Abstract
Modulation of excited-state energy-level structures through controlled molecular-stacking arrangement provides an effective strategy to tuning fluorescence and thermally activated delayed fluorescence (TADF) amplified spontaneous emission (ASE), but remains a critical challenge. Herein, we demonstrated the regulation of fluorescence and TADF ASE in organic crystalline polymorphs by controlling the excited-state dynamics through aggregate effects. Experimental and theoretical studies show that green crystals emit green fluorescence ASE while red crystals emit red TADF ASE, because higher degree of J-aggregation in red crystals significantly results in a substantial decrease of energy gaps between singlet and triplet to 0.24 eV for the realization of reverse intersystem crossing process. Our results suggest that molecular packing presents a powerful approach to tailor the radiative channels and fundamentally important to tuning fluorescence and TADF ASE in pure organic crystals.
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