Thermally activated delayed fluorescence emitters showing wide-range near-infrared piezochromism and their use in deep-red OLEDs

Organic small molecules exhibiting both thermally activated delayed fluorescence (TADF) and wide-ranging piezochromism (Delta lambda > 150 nm) in the near-infrared region have rarely been reported in the literature. We present three emitters MeTPA-BQ, tBuTPA-BQ and TPPA-BQ based on a hybrid acceptor, benzo[g]quinoxaline-5,10-dione, that emit via TADF, having photoluminescence quantum yields, Phi(PL), of 39-42% at photoluminescence (PL) maxima, lambda(PL), of 625-670 nm in 2 wt% doped films in 4,4 '-bis(N-carbazolyl)-1,1 '-biphenyl (CBP). Despite their similar chemical structures, the PL properties in the crystalline states of MeTPA-BQ (lambda(em) = 735 nm, Phi(PL) = 2%) and tBuTPA-BQ (lambda(em) = 657 nm, Phi(PL) = 11%) are significantly different. Further, compounds tBuTPA-BQ and TPPA-BQ showed a significant PL shift of similar to 98 and similar to 165 nm upon grinding of the crystalline samples, respectively. Deep-red organic light-emitting diodes with MeTPA-BQ and tBuTPA-BQ were also fabricated, which showed maximum external quantum efficiencies, EQE(max), of 10.1% (lambda(EL) = 650 nm) and 8.5% (lambda(EL) = 670 nm), respectively.