SHORT COMMUNICATION 1,3-Di(2-pyrrolyl)azulene: An Efficient Luminescent Probe for Fluoride Husein Salman, [a] Yael Abraham, [a] Shay Tal, [a] Shai Meltzman, [a] Moshe Kapon, [a] Nir Tessler, [b] Shammai Speiser, [a] and Yoav Eichen* [a] Keywords: Anions / Anion sensing / Host-Guest systems / Receptors / Supramolecular chemistry Photo-induced Energy Transfer (PET) based chemosensing is a very elegant way for reporting the presence of an analyte in solution. This method was successfully applied to the de- tection of many cationic species in solutions and already ap- pears in interesting commercial applications. In this paper we report on the preparation and host–guest chemistry of Introduction Halides, especially fluoride, chloride and iodide, are of biological relevance. They play important roles in the for- mation and prevention of severe diseases such as cancer, [1] cystic fibrosis, [2] caries [3] and osteoporosis. [4] Their bio-rele- vance raises a need for a better understanding of the host– guest chemistry of such ions, especially in polar and aque- ous media. From the practical point of view, there is also a need for new, simple, efficient and selective methods for the detection of such ions in solutions. Of particular interest would be systems that can recog- nize a target anion in solution and signal its presence through an easy-to-detect optical signature, such as a color change or increase in the luminescence of the sensing moi- ety. In the past few years, different groups have proposed a wide range of anion binding systems based on guanidinium derivatives, [5] oligoamines, [6] sapphyrins, [7] calixpyrroles, [8] cyclopyrroles, [9] quinoxalines-pyrroles, [10] etc. [11,12] Intro- duced by the group of Sessler, the quinoxaline-pyrrole sys- tems are of particular interest since they present a general approach to efficient anion binders which are coupled to moieties that respond to the presence of some anions by a color change. [10] These systems have been shown to be both effective and specific. A very elegant way that was developed for re- porting the presence of cations is the Photo-induced Energy Transfer (PET) signaling approach, first proposed by Wel- [a] Department of Chemistry, Solid State Institute, Technion – Is- rael Institute of Technology, Technion City, 32000, Haifa, Israel Fax: +927-4-8295307 E-mail: chryoav@techunix.technion.ac.il [b] Department of Electrical Engineering, Nanoelectronics Center, Technion – Israel Institute of Technology, Technion City, 32000, Haifa, Israel Eur. J. Org. Chem. 2005, 2207–2212 DOI: 10.1002/ejoc.200500012 © 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2207 1,3-di(2-pyrrolyl)azulene (5), a new azulene-based selective PET-like chemosensor that turns fluorescent upon binding the fluoride anion. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) ler [13] and brought to perfection by De Silva [14] and others. [15] This method was successfully applied to the de- tection of many cationic species in solutions and already appears in interesting commercial applications. PET chemosensors consist of a luminescent species attached to a recognition group. In the unbound dark state, the binding group quenches the excited state of the luminescent part, usually by its lone pair electrons of the unoccupied metal binding site. Upon binding, the metal ion is attached to the recognition group through the lone pair electrons. Conse- quently, the binding group can no longer serve as an ef- ficient quencher and the luminescence is regained, thus sig- naling the capture of the guest. The use of such approach to signal the binding of an anion is not straightforward since anions are usually coordinated by efficient hydrogen donors. These usually do not decrease their tendency to quench the excited state of the luminophore upon bind- ing. [16] One possible approach to the preparation of a PET- like chemosensor for anions is to couple an efficient anion receptor to a luminescent moiety in such a way that binding of an anion forces the receptor out of the plane of the lumi- nescent group and by that decreases its electronic coupling with the luminescent moiety. In such a system the fluores- cence is expected to increase upon binding of the target anion. In this paper we report on the preparation, crystal struc- ture characterization and anion binding properties of an azulene-based selective chemosensor that turns fluorescent upon binding the fluoride ion. Azulene was chosen as the luminophore due to its exceptionally short-lived emission from S 2 . The very short fluorescence lifetime of the new azulene derivative, τ = 50 ± 5 ps and τ = 30 ps in dichloro- methane and DMSO, respectively, excludes the interference of any diffusion-controlled process between the azulene de- rivative and unbound species in solution.