Photophysical Properties of Pyrene‐Substituted nido‐Carborate Anion and the Change of Luminescence Species by Counter Cations in Solution and Solid States

Abstract This study reports unique luminescent properties of pyrene‐substituted nido‐carborate anion (PYnCA) with two counter cations, tetrabutylammonium (NBu₄⁺) and tetraphenylphosphonium (PPh₄⁺). It is revealed that PYnCA shows twisted intramolecular charge transfer (TICT) emission by the change of the dihedral angle between pyrene and the C_cageC_cage bond in the carborane unit in the excited state. Although the structure of nCA is almost same as that of ortho‐carborane (oC), the photophysical processes of both pyrene‐substituted compounds are completely different. The study also succeeds in modulating the luminescence species by counter cations in both solution and solid states. To the best of the current knowledge, this is the first report that anion‐based luminescent materials can show the different luminescent species not only in solution but also in the mechanochromic behaviors depending on the type of counter cations. This fundamental research on nCA demonstrates anion‐based emitters as a new platform for stimuli‐responsive materials.