Rational Control of Charge Transfer Excitons Toward High‐Contrast Reversible Mechanoresponsive Luminescent Switching

Abstract

A practicable strategy to rationally obtain the reversible mechanochromic luminescent (MCL) material with high‐contrast ratio (green versus red) has been established. By introducing a volatile third party (small‐sized solvent molecules) into the lattice of charge transfer (CT) cocrystal of mixed‐stacking 1:1 coronene (Cor.) and napthalenetetracarboxylic diimide (NDI), a noteworthy reconfigurable molecular assembly is ingeniously achieved owing to the loosely packing arrangement as well as weakened intermolecular interactions. Accordingly, the CT excited state, strongly corresponding to the molecular stacking modes, can be intentionally tailored through external stimulus (heating, grinding, or solvent), accompanying distinct changes in photophysical properties. Subsequently, a high‐contrast reversible MCL with highly sensitive and good reproducibility is realized and the underlying mechanism is thoroughly revealed.

Red to Near‐Infrared Mechanochromism from Metal‐free Polycrystals: Noncovalent Conformational Locks Facilitating Wide‐Range Redshift

“…[3] Surprisingly, there have been few reports on PCMs that indicate the wide-range redshift (Dl > 150 nm) in the deep-red/near-IR regions before or after exposure to mechanical stimuli. [1,4] As previously reported, [5] the coplanar fused-ring geometry facilitates p-electron delocalization and decreases the energy band gap, leading to near-IR fluorescence. Nevertheless, the aggregated state of ground powder with a high degree of coplanarity is rather unstable due to the large rotational barrier (see Figure 1 a) [6] that exists between two the conjugated unsaturated and/or (hetero)arene moieties.…”

“…To the best of our knowledge, the magnitude of the spectral shift for Re-phase upon mechanical grinding (Dl em , max = 160 nm) is rarely observed in metal-free organic PCMs. [1,4] Interestingly, after the addition of several drops of DCM to the ground sample, the PL colour could reversibly switch back to bright red ( Figure S8a). The PL changes of Re-phase were ascribed to the phase transition from the crystalline state to the amorphous state during the grinding process, as confirmed by Xray diffraction measurements.…”

Organic molecular aggregates: From aggregation structure to emission property

“…[ 95 ] Interesting work has been done by Wang and coworkers by introducing a volatile third party THF to obtain a reconfigurable molecular assembly in a ternary solvated cocrystal (NDI‐Cornene)·THF. [ 96 ] Due to the weakened intermolecular interactions, as shown in Figure 12C, the loosely stacked modes can be deliberately adjusted by external stimuli (heating, grinding, or solvent), accompanied by obvious changes in photophysical properties. It is not difficult to conclude from the above examples that cocrystal strategy is an effective method to control luminescent performance.…”

“…(C) Diagram of the transformation of molecular packing in the lattice based on ternary cocrystal (NDI‐Cornene)·THF under multistimuli. [ 96 ] Copyright 2020, John Wiley and Sons…”

Recent Advances in Mechanochromic Luminescence of Organic Crystalline Compounds

“…Very recently, Hu, Zhang, and coworkers reported a rational strategy to prepare MCL crystals (Figure 9e). 90 A loosely packed arrangement of naphthalenetetracarboxylic diimide 43 and coronene (44) was constructed by introducing small solvent molecules into the lattice of their cocrystals. The red-emissive ternary cocrystal 43/44/THF ( em = 640 nm) could easily be converted to loosely packed co-assembled structures by removing THF molecules on heating.…”