Long Persistent Phosphorescence of Crystalline Phenylboronic Acid Derivatives: Photophysics and a Mechanistic Study

Abstract Room‐temperature phosphorescence (RTP) of crystalline isophthalic acid (IPA) through hyperfine‐coupling‐driven (HFC‐driven) intersystem crossing in charge‐transfer (CT) complexes has been discovered recently, giving rise to a new design method for phosphorescence materials and photosensitizers. In this study, we found that crystalline phenylboronic acid (PB) derivatives also demonstrated RTP, as recorded for IPA. Magnetic‐field and spin‐isotope effects on the emission strength intensities of PBs were typical of HFC‐driven phosphorescence. p ‐Halogenated PB esters exhibited a heavy‐atom effect with a shortened emission lifetime as expected from the study of IPA. Unlike IPA, the esters of PBs also showed RTP, which facilitates a study of the structure/photophysical property relationships by exchanging ester head groups. We then addressed a question: do PB esters with a bulky head group prohibit CT formation and thus phosphorescence? As a result, a bulky PB ester with large intermolecular distances in its crystalline form showed neither CT absorptions nor phosphorescence, confirming the significance of CT formation in the HFC mechanism.