NLO Properties Of Novel Functionalized Porphyrin Compounds And Azobenzene Polymers

NLO materials are the basic materials of optoelectronic technology. Research on organic materials nonlinearity has provided valuable applications such as storage, modem and switch. The porphyrin compound is a kind of macrocyclic conjugated organic molecules which have an extensive system of delocalizedπelectrons. It can be applied in many fields, such as macromolecule materials, chemical catalysis, electroluminescence materials and photodynamic therapy. Porphyrin oligomers have better properties in electron transfer, energy conversion, molecular devices and nonlinear optical materials than monomers. So there become very popular realm in synthesis and investigation of porphyrin oligomers. Polymers bearing azobenzene moieties (azo-polymers) are fascinating materials and have attracted considerable attention in the past few years because of their unique reversible photoisomerization and photoinduced anisotropy of the azobenzene chromophores. The photoinduced isomerization and photoinduced anisotropy can cause significant bulk, surface property variation and polarity of the polymers, such as photoinduced phase transition, photoinduced birefringence and photoinduced surface relief gratings (SRGs) et al. They can be used in the fields of optical data storage, optical switch, electro-optical (EO) modulators, and nonlinear optical materials, etc. Based on these reasons, we have done some researches as below:1: We tested the third order optical nonlinearity of a series of porphyrin compounds which contains the same substituents. But the numbers and the positions of these substituents are quite different. The substituent is the hydroxyl group which is an electron donating group. The results gave the relationship of the NLO properties and the structures of these porphyrin compounds. The open aperture results of z-scan indicated that all the samples showed the RSA (Reverse Saturable Absorption). The nonlinear refractive indexes of these samples were quite different. The density of the electron cloud was increased because of the electron donating group. So the third order nonlinear refractive index became larger. However, when we introduced another or more electron donating groups to the compound, the third order nonlinear refractive index did not increased effectively. This was because of the repulsion between the substituents. For the compound with two substituents, the nonlinear refractive index of the trans structure was quite different from that of the cis structure. It was also because of the effect of the repulsion. This conclusion told us that if we wanted to achieve large third order nonlinear refractive index of these compounds through increasing the number of the substituents, we must consider the repulsion effects between these groups.2: We also try another substituent carboxyl group which is an electron withdrawing group. The results showed that a withdrawing group could reduce the density of the electron cloud. We also tested the third order optical nonlinearity of a series of porphyrin dimers which contains the different bridging groups. The results gave the relationship of the NLO properties and the different bridging groups. The bridging group was an important part of the porphyrin dimer. When the dimer was linked by a phenyl group, the whole dimer was composed by three conjugated systems. So the third order nonlinear refractive index of this dimer was the largest. However, if there was an electron withdrawing group in the bridging group, the density of the electron cloud would be reduced. It would also cause different effects if the position of the substituent is different. For example, the electron density of two porphyrin rings was reduced for one sample, and the density of the bridging group was reduced for another, so it would lead to different results.3: The optical limiting behaviors of some porphyrin compounds were investigated by using the pulsed laser and CW laser respectively. The optical limiting behavior was based on RSA. When intensity of the input laser was weak, the output intensity of the laser increased linearly. As the intensity of the laser increased, the molecules were pumped to the excited state, and the absorption was changed to RSA of the excited state, which leaded to the optical limiting behavior for the pulsed laser. The optical limiting experiment for the CW laser is based on self-defocusing. When the input laser power was very small, the laser beam can pass through the aperture freely, with out any obstacle. As the power increased, the sample could absorb the energy from the laser beam and the sample got heated. So the refractive index of the sample partly changed, leading the output laser beam disperse, and the area of the beam at the aperture plane became larger than the aperture. So the power detected behind the aperture changed nonlinearly.4: The experiments of photoinduced birefringence on two kinds of Poly(aryl ether)s (PAEs) were investigated. Through the investigation on the PAEs with the azobenzene chromophores in the side chain, we concluded that the amounts of the chromophores affected the values of the birefringence greatly. We also tested the photoinduced birefringence of hyperbranched poly(arylene ether)s (HPAEs) with the azobenzene chromophotes. For the structure of the azobenzene group located in the main chain, the restriction made the reorientation of the chromophotes harder. So the values that they could achieve became smaller. On the other hand, the structure of side chain made the movements of the chromophotes easier, and the value of the photoduced birefringence became larger.5: The growth and decay processes of the optically induced birefringence in two novel azobenzene polymers, one is a hyper-branched poly (aryl ether) containing azobenzene groups and the other is a hydrogen-bonded complex, have been studied. Both the saturated value and the relaxed value decreased with the increase of the temperature as thermal relaxation of the chromophores was facilitated. Both growth and relaxation processes involved a"fast"and a"slow"process caused by different mechanisms. The curves for buildup and decay of birefringence were fitted well with bi-exponential functions. The dependence of the fitted time constants on the temperature was also discussed. For the HPAE structure, the fraction of the induced birefringence that preserved was much better when the pump light turned off, however it took a long time to achieve the saturated value. On the other hand, the time of response was much less for the host- guest structure, but the fraction of the induced birefringence that could preserved was much smaller.The results of our investigations showed that both of these two kinds of materials were good nonlinear optical materials with potential applications. And the research in these areas would attract more attentions in the future. We hope that our conclusions in this thesis would be a helpful guidance to develop further practical materials in these fields.