Hot exciton transition for organic light-emitting diodes: tailoring excited-state properties and electroluminescence performances of donor–spacer–acceptor molecules

Jayabharathi, Jayaraman; Panimozhi, Sekar; Thanikachalam, Venugopal

doi:10.1039/c8ra07891b

Cited by 20 publications

(12 citation statements)

References 95 publications

(68 reference statements)

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“…52 As seen, o-2CzBT, m-2CzBT, and p-2CzBT present two independent slopes, and the lower slope in low-polarity solvents is ascribed to the LE state while the larger slope in high-polarity solvents is attributed to the CT state, which is a typical behavior of HLCT materials. 32,[53][54][55] Therefore, based on the aforementioned theoretical calculations (electronic structures and properties of excited states) and photophysical studies (nanosecond-scaled lifetimes and solvatochromic effects), the HLCT feature is verified for o-2CzBT, m-2CzBT, and p-2CzBT through the coupling and intercrossing between the LE and CT states. 56…”

Section: Photophysical Propertiesmentioning

confidence: 98%

Hybridized Local and Charge-Transfer Excited-State Fluorophores through the Regulation of the Donor–Acceptor Torsional Angle for Highly Efficient Organic Light-Emitting Diodes

Chen

Liu

et al. 2022

CCS Chem

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Hybridized local and charge-transfer (HLCT) excited-state fluorophores, which enable full exciton utilization through a reverse intersystem crossing from high-lying triplet states to singlet state, have attracted increasing attention toward organic light-emitting diodes (OLEDs) application. Herein, we report three D-π-A-π-D-type isomers o-2CzBT, m-2CzBT, and p-2CzBT by adjusting the donor (D) units from ortho-, meta-, to para-substituted positions with the acceptor (A) core unit, respectively. The HLCT properties of the three compounds are evidently confirmed by theoretical calculations, solvatochromic behaviors, and transient decay lifetimes analyses. As the substituted position changes from the ortho-, meta-, and para-positions, the reduced steric hindrance brings about decreased torsional angle between D and A moieties, resulting in increased oscillator strength. Accordingly, the para-substituted p-2CzBT is endowed with a more locally excited component that accounts for faster radiative decay, leading to a higher fluorescent efficiency than that of o-2CzBT and m-2CzBT. As expected, p-2CzBT enables its nondoped and doped OLEDs with higher external quantum efficiencies (EQEs) of 12.3% and 15.0%, respectively, which are among the state-of-the-art efficiencies of HLCT-based OLEDs. Moreover, o-2CzBT and m-2CzBT are also utilized as host materials for high-performance OLEDs, thus extending the application of HLCT materials.

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Section: Photophysical Propertiesmentioning

confidence: 98%

Hybridized Local and Charge-Transfer Excited-State Fluorophores through the Regulation of the Donor–Acceptor Torsional Angle for Highly Efficient Organic Light-Emitting Diodes

Chen

Liu

et al. 2022

CCS Chem

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“…While 2EHO-CNPE retains the LE-dominated HLCT behavior in this high polarity region, 2EHO-TPA-CNPE and 2EHO-TPA-PE OPE π-systems showed much larger slopes (∼27 170−29 700) in the Lippert−Mattaga model yielding very large μ e 's of 33.1−35.3 D. Note that these significantly large μ e values are on the order of some of the highest excited state dipole moments predicted for CT compounds in the literature. 59,60 Considering that 2EHO-TPA-PE and 2EHO-TPA-CNPE molecular lengths are ∼3.55 nm, these μ e 's correspond to a large degree of charge separation (∼0.70−0.75 charge•nm) along the molecular π-backbone. These findings reveal that in low-to-medium polarity region there is a common LE-dominant HLCT excited state character for all three OPE molecules.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

Understanding and Tailoring Excited State Properties in Solution-Processable Oligo(p-phenyleneethynylene)s: Highly Fluorescent Hybridized Local and Charge Transfer Character via Experiment and Theory

2021

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Rod-shaped oligo(p-phenyleneethynylene) (OPE) offers an attractive π-framework for the development of solution-processable highly fluorescent molecules having tunable hybridized local and charge transfer (HLCT) excited states and (reverse) intersystem crossing ((R)ISC) channels. Herein, an HLCT oligo(pphenyleneethynylene) library was studied for the first time in the literature in detail systematically via experiment and theory. The design, synthesis, and full characterization of a new highly fluorescent (Φ PL-solution ∼ 1) sky blue emissive 4′,4‴-((2,5bis((2-ethylhexyl)oxy)-1,4-phenylene)bis(ethyne-2,1-diyl))bis(N,N-diphenyl-[1,1′-biphenyl]-4-amine) (2EHO-TPA-PE) was also reported. The new molecule consists of a D′−Ar−π−D−π−Ar−D′ molecular architecture with an extended π-spacer and no acceptor unit, and detailed structural, physicochemical, single-crystal, and optoelectronic characterizations were performed. A high solid-state quantum efficiency (Φ PL-solid state ∼ 0.8) was achieved as a result of suppressed exciton−phonon/vibronic couplings (no π−π interactions and multiple (14 per dimeric form) strong C−H•••π interactions). Strong solution-phase/solid-state dipole-dependent tunable excited state behavior (local excited (LE) → HLCT → charge transfer (CT)) and decay dynamics covering a wide spectral region were demonstrated, and the CT state was observed to be highly fluorescent despite extremely large Stokes shift (∼130 nm)/fwhm (∼125 nm) and significant charge separation (0.75 charge• nm). Employing the Lippert−Mataga model, along with detailed photophysical studies and TDDFT calculations, key relationships between molecular design−electronic structure−exciton characteristics were elucidated with regards to HLCT and hot exciton channel formations. The interstate coupling between CT and LE states and the interplay of this coupling with respect to medium polarity were explored. A key relationship between excited-state symmetry breaking process and the formation of HLCT state was discussed for TPA-ended rod-shaped OPE π-systems. (R)ISC-related delayed fluorescence (τ ∼ 2−6 ns) processes were evident following the prompt decays (∼0.4−0.9 ns) both in the solution and in the solid-state. As a unique observation, the delayed fluorescence could be tuned and facilitated via small dielectric changes in the medium. Our results and the molecular engineering perspectives presented in this study may provide unique insights into the structural and electronic factors governing tunable excited state and hot-exciton channel formations in OPEs for (un)conventional solution-processed luminescence applications.

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“…Researchers have used the styryl moiety as a spacer in the design strategy of the molecular structure, and it will increase the twisting nature of the molecules, and more twisting nature of the molecules push emission toward lower wavelength, due to restricting p-conjugation in the molecules. [126][127][128][129] In 2017, V. Thanikachalam et al designed and synthesized three blueemitting fluorophores, TPA-BDIS 74, TPA-BPIS 75, and Cz-BPIS 76, by connecting donor TPA/Cz and acceptor BI/PI through the styryl as a spacer (Fig. 21).…”

Section: Phenanthroimidazole-styryl Spacer Based Hlct Fluorophoresmentioning

confidence: 99%

Recent progress in imidazole based efficient near ultraviolet/blue hybridized local charge transfer (HLCT) characteristic fluorophores for organic light-emitting diodes

Hong

Tagare

2022

J. Mater. Chem. C

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Hot exciton transition for organic light-emitting diodes: tailoring excited-state properties and electroluminescence performances of donor–spacer–acceptor molecules

Abstract: SPNCN-Cz device exhibits ηex (3.15%), ηc (2.56 cd A−1), ηp (2.45 lm W−1) with CIE (0.15, 0.07).

Cited by 20 publications

References 95 publications

Hybridized Local and Charge-Transfer Excited-State Fluorophores through the Regulation of the Donor–Acceptor Torsional Angle for Highly Efficient Organic Light-Emitting Diodes

Hybridized Local and Charge-Transfer Excited-State Fluorophores through the Regulation of the Donor–Acceptor Torsional Angle for Highly Efficient Organic Light-Emitting Diodes

Understanding and Tailoring Excited State Properties in Solution-Processable Oligo(p-phenyleneethynylene)s: Highly Fluorescent Hybridized Local and Charge Transfer Character via Experiment and Theory

Recent progress in imidazole based efficient near ultraviolet/blue hybridized local charge transfer (HLCT) characteristic fluorophores for organic light-emitting diodes

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