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Conformational Effect of Catechol-Terephthalonitrile Emitters Leading to Ambient Violet Phosphorescence
Organic
ambient violet phosphorescent (AVP) materials are of great
interest due to their involvement of high energy and longer-lived
triplet excitons. Here, we show three fused ring functionalized donor–acceptor–donor
(D–A–D/D–A–D′) emitters (BPT1–BPT3), in which two catechol-based donors (3,4-dihydroxybenzophenone,
catechol, or 3,5-ditert-butylcatechol) are covalently
fused to the terephthalonitrile acceptor via four O–C single
bonds. Spectroscopic analysis revealed that all the molecules show
AVP (∼390–394 nm, τAVP = 73–101
μs) with phosphorescence quantum yields (ϕP) of 1.8–27.4% due to low singlet–triplet gaps (0.036–0.046
eV) and conformational effects. BPT3 with bulky tert-butyl groups increases AVP (Ï•P = 27.4%). Quantum
chemistry calculations reveal flat (F1) and twisted (F2) conformers
(ground state) with a low energy difference (∼4–5 kcal/mol)
for all molecules; the F1 conformer is responsible for efficient AVP,
while weak blue thermally activated delayed fluorescence with longer-lived
delayed components is realized from the F2 conformer. This approach
may provide important clues for the design of high-energy organic
phosphorescent materials