Novel rhodamine 6G fluorescent chemosensors 1 and 2 for the detection of transition metal cations were synthesized through the condensation of rhodamine 6G ethylenediamine with each of 2-hydroxy-1-naphthaldehyde and 2,6-pyridinedicarbaldehyde, respectively. 1 and 2 were characterized using 13C NMR, 1H NMR and mass spectroscopy. Fluorometric and colorimetric measurements involving various metal ions revealed the ring opening of the rhodamine 6G spirocycle framework. In the absence of metal cations, 2 was colorless and non-fluorescent, whereas the addition of metal cations (Hg2+ and others) changed the color to pink, accompanied by the appearance of an orange fluorescence. The chemosensors exhibited high selectivity for Hg2+ over other divalent first-row transition metals. The complexes of Hg2+ with 1 and 2 were successfully isolated. A huge enhancement in the fluorescence for both one- and two-photon excitations makes these compounds suitable candidates to be used for fluorescent labeling of biological systems.

Abstract
 1. Introduction
 2. Experimental
  2.1. General methods
  2.2. Synthesis of compound 3
  2.3. Synthesis of compound 1
  2.4. Synthesis of compound 2
 3. Results and discussion
 4. Conclusion
 References

• Seung Hyun Chang(Department of Chemistry and Applied Chemistry, Daegu University) Corresponding author
• Jin-Wook Choi(Department of Chemistry and Applied Chemistry, Daegu University)
• Kwang-Bo Chung(Department of Chemical Engineering, Hanseo University)