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.

Stimuli-responsive luminescence from polar cyano/isocyano-derived luminophores via structural tailoring and self-assembly

“…Based on the crystallography data, the absence and presence of π–π interactions between adjacent phenyl dicyanos were suggested to be responsible for the distinct solid MCL properties of these isomers. 72 Further work revealed that two polymorphous meso -dicyanovinyl-substituted TPEs also followed the above rule ( 2-12 , 2-13 , Fig. 6b), where the compounds showed ultrahigh-contrast MCL (Δ λ em = 141 nm).…”

“…Nevertheless, the photoinduced cis – trans isomerism is more considerable based on many more demonstrations, which could also produce photochromism. 72 In the report of Sun's group, a simple cyanostilbene derivative also displays a complex solvatochromism, acidichromism, mechanochromism, and photochromism ( 2-24 ). This multiple stimuli response benefits from an additional dimethylamino group to afford enhanced CT specialty and protonation site.…”

Polymorphous meso-Dicyanovinyl-Substituted Tetraphenylethylene Derivative Showing Distinguishing Two-Color Emission and Ultrahigh-Contrast Mechano-Responsive Luminescence Behavior

“…The dicyanovinyl (DCV) group, owning to its strong electron-withdrawing property and good planarity, was frequently used in building red-emitting emitters. So far, a variety of donor–acceptor luminogens based on TPE and DCV have been exploited. − The DCV units of these compounds were mostly located in the para -substituted benzene ring to better represent the intramolecular charge transfer (ICT) effect. Our group recently reported a pair of isomers whose single-crystal structures showed that one para -substituted DCV group and its connected benzene ring were nearly coplanar with the assistance of intramolecular hydrogen bonding .…”

“…So far, a variety of donor–acceptor luminogens based on TPE and DCV have been exploited. − The DCV units of these compounds were mostly located in the para -substituted benzene ring to better represent the intramolecular charge transfer (ICT) effect. Our group recently reported a pair of isomers whose single-crystal structures showed that one para -substituted DCV group and its connected benzene ring were nearly coplanar with the assistance of intramolecular hydrogen bonding . As far as we know, all of these reported DCV-modified TPE luminophores exhibited only one crystalline state and moderate contrasts of MRL performance. − As an extensive study in this area, we tried to implant herein two rigid DCV units into one benzene ring of TPE at two meso -positions ( TPE- m -2DCV ) for the first time in order to avoid the intramolecular steric hindrance impacts between two DCV groups themselves ( ortho ) as well as those between DCV groups and their adjacent aromatic rings ( para ).…”

“…Our group recently reported a pair of isomers whose single-crystal structures showed that one para -substituted DCV group and its connected benzene ring were nearly coplanar with the assistance of intramolecular hydrogen bonding . As far as we know, all of these reported DCV-modified TPE luminophores exhibited only one crystalline state and moderate contrasts of MRL performance. − As an extensive study in this area, we tried to implant herein two rigid DCV units into one benzene ring of TPE at two meso -positions ( TPE- m -2DCV ) for the first time in order to avoid the intramolecular steric hindrance impacts between two DCV groups themselves ( ortho ) as well as those between DCV groups and their adjacent aromatic rings ( para ). The 1,3-di­(2,2-dicyanovinyl)­benzene ( P- m -2DCV ) unit could not maintain the planar conformation under the synergistic interactions of hydrogen bonds, steric hindrance among two planar DCV units and their mother phenyl ring, and corresponding packing modes.…”