Aggregation-enhanced emission in tetraphenylpyrazine-based luminogens: theoretical modulation and experimental validation

Abstract

Aggregation-induced/enhanced emission (AIE/AEE) materials have aroused wide attention for their potential applications in optoelectronic devices and biosensors. In this paper, the working mechanism of a new kind of AEE-active luminogen, tetraphenylpyrazine (TPP) and its derivatives, is studied by first-principles calculations contrasting their pyrazine core with an inherent n → π* transition. Our results indicate that the attachment of phenyl rings to the pyrazine core can change the n → π* transition component to π → π* via the electronic conjugated effect and evoke restriction of intramolecular motion (RIM) that is responsible for the AEE characteristics. Decoration of the TPP peripheries with electron-withdrawing CF₃ groups hardly affects the electronic transition properties and barely improves the solid-state luminescence. Substitution of the peripheral phenyls with electron-donating OCH₃ groups thoroughly opens up the radiative channel and greatly enhances the emission quantum efficiency. Moreover, CF₃ and OCH₃ substituents can respectively generate extra CH⋯F/F⋯F/CF⋯π and CH⋯O interactions, which could promote RIM and slow down the non-radiative decay process. The experimental results confirm the theoretical predictions well. The above modulation guideline can be summarized from a molecular picture: the phenyl attachment reduces the n → π* transition component and evokes RIM, while the substituent decoration imparts altered electronic conjugation and promotes RIM. Our work not only offers valuable insight into AEE phenomena, but also provides a strategy for the rational design of efficient heterocycle-containing emitters.