Azaborine versus Azaborine with a Spacer: Structural Effects on the Photophysical Properties of Tunable Azaborine Chromophores

Azaborines are fascinating compounds because of their valuable and interesting optical properties making them suitable to be utilized in many optoelectronic devices. We have designed, synthesized, and investigated a series of novel conjugated thermally stable azaborine chromophores by incorporating a phenyl ring as a spacer linking the chromophore to different electronic moieties as easily tunable high-luminescent organic materials. We investigated the effect of the phenyl spacer on the azaborine unit. The substituent effects of different electronic moieties were investigated by the insertion of electron –withdrawing and –donating moieties to the phenyl spacer. We examined the role of the electron –donating and –withdrawing substituents on the HOMO and LUMO energies to aid in understanding the fluorescence tunability

Solvatochromic and aggregation-induced emission active nitrophenyl-substituted pyrrolidinone-fused-1,2-azaborine with a pre-twisted molecular geometry

Boron–nitrogen-containing heterocycles with extended conjugated π-systems such as polycyclic aromatic 1,2-azaborines, hold the fascination of organic chemists due to their unique optoelectronic properties. However, the majority of polycyclic aromatic 1,2-azaborines aggregate at high concentrations or in the solid-state, resulting in aggregation-caused quenching (ACQ) of emission. This practical limitation poses significant challenges for polycyclic aromatic 1,2-azaborines’ use in many applications. Additionally, only a few solvatochromic polycyclic aromatic 1,2-azaborines have been reported and they all display minimal solvatochromism. Therefore, the scope of available polycyclic 1,2-azaborines needs to be expanded to include those displaying fluorescence at high concentration and in the solid-state as well as those that exhibit significant changes in emission intensity in various solvents due to different polarities. To address the ACQ issue, we evaluate the effect of a pre-twisted molecular geometry on the optoelectronic properties of polycyclic aromatic 1,2-azaborines. Specifically, three phenyl-substituted pyrrolidinone-fused 1,2-azaborines (PFAs) with similar structures and functionalized with diverse electronic moieties (–H, –NO2, –CN, referred to as PFA 1, 2, and 3, respectively) were experimentally and computationally studied. Interestingly, PFA 2 displays two distinct emission properties: (1) solvatochromism, in which its emission and quantum yields are tunable with respect to solvent polarity, and (2) fluorescence that can be completely “turned off” and “turned on” via aggregation-induced emission (AIE). This report provides the first example of a polycyclic aromatic 1,2-azaborine that displays both AIE and solvatochromism properties in a single BN-substituted backbone. According to time-dependent density functional theory (TD-DFT) calculations, the fluorescence properties of PFA 2 can be explained by the presence of a low-lying n–π* charge transfer state inaccessible to PFA 1 or PFA 3. These findings will help in the design of future polycyclic aromatic 1,2-azaborines that are solvatochromic and AIE-active as well as in understanding how molecular geometry affects these compounds’ optoelectronic properties

Effect of Nitro Group Position on the Optical Properties of Pyrrolidinone-Fused-1,2-Azaborine Chromophores

Due to the nitro (NO2) group\u27s strong electron-accepting capability, aromatic compounds with NO2 groups are frequently used in n-type organic conjugates. However, adding NO2 groups to the chromophores\u27 cores can reduce or quench the fluorescence, particularly in heterocyclic aromatic compounds containing three coordinate borons, such as pyrrolidinone-fuse-1,2-azaborines (PFAs). Due to strong intermolecular π-π stacking interactions, these NO2-PFAs tend to aggregate at high concentrations, causing emission quenching, also known as aggregation-caused quenching (ACQ). In this work, we synthesized a series of PFAs substituted with a NO2 group at positions (2-, 3-, and 4-) to investigate the influence of the location of the NO2 group on the optical characteristics of PFAs. Surprisingly, changing the placement of the nitro group on the pyrrolidinone hemisphere of PFAs resulted in distinct optical properties. Compared to 2-substituted NO2-PFA, substitution of the NO2 group to position 3- resulted in significant blue-shifted emission, with no signs of intramolecular charge transfer and increased in fluorescence. 4-substituted NO2-PFA is also blue shifted but at a longer wavelength with respect to 3-, resulting in less fluorescence

Fine Tuning the Electronic Properties of Non-Conjugated Mono-Azo-Chromophores Incorporated Azaborine

Azobenzene (azo) and azaborine are fascinating compounds because of their valuable and interesting optical properties making them suitable to be utilized in many optoelectronic devices. We have designed, synthesized and will investigate a series of novel non-conjugated thermally stable mono-azo-azaborine chromophores by linking the two chromophores (azo and azaborine) together as easily tunable high-luminescent organic materials. The substituent effects on the combined chromophores will be investigated by the addition of electron –withdrawing and –donating moieties to the core of the azobenzene. We will investigate the role of the electron –donating and –withdrawing substituents on the HOMO and LUMO energies to aid in understanding the fluorescence tunability. We will also investigate the effect of the photoinduced isomerization of the azobenzene on the aggregation and fluorescence of the mono-azo-azaborine chromophore