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

doi:10.1021/acsnano.7b06529

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

ACS Nano. 2018 Jan 23;12(1):385-391. doi: 10.1021/acsnano.7b06529. Epub 2017 Dec 26.

Authors

Chen Chen¹, Trinity Joshi², Huifang Li³, Anton D Chavez^{4

5}, Zahra Pedramrazi², Pei-Nian Liu⁶, Hong Li^{3

7}, William R Dichtel⁴, Jean-Luc Bredas^{3

7}, Michael F Crommie^{2

8

9}

Affiliations

¹ Department of Chemistry, University of California at Berkeley , Berkeley, California 94720, United States.
² Department of Physics, University of California at Berkeley , Berkeley, California 94720, United States.
³ Laboratory for Computational and Theoretical Chemistry of Advanced Materials, Physical Science and Engineering Division, King Abdullah University of Science and Technology , Thuwal 23955-6900, Kingdom of Saudi Arabia.
⁴ Department of Chemistry, Northwestern University , Evanston, Illinois 60208, United States.
⁵ Department of Chemistry and Chemical Biology, Cornell University , Ithaca, New York 14853, United States.
⁶ Shanghai Key Laboratory of Functional Materials Chemistry and School of Chemistry & Molecular Engineering, East China University of Science and Technology , 130 Meilong Road, Shanghai 200237, China.
⁷ School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology , 901 Atlantic Drive NW, Atlanta, Georgia 30332-0400, United States.
⁸ Materials Sciences Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.
⁹ Kavli Energy NanoSciences Institute at the University of California Berkeley and the Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.

PMID: 29261279
DOI: 10.1021/acsnano.7b06529

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.

Keywords: covalent organic framework; density functional theory; electronic structure; porphyrin; scanning tunneling microscopy and spectroscopy.

Publication types

Research Support, U.S. Gov't, Non-P.H.S.