Trifluorocoumarino cryptands as photoprotonic molecules:: Basic features and theoretical considerations

The coupling between the fluorescence properties of the (trifluoromethyl)coumarino fluorophore and the protolytic state of the ion binding moiety of two fluorescent cryptands, F221 and F222, is investigated experimentally by carrying out steady-state and time-resolved fluorescence measurements. The high-intensity fluorescence emission of the diprotonated state of the these alkali ion-selective indicators, characterized by quantum yields of 0.6 and 0.83 as well as by lifetimes of 5.3 and 5.6 ns, are markedly quenched upon deprotonation, which leads to the monoprotonated state with quantum yields of 0.07 and 0.02 as well as lifetimes of 1.0 and 0.19 ns, respectively. The corresponding pK(a1) values are 7.07 for F221 and 5.85 for F222. The formation of the fully deprotonated state of the fluorescent cryptands, characterized by pK(a2) values of 10.6 and 9.3, respectively, is accompanied by a comparatively small additional reduction of the fluorescence quantum yield and lifetime. As a framework for the understanding of the pH-dependent fluorescence parameters, we suggest the concept of fluorescence quenching via photoinduced electron transfer (PeT), where the quenching process is assumed to be controlled by the pH-dependent availability of nonprotonated bridgehead N-atoms of the cryptand. These N-atoms act as electron donors with respect to the excited fluorophore, which functions as electron acceptor. In order to quantify the PeT energetics in the case of F221 and its monoprotonated state, one-electron oxidation and reduction potentials are determined by cyclic voltammetry for the parent cryptand [2.2.1] and the fluorophore derivative I as suitable model compounds, respectively. Experimental redox data are supplemented by simple estimations of the electrostatic energy contributions for the intramolecular radical ion pair produced through photoinduced charge separation and by corrections for the hydration energies. The resulting thermodynamic driving forces for PeT in the deprotonated F221 and its monoprotonated form show that PeT is favored for both of these species in water, where only a minor endergonic shift is observed for the monoprotonated as compared to the fully deprotonated compound. Therefore, the fully de- and the monoprotonated state of these fluorescent cryptands are regarded as being responsible for the reduction of the fluorescence quantum yield and the appearance of the second lifetime tau(2) at high pH. In contrast, PeT is expected to be completely blocked for the diprotonated state of these (trifluoromethyl)coumarino cryptands.