Reversible Crystal‐to‐Crystal Phase Transitions with High‐Contrast Luminescent Alterations for a Thermally Activated Delayed Fluorescence Emitter

Abstract

Merging thermally activated delayed fluorescence (TADF) and mechanochromic luminescence (MCL) into one single molecule is a promising strategy for developing multifunctional organic materials. Herein, a unique multifunctional molecule TPA‐DQP, comprising a large π‐conjugated diquinoxalino[2,3‐a:2′,3′‐c]phenazine (DQP) as the acceptor and triphenylamine (TPA) as the donor, is designed and synthesized. TPA‐DQP possesses polymorphism, efficient TADF emission as well as MCL property with high‐contrast in emission colors from 576 to 706 nm. Reversible crystal‐to‐crystal phase transitions in response to external stimuli such as vapor fuming and heating are realized on the basis of the two polymorphs of TPA‐DQP. The distinct crystal‐to‐crystal phase transition is attributed ultimately to the change of packing arrangements and intermolecular interactions of the two polymorphs under stimuli. Furthermore, TPA‐DQP‐based organic light emitting diode (OLED) device achieves external quantum efficiency as high as 18.3% at 676 nm, which represents the best performance for deep‐red OLEDs based on MCL‐active TADF emitters. This study reports a novel MCL‐active TADF material that exhibits crystal‐to‐crystal phase transition and brings insight into the underlying relationship between molecular packing modes and the photoluminescent behavior.

A high contrast mechanochromic luminescent diacetylene-linked bis-benzothiadiazole derivative

“…All new compounds were purified by silica gel column chromatography, using a mixture of hexane/CH 2 Cl 2 (3:1) as eluent and characterized by 1 H NMR, 13 C NMR, and mass spectrometry (see the ESI † for experimental details).…”

“…Single crystals diffraction analysis is probably the most powerful tool to attain such understanding, as it provides accurate information on the relative arrangements of neighbouring molecule and the interactions involved among them. 9,10 However, although there are remarkable examples in the literature of molecular materials that retain their crystal integrity after a mechanical operation, [11][12][13][14] the emission switching between polymorphs is rather exceptional as it involves the transformation of the molecular structure in the constrained environment of the crystal. Notwithstanding, some design hints have been already proposed to induce a mechanochromic behaviour 15,16 among them the presence of weak interaction planes which allow the relative slippage of the molecules affecting the interactions with their next neighbours, and consequently their optical properties.…”

Recent Advances in Mechanochromic Luminescence of Organic Crystalline Compounds

“…Li and coworkers reported the high contrast MCL by a polymorphic crystal of triphenylaminesubstituted diquinoxalino[2,3-a:2¤,3¤-c]phenazine 32 with thermally activated delayed fluorescence (TADF) (Figure 8b). 72 Depending on the crystallization conditions, the TADF emitter 32 crystalized as yellow-emissive or red-emissive polymorphic crystals (crystal-Y and crystal-R). The difference of the emission color could be explained by their herringbone or lamellar structures.…”

Molecular copper iodide clusters: a distinguishing family of mechanochromic luminescent compounds

“…Mechanochromic luminescent materials displaying switchable luminescence properties in response to external mechanical force are currently attracting wide interest for potential applications in strain detection, optical recording, storage and anti-counterfeiting, among others. [1][2][3][4][5][6][7] Regarding their potential applications, one of the most attractive feature of these stimuli-responsive materials is the straightforward detection of the emission changes by simple non-invasive spectroscopic means or even with the naked eye. [8][9][10][11] Even though the number of reported compounds exhibiting luminescence mechanochromism has greatly increased over the past decade, in-depth studies analysing the underlying mechanisms remain relatively rare.…”