New Type Of Strong Two-photon Polymerization Initiator Of The Design, Synthesis And Periodic Micro-structure Of Production

Recently, extensive researches have been paid on the photochemical and photophysical processes induced by two-photon absorption due to their potential applications for such as in optical power limiting, two-photon upconversion lasing, two-photon fluorescence excitation microscopy, three-dimensional optical data storage and microfabrication. These applications take advantage of the fact that the two-photon absorption (TPA) probability depends quadratically on intensity, so under tight-focusing conditions, the absorption is confined at the focus to a volume of order λ³ (where λ is the laser wavelength). This characteristic of two-photon processes has made TPA-based photo-polymerization an effective tool for processing photonic devices, micromachines, zero-threshold lasers and integrated optical waveguides in three dimensions with high spatial resolution.For two-photon free radical photo-polymerization, the polymerization rate and photo-initiation threshold is closely related to the absorption behavior of the initiator at the excitation wavelength (generally twice its single-photon absorption wavelength). Although 3D microfabrication has been illustrated using two-photon-initiated polymerization of resins incorporating conventional ultraviolet-absorbing initiators, the TPA cross sections 8 of these initiators are typically very small (δ ≤ 10×10^-50 cm⁴ s per photon), and as a result they exhibit low two-photon sensitivity. At present, effective initiators are still rare because the photopolymerization mechanism and relationships of structure/property for the initiator remained unclear. Some approaches to improve the photosensitivity of photoinitiator molecules that have been reported are based on the idea of increasing their TPA cross-sections. According to Albota and Reinhardt et al., the conjugation length, donor and acceptor strength, and planarity of the n center are the important parameters for enhancing the TPA. Based on this design strategy, some one-dimensional two-photon polymerization initiators with charge-transfer conjugated character were synthesized. Recently, ab initio calculations have been carried out for increased dimensionality of the charge-transfer networks, showing that the TPA cross sections of multi-branched molecules can be strongly enhanced compared with the one-branched counterparts. Experimental measurements have also confirmed this conclusion.In this thesis, the linear and nonlinear optical properties of three type molecules such as D-Ï€-D, D-Ï€-A, D-Ï€-D-Ï€-D and A-Ï€-D-Ï€-A have been systematically studied and some related rules have been obtained by a great deal of synthesizing and measuring work. The experimental results confirmed that these compounds are all good two-photon absorbing chromophores and operative two-photon photopolymerization initiators. The wavelengths for initiating two-photon polymerization reaction of those molecules are ranged from 600 to 830 nm and the corresponding two-photon fluorescence wavelengths are ranged from blue-violet to yellow. The possible mechanism is discussed by the charge-transfer process when the laser irradiation is applied. The concrete work is listed in the following paragraphs.1. Synthesis and characteristic of D- Ï€ -D, D- Ï€ -A and A-Ï€-D-Ï€-A type compounds.By using Wittig and Heck reaction, all symmetric and asymmetric chromophores have been synthesized. These series of newly designed chromophores, on one hand, change the structural type of it -center such as stilbene, 1,4-Divinyl-benzene and l,4-Dimethoxy-2,5-divinyl-benzene, on the other hand, vary the donor strengths in the order of carbazolyl < diphenylamino < pyrrolidineyl ^ diethlylamino < ethyl-hydroxyethylamino ?* methyl-hydroxyetheylamino. Meanwhile, the acceptor structural feature (A) was selected for pyridine. All compounds synthesized have been characterized by *H NMR spectra, 13C NMR spectra, elemental analyses and thermal analyses.2. X-ray crystallographic study.Six crystal structures, in which every crystal structure represents the characteristic of their type molecule respectively, were determined by X-ray single crystal diffraction analysis. The X-ray diffraction data of single crystals were collected on a Bruker P4 four-cycle diffractometer. Using SHELXL-97 programs, their crystal structures were solved by direct methods and refined by Full-matrix least-squares on F2. The planarity and the average degree of the bond of the initiators are discussed by the result of the full-matrix least-squares on F2. Based on this idea, the relationships between the structures and properties of them were discussed in detailed.3. Study of the linear and nonlinear optical properties.The linear absorption, one- and two-photon induced fluorescence is experimentally studied. Some conclusions were listed as following: (1) the maximum fluorescence peaks including one- and two-photon fluorescence clearly show a slight red-shift and the fluorescence intensity show a slight decrease with the increase of the polarity of the solvent except for benzyl alcohol for each compound. (2) in protonic solvents, such as benzyl alcohol and methanol, the linear absorption and one- and two-photon fluorescence peaks show a red-shift and show a relatively low fluorescence intensity; (3) the maximum peak positions of the two-photon fluorescence spectra for each compound show a slightly red-shift compared to that of one-photon fluorescence which can be explained by the reabsorption of the dye; (4) increasing of the donor strength, the position of linear absorption peak show obvious change; (5) increasing the dimensionality of the charge-transfer networks with the one-branched counterparts, the absorption and emission peaks show a slightly red-shift and the fluorescence intensity show a gradually enhancement. 3. The study of two-photon absorption cross-section.Some conclusions can be drawn by the measurement of two-photon absorption cross-section using an open aperture Z-scan technique and calculation using the time-dependent density functional theory. (1) increasing the strength of the donor and planarity of the molecule, the value of two-photon absorption cross-section show a gradually enhancement; (2) The experimental results indicate that the two-photon absorption cross-section of three new initiators with respective one-, two-, and three-branched structures with the one-branched counterparts display the enhancement with increasing the dimensionality of molecule, which is accord with the result of Macak et al. But from the theoretical calculated results, we don't display the enhancement of the two-photon absorption cross section with increasing the dimensionality of molecule. The reason is that we here only consider the electronic contribution to the two-photon absorption process. However, according to the result given by Macak et al, the vibronic contributions can play an important role for the multi-branched molecules, which definitely indicates that a full description for the two-photon absorption properties of multi-branched molecules needs to consider the vibronic contributions.4. Three-dimensional microfabrication.The wavelengths for initiating two-photon polymerization reaction of these intiators are range from 600 to 830 nm. Some three dimensional lattices were created by two-photon polymerization of an acrylic eater oligomer using these initiators. The film of the mixture of initiator and oligomer with a weight ratio of 4 5% (a little dichloroethane was added to make a solution of initiator and oligomer) was prepared by spin-coating onto glass plates. A pulse duration of 200 fs and repetition rate of 76 MHz, mode-locked Ti:sapphire laser was used for two-photon microfabircation. The corresponding lasing source was tightly focused via an objective lens (x40, NA = 0.65), and the focal point was focused on the sample film on the xy-step monitorized stage controlled by computer. The pulse energy before being focused by the objective lens was -1.2W. The polymerized solid skeleton was obtained after the unreacted liquid mixture had been washed out. The lattice fabricated was observed through a polarization microscope (Olympus, BX-51).5. Explaination of the photopolymerization mechanisms.The photopolymerization mechanisms of these new initiators are still unknown. According to Cumpston et al, strong donor substituents would make the conjugated system electron rich, and after one- or two-photon photoexcitation, these chromophores would be able to transfer an electron even to relatively weak acceptors, and this process could be used to activate the polymerization reaction. The photopolymerization mechanisms of these new initiators are explained by x-Ray crystallographic study and quantum chemistry calculations. The planarity and the average degree of the bond of the initiators are discussed by the full-matrix least-squares on F2. And the trend of intra-molecular charge transfer and possible photopolymerization mechanisms- is explained systematically. Our ab initio calculation at time-dependent hybrid density functional theory B3LYP level coded in GAUSSIAN98 package for these molecule show that the first excited state are the charge-transfer (CT) states. When a suitable laser irradiates the molecule, one can be expected that the molecule will simultaneously absorb two photons and is excited to the first excited state (the CT state). For a better understanding of the charge-transfer process, we have plotted the charge density difference between the ground and the CT states for these initiators in gas phase, which is visualized by use of MOLEKEL program. It can be seen that upon the excitation, charges are mainly...