Modulating Room-Temperature Phosphorescence-To-Phosphorescence Mechanochromism by Halogen Exchange

Abstract

Modulating the stimulus-responsiveness of a luminescent crystal is challenging owing to the complex interdependent nature of its controlling factors, such as molecular structure, molecular conformation, crystal packing, optical properties, and amorphization behavior. Herein, we demonstrate a halogen-exchange approach that disentangles this problem, thereby realizing the modulation of room-temperature phosphorescence-to-phosphorescence mechanochromism. Replacing the bromine atoms in a brominated thienyl diketone with chlorine atoms afforded isostructural crystals; i.e., molecules with different halogen atoms exhibited the same molecular conformation and crystal packing. Consequently, amorphization behavior toward mechanical stimulation was also the same, and the phosphorescence of amorphous states originated from the same conformer of each diketone. In contrast, the phosphorescence properties of each conformer were modulated differently, which is ascribable to heavy atom effects, resulting in the modulation of the mechanochromism. Thus, halogen exchange is a promising approach for modulating the stimulus-responsive photofunctions of crystals involving spin-forbidden processes.

Photoinduced Crystal Melting with Luminescence Evolution Based on Conformational Isomerisation

“…Most notably, while the crystal consists of the poorly emissive skew conformer, the planar conformer is more stable in the excited state and is responsible for the RTP in the SCL state. 31,[37][38][39][40] Such conformation-dependent RTP would be promising to visualise PCMT.…”

“…S8). 31,[37][38][39] Therefore, the luminescence evolution reflects the local melting process, which involves the conformation change from skew to planar. [50][51][52]…”

Light-melting Diketone Crystal with ON-OFF-ON Luminescence Evolution

“…Most notably, while the crystal consists of the poorly emissive skew conformer, the planar conformer is more stable in the excited state and is responsible for the RTP in the SCL state. 42,[46][47][48][49] Herein, using SO crystals, we demonstrate for the first time the phenomenon of PCMT with luminescence, which is reflective of the molecular environment and allows for the visualization of the local melting process in real time.…”

“…Considering that SO exhibits the skew conformation in crystal form (see below), 42 the initial green emission is caused by the skew conformer (Figure S6). 42,[46][47][48] Therefore, the changes in the luminescence reflect the local melting process, namely, how the molecular conformation changes from skew to planar. [56][57][58] Hence, the intermediacy of the nonemissive regime must be the key to understand the process.…”

“…46 Meanwhile, as the second step, a larger conformational change is allowed, resulting in the planar conformer: the yellow RTP occurs despite the loosened environment, because the planar conformer is inherently more emissive than the skew conformer. 42,[46][47][48] This loosening and the corresponding conformational changes occur repeatedly in the crystal during continuous UV irradiation, finally leading to greater disordering in the longer range and the eventual macroscopic melting of the bulk crystal (the third step). Thus, the stepwise changes in the luminescence of the PCMT-active crystal allowed the visualization of its molecular-level melting process in real time.…”