Systematic Tuning of Acridine ICT: Multi‐Phase Polarity Sensitivity and Tailored Applications in Encryption and Sensing

ABSTRACT Precise regulation of the electronic structures of functional molecules remains a key challenge in the development of multifunctional materials. Herein, we report the systematic tuning of intramolecular charge transfer (ICT) characteristics in a series of acridine derivatives (ACR‐CHO, ACR‐SA, and ACR‐FCN) by introducing electron‐withdrawing groups of varying strengths. Elevating the electron‐withdrawing ability narrows the HOMO‐LUMO gap, enhances ICT, and endows the three fluorophores with distinct solvatochromic and vapochromic behaviors. Strikingly, these fluorophores also exhibit solid‐state chromism with high fluorescence quantum yields when doped into polymer matrices of different polarities, which effectively suppresses aggregation‐caused quenching. Such multiphase polarity sensitivity enables a broad emission range spanning 450 ∼ 660 nm, facilitating versatile applications in smart sensing, multicolor displays, and advanced information encryption. Notably, ACR‐FCN, bearing malononitrile units as acceptor, displays a unique nonmonotonic response toward biogenic amines due to the alternation of its ICT properties. This allows visual, real‐time monitoring of food freshness via a portable sensor, with a naked‐eye detection limit of 1 ppm for putrescine. This work elucidates the critical role of acceptor modulation in tuning ICT properties and establishes a comprehensive workflow from molecular design to multifunctional applications.