Steric Control of Molecular Packing Governs Excimer Emission in Fluorene-Based Molecular Solids

For the rational design of solid-state organic luminophores, achieving predictable control over excimer emission remains a central challenge, as it is governed by subtle variations in molecular packing. Here, we demonstrate that excimer emission in fluorene-based molecular solids is a sterically programmable excited-state pathway that can be selectively enabled or suppressed through molecular design. A systematic series of fluorene derivatives bearing symmetric (2,7-dmF, 2,7-dtbF, and 3,6-dtbF) and asymmetric (2-tbF) alkyl substituents was investigated to elucidate how steric effects control supramolecular organization and excited-state behavior. Combined photophysical characterization, single-crystal X-ray diffraction, and DFT/TDDFT calculations establish a direct structure-packing-property relationship governing emission pathway. Planar fluorene and 2,7-dimethyl derivatives adopt antiparallel π-π stacked arrangements that stabilize excimer states, resulting in intense solid-state emission with high photoluminescence quantum yields (up to ∼60%) and nanosecond-scale lifetimes (up to ∼12 ns), accompanied by delayed components indicative of packing-controlled excimer formation. In contrast, tert-butyl substitution disrupts cofacial π-π interactions, suppressing excimer formation and yielding predominantly monomer-like emission. This steric control is further validated in fluorene: diamantane mixtures, where diamantane acts as a rigid three-dimensional spacer that increases intermolecular separation and inhibits excimer formation. Complementary dimer-based electronic structure calculations and charge-density analyses provide molecular-level insight into how steric constraints modulate intermolecular electronic coupling and excited-state character. These findings establish excimer emission as a packing-controlled and designable excited-state phenomenon and provide general design principles for tuning solid-state luminescence through steric engineering of molecular assemblies.