Fluorimetric detection of Mg2+ and DNA with 9-(alkoxyphenyl)benzo[b]quinolizinium derivatives

A benzo[b] quinolizinium-benzo-15-crown-5 ether conjugate 2a is presented that enables the fluorimetric detection of Mg2+ and DNA by a significant light-up effect, along with a change of the emission wavelength with different analytes (Mg2+: 495 nm; DNA: 550 nm). The mechanism of the excited-state deactivation of 2a was investigated by steady-state fluorescence spectroscopy in media of varied viscosity and compared with the photophysical properties of methoxyphenyl-substituted benzo[b] quinolizinium 2b (m,p-diOMe), 2c (m-OMe), and 2d (p-OMe) as reference compounds. Compounds 2a-c, which share the m-alkoxyphenyl substituent as the common feature, have low emission quantum yields (Phi(F) < 10(-2) in water) but exhibit a significant increase of their fluorescence intensity in viscous glycerol solutions. In contrast, the viscosity of the medium does not influence the emission properties of the parent phenyl-substituted benzo[b] quinolizinium 2e and of the p-methoxyphenyl-substituted derivative 2d. Based on these observations it is concluded that the excited-state deactivation in 2a-c is mainly due to the rotation of the m-alkoxy group about the C-ar-O bond. The interaction of 2a-c with DNA or Mg2+ ions was studied by spectrophotometric titrations and CD spectroscopy. Notably, the association of 2a or 2b with DNA or 2a with Mg2+ ions induces a strong fluorescence enhancement (15- and 40-fold for DNA, 450-fold for Mg2+), which is rationalized by the suppression of the torsional-relaxation of the alkoxy-substituent in the excited state. Additionally, the cation-induced light-up effect of 2a is selective towards Mg2+ ions as compared with other cations such as NH4+, Li+, Na+, K+ and Ba2+.