Synthesis and sensor propeties of crown-containing derivatives of 4-(1,5-diphenyl-Δ²-pyrazolin-3-yl)-1,8-naphthalimide

Abstract

Previously undescribed derivatives of 4-(1,5-diphenyl-Δ²-pyrazoline-3-yl)-1.8-naphthalimide that contain a moiety of benzo-15-crown-5- and N-phenylaza-15-crown-5-ether in the composition of the N-aryl substituent at the imide nitrogen atom of the naphthalimide nucleus have been synthesized. The derived compounds have a long-wavelength band in the absorption spectra in the region of 480 nm owing to charge transfer from the electron-donor pyrazoline moiety onto the carbonyl groups of the carboxyimide group. The fluorescence peaks are located in the region of 670 nm. In the series of the synthesized pyrazolinylnaphthalimides, the effect of the nature of the N-aryl moiety on the spectral-luminescent properties has been analyzed. It has been shown that the annelation of the 15-crown-5-ether moiety with the benzene ring of the N-phenyl substituent of 4-pyrazolinyl-N-phenyl naphthalimide does not change the position of bands in the absorption and fluorescence spectra and has little effect on the fluorescence quantum yield. At the same time, the presence of the N-phenylaza-15-crown-5-ether group in the composition of the N-aryl moiety leads to a small hypsochromic shift of the peaks in the absorption and emission spectra and to a decrease in fluorescence intensity attributed to the occurrence of a nonradiative process of electron transfer from the N-aryl substituent onto the photoexcited naphthalimide chromophore. The proposed explanation for the observed spectral effects has been confirmed by the data of semi-empirical quantum-chemical calculations using the PM6 method. The complexation of crown-containing 4-pyrazolinylnaphthalimides was studied employing alkaline earth metal cations (Mg²⁺ and Ca²⁺) in acetonitrile solutions. ¹H NMR spectroscopy has revealed that the coordination of the cation occurs through the crown-ether receptor. Upon complexation, in the absorption and fluorescence spectra, a bathochromic shift of the peaks of the long-wavelength bands by 6–15 nm was observed. The binding of the Mg²⁺ cations by the benzo-15-crown-5-ether derivative of naphthalimide was accompanied by fluorescence quenching, while in the case of 4-pyrazolinylnaphthalimide containing an N-phenylaza-crown-ether moiety in the N-aryl substituent, during the complexation with the Ca²⁺ cations, fluorescence buildup was observed. The spectrofluorometric titration data were used to determine the stability constants of complexes of crown-containing naphthalimides with alkaline earth metal cations with a composition of 1: 1 in acetonitrile. These studies have shown that the synthesized compounds are promising from the standpoint of developing fluorescent sensors for cation analysis.