Fluorescent phenanthridine-based calix[4] arene derivatives: synthesis and thermodynamic and computational studies of their complexation with alkali-metal cations

New fluorescent calix[4] arene derivatives 1, 2, and 3 were synthesized by introducing phenanthridine moieties at a lower calixarene rim. It was shown that due to the prominent fluorescence of compounds 1 and 3, they could be considered as potential sensitive fluorimetric cation sensors. Complexation of the prepared compounds with alkali-metal cations was studied at 25 degrees C in acetonitrile-dichloromethane and methanol-dichloromethane solvent mixtures (phi = 0.5) by means of fluorimetric, spectrophotometric, potentiometric, and microcalorimetric titrations as well as NMR spectroscopy. The stability constants of the corresponding complexes were determined, as were the enthalpies and entropies of the complexation reactions. In addition, equilibrium constants of ion-pairing reactions between alkali-metal cations and several anions in the solvents used were measured conductometrically. It was found that the cation-binding affinity of ligand 1 with four phenanthridine subunits was much higher than that of 2 and 3, with the complex stabilities in all cases being significantly lower in methanol-dichloromethane mixture compared to that in acetonitrile-dichloromethane. These findings were thoroughly discussed by taking into account the determined thermodynamic complexation data, structural properties of the ligand and free and complexed cations, as well as the solvation abilities of the solvents examined. The conclusions made in that way were corroborated by the results of the molecular dynamics simulations of the systems studied. An attempt to get an insight into the possible structures of the alkali-metal cation complexes with ligand 1 was made by carrying out the corresponding density functional theory calculations.