Thermally Activated Delayed Fluorescence and Room-Temperature Phosphorescence in Asymmetric Phenoxazine-Quinoline (D2–A) Conjugates and Dual Electroluminescence

Triplet
energy harvesting via either thermally activated delayed
fluorescence (TADF) or room-temperature phosphorescence (RTP) from
pure organic systems has attracted great attention in the field of
organic light-emitting diodes, sensing, and bioimaging. However, the
realization of dual electroluminescence via TADF and RTP in single
molecules remains elusive. Herein, we report two phenoxazine-quinoline
conjugates (DPQ and DPQM) in which two phenoxazine
donors are covalently attached to the 6,8-positions of 2,4-diphenylquinoline
and/or 7-methyl-2,4-diphenylquinoline acceptors. Experimental and
quantum chemistry calculations combining reference compounds (o-PQP, p-PQP, Phox, and QPP) reveal that both conjugates
show TADF (with different rate constants of reverse intersystem crossing, krISC = 0.43–1.30 × 106 s–1) via reverse intersystem crossing from the
charge transfer triplet (3CT) to singlet (1CT)
states mediated by vibronic coupling among 1CT, local triplet
(3LE), and 3CT states due to close energy gaps.
Further, RTP with quantum yields (ϕP) of ca. 21–24%
features was also observed due to the radiative decay of 3LE states. Phosphorescence measurements of DPQM at low
temperatures (T = 77, 10 K) ensure a distinct zero-field
splitting of T1(CT) into substates. Both
compounds showed dual electroluminescence with external quantum efficiencies
of ca. 11–12% due to the efficient triplet harvesting from
both TADF and RTP channels.