Optically Triggered Planarization of Boryl-Substituted Phenoxazine: Another Horizon of TADF Molecules and High-Performance OLEDs

We report the unprecedented dual properties of excited state structural planarization and thermally activated delayed fluorescence (TADF) of 10-dimesitylboryl phenoxazine, i.e., PXZBM. Bearing a nonplanar phenoxazine moiety, PXZBM shows the lowest lying absorption onset at similar to 390 nm in nonpolar solvents such as cyclohexane but reveals an anomalously large Stokes-shifted (similar to 14 500 cm(-1)) emission maximized at 595 nm. In sharp contrast, when a phenylene spacer is added between phenoxazine and dimesitylboryl moieties of PXZBM, the 10-(4-dimesitylborylphenyl)phenoxazine PXZPBM in cyclohexane reveals a much blue-shifted emission at 470 nm despite its red-shifted absorption maximized at 420 nm (cf. PXZBM). The emission of PXZBM further reveals solvent polarity dependence, being red-shifted from 595 nm in cyclohexane to 645 nm in CH2Cl2. For rationalization, the steric hindrance between phenoxazine and the dimesitylboryl unit in PXZBM caused a puckered arrangement of phenoxazine at the ground state. Upon electronic excitation, as supported by the femtosecond early relaxation dynamics, spectral-temporal evolution and energetics calculated along the reaction potential energy surfaces, the diminution of N -> B electron transfer reduces pi-conjugation and elongates the N-B bond length, inducing the fast phenoxazine planarization with a time constant of 890 +/- 100 fs. The associated charge-transfer reaction from phenoxazine (donor) to dimesitylboryl unit (acceptor) results in a further red-shifted emission in polar solvents. In stark contrast, PXZPBM shows a planar phenoxazine and undergoes excited-state charge transfer only. Despite the distinct difference in excited-state relaxation dynamics, both PXZBM and PXZPBM exhibit efficient TADF capable of producing highly efficient orange and green organic light emitting diodes with peak efficiencies of 10.9% (30.3 cd A(-1) and 18.7 lm W-1) and 22.6% (67.7 cd A(-1) and 50.0 lm W-1).