Rationally designed fluorescence turn-on sensors: A new design strategy based on orbital control

Herein, we explore a new strategy in the chemo-sensor field for fluorescence amplification upon binding with metal ions based on controlled participation of the nitrogen lone pair orbital. The basic architecture of the sensor entails a fluorophore, the sp² hybridized nitrogen lone pair (-C=N-), and a chelator site referred to as the control part. Though nonplanar and nonfluorescent, compound IC1 achieved pseudo planarity from binding with Zn²⁺ as indicated by the increased fluorescence signal. Its other analogue (IC2) is also planar, and unlike IC1-Zn²⁺ was fluorescent with a lack of binding affinity to metal ions. The time-dependent density functional theory (TDDFT) calculations revealed that the fluorescence amplification was due to the blocking of the nitrogen lone pair orbital; unlikely geometrical rearrangements were insignificant. This could indicate a breakthrough concept in the future design of fluorescent turn-on sensors.