Abstract
Semiconducting π-conjugated polymers have attracted significant interest for applications in light-emitting diodes, field-effect transistors, photovoltaics, and nonlinear optoelectronic devices. Central to the success of these functional organic materials is the facile tunability of their electrical, optical, and magnetic properties along with easy processability and the outstanding mechanical properties associated with polymeric structures. In this work we characterize the chemical and electronic structure of individual chains of oligo-(E)-1,1′-bi(indenylidene), a polyacetylene derivative that we have obtained through cooperative C1–C5 thermal enediyne cyclizations on Au(111) surfaces followed by a step-growth polymerization of the (E)-1,1′-bi(indenylidene) diradical intermediates. We have determined the combined structural and electronic properties of this class of oligomers by characterizing the atomically precise chemical structure of individual monomer building blocks and oligomer chains (via noncontact atomic force microscopy (nc-AFM)), as well as by imaging their localized and extended molecular orbitals (via scanning tunneling microscopy and spectroscopy (STM/STS)). Our combined structural and electronic measurements reveal that the energy associated with extended π-conjugated states in these oligomers is significantly lower than the energy of the corresponding localized monomer orbitals, consistent with theoretical predictions.
Highlights
The enediyne precursor 1,2-bis(2-ethynylphenyl)ethyne (1) (Figure 1) used in this study was synthesized through iterative Sonogashira cross-coupling reactions. 1 was deposited in ultrahigh vacuum onto a Au(111) surface held at room temperature (T = 293 K)
While 2 (Figure 2b, dashed box) is representative of the dominant monomer incorporated into extended oligomer chains, other monomer subunits were observed within the chains
An AFM image of a representative oligomer chain on the Au(111) surface is depicted in Figure 3a along with the scanning tunneling microscopy (STM) dI/dV spectrum (Figure 3d) measured at one point along the Letter backbone of the oligomer chain (dI/dV measurement reflects the electronic local density of states (LDOS) at the energy selected by the tip−sample bias; spectra taken at different points on the oligomer differ only in the intensity of the observed resonance)
Summary
Alexander Riss,† Sebastian Wickenburg,†,‡ Patrick Gorman,§ Liang Z. Tan,†,‡ Hsin-Zon Tsai,† Dimas G. de Oteyza,†,∥ Yen-Chia Chen,†,‡ Aaron J.
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