Changing the Behavior of Chromophores from Aggregation‐Caused Quenching to Aggregation‐Induced Emission: Development of Highly Efficient Light Emitters in the Solid State

The development of efficient luminescent materials in the solid state is of both scientific and technological interest. An obstacle to their development is the notorious aggregation-caused quenching (ACQ) effect: the emission of conventional luminophores is often weakened in the solid state in comparison to in solution, due to aggregate formation in the condensed phase. [1‐3] The ACQ problem must be properly tackled, because the luminophores are commonly used as solid films in their practical applications. Various chemical, physical, and engineering approaches have been taken to frustrate luminophore aggregation. [4,5] The attachment of bulky alicyclics, encapsulation by amphiphilic surfactants, and blending with transparent polymers are widely used methods to impede aggregate formation. These processes, however, are often accompanied by severe side effects. The steric effects of bulky alicyclics, for example, can twist the conformations of the chromophoric units and jeopardize the electronic conjugation in the luminophores, and the electronic effects of the saturated surfactants and nonconjugated polymers can dilute the luminophore density and obstruct the charge transport in electroluminescence (EL) devices. The current approaches to the problem are thus far from ideal, because the ACQ effect is alleviated at the expense of other useful properties of the luminophores. A win‐win strategy would be the elimination of the ACQ effect without sacrificing other functional properties of the luminophores. In the work reported here, we have developed such a new approach. Triphenylamine (TPA) and its derivatives are luminescent when dissolved in good solvents [6] for them but become less emissive when aggregated in the solid state, and are therefore typical ACQ luminophores. [7] For