CN-Modified Imidazopyridine as a New Electron Accepting Unit of Thermally Activated Delayed Fluorescent Emitters.

Two efficient thermally activated delayed fluorescent (TADF) emitters were developed by utilizing CN-modified imidazopyridine as an acceptor unit. The CN-modified imidazopyridine acceptor was combined with either an acridine donor or a phenoxazine donor through a phenyl linker to produce two TADF emitters, Ac-CNImPy and PXZ-CNImPy. The acridine-based Ac-CNImPy emitter exhibited sky-blue emission with a CIE coordinate of (0.18, 0.38), whereas the phenoxazine-donor-based PXZ-CNImPy showed greenish-yellow emission with a CIE coordinate of (0.32, 0.58). A high photoluminescence quantum yield of 80 % was observed for the PXZ-CNImPy emitter compared with 40 % for the Ac-CNImPy emitter. Organic light-emitting diodes based on the PXZ-CNImPy emitter demonstrated high external quantum efficiency of 17.0 %. Hence, the CN-modified imidazopyridine unit can be considered as a useful electron acceptor for the future design of highly efficient TADF emitters.

Proton Induced Modulation of ICT and PET Processes in an Imidazo-phenanthroline Based BODIPY Fluorophores.

BODIPY fluorophores linked with an imidazo-phenanthroline donor at α and ß positions have been synthesized. Intriguing intramolecular charge transfer phenomenon is observed in both the dyes which has been extensively investigated using UV-vis absorption, steady-state and time-resolved fluorescence measurements. H-bonding and intrinsic polarity of the solvents has modulated the absorption and emission bands of these fluorophores strongly causing significant increase in the Stokes shifts. In spite of having difference only in terms of the position of donor subunit, the photophysics of these dyes are not only significantly different from each other, but contradictory too. Interestingly, acidochromic studies revealed the shuttling mechanism between ICT and PET processes for BDP 2. Quantum chemical calculations have been employed further to support experimental findings. DFT and TD-DFT method of analysis have been used to optimize ground and excited state geometries of the synthesized dyes.

Synthesis of novel carbazole based styryl: rational approach for photophysical properties and TD-DFT.

The synthesis and solvatochromic behavior of four novel carbazole based fluorescent styryl dyes were explained. In chlorinated solvents such as DCM and chloroform, these dyes show bathochromic shift in their absorption as well as emission. The styryl dyes 6b and 6c show solid state yellow fluorescence. DFT and TD-DFT computations were performed to study structural, molecular, electronic and photophysical properties of these dyes. The computed absorption and emission wavelength values are found to be in good agreement with the experimental results. The photophysical properties of these 1-styryl carbazole dyes are also compared with the recently reported 3-styrl carbazole dyes. The unique behavior of dye 6d is well explained by its optimized geometry found in the excited state. Ratio of ground to excited state dipole moment of the synthesized novel styryl compounds were calculated by Bakhshiev and Bilot-Kawski correlations.

Solution-Processed Pure Red TADF Organic Light-Emitting Diodes With High External Quantum Efficiency and Saturated Red Emission Color.

In spite of recent research progress in red thermally activated delayed fluorescence (TADF) emitters, highly efficient solution-processable pure red TADF emitters are rarely reported. Most of the red TADF emitters reported to date are designed using a rigid acceptor unit which renders them insoluble and unsuitable for solution-processed organic light-emitting diodes (OLEDs). To resolve this issue, a novel TADF emitter, 6,7-bis(4-(bis(4-(tert-butyl)phenyl)amino)phenyl)-2,3-bis(4-(tert-butyl)phenyl)quinoxaline-5,8-dicarbonitrile (tBuTPA-CNQx) is designed and synthesized. The highly twisted donor-acceptor architecture and appropriate highest occupied molecular orbital/lowest unoccupied molecular orbital distribution lead to a very small singlet-triplet energy gap of 0.07 eV, high photoluminescence quantum yield of 92%, and short delayed fluorescence lifetime of 52.4 µs. The peripheral t-butyl phenyl decorated quinoxaline acceptor unit and t-butyl protected triphenylamine donor unit are proven to be useful building blocks to improve solubility and minimize the intermolecular interaction. The solution-processed OLED based on tBuTPA-CNQx achieves a high external quantum efficiency (EQE) of 16.7% with a pure red emission peak at 662 nm, which is one of the highest EQE values reported till date in the solution-processed pure red TADF OLEDs. Additionally, vacuum-processable OLED based on tBuTPA-CNQx exhibits a high EQE of 22.2% and negligible efficiency roll-off.

Borylation-Reduction-Borylation for the Formation of 1,4-Azaborines.

Given the current interest in materials containing 1,4-azaborine units, the development of new routes to these structures is important. Carbonyl directed electrophilic borylation using BBr3 is a facile method for the ortho-borylation of N,N-diaryl-amide derivatives. Subsequent addition of Et3SiH results in carbonyl reduction and then formation of 1,4-azaborines that can be protected in situ using a Grignard reagent. Overall, borylation-reduction-borylation is a one-pot methodology to access 1,4-azaborines from simple precursors.

Rational Molecular Design of Highly Efficient Yellow-Red Thermally Activated Delayed Fluorescent Emitters: A Combined Effect of Auxiliary Fluorine and Rigidified Acceptor Unit.

Molecular design strategies are crucial to develop highly efficient and long-wavelength thermally activated delayed fluorescent (TADF) emitters because the inherent limitation of the energy gap law degrades the efficiency of the red or orange TADF emitters. To resolve the low efficiency issue, we designed and synthesized two TADF emitters, 4,4'-(6-(9,9-dimethylacridin-10(9H)-yl)-7-fluoroquinoxaline-2,3-diyl)dibenzonitrile (FDQCNAc) and 11-(9,9-dimethylacridin-10(9H)-yl)-12-fluorodibenzo[a,c]phenazine-3,6-dicarbonitrile (FBPCNAc), by utilizing fluorine and peripheral cyano-substituted quinoxaline and phenazine acceptors of 4,4'-(6-fluoroquinoxaline-2,3-diyl)dibenzonitrile (FDQCN) and 11-fluorodibenzo[a,c]phenazine-3,6-dicarbonitrile (FBPCN), respectively. A fluorine atom at the ortho position of the acridine donor acts as an auxiliary acceptor to minimize the singlet-triplet energy gap (ΔEST) below 0.1 eV and promotes the reverse intersystem crossing (RISC) process. Organic light-emitting diodes (OLEDs) fabricated with FDQCNAc and FBPCNAc emitters demonstrated high external quantum efficiencies (EQEs) of 27.6 and 23.8% in the yellow-red TADF OLEDs, respectively. In particular, the combination of the F auxiliary acceptor unit and the rigidified FBPCN acceptor unit enabled red-shifted emission by about 58 nm without much sacrifice of the EQE in the red region.

Molecular Design Strategy of Thermally Activated Delayed Fluorescent Emitters Using CN-Substituted Imidazopyrazine as a New Electron-Accepting Unit.

Thermally activated delayed fluorescence (TADF)-based organic light-emitting diodes (OLEDs) have attracted enormous attention recently due to their capability to replace conventional phosphorescent organic light-emitting diodes for practical applications. In this work, a newly designed CN-substituted imidazopyrazine moiety was utilized as an electron-accepting unit in a TADF emitter. Two TADF emitters, 8-(3-cyano-4-(9,9-dimethylacridin-10(9H)-yl)phenyl)-2-phenylimidazo[1,2-a]pyrazine-3-carbonitrile (Ac-CNImPyr) and 8-(3-cyano-4-(10H-phenoxazin-10-yl)phenyl)-2-phenylimidazo[1,2-a]pyrazine-3-carbonitrile (PXZ-CNImPyr), were developed based on the CN-substituted imidazopyrazine acceptor combined with acridine and phenoxazine donor, respectively. A CN-substituted phenyl spacer was introduced between the donor and acceptor for a sufficiently small singlet-triplet energy gap (ΔEST ) and molecular orbital management. Small ΔEST of 0.07 eV was achieved for the phenoxazine donor-based PXZ-CNImPyr emitter. As a result, an organic light-emitting diode based on the PXZ-CNImPyr emitter exhibited a high external quantum efficiency of up to 12.7 %, which surpassed the EQE limit of common fluorescent emitters. Hence, the CN-modified imidazopyrazine unit can be introduced as a new acceptor for further modifications to develop efficient TADF-based OLEDs.

Isomeric Quinoxalinedicarbonitrile as Color-Managing Acceptors of Thermally Activated Delayed Fluorescent Emitters.

Highly efficient yellow and red thermally activated delayed fluorescent (TADF) organic light-emitting diodes were developed using two quinoxalinedicarbonitrile units, quinoxaline-6,7-dicarbonitrile (6,7-DCQx) and quinoxaline-5,8-dicarbonitrile (5,8-DCQx) as electron acceptors, and t-butylcarbazole or 9,9-dimethylacridan as electron donors. The strong electron-withdrawing capability of the 5,8-DCQx and 6,7-DCQx acceptors enabled from yellow to red emission in the TADF emitters. Comparing 5,8-DCQx and 6,7-DCQx, a red-shifted emission by about 40 nm was observed in the 5,8-DCQx acceptor-based emitters, but the external quantum efficiency of the TADF devices was over 20% irrespective of the acceptor, proving the usefulness of the two acceptor moieties as the building blocks of the TADF emitters. Therefore, the quinoxalinedicarbonitrile acceptors are promising as the acceptor moiety for designing yellow or red TADF emitters.