New intrinsic fluoroionophores with dual fluorescence: DMABN‐Crown4 and DMABN‐Crown5

Abstract

AbstractTwo new intrinsic fluoroionophores, with the electronic structure of 4‐(dimethylamino)‐ benzonitrile (DMABN): 4‐(1‐aza‐4,7,10‐trioxaclyclododecyl)benzonitrile (DMABN‐Crown4) and 4‐(1‐aza‐4,7,10,13‐tetraoxacyclopentadecyl) benzonitrile (DMABN‐Crown5) were synthesized and their absorption, fluorescence spectra and quantum yields of fluorescence measured and discussed. Comparison of X‐ray data and calculations from the density functional theory (DFT) shows that from DMABN to DMABN‐Crown5 the twist angle of the amino group with respect to the phenyl plane increases while the pyramidalization of the nitrogen is not significantly affected. The activation energies (Ea 8.9 kJ · mol−1, Ed 23.7 kJ · mol−1) and the enthalpy (ΔH‐14.8 kJ · mol−1) involved between the LE and TICT states of DMABN‐Crown5 were obtained in toluene and compared with those previously published for DMABN. The greater relief of strain (larger ΔH) which accompanies the formation of the TICT state in the crown derivatives, accounts for the higher proportion of TICT emission recorded at room temperature. The fluorescence of DMABN‐Crown5 in acetonitrile, is altered significantly at 3 · 10−4 mol · 1−1 of Ca2+: the long‐wavelength emission from the TICT state decreases while the short‐wavelength emission increases. From the wholly complexed probes, the two emissions are blue‐shifted relative to those of the free probe and originate from contact‐ and/or solvent‐separated probe‐cation pair with the probe in the LE and in the TICT state. Similarly, the fluorescence spectrum of DMABN‐Crown4, in acetonitrile, is altered significantly when Mg(CIO4)2 is added in concentrations of 5 · 10−3 mol · 10−1 or above.