A New Class of Supramolecular Wires

B. G. Sumpter¹, V. Meunier¹, E. F. Valeev², A. J. Lampkins³, H. Li³,
A. Vazquez-Mayagoitia⁴, and R. K. Castellano³

¹Center for Nanophase Materials Sciences, Oak Ridge National Laboratory
²Department of Chemistry, Virginia Tech; ³Department of Chemistry, University of Florida;
⁴Computer Science and Mathematics Division, Oak Ridge National Laboratory

Achievement
A new approach, emerging from experimental and theoretical studies of donor-σ-acceptor molecules, has been developed to construct one-dimensional supra-molecular wires that have easily tunable electronic and physical properties [1]. This concept is established using one class of these systems, 1-aza-adamantanetriones (AATs), which are well characterized in terms of their solution and solid-state self-assembly and chemical manipulation, (see Fig. 1). Following self-assembly of the molecules into 1-D periodic arrays [2] -- a process driven by dipolar and dispersion forces -- the frontier molecular orbitals are delocalized, spanning the entire system, through the saturated tricyclic cores of the monomers, (see Fig. 2, Top). The electronic band structure for the wire reveals significant dispersion and can be tuned from the insulating regime to the semiconducting regime by suitable chemical functionalization of the core [1], (Compare top and bottom images in Fig. 2)

Significance
Understanding and controlling the forces that drive self-assembly processes provides a viable route for the design of functional materials with desired properties. Moving toward this goal, π-conjugated systems are at the forefront of the organic electronics field due to their synthetic accessibility, tunable electronic properties with substitution and conjugation length, and predictable self-assembly into nanoscale architectures through π-stacking (often together with other non-covalent interactions). Our new theoretical understanding, highlighting the fundamental importance of the intrinsic dipolar and dispersion interactions in the self-assembly process, for a new class of supra-molecular structures based on unconventional donor-σ-acceptor molecules [1-3] and the theoretical characterization of the subsequent electronic structure and properties, sets the stage for the tailored design of novel functional materials that are an alternative to those comprised of traditional π-conjugated systems. Continued recognition that electronic, optical, and transport properties similar to traditional π-conjugated materials may be accessible to alternative organic architectures, particularly those that spontaneously organize, should encourage the development of new approaches to functional materials design such as materials for organic-based optoelectronic devices.

Summary
The resulting structure from subsequent self-assembly of multiple molecules of these two AATs into stable periodic wires is shown in the Fig. 2. The binding energy (dimer dissociation energy) and the full electronic band structure are also highlighted in these frames, showing surprisingly strong intermolecular interactions and electronic states with relatively large dispersion. As can be seen, the electronic bandgap for the two different AATs is highly dependent on the substitution of the AAT core, ranging from 2.3 eV for the tribenzyl AAT (Top, Fig. 2) to 1.5 eV for the triamide AAT (Bottom, Fig. 2), demonstrating the ability to control the electronic structure. The nature of the dispersion can be seen from the frontier molecular orbitals, which show delocalization through the saturated tricyclic cores of the monomers (insert on the band structure plot in Fig. 2, Top). The simulated XRD pattern obtained from the optimized wire structure for the tribenzyl AAT compares favorably to the experimentally measured XRD pattern and this agreement provides strong support for the concept of the formation of the 1-D supramolecular wires [2]. Additional support is available from NMR, IR, DSC and SEM images.

Acknowledgment of Support
This work was a culmination of research from both the science and user programs. BGS and VM were supported by the Center for Nanophase Materials Sciences (CNMS), sponsored by the Division of Scientific User Facilities, U.S. Department of Energy. This research was part of a CNMS User Program (CNMS2004-016 and CNMS2007-029). AJL, HL, RKC were supported by the National Science Foundation (CHE-0548003). EFV was supported by the National Science Foundation CRIF Grant CHE-0443564.

References

1. B. G. Sumpter, V. Meunier, E. F. Valeev, A. J. Lampkins, H. Li, and R. K. Castellano, J. Phys. Chem. C 111(5), 18912-18916 (2007).
2. B. G. Sumpter, V. Meunier, A. Vazquez-Mayagoitia, R. K. Castellano, Int. J. Quant. Chem. 107, 2233-2242 (2007).
3. L. Yuan, B. G. Sumpter, K. A. Abbound, R. K. Castellano, New J. Chem. 32, 1924-1934 (2008).