Facile color tuning of thermally activated delayed fluorescence by substituted ortho-carbazole-appended triarylboron emitters

We report the facile tuning of the emission color of thermally activated delayed fluorescence (TADF) emitters based on an ortho-carbazole-appended triarylboron. A series of ortho-carbazole-appended triarylboron compounds (2–7) are prepared by introducing various electron-accepting substituents, such as phenyl, pyridyl, pyrimidyl, diphenylphosphine oxide, cyano, and dimesitylboryl groups, to the phenylene ring of the dimesitylphenylboryl (PhBMes2) acceptor moiety in the parent ortho-carbazole-appended triarylboron (CzoB, 1). The X-ray crystal structure of the cyano-substituted compound 6 confirms the twisted connectivity between the Cz and phenylene rings. All the compounds exhibit strong TADF (ΦPL = 48–93% in toluene) with large delayed portions. In particular, the emission bands gradually undergo bathochromic shifts from blue (λPL = 463 nm for 1) to greenish yellow (λPL = 532 nm for 7) depending on the electron-accepting substituents. Electrochemical studies show that the greater stabilization of the LUMO level compared to the HOMO is responsible for the red shifts of the emission. Theoretical studies further support the observed bathochromic shifts in the emission, as well as the small energy splittings (ΔEST) between the excited singlet and triplet states that afford the efficient TADF.