Photo-Induced Structure Transformation And Mechano-Chromic Luminescence Based On The Anthracene-Acylhydrazone Organogels

Low molecular weight organogels are a type of soft material that have the ability to achieve the aggregation of self-assembled formed through weak non-covalent interactions, which has caused public concern. We can introduce a variety of functional groups into the structure to gain the smart responsive gels due to its features of easy modification. With the constant developments of supramolecular chemistry, researchers have not been satisfied with mono-responsive gel. It has increasingly attracted attention that one sample has multi-response. Organogels present different physical and chemical properties by external stimuli, such as mechanical strain, light, ultrasound, electric field, magnetic field, metal ions, anion, acid etc. On the basis of our group’s previous work, this paper is devoted to study the photo-induced structure transformation and mechano-chromic luminescence based on the anthracene-acylhydrazone organogels. The main contents are as follows:1. The gelator AHP-m B8 has excellent gel abilities, which can be form stable gel in many solvents, such as ethanol and cyclohexane, dimethyl sulfoxide. And it has not collapsed after being placed at room temperature for a few months. The morphology with three-dimensional fiber network was observed through SEM; and the structure of the gel systems was characterized by FT-IR, 1H NMR, UV-visible spectroscopy and X-ray diffraction(XRD). The results confirmed that the main driving forces of the gel formation are intermolecular hydrogen bonds between amide group, and π-π interactions between anthracene groups. The AHP-m B8 organogel has the property of aggregation-induced emission enhancement. In addition, AHP-m B8 was suitable as an efficient room-temperature phase-selective gelator and recovery of various aromatic solvents. The rhodamine B can be extracted by AHP-m B8 and the purification ability is as high as 99.3%.2. AHP-m B8 organogels have the properties of photo-induced structure transformation. The AHP-m B8 gel was found to collapse gradually and finally turned into transparent solutions upon irradiation by Visible light(450 nm) at room temperature. The gel phase could be restored through heated to the boiling point of cyclohexane and then cooled to room temperature. The AHP-m B8 gel showed a photo-induced fiber–globular morphology change. And the samples were tested by UV-visible spectra, fluorescence spectra and mass spectrometer. The experiments demonstrated the mechanism of gel-sol transition under the condition of visible light may be due to Visible light irradiation leading to C = N groups from stable cis-isomer to unstable trans-isomer. Due to anthracene group larger space steric hindrance, cis-isomer hinders the C = O and NH intermolecular hydrogen bond and induces the solubility of the gelator increasing, which further weaken the anthracene π-π interaction. Thus the photoisomerization of the C = N group both weakened and destroyed the intermolecular hydrogen bonding and π-π interactions, which is the main driving force of the gel formation, so it induced the transition of the gel into a solution.3. AHP-m B8 xerogel from DMF / H2 O = 3/1, cyclohexane, and ethanol has the properties of mechano-chromic luminescence. Compared with the sample before grinding, the fluorescence emission peaks after grinding have different degrees of red-shift. And the grinding samples can restore initial color through solvent-treated or spontaneously within 2 hours. IR results confirm that the intermolecular interactions of the samples has a significant change before and after grinding; XRD and DSC results confirm the aggregate structure of the sample after grinding has a big change. Combined with the literature, we propose the mechanisms of mechano-chromic luminescence in AHP-m B8 : The maximal emission peak of after grinding the sample occurs red shift in this study is due to the increase of degree of overlap between π-π anthracene chromophore, and the change of the overlap degree is attributed to the structure change of the aggregate led by grinding the sample.