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 ∼390 nm in nonpolar solvents such as cyclohexane but reveals an anomalously large Stokes-shifted (∼14 500 cm) 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 CHCl. 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 π-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 and 18.7 lm W) and 22.6% (67.7 cd A and 50.0 lm W).

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We report here the structure-property correlations of a series of anthracene/pyrene derivatives grafted on a rigid, planar, and well conjugated endo-cyclic alkenyl lactone (thienodipyrandione) that exhibited aggregation induced emission. Unlike typical vinylene based solid-state fluorescence materials, rigid thienodipyrandione moieties can bestow a higher order of conformational twist as well as enhanced intermolecular interactions by C-H/O interactions due to the presence of a carbonyl group that leads to enhanced thermal stability. Compounds TDAn and TDAnPh exhibit distinct piezofluorochromic properties in which emission colors can be switched by grinding and heating. Based on a variety of spectroscopic techniques including XRD, DSC, and NMR, the observed piezochromic emission is attributed to the result of phase transition from structural crystalline (order) to amorphous (disorder) state or vice versa. Most uniquely, one of the anthracene derivatives, TDAn, displayed specific acidofluorochromism whereas the other congener, TDAnPh, did not, even though they are structurally very similar. Crystal structure analysis revealed that a combination of various secondary interactions such as C-H/O, p-p and C-H/p interactions all play key roles in the supramolecular assemblies to display such distinctive features. The current study has disclosed a class of new stimuli-responsive materials with great potential in applications for micro-environmental sensing, optical switching and recording.

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