A Naphthalimide‐Based Cd 2+ Fluorescent Probe with Carbamoylmethyl Groups Working as Chelators and PET‐Promoters under Neutral Conditions

We have developed a novel naphthalimide‐based Cd 2+ fluorescent probe ( 1 ), featuring almost no background response, high sensitivity and selectivity toward Cd 2+ through its high association constant [ K =(2.10±0.423)×10 6 ], 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 Cd 2+ 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 Cd 2+ 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 Cd 2+ probes under neutral conditions.