A hybridized local and charge transfer excited state for solution-processed non-doped green electroluminescence based on oligo(-phenyleneethynylene)

We herein report a new highly efficient green emissive hot-exciton molecule, 1,4-bis((4′-diphenylamino-3-cyano-[1,1′-biphenyl]-4-yl)ethynyl)-2,5-bis(2-ethylhexyloxy)benzene ( 2EHO-TPA-CNPE ) that consists of an extended D′-π′-A-π-D-π-A-π′-D′ molecular π-system with diphenylamino end units (D′) and ethynylene/phenylene spacers (π/π′). The new molecule exhibits high photoluminescence (PL) quantum efficiencies ( Φ PL = 0.95 (solution) and 0.45 (spin-coated neat thin-film)), and a strong PL solvatochromic behavior revealing significant changes in excited state energies/characteristics (locally excited (LE) → hybridized local and charge transfer (HLCT) → charge-transfer (CT)) depending on solvent polarity. Highly efficient (radiative exciton yield ( η r ) = 50-59% > 25%) green-emitting OLEDs were fabricated in a conventional device architecture by employing (non-)doped thin-films reaching a maximum current efficiency (CE max ) of 12.0 cd A −1 and a maximum external quantum efficiency (EQE max ) of 5.5%. The emission profile of the non-doped OLED has CIE 1976 ( u ′, v ′) chromaticity coordinates of (0.10, 0.55) corresponding to a night vision imaging system (NVIS) compatible Green A region. 2EHO-TPA-CNPE -based OLED devices of industrial relevance were also fabricated by ink-jet printing the emissive layer and by fabricating an inverted architecture, which possessed respectable device performances of 2.4-6.1 cd A −1 . The solid-state solvation effect in OLED devices yields HLCT electronic behavior resulting in high η r 's, which is confirmed by TDDFT to originate from energetically/spatially favorable reverse intersystem crossings (RISCs) (T 2/3 → S 1 ). As a unique observation, delayed fluorescence due to this RISC was evident in the PL decay lifetime measurement with a ns-scale lifetime of ∼10 ns. These results clearly allow a better understanding of the structure-photophysical property-electroluminescence relationships in this new class of oligo( p -phenyleneethynylene)-based hot-exciton molecules, and it could open up new opportunities for high-performance solution-processed optoelectronic/sensing applications. A novel oligo( p -phenyleneethynylene)-based hot-exciton molecule with hybridized local and charge transfer (HLCT) excited states was developed to yield high radiative exciton yields in OLEDs.