Opposite behavior with an emission enhancement upon aggregation of the fluorophores is also known, but early reports are rare. [7] This phenomenon is coined aggregationinduced-emission (AIE) and became more popular through the work of Tang and co-workers since the early 2000s. [7,8,9] A plethora of molecules with AIE-behavior was investigated and a general understanding of their photophysical properties was achieved. [10,11] The emission quenching is referred to internal motion of suitable substituents like tetra phenylethylene and hexaphenylsilole at the fluorophore. [6,12] Rotation and vibration of these groups open non-radiative pathways which result in radiationless decay in solution. [9,13] Aggregation confines the flexibility of the substituents to restrict the internal motion. [14] Consequently, the non-radiative pathways are blocked, and the emission enhances in highly aggregated-solutions and in the solidstate. [11,15] Further strategies for obtaining solid-state luminescent materials aim at the introduction of bulky substituents at the fluorophores to prevent strong interchromophoric interactions. [16] These approaches are mainly based on intramolecular modifications and therefore the resulting fluorescence of the aggregates can be considered to be close to the emission from a monomeric state of the fluorophores.Controlling photophysical properties through non-covalent interactions (NCIs) [17] is more challenging. In early works, Jelley and Scheibe observed the appearance of new absorption and emission bands in pseudoisocyanine solutions with increasing dye-concentration, which were attributed to the formation of aggregates. [18] More recently a different approach for controlling and optimizing the photophysical properties of polyaromatic hydrocarbons in the solid-state has been successfully introduced. Instead of avoiding intermolecular interactions, molecules are designed, which allow specific interactions between their fluorophores in the aggregated state. The occurring π-π interactions lead to the formation of an excited dimer (excimer) after irradiation. [19] The formation of excimers in solution is well known since the investigations of the pyrene excimer by Förster [20] and Birks. [21] The typical features of spectral broadening and increased lifetime are also well investigated. In recent publications, this approach was successfully transferred to efficient solid-state emitters. Through control of the NCIs, the formation of excimers or exciplexes in the solid-state is possible and allows tuning of the photophysical properties. [22][23][24][25][26] Solid-state luminescent materials are essential for the development of optoelectronic devices like lasers, sensors, and organic light-emitting diodes. Organic molecules reveal several benefits like stability, costs, and environmental compatibility compared to metal-based materials. As common organic fluorophores often suffer from aggregation-caused quenching, different strategies have been established to overcome this quenching, which are mainly based...
