Abstract
The three-dimensional construction of arrays of functional molecules on an electrode surface, such as organic semiconductors and redox-active molecules, is a considerable challenge in the fabrication of sophisticated junctions for molecular devices. In particular, well-defined organic layers with precise molecular gradients are anticipated to function as novel metal/organic interfaces with specific electrical properties, such as a space charge layer at the metal/semiconductor interface. Here, we report a strategy for the construction of a three-dimensional molecular array with an electrical connection to a metal electrode by exploiting dendritic molecular architecture. Newly designed dendritic molecules consisting of viologens (1,1′-disubstituted-4,4′-bipyridilium salts) as the framework and mercapto groups as anchor units form unique self-assembled monolayers (SAMs) on a gold surface reflecting the molecular design. The dendritic molecules exhibit a conical shape and closely pack to form cone arrays on the substrate, whereas, in solution, they expand into more flexible conformations. Differences in the introduction position of the anchor units in the dendritic structure result in apical- and basal-type cone arrays in which the spatial concentration of the viologen units can be precisely configured in the cones. The concentration in apical-type SAMs increases away from the substrate, whereas the opposite is true in basal-type SAMs.
Highlights
Dendritic architecture, which is often observed in living organisms, plays critical roles in fundamental functions for life and growth
Designed molecules with several generations were successfully synthesized via a microwave-heating technique (Supplementary Scheme S1 and Fig. S1), and their structures were confirmed by elemental analysis, 1H NMR, and electrospray ionization mass (ESI-MS) measurements
The spatial control based on the conical shape induced by the dendritic structure is often found in supramolecular materials, such as melamine-barbiturate-based rosettes[21] and discotic liquid crystals[22,23], as well as in neurons
Summary
Dendritic architecture, which is often observed in living organisms, plays critical roles in fundamental functions for life and growth. Neurons are connected to each other at this increased surface through robust multiple synapse formation. Such a conical structure is observed in the retina of the eyes. Cone cells, which are one of the photoreceptors in the retina, contain a conical outer segment that includes photopigments. Inspired by the spatial control, robust multiple connections, signal transmission, and formation of unique assembled layers observed in neurons, we were motivated to develop a new molecular-assembled system based on dendritic architecture. Structured molecules composed of redox units as the framework organized on a substrate under spatial control result in three-dimensional arrays of the functional units. We report the first construction of three-dimensional molecular arrays with controlled gradient structures on gold surfaces by utilizing self-assembly of dendritic viologen-arranged molecules with mercapto groups at the apex and periphery of the dendritic structure
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