Local Electronic Structure of a Single-Layer Porphyrin-Containing Covalent Organic Framework

Chen Chen, Trinity Joshi, Huifang Li, Anton D. Chavez, Zahra Pedramrazi, Pei Nian Liu, Hong Li, William R. Dichtel, Jean Luc Bredas, Michael F. Crommie

Research output: Contribution to journalArticlepeer-review

38 Scopus citations

Abstract

We have characterized the local electronic structure of a porphyrin-containing single-layer covalent organic framework (COF) exhibiting a square lattice. The COF monolayer was obtained by the deposition of 2,5-dimethoxybenzene-1,4-dicarboxaldehyde (DMA) and 5,10,15,20-tetrakis(4-aminophenyl) porphyrin (TAPP) onto a Au(111) surface in ultrahigh vacuum followed by annealing to facilitate Schiff-base condensations between monomers. Scanning tunneling spectroscopy (STS) experiments conducted on isolated TAPP precursor molecules and the covalently linked COF networks yield similar transport (HOMO-LUMO) gaps of 1.85 ± 0.05 eV and 1.98 ± 0.04 eV, respectively. The COF orbital energy alignment, however, undergoes a significant downward shift compared to isolated TAPP molecules due to the electron-withdrawing nature of the imine bond formed during COF synthesis. Direct imaging of the COF local density of states (LDOS) via dI/dV mapping reveals that the COF HOMO and LUMO states are localized mainly on the porphyrin cores and that the HOMO displays reduced symmetry. DFT calculations reproduce the imine-induced negative shift in orbital energies and reveal that the origin of the reduced COF wave function symmetry is a saddle-like structure adopted by the porphyrin macrocycle due to its interactions with the Au(111) substrate.

Original languageEnglish (US)
Pages (from-to)385-391
Number of pages7
JournalACS Nano
Volume12
Issue number1
DOIs
StatePublished - Jan 23 2018
Externally publishedYes

Keywords

  • covalent organic framework
  • density functional theory
  • electronic structure
  • porphyrin
  • scanning tunneling microscopy and spectroscopy

ASJC Scopus subject areas

  • Materials Science(all)
  • Engineering(all)
  • Physics and Astronomy(all)

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