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Effect of a Pendant Acceptor on Thermally Activated Delayed Fluorescence Properties of Conjugated Polymers with Backbone-Donor/Pendant-Acceptor Architecture.

Three sets of conjugated polymers with backbone-donor/pendant-acceptor architectures, named PCzA3PyB, PCzAB2Py, and PCzAB3Py, are designed and synthesized. The three isomeric benzoylpyridine-based pendant acceptor groups are 6-benzoylpyridin-3-yl (3PyB), 4-((pyridin-2-yl)carbonyl)phenyl (B2Py) and 4-((pyridin-3-yl)carbonyl)phenyl (B3Py), whereas the identical backbone consists of 3,6-carbazolyl and 2,7-acridinyl rings. One acridine ring and each acceptor group constitute a definite thermally activated delayed fluorescence (TADF) unit, incorporated into the main chain of the polymers through the 2,7-position of the acridine ring with the varied content. All of the polymers display legible TADF features with a short microsecond-scale delayed lifetime (0.56-1.62 μs) and a small singlet/triplet energy gap (0.10-0.19 eV). Progressively redshifted emissions are observed in the order PCzAB3Py, PCzA3PyB, and PCzAB2Py owing to the different substitution patterns of the pyridyl group. Photoluminescence quantum yields can be improved by regulating the molar content of the TADF unit in the range 0.5-50 %. The non-doped organic light-emitting devices (OLEDs) fabricated by solution-processing technology emit yellow-green to orange light. The polymers with 5 mol % of the TADF unit exhibit excellent comprehensive electroluminescence performance, in which PCzAB2Py5 achieves a maximum external quantum efficiency (EQE) of 11.9 %, low turn-on voltage of 3.0 V, yellow emission with a wavelength of 573 nm and slow roll-off with EQE of 11.6 % at a luminance of 1000 cd m-2 and driving voltage of 5.5 V.