (blue) and of the plasmon (grey) spectrum. Grey area: Savitzky-Golay smoothed plasmon spectrum. Blue line: Savitzky-Golay-smoothed exciton spectrum. Green line: same as blue line but after subtraction of the contribution from the smo...

    work page

  8. [8]

    Versatile optical access to the tunnel gap in a low-temperature scanning tunneling microscope

    References (1) Kuhnke, K.; Kabakchiev, A.; Stiepany, W.; Zinser, F.; Vogelgesang, R.; Kern, K. Versatile optical access to the tunnel gap in a low-temperature scanning tunneling microscope. Review of Scientific Instruments 2010, 81 (11). (2) Stadler, J.; Schmid, T.; Zenobi, R. Nanoscale Chemical Imaging Using Top-Illumination Tip- Enhanced Raman Spectrosc...

    work page 2010