Aggregation-Enhanced Emission in Tetraphenylpyrazine-Based Luminogens: Theoretical Modulation and Experimental Validation

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 luminogens, tetraphenylpyrazine (TPP) and its derivatives, is studied by first-principles calculations contrasting their pyrazine core with inherent n→π* transition. Our results indicate that attachment of phenyl rings to pyrazine core can change n→π* composition to π→π* via electronic conjugated effect and evoke restriction of intramolecular motion (RIM) that is responsible for the AEE characteristics. Decoration to the TPP peripheries with electron-withdrawing CF3 groups hardly affects the electronic transition properties and less 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 n→π* composition 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 strategy for the rational design of efficient heterocycle-containing emitters.