Mechanochromism of π-conjugated borate derivatives and the effect of intermolecular non-covalent forces on molecular packing and mechanochromic property

Abstract

We prepared three π-conjugated pinacol borate derivatives anthryl dioxaborolane (ANB), pyrenyl dioxaborolane (PYB) and perylenyl dioxaborolane (PEB) based on anthracene, pyrene and perylene moieties, and studied their optical properties in solid and solution. Interestingly, the three molecules were structurally similar, yet exhibited different optical properties and mechanochromic behavior. Crystal structure studies showed that the mechanical fluorescent response of ANB, PYB and PEB were closely related to the molecular packing modes. Although there were multiple π-π and σ-π interactions in all three molecules, the conjugation degree of ANB was relatively low, resulting in a weak π-π interaction. Therefore, the effective partial parallel packing between adjacent ANB could not form in the initial state, and external mechanical stimulation could not induce the change of optical properties. In PYB and PEB molecules with a high conjugation degree, the π-π interaction was stronger. A partial parallel packing state was formed between the molecules under the synergistic action of π-π and σ-π interaction. Under external disturbance, the relative slippage between molecules was easy to occur. Thus, the characteristic static excimer fluorescence was displayed due to the enhancement of parallel packing. Since the intermolecular interactions of PYB and PEB were a kind of weak non-covalent forces, the packing state changed by external force could return to the initial state at room temperature, showing reversible mechanochromic characteristic. This kind of room temperature reversible force sensitive material constructed by weak non-covalent interactions was expected to be applied and popularized in the fields of encrypted communication and anti-counterfeiting display.