Preparation And Photoresponsive Properties Of Supramolecular Azopolymers

Azobenzene chromophore is noncovalently linked to polymer chains and supramolecular azopolymers are obtained. The characteristic of supramolecular azopolymers is easy-to-prepare and modular tunability of the structure. The photoresponsive property of supramolecular azopolmers is driven by components and nonconvalent effect between them, and therefore has unexpected changes.A series of supramolecular azopolymers are prepared by connecting components via hydrogen bonding. Azobenzene compounds with different substituted groups are prepared as hydrogen bonding donor and poly (4-vinylpyridine) is prepared as hydrogen bonding acceptor. The photoisomerization behavior and nonlinear photoresponsive property of supramolecular azopolymers are studied. Polarization storage and multilayered data storage have been demonstrated in supramolecular azopolymers based on two-photon process. All the details are shown as follows:1. A series of hydroxyl-azobenzene compounds with different para-subsituted groups were synthesized, containing-H(AzoOH),-CH3(CAzoOH),-OCH3(COAzoOH),-NO2(NAzoOH), respectively. Supramolecular azopolymrs containing azobenzene compound as hydrogen bonding donor and poly (4-vinylpyridine) as hydrogen bonding acceptor were formed via hydrogen bonding between pyridine group and phenol hydroxyl group. The molar ratio of feed pyridine group to hydroxyl group was set as1.0. The form of hydrogen bonding was verified by FT-IR spectropy. The photoisomerization behavior of azobenzene compound and supramolecular azopolymers were studied by UV-vis absorption spectra. The results indicated that azobenzene compound possessing push-and-pull structure quickly reached balance due to its fast thermal relaxation. It took longer for supramolecular azopolymer to complete photoisomerization. The photoisomerization behavior of supramolecular azopolymers was separated as two stages:the photoisomerization rate constant was slower in the first stage due to the restrict of polymer chains; in the second stage, the photoisomerization rate constant was faster owing to the coordination motion of hydrogen bonding.2. Z-scan technique was used to measure the third order nonlinear photoresponsive properties of azobenzene compounds and supramolecular azopolymers.The nonlinear absorption coefficient and nonlinear refractive index were calculated according to formulas. The results indicated that the azobenzene compound possessing push-and-pull structure exhibited larger nonlinear absorption coefficient and nonlinear refractive index. It was mainly due to an easy flow of the charge-clouds and a high degree of charge transfer. The effect of hydrogen bonding between azobenzene chromophore and solvent molecules on the nonlinear photoresponsive properties was also investigated. The enhancement of nonlinear photoresponsive properties was observed due to the improvement of electron-donor ability and the increase of azo molecule alligenment order degree. The nonlinear coefficient of supramolecular azopolymers was decreased compared to corresponding azobenzene compounds. It was mainly because that the concentration of azochromophore decreased and meanwhile the motions of azochromophore were confined owing to the entanglement of polymer chains.3. Two-photon-induced data storage in supramolecular azopolymer P4VP/(NAzoOH)x (x=0.25,0.5,0.75,1.0) films were demonstrated. The data were recorded by linearly polarized Ti:Sapphire laser (wavelength:800nm, pulse duration:80fs, and repetition rate:80MHz) due to two-photon-induced birefringence resulted from reorientation caused by photoselcctive isomerization of azochromophores. The data were read by transmission confocal microscope and it was found that the recorded bits were bright dots when the polarization of the reading beam was parallel to the recording beam, the recorded bits "faded" completely when the polarization of the reading beam was deviated with an angle of45°, and the recorded bits became dark dots when the polarization of the reading beam and the recording beam was perpendicular to each other. Based on this polarization characteristic, polarization-binary storage was performed. That was to say two kinds of data were recorded in the same region of a given layer by separating the two polarization directions of recording beams at an angle of45°. This polarization characteristic was attributed to photoinduced birefringence and optical anisotropy of the supramolecular azopolymers containing azobenzene chromophores under two-photon excitation. Moreover, as the content of AzoOH increased, the threshold for data recording was found to be decreased and the stability of recorded data was enhanced, which could be attributed to the interplay of hydrogen bonding and chromophore-chromophore interaction in the supramolecular azopolymer films. Furthermore, a multilayered composite film with supramolecular azopolymers as a recording layer and PVA as spacing layer was prepared through alternately stacking. Polarization-multipexed and multilayered four-dimensional optical storage were successfully realized, and the storage density was0.6Gbits/cm3,15times more than conventional DVD.