Structural\nRigidification Strategy Based on Self-Assembly\nEnabled Reversible Excited-State Conversion of Iridium(III) Complexes\nfor Multiple-Stimulus-Responsive Data Encryption

Stimulus-responsive\nchromic materials exhibit color-switching properties\nunder specific external stimuli and have been widely used in various\nfields. Transition-metal complexes show great potential applications\nas promising candidates for stimulus-responsive chromic materials,\nas their excited states not only depend on the chemical composition\nbut are also affected by the intermolecular stacking modes. Owing\nto the intrinsic difficulty in the ordered stacking of the octahedral\nconfiguration, changing the stacking modes of iridium(III) complexes\nfor multiple-stimulus responsiveness remains a significant challenge.\nIn this work, we propose a structural rigidification strategy based\non self-assembly to reversibly regulate the excited states of iridium(III)\ncomplexes, therefore achieving color switch under different stimulus\nconditions. We prepare cationic iridium(III) complexes by using tetrakis(perfluorophenyl)-borate\n([B(PhF<sub>5</sub>)<sub>4</sub>]<sup>−</sup>) as the counterion,\nwhose matching tetrahedral configuration and electron-deficient aromaticity\nenables polar-π interaction with the octahedral iridium(III)\ncations, inducing self-assembly to form structural rigidification.\nThe structural rigidity restricts the large conformational changes\nof the metal-to-ligand charge transfer (<sup>3</sup>MLCT) excited\nstate, and facilitates the conversion from the <sup>3</sup>MLCT to\nthe ligand-center (<sup>3</sup>LC) excited state in aggregated states.\nThe excited-state conversion results in a 54 nm blue shift (from yellow\nto sky blue) in the photoluminescence spectra. As a result, we report\na series of cationic iridium(III) complexes with different responses\nto low temperature, vapor fuming, and mechanical force, therefore\nachieving multiple-stimulus-responsive data encryption. Our work provides\na novel strategy to achieve ordered stacking of octahedral complexes,\nshows a deeper understanding of the photophysical processes of transition-metal\ncomplexes, and offers a new perspective to develop multiple-stimulus-responsive\nchromic materials.