A Trapezoidal Octacyanoquinoid Acceptor Forms Solution and Surface Products by Antiparallel Shape Fitting with Conformational Dipole Momentum Switch

Abstract

A new compound (1) formed by two antiparallelly disposed tetracyano thienoquinoidal units has been synthesized and studied by electrochemistry, UV/Vis-NIR, IR, EPR, and transient spectroscopy. Self-assembly of 1 on a Au(111) surface has been investigated by scanning tunneling microscopy. Experiments have been rationalized by quantum chemical calculations. 1 exhibits a unique charge distribution in its anionic form, with a gradient of charge yielding a neat molecular in-plane electric dipole momentum, which transforms out-of-plane after surface deposition due to twisted! folded conformational change and to partial charge transfer from Au(111). Intermolecular van der Waals interactions and antiparallel trapezoidal shape fitting lead to the formation of an optimal dense on Au(111) two-dimensional assembly of 1. The realization of novel properties emerging upon electronic covalent coupling between chromophores (i.e., bichromophoric systems) is a critical issue for the development of photo- and electrically active systems.[1–3] In this regard, the relative topology and orientation of the p-subchromophores, such as in A + B type p-systems in Figure 1, with 1D linear conjugation, 2D parallel conjugation,[4] 3D orthogonal spiroconjugation[5] or 3D conjugation[6] are key factors. On the other hand, studies of the distribution of the excess of charge in p-conjugated moieties in post-electron transfer events are central issues in photophysics and photochemistry, in energy storage[7] and in organic electronics.[8] In addition to this electronic provision, its embedment in different molecular forms is of relevance as these can define unique ways of shape fitting in supramolecular and surface assemblies. Joint electronic and molecular shape designs thus allow to build molecular-based synthons in a tailored manner towards new bulk and nano organized materials. (...)