Structural Rigidification Strategy Based on Self-Assembly Enabled Reversible Excited-State Conversion of Iridium(III) Complexes for Multiple-Stimulus-Responsive Data Encryption

Abstract

Stimulus-responsive chromic materials exhibit color-switching properties under specific external stimuli and have been widely used in various fields. Transition-metal complexes show great potential applications as promising candidates for stimulus-responsive chromic materials, as their excited states not only depend on the chemical composition but are also affected by the intermolecular stacking modes. Owing to the intrinsic difficulty in the ordered stacking of the octahedral configuration, changing the stacking modes of iridium(III) complexes for multiple-stimulus responsiveness remains a significant challenge. In this work, we propose a structural rigidification strategy based on self-assembly to reversibly regulate the excited states of iridium(III) complexes, therefore achieving color switch under different stimulus conditions. We prepare cationic iridium(III) complexes by using tetrakis(perfluorophenyl)-borate ([B(PhF₅)₄]⁻) as the counterion, whose matching tetrahedral configuration and electron-deficient aromaticity enables polar-π interaction with the octahedral iridium(III) cations, inducing self-assembly to form structural rigidification. The structural rigidity restricts the large conformational changes of the metal-to-ligand charge transfer (³MLCT) excited state, and facilitates the conversion from the ³MLCT to the ligand-center (³LC) excited state in aggregated states. The excited-state conversion results in a 54 nm blue shift (from yellow to sky blue) in the photoluminescence spectra. As a result, we report a series of cationic iridium(III) complexes with different responses to low temperature, vapor fuming, and mechanical force, therefore achieving multiple-stimulus-responsive data encryption. Our work provides a novel strategy to achieve ordered stacking of octahedral complexes, shows a deeper understanding of the photophysical processes of transition-metal complexes, and offers a new perspective to develop multiple-stimulus-responsive chromic materials.