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

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.

Regioselective Synthesis of 5-Propyl-2-((trityloxy)methyl)thiophene-3-carbaldehyde

5-propyl-2-((trityloxy)methyl)thiophene-3-carbaldehyde was synthesized by using the concept of chemo- and regioselective Br/Li exchange reaction from 3-bromo-5-propyl-2-((trityloxy)methyl)thiophene. This is a five-step protocol starting from thiophene with an overall yield of 33%. These lithium/halogen exchange reactions were carried out at −78 °C to rt over the period of 1 to 18 h depending on the reactivity of electrophiles.

Gram-scale Preparation of Acyl Fluorides and Their Reactions with Hindered Nucleophiles

A series of acyl fluorides was synthesized at 100 mmol scale using phase transfer catalyzed halogen exchange between acyl chlorides and aqueous bifluoride solution. The convenient procedure consists of vigorous stirring of the biphasic mixture at rt, followed by extraction and distillation. Isolated acyl fluorides (usually 7 g to 20 g) display excellent purity, and can be transformed into sterically hindered amides and esters, when treated with lithium amide bases and alkoxides under mild conditions.