Synthesis And Mechanofluorochromic Properties Of Novel β-iminoenolate Boron Complexes

Mechanofluorochromic(MFC) materials are a new kind of stimuli-responsive materials, whose fluorescent colors can change in response to external mechanical forces and be restored to the original states by annealing or fuming. Due to their potential applications in fluorescent switch, mechanosensors, data storage and security ink, MFC materials have gained increasing interest. Till now, the derivatives of tetraphenylethene, 9,10-divinylanthracene and oligo(p-phenylene vinylene) are studied systematically as MFC materials. It is urgent to develop new series of MFC materials. In addition, it is well-known that N,O-chelated boron complexes often exhibit strong luminescence in solid states and have not been systematically investigated in MFC materials. Therefore, we designed and synthesized new β-iminoenolate boron complexes with different chelating and functional groups in this thesis and investigated their MFC properties. Some creative results have been obtained and outlined below:(1) Carbazole and t-butylcarbazole functionalized β-iminoenolate boron complexes CB and TCB were synthesized. They emitted strong fluorescence in solutions, and the ΦF values of CB and TCB in CH2Cl2 were 0.58 and 0.65, respectively. Notably, their solid emission was dependent not only on the morphologies but also on the molecular structures. For instance, the as-synthesized crystal of CB emitted orange light with two emission bands derived from the monomers and the excimers, respectively. After grinding the as-synthesized crystal for a while, the ground powder 1 composed of small crystals and a certain amount of monomers emitted bright yellow light since the emission intensity of excimers decreased and emission of monomers increased. On further grinding for a long time, the obtained amorphous ground powder 2 of CB emitted dark green light on account of the disappearance of the excimers. TCB emitted sky blue light under UV irradiation in the as-synthesized crystal because no excimers were formed due to the steric hindrance of t-butyl. After grinding, the amorphous ground powder of TCB emitted bright green light derived from excimers. Additionally, their emitting colors could be transformed reversibly during the repeating processes of grinding/fuming or heating.(2) β-Iminoenolate boron complexes bearing non-planar phenothiazine(P2B and P16B) and phenothiazine-S,S-dioxide(PO2B and PO16B) were synthesized. It was found that the D-π-A type P2 B and P16 B gave ICT emission, while no ICT emission was observed for PO2 B and PO16 B. Interestingly, P2 B and P16 B with ICT emission exhibited high-contrast MFC behaviors. For example, the as-synthesized crystals of P2 B gave two emission bands at 531 nm and 680 nm. After grinding, the amorphous ground powders emitted deep orange-red light. After fuming with DCM or heating, they emitted bright yellow(548 nm) or yellowish green(526 nm) light, respectively. The as-synthesized crystals of P16 B emitting yellow light centered at 533 nm could be changed into ground powders emitting orange light located at 582 nm upon grinding. The introduction of the long carbon chain reduced the phase conversion temperature and led to the self-recovery of its emitting color. In the case of PO2 B without ICT emission, low-contrast MFC properties were observed. The as-synthesized crystals emitted yellow light centered at 545 nm from the excimers. After grinding, the amorphous state emitted yellowish green light centered at 518 nm. Therefore, new MFC materials with multi-color emission or self-recovery ability of emitting color were developed. It provided new strategy to design novel MFC materials.(3) A new β-iminoenolate boron complex with a terminal triphenylamine(TP) was synthesized, and it could emit strong fluorescence in solutions and in solid states. Two types of crystals were obtained via evaporation of the solutions of TP in different solvents. A yellow ribbon-like crystal(Y-crystal) giving yellowish green emission centered at 518 nm was obtained from tetrahydrofuran(THF) solution, and a green needle-like crystal(G-crystal) emitting green light with maximum emission at 495 nm was yielded from dichloromethane(DCM)/petroleum ether solution. It was found that the molecules adopted a stacking mode of J-aggregation in Y-crystal, and molecules stacked head-to-tail in G-crystal. Due to the strong π-π interactions in Y-crystal, the absorption and emission bands of Y-crystal gave a red-shift comparing with those of G-crystal. Besides the polymorphism feature, TP exhibited MFC properties. Upon ground, Y-crystal and G-crystal could transform into the similar ground powders with yellow emission centered at 533 nm. Interestingly, when the ground powder formed from Y-crystal or G-crystal was fumed with DCM or heated, the XRD pattern and the fluorescence emission spectrum of the fumed sample would be recovered to their own initial crystal states. Such memory ability of the molecular packing mode during reversible MFC processes might be useful for our well understanding the MFC mechanism and helpful to design new mechanosensors.(4) A series of novel β-iminoenolate boron complexes(BB, B2 B, B3 B, B4 B, B24 B, B25 B and B345B) were synthesized, which were modified by methoxyl groups in different positions of benzene rings. All of them exhibited AIE properties. For B2 B, the emission intensity in THF/H2O(v/v=1/19) was 91-fold higher than that in THF. Specifically, B4 B formed nanoparticles in THF/H2O(v/v=2/8) and emitted the enhanced green fluorescence, exhibiting yellow emission in THF/H2O(v/v=1/9). Their molecular stacking modes in crystal states enormously affected their emission. B2 B emitted blue light due to the weakest π-π interaction. BB, B3 B and B24 B emitted blue-green light on account of their similar molecular packing modes. Additionally, in the crystal of B4 B, π-aggregates were generated due to the strongest π-π interaction, leading to the special yellow emission. On the other hand, B4 B exhibited multi solid emission changes during MFC processes. When B4 B was ground into ground powder 1, π-aggregates were damaged and the emitting color changed from yellow to green. If a small amount of ground powder 1 was further ground on a parchment, it would become ground powder 2 emitting blue light derived from the isolated molecules. Besides, ground powder 1 could be self-recovered or restored to the initial state after DCM fuming. Ground powder 2 could be restored to ground powder 1 after a drop of DCM was dropped on the vegetable parchment. This kind of multi-color emissive MFC materials might have potential applications in new anti-fake labels and in mechanic sensors, which can distinguish different strength of forces so as to well protect the fragile products.(5) ?-Carbonyl quinoxaline derivatives BQ1, BQ2 and BQ3 and the corresponding boron complexes BQB1, BQB2 and BQB3 were synthesized. The three boron complexes exhibited strong emission in both solutions and solid states, while the quinoxaline derivatives were non-emissive in solutions or amorphous states, but gave strong emission in crystalline state, meaning CIE activity. The fluorescence "ON/OFF" switch based on the transformation between crystalline state and amorphous state of BQ2 was obtained, and a multi-color "ON/OFF" switch of BQ1 was gained due to its MFC property.