π-Radical Formation by Pyrrolic H Abstraction of Phthalocyanine Molecules on Molybdenum Disulfide

Abstract

For a molecular radical to be stable, the environment needs to be inert. Furthermore, an unpaired electron is less likely to react chemically, when it is placed in an extended orbital. Here, we use the tip of a scanning tunneling microscope to abstract one of the pyrrolic hydrogen atoms from phthalocyanine (H2Pc) deposited on a single layer of molybdenum disulfide (MoS2) on Au(111). We show the successful dissociation reaction by current-induced three-level fluctuations reflecting the inequivalent positions of the remaining H atom in the pyrrole center. Tunneling spectroscopy reveals two narrow resonances inside the semiconducting energy gap of MoS2 with their spatial extent resembling the highest occupied molecular orbital (HOMO) of H2Pc. By comparison to simple density functional calculations of the isolated molecule, we show that these correspond to a single occupation of the Coulomb-split highest molecular orbital of HPc. We conclude that the dangling σ bond after N-H bond cleavage is filled by an electron from the delocalized HOMO. The extended nature of the HOMO together with the inert nature of the MoS2 layer favor the stabilization of this radical state.Molecular radicals hold one or more unpaired electrons in their valence orbitals. These molecules have attracted great interest for applications in organic optoelectronic [1, 2] and spintronic [3, 4] devices. Unfortunately, the open-shell nature renders these molecules highly reactive. Hence, the class of stable molecular radicals is limited. Most promising candidates for stable radicals are molecules with the unpaired electron being hosted in an extended orbital [3, 5]. Such molecules can be obtained, e.g., by dehydrogenation or dehalogenation, where the dissociation reaction involves the highest occupied molecular orbital (HOMO) [6][7][8][9][10][11]. Prime examples for H abstraction are porphyrin and phthalocyanine molecules, where one of the pyrrolic H atoms can be detached by a voltage pulse from the tip of a scanning tunneling microscope (STM) [12,13]. However, when the molecules are placed on a metal substrate the expected radical state is not preserved. Intramolecular relaxations and charge transfer with the substrate lead to a closed-shell configuration [8,14,15]. To suppress the interaction of radical molecules with the substrate, thin insulating layers of NaCl have been employed successfully for several radical species [7][8][9][10][11], but a radical state of H-abstracted Pc has not been reported to date.Here, we show that a monolayer of MoS 2 on Au(111) is an ideal system to study the H-abstraction reaction by STM. We prove the successful dehydrogenation reaction by three-level fluctuations of the tunneling current, reflecting the switching of the position of the single H atom in the pyrrolic center [12]. By mapping the spatial distribution of the singly occupied molecular orbital (SOMO) and singly unoccupied molecular orbital (SUMO), we find that the cleavage of an N-H bond does not lead to a localized dangling bond, but to an extended π rad...

Monolayers of MoS2 on Ag(111) as decoupling layers for organic molecules: resolution of electronic and vibronic states of TCNQ

“…The narrow width thus reflects that MoS 2 acts as a decoupling layer from the metal substrate. However, this resonance width is broader than what has been observed for the HOMO resonance of other organic molecules on MoS 2 on Au(111) [26,52,56]. In contrast to those cases, where the HOMO lay well inside the electronic gap of MoS 2 , the LUMO of TCNQ is located right at the onset of the conduction band.…”

“…6 fs of the excited state. This is almost one order of magnitude longer than on the bare metal surface, where the hot electron vanishes into the bulk on ultrafast timescales, but an order of magnitude shorter than for molecular resonances well separated from the band onsets [26,52,56]. Yet, the increase in the lifetime of the excited state allowed us to resolve vibronic states of the transiently negatively charged TCNQ molecule albeit only up to approx.…”

Internal Stark effect of single-molecule fluorescence

“…This phenomenon can be tracked in the variation of the tip-sample distance (Δz) versus time (Fig. 1 c) recorded at constant current where it appears as two-level fluctuations 24 , 27 – 31 . As a next step of the experiment, we located the tip on top of the center of the H 2 Pc molecule and applied a positive voltage ramp at a constant current of 10 pA while simultaneously recording the relative tip-sample distance “z”.…”

“…Based on a comparison with time-dependent density functional theory (TD-DFT) simulations, we show that these shifts can be traced back to the radial electric field generated by charges confined to the σ -orbitals of the deprotonated chromophores whereas their π -orbitals remain unchanged. As a consequence, the neutral and deprotonated compounds are iso-electronic, in contrast with scanning probe experiments in which charging a molecule alters its π -orbitals 19 – 24 .…”

Electronic Structure and Bistable Conformational Study of the Tetraphenylporphyrin Erbium(III) Acetylacetonate Complex on the CaF2/Si(100) Surface at Low Temperatures

“…7,8 The investigation of single metallated phthalocyanines and porphyrins molecules has revealed the possibility to partly tune their electronic and optical properties by changing the central metal atom. 9,10,11,12,13 So far, while self-assembled Ln-Pc or Ln-TPP molecules have been investigated in the gas phase 14 , single molecule are rarely studied on surfaces at the atomic scale. 15 In this work, we investigate the electronic structure of single tetraphenylporphyrinate Erbium (III) acethylacetonate complex, i.e.…”