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Here, we reported the photophysical studies, single-crystal X-ray crystallography, and density functional theory (DFT) calculations of two bipyridine derivative molecules. In addition, thermal gravimetric analysis and cyclic voltammetry studies were also performed for both compounds. According to crystallographic data, the π-conjugated molecules have high-quality crystal structures as a result of intramolecular and intermolecular hydrogen bonds occurring through the molecules of the compound. It was determined that when the functional groups (F– and CH3O−) were introduced to the para positions, the molecules adopted slipped stacking (J-aggregate) and antiparallel cofacial stacking (H-aggregate). It was observed that these two bipyridine derivatives disclose the relationship between molecular conformation-molecular packing modes and photophysical behavior of organic solids. The results of DFT calculations supported the structural, spectroscopic, and photophysical data and confirmed the compositions of frontier molecular orbitals in both molecules.

We designed and synthesized two new ionic thermally activated delayed fluorescent (TADF) emitters that are charged analogues of a known multiresonant TADF (MR-TADF) compound, DiKTa. The emission of the charged derivatives is red-shifted compared to the parent compound. For instance, DiKTa-OBuIm emits in the green (λPL = 499 nm, 1 wt % in mCP) while DiKTa-DPA-OBuIm emits in the red (λPL = 577 nm, 1 wt % in mCP). In 1 wt % mCP films, both emitters showed good photoluminescence quantum yields of 71% and 61%, and delayed lifetimes of 316.6 μs and 241.7 μs, respectively, for DiKTa-OBuIm and DiKTa-DPA-OBuIm, leading to reverse intersystem crossing rates of 2.85 × 103 s−1 and 3.04 × 103 s−1. Light-emitting electrochemical cells were prepared using both DiKTa-OBuIm and DiKTa-DPA-OBuIm as active emitters showing green (λmax = 534 nm) and red (λmax = 656 nm) emission, respectively.