Steady-state and time-resolved investigations of a crown thioether conjugated with methylacridinium and its complexes with metal ions

The crown thioether 9-[4-(4,7,10,13-tetrathia-1-azacyclopentadecyl]phenyl-N-methylacridinium perchlorate (TCMA) was synthesized and characterized with the aim to verify its ability to interact selectively with metal ions and substantiate the possibility to detect easily the presence of heavy metals in fluid samples. The spectroscopic properties of TCMA, alone and in the presence of metal ions, were therefore studied in polar solvents (MeCN and H2O); in particular, steady-state UV-Vis spectrophotometric and fluorimetric techniques were used together with transient absorption spectroscopy with fs time resolution to investigate the spectral and dynamic properties of the lowest excited singlet state of TCMA and of TCMA/metal ion complexes. The absorption in the Vis region is characterized by a charge-transfer nature with the methylacridinium moiety acting as the electron-acceptor and the anilic group as the electron-donor. No emission from the S1 was detected both in MeCN and H2O, while a small S2 → S0 fluorescence emission (λmax = 485 nm and ϕF = 0.0011) was detected in water. Time-resolved measurements with fs resolution of TCMA in MeCN have shown that the relaxed S1 state is reached ∼0.6 ps after the laser pulse, while the S1 → S0 time constant is 3.7 ps. Among the investigated metal ions, only Fe3+ (in MeCN) and Hg2+ (in MeCN and H2O) were able to form stable complexes (association constant, Kass = 1–11 × 104 M−1) with TCMA. The S1 state of the TCMA/Mn+ complexes emits with low quantum yield (ϕF = 0.0023–0.014) and decays with time constants much longer than TCMA itself, at least in the case of TCMA/Hg2+ in MeCN. This study showed that TCMA is a good candidate for colorimetric/fluorimetric sensing of Hg2+ in aqueous media owing to its high selectivity towards metal ions.