Abstract
Organic charge transfer (CT) complexes obtained by combining molecular electron donors and acceptors have attracted much interest due to their potential applications in organic opto-electronic devices. In order to work, these systems must have an electronic matching - the highest occupied molecular orbital (HOMO) of the donor must couple with the lowest unoccupied molecular orbital (LUMO) of the acceptor - and a structural matching, so as to allow direct intermolecular CT. Here it is shown that, when molecules are adsorbed on a metal surface, novel molecular organizations driven by surface-mediated CT can appear that have no counterpart in condensed phase non-covalent assemblies of donor and acceptor molecules. By means of scanning tunneling microscopy and spectroscopy it is demonstrated that the electronic and self-assembly properties of an electron acceptor molecule can change dramatically in the presence of an additional molecular species with marked electron donor character, leading to the formation of unprecedented core-shell assemblies. DFT and classical force-field simulations reveal that this is a consequence of charge transfer from the donor to the acceptor molecules mediated by the metallic substrate.
Highlights
Whereas all TCNQ molecules assemble in these clusters, the excess TBP molecules form homomolecular islands, the organization and structure of which follow the same herringbone arrangement observed when only TBP is deposited on Au(111)[33] (Fig. 3(a))
These TBP islands result from a delicate interplay of attractive vdW forces and repulsive electrostatic interactions, the latter caused by interfacial dipoles induced by reversible integer charge transfer (CT) from the TBP molecules to the substrate.[33]
We identify the peak observed at −1.0 V as a metal– organic state, resulting from the mixing between Au adatom states and the lowest unoccupied molecular orbital (LUMO) of the TCNQ molecule
Summary
Most of the promising applications of D–A systems are in cheap, flexible and portable opto-electronics, including the production of organic ferroelectric devices, field-emission transistors and artificial photosynthetic systems.[13,14,15,16,17,18,19,20,21,22] For these applications, the D–A materials are deposited from a liquid solution or by thermal vapor deposition as thin films onto a conductive electrode,[23,24,25,26,27,28,29,30] resulting in organic-electrode interfaces that can be highly complex, though crucial for the device performances. Paper the organic materials are preserved even after surface deposition. This assumption often neglects the fact that the adsorption of organic molecules onto a solid electrode can significantly modify the relative energies of the relevant molecular electronic orbitals, dramatically affecting the material’s optoelectronic properties and the intermolecular bonding at the interface. As consequence, both the electronic structure and the molecular organization of functional organic thin films can be significantly different from their bulk counterparts, resulting in significant variations with respect to the expected device performance. Several studies have been reported on the perturbation of the electronic structure of single molecules after surface adsorption,[31,32,33,34,35,36] the effect of solid surfaces on the electronic properties of interfacial D–A systems has received much less attention.[37,38,39,40,41] While these perturbations are often seen as challenges to be overcome, they can represent an opportunity, as unprecedented D–A assemblies can be obtained as a result of processes occurring exclusively at metal–organic interfaces
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