Mechanochromic Luminescence of 2,6-Bis(4-biphenyl)isonicotinic Acid via Interconversion of Classical/Frustrated Brönsted Pair

Abstract

A practicable strategy to a reversible mechanochromic material featuring interconversion of classical/frustrated Brönsted pairs has been established. We report the mechanochromic property of 2,6-bis­(4-biphenyl)­isonicotinic acid (1), which features a frustrated Brönsted pair in the crystalline form and a classical Brönsted pair after grinding. A large mechanochromic shift was found from 428 to 505 nm. In addition, compound 1 also exhibits acidochromic behavior, which further proves that the formation of an acid-base interaction is responsible for the mechanochromic phenomenon.

Mechanofluorochromism of 2‐Biarylyl Cinchoninic Acids with High Sensitivity and Large Mechanochromic Shift

“…After grinding, a prominent peak around 3450 cm −1 appears, which indicates the formation of hydrogen bonds. We attributed this peak to intermolecular hydrogen bonds of COOH⋅⋅⋅N interactions [12,16] . Therefore, as similar as our previous MFC model, the MFC properties of 2‐biarylyl cinchoninic acids 1 – 4 can be explained as interconversion of classical/ frustrated Brönsted pairs.…”

“…We attributed this peak to intermolecular hydrogen bonds of COOH•••N interactions. [12,16] Therefore, as similar as our previous MFC model, the MFC properties of 2-biarylyl cinchoninic acids 1-4 can be explained as interconversion of classical/ frustrated Brönsted pairs. It is worth mentioning that compound 4 has the property of multiple responses.…”

“…The large mechanochromic shift is proposed to be due to the formation of classical Brönsted pairs, which is largely different compared with the initial state (frustrated Brönsted pairs). 12 Then different substituents have been tried to tune the MCF property. In the initial state of 2, the fluorescence emission peak blue-shifts to 450 nm due to the electron-withdrawing fluoro group.…”

“…Recently, we reported 2,6‐bis(4‐biphenyl)‐isonicotinic acid I (Figure 1) which shows interesting MCF property with luminescence change from 428 nm to 505 nm with grinding. It is supposed that the MCF property is due to the interconversion of classical and frustrated Brönsted pairs [12] . In the ground state, this compound has isolated acidic (COOH) and basic sites (N of pyridine).…”

“…It is supposed that the MCF property is due to the interconversion of classical and frustrated Brönsted pairs. [12] In the ground state, this compound has isolated acidic (COOH) and basic sites (N of pyridine). With external stimulus, the newly-built interaction of acidic and basic sites leads to formation of Brönsted pairs, which results in the luminescence change.…”