ABSTRACT
Three new donor-acceptor molecular glasses were designed and synthesized linking 1,8-naphthalimide and triphenylamino groups though the different bridges. The comprehensive characterization of the compounds was carried out using theoretical and experimental approaches. The compounds showed efficient orange-red emission in solid state with photoluminescence intensity maxima in the range of 584-654 nm. The compounds showed extremely high thermal stability with 5 % weight loss temperatures up to 477 °C. They formed molecular glasses with glass-transition temperatures in the range of 161-186 °C. The fabricated organic light-emitting diodes (OLEDs) based on the developed emitters and conventional host showed maximum external quantum efficiency of 2.5 % in the best case. This value was increased up to 4.7 % by the usage of the host exhibiting thermally activated delayed fluorescence (TADF). OLED containing the TADF host displayed orange emission peaking at 589 nm with colour coordinates x of 0.53 and y of 0.45 combined with power efficiency of 6.7 lm·W-1 and current efficiency of 11.8 cd·A-1. Time-resolved electroluminescence technique was used to study the effect of the different guest-host systems on exciton utilization efficiency in devices based on the same emitter exhibiting prompt fluorescence and on the conventional or TADF hosts.
ABSTRACT
Aiming to design bipolar organic semiconductors with high electron mobility and efficient red thermally activated delayed fluorescence (TADF), three donor-acceptor compounds were designed and synthesized selecting 1,8-naphthalimide as an acceptor and phenoxazine, 3,7-di-tert-butylphenothiazine or 2,7-di-tert-butyldimethyl-9,10-dihydroacridine as donor moieties. Aggregation induced emission enhancement was detected for the compounds causing efficient TADF in the solid-state. Photoluminescence quantum yields up to 77% were observed for the films of the compounds doped in a host. The compounds exhibited small singlet-triplet splitting (0.03-0.05 eV), and high reverse intersystem crossing rates of 2.08 × 105-1.13 × 106 s-1. The compounds were characterized by satisfactory hole and electron-injecting properties with ionization potentials of 5.72-5.83 eV and electron affinities of 2.79-2.91 eV. Bipolar charge transport was revealed by time of flight measurements. Electron transport with low dispersity and mobilities exceeding 2 × 10-3 cm2 V-1 s-1 was observed at an electric field of 4.6 × 105 V cm-1. The compounds were used as emitters in red electroluminescent devices, which showed maximum external quantum efficiencies up to 8.2%. Utilization of host-guest systems as light-emitting materials with hosts preferably transporting holes and TADF guests which preferably transport electrons allowed maximum efficiencies to be achieved at a practical brightness of 700-2200 cd m-2. DFT calculations of the geometry, electronic structure, absorption and photoluminescence spectra of all compounds were carried out to prove the conclusions drawn from the experiment. The results of the calculations clearly show that the first excited state for all compounds is the intramolecular charge transfer state. Quantitative analysis of the separation degree of electronic density during excitation allows the observed dependence of the blue shift value in the absorption and emission spectra on the increasing polarity of the solvent to be explained.
ABSTRACT
Four aryl-substituted acridan derivatives were designed, synthesized and characterized as electroactive materials for organic light emitting diodes based on emitters exhibiting thermally activated delayed fluorescence. These compounds possessed relatively high thermal stability with glass-transition temperatures being in the range of 79-97 °C. The compounds showed oxidation bands arising from acridanyl groups in the range of 0.31-038 V. Ionization potentials of the solid films ranged from 5.39 to 5.62 eV. The developed materials were characterized by triplet energies higher than 2.5 eV. The layer of 10-ethyl-9,9-dimethyl-2,7-di(naphthalen-1-yl)-9,10-dihydroacridine demonstrated hole mobilities reaching10-3 cm2/V·s at electric fields higher then ca. 2.5 × 105 V/cm. The selected compounds were used as hosts in electroluminescent devices which demonstrated maximum external quantum efficiencies up to 3.2%.
