Binding studies of tetrathiafulvalene-calix[4]pyrroles with electron-deficient guests Kent A. Nielsen a , Luis Martı ´n-Gomis b , Ginka H. Sarova c , Lionel Sanguinet d , Dustin E. Gross e , Fernando Ferna ´ ndez-La ´ zaro b , Paul C. Stein a , Eric Levillain d , Jonathan L. Sessler e, * , Dirk M. Guldi c, * ,A ´ ngela Sastre-Santos b, * , Jan O. Jeppesen a, * a Department of Physics and Chemistry, University of Southern Denmark, Campusvej 55, DK-5230, Odense M, Denmark b Divisio ´n de Quı ´mica Orga ´nica, Instituto de Bioingenierı ´a, Universidad Miguel Herna ´ndez, Elche 03202, Alicante, Spain c Institute for Physical and Theoretical Chemistry, Universita ¨t Erlangen-Nu ¨rnberg, Egerlandstr. 3, 91058 Erlangen, Germany d Universite´ d’Angers, CIMA CNRS, UFR Sciences, 2, bd Lavoisier, 49045 Angers Cedex, France e Department of Chemistry and Biochemistry, The University of Texas at Austin,1 University Station, A5300, Austin, TX 78712-0165, USA article info Article history: Received 1 April 2008 Received in revised form 19 May 2008 Accepted 30 May 2008 Available online 5 June 2008 Keywords: Calix[4]pyrroles Supramolecular chemistry Tetrathiafulvalenes Fullerenes Molecular switches abstract The neutral meso-octamethylporphyrinogen derivative, tetraTTF-calix[4]pyrrole 1 (TTF¼tetrathia- fulvalene), acts as a multi-faceted receptor in that it interacts with an assortment of different guests in different ways. The conformation of receptor 1 can be reversibly switched between the 1,3-alternate conformation (i.e., 1 , Fig. 1) and the cone conformation (i.e., 1$Cl  , Fig. 2) by the repetitive addition of chloride and sodium ions. In this paper, the results of detailed and systematic complexation studies involving both 1 and its chloride-bound complex, 1$Cl  , with a variety of guests are described. Receptor 1 binds quasi-planar nitroaromatic guests in its 1,3-alternate conformation, while release of these guests takes place upon addition of chloride anions. On the other hand, spherical fullerene guests are strongly bound by 1$Cl  . Finally, it was found that a bidentate guest, consisting of a quasi-planar 2,5,7-trinitro- 9-dicyanomethylenefluorene moiety tethered to a spherical C 60 fullerene, could be recognized by receptor 1 in either its 1,3-alternate or its chloride-bound cone conformation, albeit through very different binding modes. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Calix[4]pyrrole (meso-octamethylporphyrinogen) was first made by Baeyer in 1886 1 and reintroduced by Sessler and co- workers 2 as an anion binding agent in 1996. It displays good anion binding affinity and selectivity in apolar solvents and can be pre- pared via a simple, one-step reaction. This has made it an attractive platform for further modification. For instance, ongoing efforts to improve the anion binding affinity and selectivity of cal- ix[4]pyrroles have resulted in various structural modifications to the basic tetrapyrrolic skeleton, 3,4 including functionalized deri- vatives that act as rudimentary colorimetric chemosensors for various negatively charged analytes. 5 Although some efforts have been carried out to combine calix[4]pyrroles with redox-active units, such as ferrocene, 6 in order to generate electrochemically active sensors, they were not fully successful until 2003 7 when a chemosensor based on a redox-active tetrathiafulvalene 8,9 (TTF) unit in combination with a pseudocalix[4]pyrrole 10 system was reported. In the context of this work, it was found that attaching one redox-active TTF unit directly to the pseudocalix[4]pyrrole core produced a receptor with enhanced binding affinities toward Br  , Cl  , and F  ions. However, the inherent instability of the underlying pseudocalix[4]pyrrole system precluded further studies of this first generation system. Accordingly, meso-octamethylcalix[4]pyrrole derivatives bearing TTF units annulated to the b pyrrolic positions of the meso-octamethylcalix[4]pyrrole core were prepared. 11 Since TTF and its derivatives are good electron donors capable of forming donor/acceptor complexes, 8 it was expected that the resulting modified calix[4]pyrroles, such as the tetraTTF-calix[4]pyrrole host 1 (Fig. 1), would be capable of interacting with electron-deficient guests. As detailed in two recent communications, 12 not only was this expectation met, it was also found that the complexation and decomplexation processes (Fig. 2) could be controlled through the absence/presence of Cl  ions. In this follow-up paper, we elaborate on this fundamental finding and detail the manner in which the tetraTTF-calix[4]pyrrole * Corresponding authors. Tel.: þ1 512 471 5009; fax: þ1 512 471 7550 (J.L.S.). E-mail addresses: sessler@mail.utexas.edu (J.L. Sessler), guldi@chemie.uni- erlangen.de (D.M. Guldi), asastre@umh.es (A ´ . Sastre-Santos), joj@ifk.sdu.dk (J.O. Jeppesen). Contents lists available at ScienceDirect Tetrahedron journal homepage: www.elsevier.com/locate/tet 0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2008.05.141 Tetrahedron 64 (2008) 8449–8463