A Naphthalimide-Based Cd2+ Fluorescent Probe with Carbamoylmethyl Groups Working as Chelators and PET-Promoters under Neutral Conditions

We have developed a novel naphthalimide-based Cd2+ fluorescent probe (1), featuring almost no background response, high sensitivity and selectivity toward Cd2+ through its high association constant [K=(2.10±0.423)×106], and a practical working pH range. Membrane-permeability was conferred on 1 by replacing the imide and amide substituents with n-butyl groups, and hence the derivative (4) has found practical utility on fluorescent imaging of Cd2+ in HeLa cells. Comparison of fluorescent properties between various compounds derived from 1 has demonstrated that the carbamoylmethyl groups in 1 function not only as Cd2+ chelators but also as promoters for photoinduced electron transfer (PET) by lowering the basicity of the two tertiary amino groups. As a result, 1 and 4 exhibited highly practical performance as Cd2+ probes under neutral conditions.