4,7‐Bis[3‐(dimesitylboryl)thien‐2‐yl]benzothiadiazole: Solvato‐, Thermo‐, and Mechanochromism Based on the Reversible Formation of an Intramolecular B−N Bond

Abstract

4,7-Bis-[3-(dimesitylboryl)thien-2-yl]benzothiadiazole (1) and monoborylated derivative 2 were synthesized and their chromic behavior was investigated. Photophysical measurements, single-crystal XRD analysis, and theoretical calculations revealed that an intramolecular B-N coordination bond formed reversibly. The equilibrium of this reversible bond formation depends on the solid-state structure, solvent, temperature, and mechanical forces, and leads to significant changes in the electronic structure and chromic behavior of these molecules. The responsiveness toward external stimuli, resulting in the reversible formation of open and closed forms of this system, is achieved through weak intramolecular B-N coordination bonds induced by the steric bulk of the mesityl groups on the boron centers.

Locking the Coplanar Conformation of π‐Conjugated Molecules and Macromolecules Using Dynamic Noncovalent Bonds

“…Compared to the strong N→B coordinative bonds in 14 and 15 that shared a similar six‐member ring architecture, the N→B coordination in 18 was much weaker because of the bulky mesitylene ligand. As a result, the energy difference between the open conformer and the “mono‐closed” conformer was as small as 1.02 kcal/mol according to DFT calculations . The coplanar and non‐coplanar conformers co‐existed in solution, and the temperature variation affected the equilibrium of forming N→B coordinative bonds and changed the population of each conformer (Figure d).…”

“…In addition to Lewis basic molecules, the reversible formation/dissociation of N→B coordination bonds can be driven by the change of temperature. Wakamiya et al developed thermochromic materials based on a N→B coordination bridged π‐ conjugated molecule 18 (Figure d) . Compared to the strong N→B coordinative bonds in 14 and 15 that shared a similar six‐member ring architecture, the N→B coordination in 18 was much weaker because of the bulky mesitylene ligand.…”

n‐Channel Organic Semiconductors Derived from Air‐Stable Four‐Coordinate Boron Complexes of Substituted Thienylthiazoles

“…In this group of compounds a choice of the ligand and boron substituents is, however, also critical for stability and properties. For instance, some BF 2 complexes of diketonates undergo hydrolysis in contact with moisture, while sterically demanding mesityl (Mes) substituents on the boron center can induce stimulus‐responsive material behavior . Yet, there are plenty of molecules whose exciting photophysical features can be explored in air‐ and moisture‐stable devices, mainly in OLEDs, or as imaging materials.…”

Synthesis of Azole-fused Benzothiadiazoles as Key Units for Functional π-Conjugated Compounds

“…Moreover, in thiazole-fused BT units, the electronic structure could be further modulated by varying the oxidation state of the sulfur atom in methylthio group at the fused thiazole ring. A variety of D-A molecules and polymers would be developed using these electron-accepting units by combining with various electron-donating and -conjugated units [25][26][27][28][29][30][31][32], which is currently in progress in our laboratory.…”