Supramolecular Liquid Crystals Based on Cyclo[8]pyrrole

摘要

Expanded porphyrins,[1] larger analogues of naturally occurring tetrapyrrolic pigments, have attracted considerable interest in recent years because of their potential utility in areas as diverse as anticancer drug development,[2], [3] nonlinear optics,[4]-[6] and anion recognition.[7], [8] They have also seen use in the areas of cation coordination[1], [9] and as test systems for the study of aromatic properties in large heteroannulenes.[1], [6], [10]-[13] As a class, expanded porphyrins possess electronic, structural, and spectral properties that are distinct from those of their far better studied porphyrin progenitors. However, in spite of these potentially attractive features, reports of liquid crystals (LCs) derived from expanded porphyrins are exceedingly rare. Indeed, we are aware of only two expanded porphyrin mesophases: both are derived from imine-linked oligopyrrolic macrocycles and neither is aromatic.[14]-[16] We have thus sought to produce a liquid-crystalline mesophase based on an all-carbon-linked, aromatic expanded porphyrin framework and report here the synthesis of new cyclo[8]pyrrole derivatives 1 a-c that form hexagonal columnar (Colh) mesophases when combined with electron-deficient acceptors, such as trinitrofluorenone (TNF), trinitrobenzene (TNB), trinitrophenol (TNP), and trinitrotoluene (TNT). The resulting systems are structurally interesting because of the presence of hydrogen-bonded sulfate ions. Furthermore, the optical properties of the cyclo[8]pyrrole ring make the new mesophases potentially interesting materials for molecular electronic and photovoltaic applications.[17]-[20] Finally, the possibility of producing liquid crystallinity upon exposure to TNT makes the new cyclo[8]pyrroles described herein of interest as a possible explosives-sensing material.