Exploiting the deprotonation mechanism for the design of ratiometric and colorimetric Zn 2+ fluorescent chemosensor with a large red-shift in emission

Abstract

The design, synthesis, and photophysical evaluation of a new naphthalimide-based fluorescent chemosensor, N-butyl-4-[di-(2-picolyl)amino]-5-(2-picolyl)amino-1,8-naphthalimide ( 1), were described for the detection of Zn 2+ in aqueous acetonitrile solution at pH 7.0. Probe 1 showed absorption at 451 nm and a strong fluorescence emission at 537 nm ( Φ F=0.33). The capture of Zn 2+ by the receptor resulted in the deprotonation of the secondary amine conjugated to 1,8-naphthalimide so that the electron-donating ability of the N atom would be greatly enhanced; thus probe 1 showed a 56 nm red-shift in absorption (507 nm) and fluorescence spectra (593 nm, Φ F=0.14), respectively, from which one could sense Zn 2+ ratiometrically and colorimetrically. The deprotonated complex, [( 1-H)/Zn] +, was calculated at m/ z 619.1800 and measured at m/ z 618.9890. In contrast to these results, the emission of 1 was thoroughly quenched by Cu 2+, Co 2+, and Ni 2+. The addition of other metal ions such as Li +, Na +, K +, Mg 2+, Ca 2+, Fe 3+, Mn 2+, Al 3+, Cd 2+, Hg 2+, Ag +, and Pb 2+ produced a nominal change in the optical properties of 1 due to their low affinity to probe 1. This means that probe 1 has a very high fluorescent imaging selectivity to Zn 2+ among metal ions.