Activating Electroluminescence of Charged Naphthalene Diimide Complexes Directly Adsorbed on a Metal Substrate

Electroluminescence from single molecules adsorbed on a conducting surface imposes conflicting demands for the molecule-electrode coupling. To conduct electrons, the molecular orbitals need to be hybridized with the electrodes. To emit light, they need to be decoupled from the electrodes to prevent fluorescence quenching. Here, we show that fully quenched 2,6-core-substituted naphthalene diimide derivative in a self-assembled monolayer directly deposited on a Au(111) surface can be activated with the tip of a scanning tunneling microscope to decouple the relevant frontier orbitals from the metallic substrate. In this way, individual molecules can be driven from a strongly hybridized state with quenched luminescence to a light-emitting state. The emission performance compares in terms of quantum efficiency, stability, and reproducibility to that of single molecules deposited on thin insulating layers. Quantum chemical calculations suggest that the emitted light originates from the singly charged cationic pair of the molecules.

Figure 1

Adsorption configuration of Tpd-sNDI molecules on Au(111). (a) Chemical structure of Tpd-sNDI as synthesized, comprising a chromophore (red) and a tripodal anchor (black). Long and short axis of the molecule are represented by red and blue arrows, respectively. (b) STM image of ordered domains of Tpd-sNDI molecules ($U=1.8\mathrm{V}$, $I=2\mathrm{pA}$). Green arrows show the orientation of the molecular islands, red arrows show the $⟨1\overline{1}0⟩$ directions of Au(111). (c) Enlarged STM image ($U=-2.5\mathrm{V}$, $I=12\mathrm{pA}$) of a molecular island. (d) Molecular structure superimposed to scale on (c). A monoclinic unit cell ($3.24\mathrm{nm}\times 4.79\mathrm{nm}=11.44{\mathrm{nm}}^2$, $\alpha ={47.5}^0$) of the island is represented in red containing four molecules. (e) Atomic lattice of the Au(111) surface with the suggested molecular arrangement superimposed to scale.

Figure 2

Switching of Tpd-sNDI molecules. (a) Flat lying pair of Tpd-sNDI molecule with molecular structure superimposed to scale ($U=-2.5\mathrm{V},I=2\mathrm{pA}$). (b) Topography after switching both the molecules in the pair ($U=-2.6\mathrm{V},I=3\mathrm{pA}$). (c) Difference of the topographies [shown in (a) and (b)] before and after switching. (d) STM image of an ordered island ($U=1.8\mathrm{V}$, $I=2.4\mathrm{pA}$). (e),(f) Enlarged image of the rectangular area marked in (a) recorded at positive ($U=2.5\mathrm{V}$, $I=2.1\mathrm{pA}$) and negative bias ($U=-2.4\mathrm{V}$, $I=2.1\mathrm{pA}$) voltages after switching. Molecular motifs of the switched molecules (labeled 1, 2, and 3) are marked with black and white contours. (g) STM image of an ordered island with some switched molecules showing a high feature at the position of the chromophore ($U=-2.3\mathrm{V}$, $I=2\mathrm{pA}$). (h) Optical emission spectra recorded by placing the STM tip above different molecular motifs or Au(111). Positions of the spectra are marked with colored crosses in (b). Measurement conditions for all the spectra are $U=-2.50\mathrm{V}$, $I=30\mathrm{pA}$, $t=3\mathrm{s}$. PL spectrum is shown by yellow shaded region, recorded in dichloromethane solution with a concentration of $25\mathrm{\mu }\mathrm{M}$ at ambient temperature.

Figure 3

Normalized $dI/dU$ spectra measured on Au(111) (black line), on a pristine molecule (green line), on a switched dark molecule (blue line), and on a switched fluorescent molecule (red line). The low voltage, low current range has been cut out due to noise leading to a divergence of the normalized signal.

Figure 4

Excitation of $X^{+}$ emission line. (a) Topography of switched Tpd-sNDI during the recording of the photon map shown in (b). The molecular structure is superimposed to scale. (b) Simultaneously recorded intensity of the $X^{+}$ line (grid of $40\times 40$ points, integrated photon count in the energy range of 1.82–1.90 eV), $U=-2.3\mathrm{V}$, $I=5.1\mathrm{pA}$, $t=1\mathrm{s}$. (c) Optical emission spectra at positive (blue line) and negative (black lines) sample biases ($I=6\mathrm{pA}$, $t=60\mathrm{s}$). (d) Optical emission spectrum with the STM tip positioned above the switched Tpd-sNDI (see red cross in the inset topography; $U=-2.35\mathrm{V}$, $I=6\mathrm{pA}$, $t=60\mathrm{s}$). Scale bar in the inset image is 1 nm. (e) Simulated TD-CAM-B3LYP photoemission spectrum of charged pair of Tpd-sNDI showing pure electronic transitions in the indicated energy range.

Figure 5

Hole (left) and particle (right) natural transition orbitals (NTOs) involved in the main optical transitions of the charged Tpd-sNDI pair obtained at the TD-CAM-B3LYP/def2-TZVP (def2-SVP for H) level of theory [41].

Figure 6

Hole (left) and particle (right) NTO for the first excited state of the Tpd-sNDI monomer obtained at the TD-CAM-B3LYP/def2-TZVP (def2-SVP for H) level of theory. The excitation energy corresponding to this set of NTOs is found at 2.72 eV, ruling it out as possible source of the light emission observed in the STM experiments.

Figure 7

Front view (left) and side view (right) of the optimized noncovalently bonded assembly of two Tpd-sNDI molecules.