Stability constants Coumarin

In contrast to the above-described systems, there are only few systems in which the bound cation can interact with the acceptor part of charge-transfer probes. The case of coumarins linked to crowns (Figure 10.24) is of special interest because the cation interacts directly with the electron-withdrawing group, i.e. the carbonyl group, in spite of the spacer between the fluorophore and the crown. An important consequence is the increase in stability constant of the complexes with respect to the same crown without external complexing atoms. 

Lis et al. [70] have shown recently by using AI/AI2O3 electrodes that a 1 2 chelate of Eu(lll) and coumarin-3-carboxylate is electrochemiluminescent in aqueous solution. The same group has also smdied some other weak complexes that show ECL but cannot be used as labels due to their too small stability constants [71]. 

Sandanayake et al. made the first systematic attempt to couple a donor and acceptor in the construction of an ICT system using the coumarine boronic acid 7 [44]. Here both fluorescence intensity and wavelength are affected since the nitrogen is directly connected with the chromophore. Sadly, this system shows small shifts in intensity and wavelength, in spite of its clever molecular design. The observed stability constant (f pp) for 7 was 27 for D-fructose in 1 1 methanol-water. 

The first concerted effort to couple donor and acceptor fragments to produce an ICT sensor for saccharides was by Sandanayake, sensor 71. In this case coumarin was used as the fluorophore, which led to the redshifting of the emission wavelength and a decrease in the emission intensity on saccharide binding. Unfortunately, these changes were rather modest with an observed stability constant (ATobs) of 27 for D-fructose in 1 1 (v/v) methanol/water. 

---