Excited‐State Modulation for Controlling Fluorescence and Phosphorescence Pathways toward White‐Light Emission

Abstract Luminescence originated from singlet (S₁) and triplet (T₁) states is categorized as fluorescence and phosphorescence, respectively. Modulation of fluorescence and phosphorescence pathways plays a central role in developing organic luminescent materials, but remains difficult because of the lack of control ways. Here, luminescence of cocrystals of 1,7‐phenanthroline (PR) and 1,4‐diiodotetrafluorobenzene (DITFB) can be switched by adjusting their stoichiometry, from bluish fluorescence for 1:0 pure PR crystal (P1D0) to yellowish phosphorescence for 1:1 PR:DITFB cocrystal (P1D1). More importantly, 2:1 PR:DITFB cocrystal (P2D1) is found to exhibit dual fluorescence and phosphorescence simultaneously, thus giving rise to white‐light emission. Experimental and time‐dependent density‐function‐theory results reveal that although the S₁ and T₁ energies keep invariable, high‐lying T_n states introduced in cocrystals decreases the S₁–T_n energy gap, meanwhile multiple intermolecular halogen bonding enhances the spin‐orbital coupling. As a result, the S₁ → T_n intersystem crossing rate (k_ISC) is accelerated by 2 orders of magnitude, making k_ISC comparable and faster than the fluorescence decay rate k_Fl for P1D0 (fluorescence), P2D1 (dual emissions), and P1D1 (phosphorescence), respectively. The results provide not only a quantum‐mechanical understanding but also a novel strategy to modulate the excited‐state dynamics toward fluorescence and/or phosphorescence emissions for luminescent materials.