Exploiting the deprotonation mechanism for the design of ratiometric and colorimetric Zn2+ fluorescent chemosensor with a large red-shift in emission

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 Zn2+ in aqueous acetonitrile solution at pH 7.0. Probe 1 showed absorption at 451 nm and a strong fluorescence emission at 537 nm (Phi(F)=0.33). The capture of Zn2+ by the receptor resulted in the deprotonation of the secondary amine conjugated to 1,8-naphthatimide so that the electron-donating ability of the N atom would be greatly enhanced; thus probe 1 showed a 56 run red-shift in absorption (507 nm) and fluorescence spectra (593 nm, Phi(F)=0.14), respectively, from which one could sense Zn2+ 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 Cu2+, Co2+, and Ni2+. The addition of other metal ions such as Li+, Na+, K+, Mg2+, Ca2+, Fe3+, Mn2+, Al+, Cd2+, Hg2+, Ag+, and Pb2+ produced a nominal change in the optical properties of I due to their low affinity to probe 1. This means that probe 1 has a very high fluorescent imaging selectivity to Zn2+ among metal ions. (c) 2006 Elsevier Ltd. All rights reserved.