Isomeric Pair of E/Z Tetraphenylethene-Cored Luminogens Showing Distinguishing Mechanoresponsive Luminescence Turn-On and Two-Color Behavior

Abstract

A pair of tetraphenylethylene (TPE)-based E/Z isomers (E-TPE2PD and Z-TPE2PD) using two dicyanovinyl groups as the electron-accepting moieties have been synthesized and separated successfully. Reversible E/Z isomerization for each isomer is achieved, in which the isomerization speed from E to Z is higher than that from Z to E. This pair of pure E/Z isomers exhibits obvious AIEE properties. Z-TPE2PD shows highly morphology-dependent solid-state photoluminescent properties, whose solid-state emission is red-shifted from 564 to 588 nm with an enhancement of crystalline degree together with a decrease in the emission intensity. In contrast, E-TPE2PD leads to a slightly blue-shifted emission of 5 nm dependent on its crystalline degree. To the best of our knowledge, the discrepancy in emissive color between TPE-based E and Z isomers is much larger than most of the reported ones. The absence and presence of π–π interactions between adjacent PDCV units observed in their crystal packing structures are suggested to be responsible for the distinct solid fluorescence of E and Z isomers. More interestingly, Z-TPE2PD exhibits blue-shifted mechanoresponsive luminescence (MRL) turn-on behavior when subjected to grinding, while E-TPE2PD shows a remarkable two-color photoluminescence (PL) nature. This paper offers a synthetic strategy by introducing strong polar dicyanovinyl groups into the TPE core to increase the molecular polarity difference of E and Z isomers and facilitate their separation, which makes the exploration of distinguishing morphology-dependent solid-state PL properties possible. This is the first report on an isomeric pair of E/Z tetraphenylethene-cored luminogens displaying different MRL turn-on and dichromic behavior.