Optoelectronic, Aggregation, and Redox Properties of Double‐Rotor Boron Difluoride Hydrazone Dyes

Abstract

We develop the chemistry of boron difluoride hydrazone dyes (BODIHYs) bearing two aryl substituents and explore their properties. The low‐energy absorption bands (λmax=427–464 nm) of these dyes depend on the nature of the N‐aryl groups appended to the BODIHY framework. Electron‐donating and extended π‐conjugated groups cause a redshift, whereas electron‐withdrawing groups result in a blueshift. The title compounds were weakly photoluminescent in solution and strongly photoluminescent as thin films (λPL=525–578 nm) with quantum yields of up to 18 % and lifetimes of 1.1–1.7 ns, consistent with the dominant radiative decay through fluorescence. Addition of water to THF solutions of the BODIHYs studied causes molecular aggregation which restricts intramolecular motion and thereby enhances photoluminescence. The observed photoluminescence of BODIHY thin films is likely facilitated by a similar molecular packing effect. Finally, cyclic voltammetry studies confirmed that BODIHY derivatives bearing para‐substituted N‐aryl groups could be reversibly oxidized (Eox1=0.62–1.02 V vs. Fc/Fc+) to their radical cation forms. Chemical oxidation studies confirmed that para‐substituents at the N‐aryl groups are required to circumvent radical decomposition pathways. Our findings provide new opportunities and guiding principles for the design of sought‐after multifunctional boron difluoride complexes that are photoluminescent in the solid state.

Optoelectronic Properties of Carbon‐Bound Boron Difluoride Hydrazone Dimers

“…None of the complexes 11 a – d described exhibited significant solvatochromism that would be expected for transitions with significant charge‐transfer character. The low Φ em values estimated for 11 a – 11 d are likely due to the free rotation of their N ‐aryl substituents in solution, which was observed previously for structurally related BODIHY monomers …”

“…The incorporation of electron‐donating substituents in 11 b (R 1 , R 2 =OMe) led to a red shift (Δ λ max =+20 nm) in the low‐energy absorption band, while the introduction of electron‐withdrawing substituents in 11 c (R 1 , R 2 =CF 3 ) resulted in a blue shift (Δ λ max =−10 nm). The molar extinction coefficients ( ϵ ) of 11 a – d in THF range between 20 500 and 33 800 m −1 cm −1 and are larger than those obtained for structurally related BODIHY monomers . Dimer 11 a had a low‐energy emission maximum ( λ em ) at 557 nm and Φ em =1 %, while 11 b (Δ λ em =+29 nm, Φ em <1 %) and 11 c (Δ λ em =−24 nm, Φ em =2 %) exhibited similar characteristics that followed the trends in absorption properties discussed above.…”

“…Hydrazone dimers 10 a – d were prepared by adapting a literature procedure previously used for monomeric derivatives . This procedure involves condensation reactions of the respective aryl hydrazines and 2,2′‐pyridil (Scheme , Figures S1–S8).…”

“…The Aprahamian group has shown that BODIHYs bearing a single molecular rotor (e.g., 3 a ) exhibit aggregation‐induced emission (AIE) and fluoresce from the third singlet excited state, S 3 , rather than the first singlet excited state, S 1 , thus violating Kasha's rule . Recently, we reported the photophysical and AIE properties of double‐rotor BODIHYs (e.g., 3 b ) and explored their redox chemistry . The incorporation of a fourth nitrogen atom into the ligand framework of BF 2 formazanates 4 has yielded dyes that found application in fluorescence cell imaging and materials that exhibit near‐infrared emission and electrochemiluminescence …”

A Boron Difluoride Hydrazone (BODIHY) Polymer Exhibits Aggregation‐Induced Emission

“…[ 12 ] We have recently found that dual‐rotor BODIHYs 8b and BODIHY dimers 9 exhibit tunable redox properties dependent on the electronic nature of the N ‐aryl substituents and exhibit AIE (Figure 1). [ 13 ] Motivated by these findings, we have developed the first polymer containing BODIHY units in an effort to expand the functionality of these unique molecular, and now macromolecular, materials. It is envisioned that the incorporation of BODIHYs into polymers will alter their film‐forming properties and solution characteristics, thus providing an outlet for their potential use in organic electronics and biological applications.…”