A fluorescent ammonia sensor based on a porphyrin cobalt(II)–dansyl complex Jiyeon Kim a , Si-Hyung Lim b , Yeoil Yoon c , T. Daniel Thangadurai a,⇑ , Sungho Yoon a,⇑ a Department of Chemistry, Kookmin University, 861-1 Jeongneung-dong, Seongbuk-gu, Seoul 136-702, Republic of Korea b Department of Mechanical Engineering, Kookmin University, 861-1 Jeongneung-dong, Seongbuk-gu, Seoul 136-702, Republic of Korea c Greenhouse Gas Research Center, Korea Institute of Energy Research, 71-2, Jangdong, Yuseonggu 305-343, Republic of Korea article info Article history: Received 17 January 2011 Revised 7 March 2011 Accepted 10 March 2011 Available online 16 March 2011 Keywords: Cobalt(II)porphyrin Dansylpiperazine Axial ligand Ammonia Fluorescence enhancement abstract Detecting and measuring the concentration of ammonia is of interest in many scientific and technological areas. A porphyrin based cobalt(II) complex with a dansyl fluorophore has been synthesized and investigated as a ‘turn-on’ fluorescent ammonia sensor. Over sixfold increase in fluorescence emission occurs upon the treatment of NH 3 to [Co(TPP)(Ds-pip)] sensor solution, resulting from NH 3 -induced displacement of the axially coordinated fluorophore. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Ammonia has been widely used in the production of explosives, fertilizers, and as an industrial coolant, 1 the excess presence of ammonia in the atmosphere, however, creates potential hazards to human being and ecosystems. Ammonia is toxic to many aquatic organisms even in very low concentrations and inhalation of only a small dose of ammonia vapor causes acute poisoning to people. Detecting and measuring the concentration of ammonia is of inter- est in many scientific and technological areas. Although electro- chemical ammonia sensors are widely used in environmental monitoring, automotive and chemical industry, 2 in recent years, several new approaches have been reported, including optical gas- eous ammonia sensors. 1,3–9 They utilize the reaction of ammonia vapor with either a pH-dependent dye material or a pH-sensitive film, 7,10 which undergoes a suitable color change. Our method in developing sensors that utilize the formation of transition-metal ammonia complexes to trigger a change in fluorescence is based on a strategy that takes advantage of the well-known fluorescence-quenching properties of transition metal complexes with partially filled d-shells. In particular, we have been exploring systems in which reaction of NH 3 with a transition-metal complex containing a coordinated fluorophore conjugate results in removal of the fluorophore from the coordination sphere with concomitant fluorescence turn-on. Fluorescence enhancement is generally preferred over fluorescence quenching when monitoring an analyte. Herein, we report that NH 3 can displace a axial fluorophore ligand bound to cobalt(II) complex with concomitant fluorescence increase (Scheme 1). The observed fluorescence response of these cobalt–fluorophore conjugates may ultimately allow their use for measuring NH 3 vapor in atmosphere. 2. Results and discussion Metalloporphyrins have been used as colorimetric gas sensors based on characteristic color changes, resulting from coordination of analytes at the metal center. 11–14 The metal center of M(II)- porphyrins and M(II)-phthalocyanines possesses coordinatively unsaturated axial sites, which are potentially used in catalysis or development of sensor. In the adsorbed state, the underlying metal surface can occupy one of the axial sites and, as an additional ligand, influence the electronic structure of the metal center. The nature of this interaction, which is the main focus of this paper, has been studied previously, in particular on the example of the coordinated cobalt ion. 15 Moreover, the axial ligation properties of the metalloporphyrins can be utilized for the self-organization of the molecules. Fluorescence quenching upon recognition of the analyte is not only disadvantageous for a high signal output but also hampers temporal separation of spectrally similar complexes with time- resolved fluorometry. 16 Thus, it is of interest that the recognition of NH 3 by the sensor does increase the fluorescence. To enhance 0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2011.03.048 ⇑ Corresponding authors. Tel.: +82 2 910 4763/5440; fax: +82 2 910 4415 (T.D.T.). E-mail addresses: danielt@kookmin.ac.kr (T.D. Thangadurai), yoona@kookmin. ac.kr (S. Yoon). Tetrahedron Letters 52 (2011) 2645–2648 Contents lists available at ScienceDirect Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet