An Ideal Molecular Construction Strategy for Ultra‐Narrow‐Band Deep‐Blue Emitters: Balancing Bathochromic‐Shift Emission, Spectral Narrowing, and Aggregation Suppression

Abstract Narrowband emissive multiple resonance (MR) emitters promise high efficiency and stability in deep‐blue organic light‐emitting diodes (OLEDs). However, the construction of ideal ultra‐narrow‐band deep‐blue MR emitters still faces formidable challenges, especially in balancing bathochromic‐shift emission, spectral narrowing, and aggregation suppression. Here, DICz is chosen, which possesses the smallest full‐width‐at‐half‐maximum (FWHM) in MR structures, as the core and solved the above issue by tuning its peripheral substitution sites. The 1‐substituted molecule Cz‐DICz is able to show a bright deep‐blue emission with a peak at 457 nm, an extremely small FWHM of 14 nm, and a CIE coordinate of (0.14, 0.08) in solution. The corresponding OLEDs exhibit high maximum external quantum efficiencies of 22.1%–25.6% and identical small FWHMs of 18 nm over the practical mass‐production concentration range (1–4 wt.%). To the best of the knowledge, 14 and 18 nm are currently the smallest FWHM values for deep‐blue MR emitters with similar emission maxima under photoluminescence and electroluminescence conditions, respectively. These discoveries will help drive the development of high‐performance narrowband deep‐blue emitters and bring about a revolution in OLED industry.

fluorescence spectra, fluorescence lifetime, and quantum efficiency were carried out with Edinburgh fluorescence spectrometer (FLS1000) with an integrating sphere.Transient spectra for prompt part were collected with a 365 nm picosecond pulsed LED (EPL365), while the delayed part was collected with a 312.8 nm pulse width tuning laser (VPL320).
Calculation Formulas for the Photophysical Parameters.The kr and kRISC rates were calculated to be 1.8 × 10 8 s -1 and 2.6 × 10 3 s -1 , respectively, using a previously reported method (Adv. Mater. 2018, 30, 1705406): where kISC is the intersystem crossing rate, kRISC is the reverse intersystem crossing rate, kr are the rate constants of the singlet radiative transition, kp is the prompt decay rate, and kd stands for the delayed decay rate, respectively.p stands for the PLQY of the prompt part while d for the delayed part.
Electrochemical measurements.The electrochemical properties of Cz-DICz were studied by cyclic voltammetry.As shown in Figure S6, the oxidation potentials calculated from the onset of the oxidation curves are -4.72 eV and -1.76 eV for Cz-DICz, vs. an Fc/Fc + standard, corresponding to the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of -5.55 eV and -2.59 eV for Cz-DICz, using ferrocene as a reference.
Device fabrication and measurement of EL characteristics.All compounds were subjected to temperature-gradient sublimation under high vacuum before use.OLEDs were fabricated on the ITO-coated glass substrates with multiple organic layers sandwiched between the transparent bottom indium-tin-oxide (ITO) anode and the top metal cathode.Before device fabrication, the ITO glass substrates were pre-cleaned carefully.All material layers were deposited by vacuum evaporation in a vacuum chamber with a base pressure of 10 -6 torr.The deposition system permits the fabrication of the complete device structure in a single vacuum pump-down without breaking vacuum.The deposition rate of organic layers was kept at 0.1 -0.2 nm s -1 .The doping was conducted by co-evaporation from separate evaporation sources with different evaporation rates.The current density, voltage, luminance, external quantum efficiency, electroluminescent spectra and other characteristics were measured with a Keithley same time.The EQE measurement system is Hamamatsu C9920-12, which equipped with Hamamatsu PMA-12 Photonic multichannel analyzer C10027-02 whose longest detection wavelength is 1100 nm.

Synthetic procedures and characterization data.
Scheme S1.Synthetic procedures of Cz-DICz.

Other supplementary figures and tables
Device λEL a) [nm]     As shown in Figure S12 and Table S4, both the TADF sensitized films exhibited similar deepblue emissions at around 463 nm with an identical small FWHM of 18 nm, which was consistent with the PL results for the pristine ones.The fluorescence decays for the TADF sensitized films were fitted with double exponential expressions.The first short time scale decay (~5.6 ns) should be assigned to the prompt fluorescence of the Cz-DICz emitters, where similar exciton lifetimes could be observed in pristine films.The decay component (1.8-3.7 μs) can be ascribed to the Forster energy transfer from m4TCzPhBN to the Cz-DICz, which was much shorter than the pure MR emitters (~426 μs) and even shorter than m4TCzPhBN (~9.2 μs, Adv.Mater. 2020, 32, 1908355).Table S5.Summary of the EL data of TADF sensitized Cz-DICz devices.

Figure S4 .
Figure S4.The absorption and emission spectra, calculated by the MOMAP package at the PBE0/6-31G(d) level.The frequency correction factor set to 0.9512.

Figure S5 .
Figure S5.Vibrational modes in the S0 state that contribute significantly to the emission spectrum of Cz-DICz, calculated by the PBE0/6-31G(d) method.

Figure S10 .
Figure S10.The performances of the non-sensitized devices.(a) The energy level diagram.(b) The EL spectra and CIE color coordinates at 10 mA/cm 2 .(c) Current density-voltage-luminance characteristics.(d) The EQEmax values versus doping concentrations.(e) Current efficiency versus luminance curves.(f) Power efficiency versus luminance curves.
12 a) Maximum electroluminescence wavelength.b) Full width at half maximum of electroluminescence. c) Turn-on voltage when brightness is 0.2 cd m -2 .d) Maximum efficiency/ efficiency at 500 cd m -2 .e) Recorded at 10 mA/cm 2 .

Figure S14 .
Figure S14.The EL spectra of TADF sensitized devices under different voltages.

Figure S15 .
Figure S15.(a) Current efficiency versus luminance curves of the TADF sensitized devices.(b) Power efficiency versus luminance curves of the TADF sensitized devices.
11 a) Maximum electroluminescence wavelength.b) Full width at half maximum of electroluminescence. c) Turn-on voltage when brightness is 0.2 cd m -2 .d) Maximum efficiency/ efficiency at 500 cd m -2 .e) Recorded at 10 mA/cm 2 .

Figure S16 .
Figure S16.Energy level and molecular structure of the TTA device.

Figure S17 .
Figure S17.Current density-voltage-luminance characteristics of the TTA devices.

Figure S18 .
Figure S18.(a) External quantum efficiency versus luminance curves of the TTA devices.

Figure S19 .
Figure S19.(a) Current efficiency versus luminance curves of the TTA devices.(b) Power efficiency versus luminance curves of the TTA devices.

Table S1 .
Summary of TD-DFT calculations for Cz-DICz at the S0 and S1 structures at the

Table S2 .
Summary of the representative narrowband deep-blue MR emitters with emission wavelength around 460 nm.

Table S6 .
Summary of the EL data of the TTA devices.

Table S7 .
Crystal Data and Structure Refinement of Cz-DICz.