Bicolor switching mechanism of multifunctional light-emitting molecular material in solid phase

Multifunctional light-emitting molecular materials have attracted great attention recently, while the amounts are still quite limited. The Cz-AQ molecule with thermally activated delayed fluorescence, aggregation-induced emission and mechanochromic luminescence properties was reported and the bicolor electroluminescence was found in film recently by Jiang's group (J. Mater. Chem. C 5 (2017) 12031–12034). In this paper, the bicolor switching mechanism of the Cz-AQ molecule in solid phase is studied based on first-principles and combined quantum mechanics and molecular mechanics calculations. Our results indicate that the emission of Cz-AQ in solid phase does not come from monomers but dimers. The emission colors have close relationship with intermolecular positions of two molecules in dimers. There is no need of conversion of lattice types for color switching, and slight variation of relative position of molecules in dimers will induce the emission color change between red and yellow. Our theoretical study not only gives reasonable explanation of experimental results but also provides some insights on the design of new type multifunctional light-emitting molecules.