Abstract
Semiconducting O-doped polycyclic aromatic hydrocarbons constitute a class of molecules whose optoelectronic properties can be tailored by acting on the π-extension of the carbon-based frameworks and on the oxygen linkages. Although much is known about their photophysical and electrochemical properties in solution, their self-assembly interfacial behavior on solid substrates has remained unexplored so far. In this paper, we have focused our attention on the on-surface self-assembly of O-doped bi-perylene derivatives. Their ability to assemble in ordered networks on Cu(111) single-crystalline surfaces allowed a combination of structural, morphological, and spectroscopic studies. In particular, the exploitation of the orbital mapping methodology based on angle-resolved photoemission spectroscopy, with the support of scanning tunneling microscopy and low-energy electron diffraction, allowed the identification of both the electronic structure of the adsorbates and their geometric arrangement. Our multi-technique experimental investigation includes the structure determination from powder X-ray diffraction data for a specific compound and demonstrates that the electronic structure of such large molecular self-assembled networks can be studied using the reconstruction methods of molecular orbitals from photoemission data even in the presence of segregated chiral domains.
Highlights
Reported semiconducting O-doped polycyclic aromatic hydrocarbons (PAHs), bearing a pyranopyranil or a furanyl core, are very appealing candidates for photoelectronic applications and as p-type semiconductors, showing exceptionally high emission yields and tunable optoelectronic properties in solution.[1−5] Photophysical and electrochemical characterization showed that complementary spectroscopic and redox properties could be tailored through fine tuning of both the π-extension of the carbon scaffold and the oxygen linkages
A combination of structural, morphological, and spectroscopic studies of the interfacial layer at metal substrates is necessary to get insight into the mechanisms governing the formation of thin films: from their assembly to the degree of electronic coupling to the substrate.[11−13] From this perspective, the use of the orbital mapping based on angle-resolved photoemission (ARPES), together with the support of complementary techniques, such as scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED), allows us to shed light on π-conjugated systems, identifying both the electronic structure of the adsorbates and their geometric arrangement.[14]
A number of experimental and theoretical works on different molecular structures have been published by following the orbital tomography methodology.[15,31,55−60] In our case studies, we partially meet the requirements for the plane wave (PW) final-state assumption since we show experimental data on quasi-planar and planar self-assembled large π-conjugated molecules
Summary
Reported semiconducting O-doped polycyclic aromatic hydrocarbons (PAHs), bearing a pyranopyranil or a furanyl core, are very appealing candidates for photoelectronic applications and as p-type semiconductors, showing exceptionally high emission yields and tunable optoelectronic properties in solution.[1−5] Photophysical and electrochemical characterization showed that complementary spectroscopic and redox properties could be tailored through fine tuning of both the π-extension of the carbon scaffold and the oxygen linkages (i.e., furanyl vs pyranopyranyl rings). We observe interface states, visible on the Fermi surface (Eb = 0 eV) from features of substrate photoelectrons diffracted by the molecular lattice These features have already been described, as a result of finalstate effects, for other π-conjugated molecules assembled in ordered networks[62,63] (see Figure S2 in Supporting Information). The two pro-chiral forms adsorb on the surface forming two distinct domains (see Figure 6) with the same superlattice parameters (BPPP column of Table 1) but with different orientations with respect to the substrate high-symmetry directions. Addition, the moderate agreement between the calculated and experimental maps together with the EDC of Figure 9 indicates that the LUMO remains empty upon adsorption
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