High-efficiency dimer fluorescence system based on π-π interaction between anthracenes: Recognition of excimer

Abstract

Excimer is a class of dimeric excited-state species, which is often formed by association of an electronically excited molecule with the same one in the ground state. In the excited state, the intermolecular interactions are attractive while they are repulsive in the ground state. Therefore, as typical spectral characteristics, excimer usually exhibits red-shifted, structureless and broadened emission spectrum relative to that of monomer. Since Forster and Kasper firstly found the pyrene excimer fluorescence in hexane solution in 1954, aromatic excimer formation has received tremendous attention because of its many potential applications in organic light-emitting diodes (OLEDs), chemo-sensors, bio-probes, high-resolution bio-imaging, organic lasers and so on. Although the spectral characteristics of excimer in rigid media (crystals, glassy matrixes, supramolecular capsules, etc.) are similar to those of excimer in fluid or gas media, the definition of excimer in rigid media is not supported by that in fluid or gas media. Therefore, we prefer to name them as dimer emission as a result of the pre-associated π-π dimer of ground state in solid state. Generally, in the solid state of planar and rigid aromatic molecules, the strong intermolecular π-π interactions tend to quench the solid-state luminescence, which commonly causes very low efficiency of solids. As a solution, the chemical modification with bulky substituents or physical doping was usually adopted to suppress the intermolecular π-π stacking and achieve the high-efficiency single-molecule luminescence. Interestingly, we have found a class of dimer fluorescence in crystal with high efficiency and long lifetime, which is based on intermolecular π-π interaction between two anthracenes in ground state. The emission spectra of these crystals are characterized by those of excimer. Experimental evidences and theoretical calculations revealed the essences for high-efficiency luminescence: on the one hand, the constringent interplanar π-π distance of dimer excited state reinforces the geometric rigidity and significantly decreases the non- radiative energy decay; on the other hand, the monodisperse dimeric π-π stacks produce the pure and uniform dimer emissive excited state to avoid the formation of energy-trapping dark state, which effectively suppresses the non-radiative energy transfer. The explanation of high-efficiency dimer fluorescence has been also evidenced by the further experimental data, i.e., the more anthracene dimers in solids, the higher fluorescence efficiency. Furthermore, based on the relation between molecular conformation and stacked structure in crystal, molecular design strategy of high- efficiency dimer fluorescence is put forward to successfully construct the monodisperse dimeric π-π stacks in solids. The molecular structure should contain two moieties, i.e., π-conjugated group and substituent group, and the spatial position of substituent group leans to the one side of π plane. When two π-conjugated units stack in dimeric form, the substituent groups play a role in isolating every dimer which makes the dimer monodisperse in crystal structure. Intermolecular dimer belongs to the simplest supermolecular system, and is significant for the study on supermolecular luminescence materials. Besides, this review concludes the latest progresses of dimer fluorescence, existent issues and application prospects of supermolecular luminescence materials. Expectedly, new-generation high-efficiency dimer and supermolecular luminescence materials could be developed on the basis of new concepts, new principles and new aplications, as well as the structure-property relationship could be established among molecular (stacking) structure, excited state property and luminous performance.