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 Tn states introduced in cocrystals decreases the S1–Tn energy gap, meanwhile multiple intermolecular halogen bonding enhances the spin‐orbital coupling. As a result, the S1 → Tn intersystem crossing rate (kISC) is accelerated by 2 orders of magnitude, making kISC comparable and faster than the fluorescence decay rate kFl 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.

Recent Advances of Cocrystals with Room Temperature Phosphorescence

“…Cocrystallization of the organic phosphors can provide a model platform to plan and accomplish new organic materials having persistent RTP with reasonable efficiency. [23,40,41] Though, there are some critical problems regarding to their preparation: [42] Such as, lack of the clear concept for interaction mechanism and principles for cocrystallization, since it is not possible to cocrystallize every pair of acceptor and donor together; how to practically choose suitable molecules for sure cocrystallization; [43,44] and how to make sure selected conformers structurally match and cocrystallize. [45] Besides, the correlation among crystal systems and their properties within cocrystals along with the mechanism of electronic interaction between repeating units until now is ambiguous.…”

“…[91,92] Here we will discuss about some specific examples of cocrystals having π-π stacking with phosphorescence. For instance, in cocrystal C1b (having 2:1 molar ratio of donor and acceptor) [41] the D32 and A14 molecules have slipped stacking along the b-axis and D32 moiety gives rise to a large spatial overlap between adjacent π-systems within π-stacking, implying strong π-π interactions (Figure 17a).…”

“…The phosphorescence decay curve of cocrystal d) C1b and e) C1c at 550 nm.f ) The molecular packing along the ac plane as well as the intermolecular interactions for cocrystals C1b and C1c. Reproduced with permission [41]. Copyright 2018, Wiley-VCH.…”

“…Figure 17. a) The longitudinal displacement and interplanar distance between two parallel adjacent D32 molecules in π-stack of cocrystal C1b (d L-L = 4.787 Å × cos46.4° = 3.30 Å and d = 3.363 Å, respectively). Reproduced with permission [41]. Copyright 2018, Wiley-VCH.…”

Rare Earth‐Free Luminescent Materials for WLEDs: Recent Progress and Perspectives

“…Moreover, dual‐channel fluorescence and phosphorescence emissions originated from both hydrogen‐ and halogen‐bonding interactions covered the whole visible region, thus giving ideal white‐light emission with CIE coordinates of (0.31, 0.31) and a quantum yield of 16.6%. [ 135 ] Table 4 was constructed to compare and analyze the various characteristics of the aforementioned materials.…”