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Real-space investigation of energy transfer in heterogeneous molecular dimers

Abstract

Given its central role in photosynthesis1,2,3,4 and artificial energy-harvesting devices5,6,7, energy transfer has been widely studied using optical spectroscopy to monitor excitation dynamics and probe the molecular-level control of energy transfer between coupled molecules2,3,4. However, the spatial resolution of conventional optical spectroscopy is limited to a few hundred nanometres and thus cannot reveal the nanoscale spatial features associated with such processes. In contrast, scanning tunnelling luminescence spectroscopy8,9,10,11,12,13,14,15,16,17,18,19 has revealed the energy dynamics associated with phenomena ranging from single-molecule electroluminescence11,12,14,17,19, absorption of localized plasmons19 and quantum interference effects19,20,21 to energy delocalization17 and intervalley electron scattering15 with submolecular spatial resolution in real space. Here we apply this technique to individual molecular dimers that comprise a magnesium phthalocyanine and a free-base phthalocyanine (MgPc and H2Pc) and find that locally exciting MgPc with the tunnelling current of the scanning tunnelling microscope generates a luminescence signal from a nearby H2Pc molecule as a result of resonance energy transfer from the former to the latter. A reciprocating resonance energy transfer is observed when exciting the second singlet state (S2) of H2Pc, which results in energy transfer to the first singlet state (S1) of MgPc and final funnelling to the S1 state of H2Pc. We also show that tautomerization22 of H2Pc changes the energy transfer characteristics within the dimer system, which essentially makes H2Pc a single-molecule energy transfer valve device that manifests itself by blinking resonance energy transfer behaviour.

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Figure 1: Excitation of molecular fluorescence through intermolecular energy transfer.
Figure 2: Energy-level alignment and the mechanism of energy transfer.
Figure 3: Backward energy transfer from H2Pc to MgPc.
Figure 4: Blinking of RET.

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Acknowledgements

This work was supported in part by MEXT/JSPS KAKENHI (Grant No. 15H02025, 26886013, 16K21623), and MEXT/JSPS Fellows (No. 15J03915). Some of the numerical computations were performed using RICC and HOKUSAI systems at RIKEN. We thank M. Trenary, H. Kuramochi, K. Inoue and H. Walen for helpful discussions.

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Authors and Affiliations

Authors

Contributions

H.I, M.I.-I., S.K. and K.K designed the experiments. H.I. performed the experiment and analysed the data. K.M. provided the theoretical analysis. Y.K. directed the project. All authors discussed the results and wrote the manuscript.

Corresponding author

Correspondence to Yousoo Kim.

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The authors declare no competing financial interests.

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Reviewer Information

Nature thanks L. Gross, G. Nazin and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Figure 1 DFT structural analysis of four possible (3.5, 2.5) MgPc–H2Pc dimer configurations.

a, DFT analysis of the total energy indicates that the structure in a is the most stable configuration, where the x axis of H2Pc is almost perpendicular to the major axis of the dimer, and MgPc is tilted towards the H2Pc. b, In the second stable configuration, the only difference from a is the direction of the x axis of the H2Pc, which is almost parallel to the major axis. The energy difference between configurations a and b is only 1.1 meV, and the tautomerization of the H2Pc is induced by the tunnelling electron22. c, d, In configurations c and d MgPc has different tilt angles, and the total energies increased by around 14 meV, indicating that the two angles ±38° are no longer equivalent in the dimer configuration, thus suppressing the shuttling motion of MgPc/NaCl.

Extended Data Figure 2 Tip position dependence of energy transfer.

The STL spectra were measured on the MgPc in the (3.5, 2.5) MgPc–H2Pc dimer at various tip positions (V = −2.1 V, It = 5 pA, t = 1 min). The measurement positions are displayed in the STM image. The Qx line arising from the energy transfer was observed at all points over MgPc, indicating that the energy transfer is not sensitive to the tip position. The Q fluorescence of MgPc was also observed at almost every point, the only exception being at the molecular centre (tip position 17). The Q fluorescence disappeared when the tip was placed at the centre of the MgPc, but the H2Pc Qx fluorescence was clearly observed. The suppression of the single-molecule STL when the tip is placed at the molecular centre was reported in previous works14,17. The appearance of the H2Pc luminescence when the tip is at the MgPc molecular centre is explained as follows. First, the Q state is excited by charge injection. Although the Q state cannot emit a far-field photon efficiently owing to the STL suppression, the state can transfer its energy to the nearby H2Pc where plasmon–exciton coupling is allowed, and the H2Pc exhibits Qx fluorescence. When the tip is off-centre on the MgPc (positions 1–16), plasmon–exciton coupling is allowed for both MgPc and H2Pc, which causes the Q and Qx lines to appear. It should be noted that the blinking behaviour of RET (Fig. 4) makes it difficult to precisely analyse the tip position dependence of the RET probability in our system. The quantitative analysis of the position dependence of RET will be realized with a rigid molecular system.

Extended Data Figure 3 DFT electronic structure analysis of the (3.5, 2.5) MgPc–H2Pc dimer.

a, b, Calculated frontier molecular orbitals of the most stable and second most stable structures of the (3.5, 2.5) MgPc–H2Pc dimer. The isosurface of charge density and the energy level of each orbital are presented. All of the molecular orbitals are localized in one of the molecules, and no clear hybridization was observed between the orbitals. However, their energy levels were slightly altered by intermolecular interactions. c, The energy gaps between the molecular orbitals at the ground state are listed. Note that the calculated energy gaps cannot be directly compared with the experimental results (Fig. 3a), because the experimentally measured peak positions correspond to energy gaps between the excited state and the ground state. However, the DFT analysis shows that energy levels of the molecular orbitals at the ground state are different in configurations a and b, suggesting that the resonance energies of the electronic transitions among them are also different.

Extended Data Figure 4 Determination of the threshold voltage required to induce single-molecular fluorescence of the MgPc molecule.

a, The bias voltage-dependent STL spectra of MgPc/3ML NaCl were measured with It = 20 pA, t = 1 min, at the red dot in the inset. This shows that the threshold voltage was −1.95 V. b, A dIt/dV spectrum of MgPc measured with the same tip used in a (at the red dot in the inset of a). As reported previously, the threshold voltage for single-molecule electroluminescence corresponds to that of the resonant tunnelling channel through the HOMO in dIt/dV spectrum17.

Extended Data Figure 5 Threshold voltages to induce single-molecular fluorescence of the H2Pc molecule.

a, The bias voltage-dependent STL spectra of H2Pc/3ML NaCl were measured with It = 25 pA and t = 1 s, at the red dot in the inset. b, A dIt/dV spectrum of H2Pc in the threshold voltage region. When , the spectrum shows only the radiation of the localized plasmon (the intensity is very weak) and no molecular fluorescence was detected. When , weak Qx fluorescence appeared at 1.81 eV and Qy fluorescence was barely seen. When , strong Qx fluorescence and weak Qy fluorescence were observed. It is clear that there are two threshold voltages, Vth1 = −1.8 V for Qx fluorescence and Vth2 = −2.3 V for both Qx and Qy fluorescence of H2Pc. Vth1 corresponds to the energy of the Qx state (1.81 eV), and Vth2 to the threshold voltage of the resonant tunnelling channel through the HOMO in dIt/dV spectrum as seen in b. The former is similar to the process reported in ref. 18, and the latter was described in ref. 17. Although another threshold voltage was expected at −1.92 V, which corresponds to the energy of the Qy state (1.92 eV), it was not clearly observed in our experiment because of the weakness of the Qy luminescence. The strong single-molecule electroluminescence of H2Pc is triggered by hole injection into the HOMO, which is similar to the case of MgPc.

Extended Data Figure 6 Direct and indirect excitation of the Q fluorescence of MgPc.

a, An STM image of the (3.5, 2.5) MgPc–H2Pc dimer and an MgPc molecule (V = −2.3 V, It = 5 pA). b, The STL spectra measured at the three different positions indicated in a were compared. The red, blue and black curves were measured at the red, blue and black points in a, respectively, with the same measurement parameters (V = −2.3 V, It = 30 pA, t = 1 min). The integrated photon intensities in the range 1.871–1.908 eV were 13,000, 5,116 and 8,823 counts for the red, blue, and black curves, respectively. The results clearly show that the excitation of the Q state of MgPc is much more efficient when induced by indirect excitation through RET from the Qy state of the nearby H2Pc than by direct excitation with the tunnelling current. It is therefore concluded that the main excitation mechanism of the Q state is RET from the Qy state under the measurement conditions of the red curve (which is the same spectrum as the red curve shown in Fig. 3).

Extended Data Figure 7 Lifetime estimation from the linewidths.

The linewidths observed in our experiment were 4.8 meV for the H2Pc Qx fluorescence and 14.4 meV for the MgPc Q fluorescence measured by single Lorentzian fitting. The linewidth of the MgPc Q fluorescence is not determined only by the lifetime of the state, because the line shape is not a simple Lorentz function and it is possible that other radiative processes are involved in the peak. In contrast, the H2Pc Qx fluorescence is reasonably fitted with a single Lorentz function, suggesting that the linewidth is mostly determined by the lifetime of the Qx state. The 4.8 meV linewidth is similar to the previously reported value (4.4 meV; ref. 12), and the lifetime of the Qx state is estimated to be approximately a few hundred femtoseconds (about 10−13 s). We believe that the Q state of MgPc also has a similar lifetime, because the magnitudes of the transition dipole moments and the spatial distributions of the molecular orbitals are similar for H2Pc and MgPc. The difference in the STL line shape might arise from different vibrational interactions with the NaCl substrate, which may be expected from the very different adsorption configurations of the two molecules.

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Imada, H., Miwa, K., Imai-Imada, M. et al. Real-space investigation of energy transfer in heterogeneous molecular dimers. Nature 538, 364–367 (2016). https://doi.org/10.1038/nature19765

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