Structural characterization of new Cd²⁺ fluorescent sensor based on lumazine ligand: AM1 and ab initio studies

6-Thienyllumazine (TLM) is synthesized as a new fluorescent sensor that is capable of indicating selectively the presence of Cd²⁺ ion via a fluorescence signal. Experiment has been performed in the presence of Ni²⁺, Co²⁺, Cu²⁺, Ag⁺, Mn²⁺, Hg²⁺, Zn²⁺, Pb²⁺, and Mg²⁺ metal ions in aqueous solutions. The product was characterized by elemental analysis, mass, and NMR spectra. The spectral characteristics (maxima, quantum yields, Stokes shift, and lifetimes) of TLM in organic and aqueous solvents have been studied with the help of absorption and fluorescence spectroscopy, as well as, using time dependent spectrofluorimetry (single photon counting technique). The fluorescence dependence of TLM on the pH has also been investigated. The experimental results indicate that TLM exists in two ionic forms: neutral (acid) and anion (base). Electronic structure calculations of TLM were carried out using Semiempirical Austin Model 1 (AM1) and ab initio Hartree-Fock (HF) with 6-31G* basis set and using Gaussian 03 program. Absorption energies for TLM have been calculated using ZINDO method. The theoretical results confirm the presence of the thiophene and pteridine rings in two conformations: twisted at angle of about 35° in the excited state and coplanar in the ground state.