High-Efficiency Excimer Luminescence from Discrete π–π Stacked Dimers

Conspectus Molecular aggregates serve as an important platform for the practical applications of organic luminescent materials. Within these aggregates, both molecular arrangements and intermolecular interactions provide crucial avenues for modulating the luminescence properties. Given that organic chromophores are typically characterized by planar and rigid π-conjugated units, π–π stacking and π–π interactions are determinative for the formation of aggregates and the regulation of photophysical properties. However, strong π–π stacking usually leads to luminescence quenching in organic aggregates, due to the forbidden radiative transition and the activated nonradiative transition. Specifically, the strong π–π stacking facilitates the formation of excited dimers, i.e., excimers, and these excimers have long been regarded as “dark” states that are detrimental to luminescence. This inherent quenching effect is in conflict with the requirement for high luminescence efficiency of organic materials in optoelectronic applications. In this Account, we highlight our recent findings: strong π–π stacked dimers can achieve high-efficiency excimer luminescence when they are spatially discrete in aggregates. The findings stem from the crystal packing structure observation of a thianthrene-modified anthracene compound, which exhibits a unique discrete π–π dimer stacking of anthracene with an unexpectedly high photoluminescence quantum yield (Φ) of 80%. Theoretical and experimental investigations reveal that this discrete π–π dimer stacking promotes the formation of a pure and single excimer state. Building on this, high-efficiency excimer luminescence can be ascribed to the special nature of this excimer state: a hybridized local and charge-transfer state, which consists of two locally excited components of the monomers and two charge-transfer components between the two monomers. Compared to the monomer, although the radiative transition rate of the excimer is reduced by 2 orders of magnitude, the suppression of nonradiative transition rate is even more significant. The combined effect of these factors enables high-efficiency excimer luminescence to be achieved in discrete π–π stacked dimers. Based on these findings, a molecular design principle was proposed to construct the discrete π–π stacked dimers. The molecular structure consists of two moieties: a planar aromatic core (π moiety) and a unilateral substituent (side moiety). On the one hand, the unilateral substituent is attached to the π moiety to guarantee the dimer formation in antiparallel π–π stacking with the aid of steric hindrance. On the other hand, two substituents can act as the spacers to isolate the formed π–π dimers from each other by means of their appropriate size and orientation. To verify the universality of this design strategy, various π and side moieties were combined together to prepare a series of crystalline materials with discrete π–π stacked dimers and tunable emission colors. Furthermore, the efficient excimer luminescence was also achieved in films and nanoparticles, indicating that they have unique application potential in stimuli-responsive systems, organic light-emitting diodes, and biological imaging. As a perspective, these discrete π–π stacked dimers, which act as “mini-excitons”, provide an ideal transitional model for understanding of the photophysical properties from single molecule to aggregates, and facilitate further exploration of supramolecular luminescence properties.