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The overwhelming majority of efficient organic light‐emitting diodes (OLEDs) are based on amorphous emitting layers. The devices with crystalline emitters usually show significantly worse performance. It is demonstrated that controlled thermal treatments of phenoxazine‐substituted acridones may lead to a spectacular improvement of photo‐ and electroluminescence of these luminophores. The three acridone derivatives studied, namely 2‐phenoxazine‐N‐hexylacridone (PHhA), 2,7‐bis‐(phenoxazine)‐N‐hexylacridone (bPHhA), and 2,7‐bis[3‐(phenoxazine)phenyl]‐N‐hexylacridone (PHPhhA), are characterized by relatively narrow photoluminescence spectra and photoluminescence quantum yields approaching 100%. These spectra can be reversibly tuned through appropriate thermal treatment of the deposited layers, undergoing shifts up to 92 nm upon transformation from amorphous to crystalline states. Strong thermally activated delayed fluorescence (TADF) observed for these compounds is manifested by a significantly higher reverse intersystem crossing rate of 1.92×106 s−1 as compared to the intersystem crossing rate of 5.68×105 s−1. These improvements are attributed to the annealing‐induced conformational equilibration due to crystallisation in combination with hosting. Adequacy of the proposed concept is demonstrated by the fabrication of devices exhibiting external quantum efficiencies reaching 20.7% – a record value for OLEDs with crystalline emitting layers. The presented approach of annealing‐induced conformational equilibration can lead to improved TADF properties for other fluorescent organic emitters presently considered inefficient. [ABSTRACT FROM AUTHOR]

Three new derivatives of carbazole were synthesized, namely two isomers disubstituted with diarylaminophenyl groups in 2,7- and 3,6- positions ( C1 and C2 ) and diarylaminophenyl symmetrically disubstituted with carbazole C3 . All three compounds showed similar electrochemical behavior with one reversible redox couple, originating from the oxidation of the diarylaminophenyl units to radical cations, followed by an irreversible anodic peak associated with the oxidation of the carbazole unit. The electrochemical results were supported by DFT calculations which predicted the same order of ionization potential values as experimentally found. Absorption and emission bands in the spectra of C1 and C3 were bathochromically shifted as compared to the corresponding bands in C2 , again consistent with DFT calculations. C1 showed a very high value of the photoluminescence quantum yield in THF solution, i.e. 90%. Moreover, the position of the emission band turned out to be solvent dependent and underwent a hypsochromic shift in toluene with simultaneous decrease of the photoluminescence quantum yield value to 68%. Blue light emitting C1 was used as an electroluminophore in a simple, guest/host-type single-layer light emitting diodes exhibiting luminance exciding 1000 cd/m 2 and luminous efficiency of 0.7 cd/A. [ABSTRACT FROM AUTHOR]