New ferrocenyl naphthoquinone fused crown ether chemosensors: Highly selective, kinetically and regio controlled colorimetric, beryllium ion recognition

New organometallic chemosensors which are based on triad molecules, ferrocenyl naphthoquinone linearly/angularly fused crown ethers (Fc–cnq–1a and Fc–cnq–1b) bearing ferrocene, quinone, and crown ether functional groups together, were synthesized and utilized for selective sensing trace amount of Be²⁺ ion (5.41 μM) among other metal cations. The UV–vis spectrophotometric titration experiments at controlled temperatures (25–60 °C) resulted in characteristic spectral changes in the intramolecular charge-transfer (CT) transitions upon addition of Be²⁺ ion into the solution of sensors Fc–cnq–1a and Fc–cnq–1b. However, no spectral changes were observed for the other metal cations used for testing at the same conditions, indicating that the sensors can selectively detect Be²⁺ among the studied metal ions even at higher temperatures. The spectral change in the absorption spectra of the sensors upon addition of Be²⁺ ion corresponds to the visible region of the spectrum, from deep green to yellow color, and this leads to observe the effect of the Be²⁺ ion with naked eye. The sensors used for the signaling of Be²⁺ ion displayed a significant shortening in response time depending on increasing temperature without any degradation of the sensors. Fc–cnq–1a displayed faster response time than Fc–cnq–1b in the temperature range of 35–60 °C. The activation energies for the pseudo-first order complexation reactions were calculated as 124.2±13.0 kJ mol⁻¹ and 151.9±18.0 kJ mol⁻¹ for Fc–cnq–1a and Fc–cnq–1b, respectively, which explains faster response time of Fc–cnq–1a for recognizing Be²⁺ ion. Density Functional Theory calculations (TD-B97D/TZVP//mPWPW91/6-31 + G(2d,2p) level) were performed on Fc–cnq–1a, Fc–cnq–1b and their Be⁺² complexes in order to elucidate their geometries and the molecular orbitals. Calculations have shown comparable results with those obtained from the experimental data.