Iptycenes in the design of high performance polymers.
This Account details the use of building blocks known as iptycene units, which are particularly useful in the design of advanced materials because of their three-dimensional, noncompliant structures. Iptycenes are built upon [2,2,2]-ring systems in which the bridges are aromatic rings, and the simplest member of this class of compounds is triptycene. Iptycenes can provide steric blocking, which can prevent strong interactions between polymeric chromophores that have a strong tendency to form nonemissive exciplex complexes. Iptycene-containing conjugated polymers are exceptionally stable and display solution-like emissive spectra and quantum yields in the solid state. This application of iptycenes has enabled new vapor detection methods for ultratrace detection of high explosives that are now used by the U.S. military. The three-dimensional shape of iptycenes creates interstitial space (free volume) around the molecules. This space can confer size selectivity in sensory responses and also promotes alignment in oriented polymers and liquid crystals. Specifically, the iptycene-containing polymers and molecules align in the anisotropic host material in a way that minimizes the free volume. This effect can be used to align molecules contrary to what would be predicted by conventional models on the basis of aspect ratios. In one demonstration, we show that an iptycene polymer aligns orthogonally to the host polymer when stretched, and these structures approximate molecular versions of woven cloth. In liquid crystal solutions, the conjugated iptycene-containing polymers exhibit greater electronic delocalization, and the transport of excited states along the polymer backbone is observed. Structures that preserve high degrees of internal free volume can also be designed to create low dielectric constant insulators. These materials have high temperature stability (>500 degrees C) and hardness that make them potential interlayer dielectric materials for integrated circuits. In cases where the iptycene structures are less densely spaced along the polymer backbones, interlocking structures can be created. These structures allow for small interpolymer motions, but at large deformations, the steric clashes between iptycenes result in the transfer of load from one polymer to another. This mechanism has the ability to impart greater modulus, strength, and ductility. It is difficult to increase modulus without adversely affecting ductility, and classical high-modulus materials have low ductility. As a result, the use of interlocking iptycene structures is a promising approach to new generations of structural materials.
Swager
Timothy M
TM
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. tswager@mit.edu