ORIGINAL PAPER A chloride-anion insensitive colorimetric chemosensor for trinitrobenzene and picric acid Dae-Sik Kim & Vincent M. Lynch & Kent A. Nielsen & Carsten Johnsen & Jan O. Jeppesen & Jonathan L. Sessler Received: 25 February 2009 / Revised: 16 April 2009 / Accepted: 21 April 2009 / Published online: 24 May 2009 # Springer-Verlag 2009 Abstract A new receptor, the bisTTF-calix[2]thiophene[2] pyrrole derivative 3, has been prepared from the Lewis acid-catalyzed condensation of 2,5-bis(1-hydroxymethy- lethyl)thiopheno-TTF and pyrrole. This new system is found to form complexes with the electron-deficient guests, trinitrobenzene (TNB) and picric acid (PA), which serve as models for nitroaromatic explosives. The binding phenom- enon, which has been studied in organic solution using proton nuclear magnetic resonance and absorption spectros- copies, results in an easy-to-visualize color change in chloroform that is independent of the presence of chloride anion, a known interferant for an earlier tetrakisTTF-calix[4] pyrrole TNB chemosensor. Support for the proposed binding mode comes from a preliminary solid state structure of the complex formed from TNB, namely TNB⊂3. A color change is also observed when dichloromethane solutions of chemosensor 3 are added to solvent-free samples of TNB, PA, and 2,4,6-trinitrotoluene supported on silica gel. Keywords Calix[4]pyrroles . Charge transfer (CT) . Colorimetric chemosensors . Explosive detection . Supramolecular chemistry . Tetrathiafulvalenes Introduction Systems allowing for the rapid and inexpensive detection of explosives are of considerable current interest given the importance associated with detecting such species in mine- fields, in military situations, and in homeland security monitoring [1–4]. Considerable attention is being devoted to this problem and, indeed, a number of methods have been developed that are capable of detecting explosives under certain conditions. These include gas chromatogra- phy coupled with mass spectrometry [5], surface-enhanced Raman spectroscopy [6], nuclear quadruple resonance, energy-dispersive X-ray diffraction [7], electron capture detection, and cyclic voltammetry [8]. While in many cases these approaches have proved effective, fairly complex instrumentation is often required, and this has limited production of small, low-power units suitable for use in the field. Recently, however, success has been encountered via the use of fluorescence modulation in appropriately modified polymers; this has allowed deployment of field- ready devices [9]. Nevertheless, there remains a need for alternative even “lower tech” solutions that would allow for explosives sensing under rather rudimentary conditions. One approach to meeting this latter challenge involves creating so-called chemosensors that produce a visible optical change in the presence of explosives. Such systems, which rely on changes in absorption as opposed to emission spectral features, can be employed in the absence of instrumentation via simple “naked eye” detection. They can also be incorporated into inexpensive and portable Anal Bioanal Chem (2009) 395:393–400 DOI 10.1007/s00216-009-2819-4 Electronic supplementary material The online version of this article (doi:10.1007/s00216-009-2819-4) contains supplementary material, which is available to authorized users. D.-S. Kim : V. M. Lynch : J. L. Sessler (*) Department of Chemistry & Biochemistry, The University of Texas at Austin, 1 University Station-A5300, Austin, TX 78712-0165, USA e-mail: sessler@mail.utexas.edu K. A. Nielsen : C. Johnsen : J. O. Jeppesen (*) Department of Physics and Chemistry, University of Southern Denmark, Campusvej 55, Odense M 5230, Denmark e-mail: joj@ifk.sdu.dk