Elaboration of the appropriate host materials proved to be not less important for the fabrication of a highly efficient OLED than the design of emitters. In the present work, we show how by simple ...variation of molecular structure both blue emitters exhibiting delayed fluorescence and ambipolar high triplet energy hosts can be obtained. The compounds with a para-junction revealed higher thermal stability (T ID up to 480 °C), lower ionization potentials (5.51–5.60 eV), exclusively hole transport, and higher photoluminescence quantum efficiencies (0.90–0.97). Meta-linkage leads to ambipolar charge transport and higher triplet energies (2.82 eV). Introduction of the accepting nitrile groups in the para-position induces intensive delayed fluorescence via a triplet–triplet annihilation up-conversion mechanism. By utilization of the para-substituted derivative as an emitter and the meta-substituted isomer as the host, a deep-blue OLED with the external quantum efficiency of 14.1% was fabricated.
Carbazolyl dicyanobenzene (CDCB) derivates exhibiting thermally activated delayed fluorescence (TADF) have shown themselves to be excellent photocatalysts over recent years, particularly 4CzIPN, ...although investigation into organic TADF compounds as photocatalysts outside of the CDCB group has been limited. Herein, we report an alternative donor–acceptor TADF structure, 9,9′-(sulfonylbis(pyrimidine-5,2-diyl))bis(3,6-di-tert-butyl-9H-carbazole), pDTCz-DPmS, for use as a photocatalyst (PC). A comparison of the electrochemical and photophysical properties of pDTCz-DPmS with 4CzIPN in a range of solvents identifies the former as a better ground state reducing agent and photoreductant, while both exhibit similar oxidation capabilities in the ground and excited state. The increased conjugation of pDTCz-DPmS relative to 4CzIPN presents a more intense CT band in the UV–vis absorption spectrum, aiding in the light absorption of this molecule. Prompt and delayed emission lifetimes are observed for pDTCz-DPmS, confirming the TADF nature, both of which are sufficiently long-lived to participate in productive photochemistry. These combined properties make pDTCz-DPmS useful in photocatalysis reactions, covering a range of photoredox oxidative and reductive quenching reactions, as well as those involving a dual Ni(II) cocatalyst, alongside energy transfer processes. The higher triplet energy and increased photostability of pDTCz-DPmS compared with 4CzIPN were found to be advantages of this organic PC.
We present a study of two isomeric thermally activated delayed fluorescence (TADF) emitters 9,9′-(sulfonylbis(pyrimidine-5,2-diyl))bis(3,6-di-tert-butyl-9H-carbazole) (pDTCz-DPmS) and ...9,9′-(sulfonylbis(pyrazine-5,2-diyl))bis(3,6-di-tert-butyl-9H-carbazole) (pDTCz-DPzS). The use of pyrimidine and pyrazine as bridging units between the electron donor and acceptor moieties is found to be advantageous compared to the phenyl- (pDTCz-DPS) and pyridine-based analogues (pDTCz-3DPyS and pDTCz-2DPyS). Conformational modulation of the donor groups as a function of the bridge results in high photoluminescence quantum yields (ΦPL > 68%) and small energy gaps between singlet and triplet excited states (ΔE ST < 160 meV). OLEDs using pDTCz-DPmS and pDTCz-DPzS as emitters exhibit blue and green electroluminescence, respectively, with higher maximum external quantum efficiencies (EQEmax of 14% and 18%, respectively) and a reduced efficiency roll-off as compared to the reference devices using pDTCz-DPS, pDTCz-3DPyS, and pDTCz-2DPyS as the emitters. Our results provide a more complete understanding on the impact of the bridge structure in D-A-D TADF systems on the optoelectronic properties of the emitter and how the balance between color purity and EQE in the devices can be controlled, advancing the design strategies for TADF emitters.
Two new deep-blue narrowband multi-resonant emitters, 1B-DTACrs and 2B-DTACrs, one of which shows thermally activated delayed fluorescence (TADF), based on boron, nitrogen, and oxygen doped ...nanographenes are reported. Devices based on 2B-DTACrs showed an EQE max of 14.8% and CIE coordinates of (0.150, 0.044), which are very close to the BT.2020 requirement for blue pixels.
Two new asymetric D–A–D′ type compounds containing benzophenone and diphenylsulfone as electron accepting core, and acridan and thianthrene as electron donating moieties were design as ...multifunctional materials. The influence of different acceptor and donor units on thermally activated delayed fluorescence and room temperature phosphorescence was investigated and discussed. The combination of different donors with benzophenone unit resulted in the compound with effective frontier orbital separation, as well as the sustainment of high radiative rate. These statements are both experimentally and theoretically supported using time-resolved photoluminescence spectroscopy and DFT calculations. Deoxygenation experiment revealed higher triplet exciton contribution in the emission of benzophenone derivative, as compared to that of diphenylsulfone-based compound. The films of molecular mixtures of these emitters with 1,3-bis(N-carbazolyl)benzene demonstrated temperature-dependent photoluminescence. The estimated energy gaps between the lowest singlet and triplet states were found to be 0.06 eV and 0.27 eV for benzophenone and diphenylsulfone derivatives, respectively. Due to the relatively high singlet-triplet energy splitting of the diphenylsulfone-based compound, triplet harvesting was detected not only via temperature-stimulated fluorescence but also via room temperature phosphorescence. Displaying effect of triplet harvesting on efficiency of all-organic blue and green OLEDs based on thianthrene and acridan-substituted benzophenone or diphenylsulfone derivatives demonstrated very different maximum external quantum efficiencies of 1.4 and 22.2% respectively.
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•Synthesis and influence of different acceptor units on photophysical properties.•Demonstration of effect of triplet harvesting via TADF and RTP on efficiency of OLEDs.•Fabrication of TADF OLEDs with maximum external quantum efficiency reaching 22.2%.
In this work, we report on the synthesis and photophysical investigation of a new star-shaped triazine-carbazole derivative 2,4,6-tris(3-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)-1,3,5-triazine. ...Comparative study of the photophysical properties of the newly synthesized emitter along with its para-substituted isomer 2,4,6-tris(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)-1,3,5-triazine was performed. While para-linkage caused higher oscillator strength of the lowest energy absorption band and high fluorescence quantum yield, the meta-linkage resulted in stronger charge transfer character as well as higher triplet energy. Delayed emission of meta-isomer was found to be 3 orders of magnitude more intense than that of para-isomer. Temperature dependent measurements of meta-isomer confirmed the thermally activated delayed fluorescence origin of its delayed fluorescence with the activation energy of 0.07 eV. Organic light emitting diode containing this emitter dispersed in bis2-(diphenylphosphino)phenyl ether oxide with emission spectrum peak at 475 nm was fabricated. Commission Internationale de l’Éclairage color coordinates corresponded to a sky-blue emission color (0.16, 0.23). The turn-on voltage of the electroluminescent device was found to be in the range of 5–6 V with a maximum external quantum efficiency of 9.5%. These results confirm the importance of the linking pattern between donor and acceptor moieties in the molecular design of thermally activated delayed fluorescence emitters.
Excited state dynamics of trinary star-shaped dendritic compounds with triphenylamine arms and different cores were studied by means of time-resolved fluorescence and transient absorption. Under ...optical excitation, nonpolar C 3 symmetry molecules form polar excited states localized on one of the molecular substituents. Conformational excited state stabilization of molecules with an electron-accepting core causes a formation of twisted internal charge transfer (TICT) states in polar solvents. A low transition dipole moment from TICT state to the ground state causes very weak fluorescence of those compounds and strong dependence on the solvent polarity. The compound formed from the triphenylamine central core and identical arms also experiences excited state twisting, however, weakly sensitive to the solvent polarity.
The development of high-performance solution-processed red organic light-emitting diodes (OLEDs) remains a challenge, particularly in terms of maintaining efficiency at high luminance. Here, we ...designed and synthesized four novel orange-red thermally activated delayed fluorescence (TADF) dendrimers that are solution-processable:
2GCzBP
,
2DPACzBP
,
2FBP2GCz
and
2FBP2DPACz
. We systematically investigated the effect of substitution position and strength of donors on the optoelectronic properties. The reverse intersystem crossing rate constant (
k
RISC
) of the emitters having donors substituted at positions 11 and 12 of the dibenzo
a,c
phenazine (BP) is more than 10-times faster than that of compounds substituted having donors substituted at positions 3 and 6. Compound
2DPACzBP
, containing stronger donors than
2GCzBP
, exhibits a red-shifted emission and smaller singlet-triplet energy splitting, Δ
E
ST
, of 0.01 eV. The solution-processed OLED with 10 wt%
2DPACzBP
doped in mCP emitted at 640 nm and showed a maximum external quantum efficiency (EQE
max
) of 7.8%, which was effectively maintained out to a luminance of 1,000 cd m
−2
. Such a device∙s performance at relevant display luminance is among the highest for solution-processed red TADF OLEDs. The efficiency of the devices was improved significantly by using 4CzIPN as an assistant dopant in a hyperfluorescence (HF) configuration, where the
2DPACzBP
HF device shows an EQE
max
of 20.0% at
λ
EL
of 605 nm and remains high at 11.8% at a luminance of 1,000 cd m
−2
, which makes this device one of the highest efficiency orange-to-red HF SP-OLEDs to date.
We report on the properties of the three positional isomers of (2,7-di-
tert
-butyl-9,9-dimethylacridin-10(9
H
)-yl)benzonitrile, which are found to have comparable donor steric environments and ...donor-acceptor dihedral angles. An unexpected intramolecular dipole interaction imparts a unique molecular geometry to the
ortho
-linked isomer, while comparison of the
meta
- and
para
-isomers uncovers how positional differences in acceptor strengths (a consequence of differences in aromatic π-system electron density) lead to very different triplet harvesting and emission properties. These positional-isomer effects on TADF follow the well-known aromatic directing rules from organic synthesis, in keeping with their common origin arising from contributions of multiple electronic resonance structures. Understanding these positional effects and methods of dihedral control is critical to the future design of efficient TADF emitters.
Rather than donor-acceptor dihedral angles, the TADF performance of DMAC-BZN positional isomers is instead controlled by differences in acceptor strength arising from π-system electron density - along with a through-space dipole interaction.
We designed and synthesized two new ionic thermally activated delayed fluorescent (TADF) emitters that are charged analogues of a known multiresonant TADF (MR-TADF) compound,
DiKTa
. The emission of ...the charged derivatives is red-shifted compared to the parent compound. For instance,
DiKTa-OBuIm
emits in the green (λ
PL
= 499 nm, 1 wt % in mCP) while
DiKTa-DPA-OBuIm
emits in the red (λ
PL
= 577 nm, 1 wt % in mCP). In 1 wt % mCP films, both emitters showed good photoluminescence quantum yields of 71% and 61%, and delayed lifetimes of 316.6 μs and 241.7 μs, respectively, for
DiKTa-OBuIm
and
DiKTa-DPA-OBuIm
, leading to reverse intersystem crossing rates of 2.85 × 10
3
s
−1
and 3.04 × 10
3
s
−1
. Light-emitting electrochemical cells were prepared using both
DiKTa-OBuIm
and
DiKTa-DPA-OBuIm
as active emitters showing green (λ
max
= 534 nm) and red (λ
max
= 656 nm) emission, respectively.
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