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Rigidity and Polarity Effects on the Electronic Properties of Two Deep Blue Delayed Fluorescence Emitters
journal contribution
posted on 2018-05-15, 00:00 authored by Christian
M. Legaspi, Regan E. Stubbs, Md. Wahadoszaman, David J. Yaron, Linda A. Peteanu, Abraham Kemboi, Eric Fossum, Yongli Lu, Qi Zheng, Lewis J. RothbergMolecules that undergo
reverse intersystem-crossing (RISC), which
enables thermally activated delayed fluorescence, represent an important
advance in the development of organic-based light-emitting diodes
(OLEDs). The current study focuses on two blue-emitting RISC molecules
employing carbazole as the donor and benzothiazole or benzoxazole
derivative as the acceptor (BTZ/BOX-carbazole (CBZ)). Although the
emission maxima of these compounds are deep blue (∼410 nm)
in hydrocarbon solvents, their spectra broaden, red shift, and decrease
in intensity with even a modest increase in solvent polarity because
of their strong charge-transfer (CT) character. These effects are
qualitatively predicted from time-dependent density functional theory
calculations using the state-specific polarizable continuum model,
though the emission spectral shifts are significantly overestimated.
The desired blue emission peak of both compounds (∼425 nm)
is recovered by rigidifying the environment, either in low-temperature
glasses or in room-temperature polymer films, independent of local
polarity. The polarity-induced emission red shift is therefore due
to the solvent orientational polarizability. The effects of an applied
electric field on the spectra (Stark effect) are used to quantify
the CT character of the absorbing and emitting states. Significantly
less field-induced emission quenching is observed in BOX-CBZ versus
that in BTZ-CBZ. Minimizing this effect is important for the performance
in the large (1–10 MV) fields present within OLED devices.
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Keywords
room-temperature polymer filmsPolarity Effectspolarity-induced emissionfield-induced emission quenchingcompoundDeep Bluetheory calculationsOLED devicesemission peakBTZ-CBZElectronic PropertiesshiftBTZhydrocarbon solventsCBZstate-specific polarizable continuum modeltime-dependent densityFluorescence Emitters MoleculesBOX-CBZnmMVStark effectemission maximaCT characterorientational polarizabilityblue-emitting RISC moleculesspectraorganic-based light-emitting diodes
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