Benzothiadiazole‐Based Green Hybridized Local and Charge‐Transfer (HLCT) Fluorophores as Solution‐Processed Non‐Doped Emitters for Simple Structured Electroluminescent Devices

Abstract

Herein, two new solution-processable HLCT fluorophores (CBzF and CBzFC) based on benzothiadiazole (Bz) derivatives were designed and synthesized (HLCT = hybridized local and charge transfer). Their optical and photophysical properties were experimentally and theoretically studied and verified by the solvatochromic effect and density functional theory calculations. The two molecules demonstrate high solubility, HLCT features, and an intense green fluorescence with solid-state photoluminescence quantum yields of 63-72 %. Both fluorophores are effectively applied as non-doped emitters for solution-processed double-layered OLEDs, which produce intense green emission colors with low turn-on voltages (3.0-3.2 V). Specifically, the CBzFC-based solution-processed device achieves a high maximum luminescence (24400 cd m À 2 ), high maximum efficiencies (EQE max = 5.59 %, CE max = 12.24 cd A À 1 , and PE max = 10.54 lm W À 1 ), and decent efficiency low-off.

Red‐Emitting Fluorophores Featuring Combined Hybridized Local and Charge‐Transfer Excited State and Aggregation‐Induced Emission as Efficient Emitters for Electroluminescent Devices

“…However, the solvatochromic effect in the PL spectra is clearly observed with a change in the solvent polarity from low‐polarity triethylamine (TEA) to high‐polarity dimethylformamide (DMF). The PL spectra of the two compounds changed to a great extent from a narrow emission band peaked at 557 nm for TTBZ and 649 nm for TTNz (in TEA) to a broad emission band peaked at 688 nm for TTBz and 729 nm for TTNz (in DMF), that is, 80–131 nm red shift with a change in the solvent polarity is noteworthy and signifies the existence of CT components in their excited states as noted in other reports [14,29] . To better understand their solvatochromic effect, the linear fitting analysis of the Stokes shift (ν a ‐ν f ) vs solvent orientational polarizability ( f ) plotted according to the Lippert–Mataga (LM) model was evaluated [44] .…”

“…The μ e of the smaller slope is slightly greater than that of common LE fluorophores but close to that of a typical HLCT molecule N,N-diphenyl-4-(9phenylnaphthothiadiazol-4-yl)aniline (TPA-NZP) (μ e = 17.50 D), [13] while the μ e of the larger slope is close to that of a typical CT molecule 4-(N,N-dimethylamino)benzonitrile (DMABN) (μ e = 23 D). [45] In fact, some HLCT molecules exhibited LM plots with only one medium slope (μ e = 10.00-17.50 D), [14,26,46,47] this situation suggested that the CT and LE states might hybridize sufficiently, and such HLCT materials exhibited quasi-equivalent hybridization of CT and LE components. Hence, both TTBz and TTNz in high-polarity solvents (f � 0.15) retain a strong CT-emitting character, while in low-polarity solvents (f � 0.15), they behave like LE-emitting species with a small CT component or HLCT state emission formed in this solvent polarity region which was also illustrated through the solvatochromic experiments as mentioned above (Figure 2a).…”

“…The PL spectra of the two compounds changed to a great extent from a narrow emission band peaked at 557 nm for TTBZ and 649 nm for TTNz (in TEA) to a broad emission band peaked at 688 nm for TTBz and 729 nm for TTNz (in DMF), that is, 80-131 nm red shift with a change in the solvent polarity is noteworthy and signifies the existence of CT components in their excited states as noted in other reports. [14,29] To better understand their solvatochromic effect, the linear fitting analysis of the Stokes shift (ν a -ν f ) vs solvent orientational polarizability (f) plotted according to the Lippert-Mataga (LM) model was evaluated. [44] LM provides information on how compounds' fluorescence properties (excited dipole moment) change when exposed to various polar solvents.…”

“…To enhance the IQE max of such OF emitters, enhancement of the transition rate by directly regulating the excited state component, known as the hybridized local and charge-transfer (HLCT) excited state, has been introduced. [13][14][15] In the HLCT molecule, the lowest excited state possesses both local excitation (LE) and charge transfer (CT) characters simultaneously. The LE component typically retains a large transition dipole moment and a large overlap of HOMO and LUMO giving rise to a more efficient radiative state, which thus effectively boosts the Φ PL .…”

“…This is caused by the extensive spatial separation between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) in the D‐A system. To enhance the IQE max of such OF emitters, enhancement of the transition rate by directly regulating the excited state component, known as the hybridized local and charge‐transfer (HLCT) excited state, has been introduced [13–15] . In the HLCT molecule, the lowest excited state possesses both local excitation (LE) and charge transfer (CT) characters simultaneously.…”