Optical Measurements And Theoretical Computations Of Strong Two-Photon Absorption Materials

Two-photon absorption (TPA) is a process in which two photons can be simultaneously absorbed via a virtual state in medium. Since the concept of two-photon absorption was raised sixty years ago, as an important nonlinear optical phenomenon, two-photon absorption has received great attentions by scientists. Due to the great potential applications in many fields such as two-photon pumped upconversion laser, two-photon induced optical limitor, two-photon fluorescence imaging, three-dimension optical data storage etc., new materials with strong two-photon absorption properties have always been explored by material scientists. Since mid last century, new organic materials with strong two-photon absorption properties have been synthesized successively by different research groups. During the research on the relationships between structure and two-photon absorption properties of the new molecules, some experience and design ideas have been realized which are very important in the synthesis of new strong two-photon absorption materials. Under the guidance of these valuable ideas, the molecular structures of two-photon absorption materials have been optimized and the two-photon absorption abilities of these materials have been improved. As one kind of nonlinear optoelectronic multifunction materials with abroad applications, materials with strong two-photon absorption properties probably will become one of the hot spots in materials research fields for a longer period of time.Both experimental measurements and theoretical computations are necessary tools for characterizing the performances of two-photon absorption materials. By characterizing different two-photon absorption abilities between different materials, by exploring the origins of materials with strong two-photon absorption properties in mechanism, experimental or theoretical results can give guidelines for the synthesis of new materials with even higher two-photon absorption performance. During the processes of exploringnew two-photon absorption materials, we have successfully synthesized series of materials with strong two-photon absorption properties, such as pyridinium dyes, organic boron compounds and organic metal complexes. In these materials, some are based on traditional types and some are developed by ourselves. In the thesis, pyridinium dyes, organic boron compounds and organic metal complexes are used as researches and the relationships between structures and one- and two-photon properties have been investigated systematically. Pyridinium dyes have already been investigated by other research groups. However these investigations are not systematical and complete as a whole. In this thesis, the influences of molecule structures on linear absorption spectra, one-photon fluorescence spectra and upconversion efficiencies of pyridinium dyes have been studied systematically. Two-photon absorption cross-sections of a series of pyridinium dyes have been measured by using nonlinear transmittance and z-scan methods. All of these have developed and completed the former research results. Organic boron compounds are new type of two-photon absorption materials that are not studied before. Based on the structure characteristics of boron compounds themselves, experimental measurements combined with theoretical computations were used to study the absorption and emission properties and the structure-property relationships of a series of boron compounds. The experimental and computational results are of great importance to the design and synthesis of boron compounds with better two-photon absorption performance. There are few reports on two-photon properties of organic metal complexes. The one- and two-photon excited fluorescence properties of organic metal complexes and their organic ligands have been measured experimentally. The experimental results can be used as references for exploring new type of two-photon absorption materials. In addition to the studies on structure and two-photon absorption properties of two-photon absorption materials, the influences of solvents on fluorescence lifetimes, the influence of solvents on spectra, upconverted efficiencies of compounds under different pump intensities and upconverted efficiencies of compounds in different solvents have been studied as well. In addition to the studies on properties of materials themselves, the mechanisms and methods used in experimental measurements and theoretical computations have been discussedin details.The main conclusions and rules of the thesis are as follows. Two-photon pumped amplified spontaneous emission (ASE) properties can be observed when some pyridinium dyes are pumped by picosend laser pulses. Amplified spontaneous emission is the single pass emission property of high gain medium under excitations of laser pulses with high energy. Comparing with lasing, ASE is free of influence from the resonant cavity. The peak wavelengths of one- and two-photon fluorescent spectra overlap in principle. However the measured peak wavelengths of one- and two-photon emission spectra may move to each other due to reabsorption effects of dyes in high concentrations.The following conclusions were obtained based on the study of the linear absorption properties of a series of pyridinium compounds. (1) For the dyes with the same anions, the stronger the electron-donating abilities of the donors is, i.e. from N, N-dimethylamino, N, N-diethylamino, N-n-propyl-N-n-propylamino to N-n-buryl-N-n - butylamino, the more red-shift the wavelengths of linear absorption peaks. (2) The wavelengths of linear absorption peaks are the same for the dyes with the same donors while different anions, which indicates that it is the cations with the backbone of styryl pyridinium that determine the wavelengths of linear absorption peaks; the anions have little influence on the wavelengths of linear absorption peaks. Based on the one-photon fluorescence spectra of pyridinium compounds in the same solvent, the following conclusions can be drawn: (1) For the dyes with the same anions and in the same solvents, the peak wavelength is red-shift with the increase of electron-donating abilities of the donor. (4) The anions have little influence on fluorescence peaks. It is the anions that mainly determine the wavelengths of fluorescence peaks.Two-photon excited fluorescence lifetimes of pyridinium dyes in different solvents have been studied. (1) The fluorescence lifetime decreases with the increase of polarities of solvents. (2) The viscosity constants of the solvents may have very great influence on the two-photon absorption fluorescence lifetime. The big viscosity limits the rotation of the dye molecules, reduces conflictions among the dye molecules themselves and conflictions between the dye molecules and solvents molecules. All of theseare in favor of the increase of two-photon fluorescence lifetime. (3) The formation of hydrogen bonding greatly reduces the two-photon fluorescence lifetime. With the formation of hydrogen bonding between the H of hydroxy in methanol and the N of pyridinium ring in the dye, the planarity of the dye molecules will be destroyed and the energy level structure of dye molecules will be changed as well, which decreases fluorescence lifetime. (4) The possible existence of define alcohol resonance structures in solvents will lead to short fluorescence lifetime of dyes in it.Based on the studies of TPA fluorescence lifetimes of all the dyes in benzyl alcohol, the following conclusions can be obtained. (1) For dyes with the same anions, the stronger the electron-donating abilities of the donors, the longer the TPA fluorescence lifetime. For the dyes with the same donor, the lifetimes of the dyes descend by the sequence of the following anions: p-toluene sulfonate, I', BF4', SCN", PFe". This indicates that the anions participate in the TPA and two photon excited fluorescence emission processes.Pyridinium dyes show higher upconverted efficiencies under picosecond laser pulses. The upconversion efficiency increases with the increase of pump intensity until the relationship between upconverted intensities and pump intensities deviates from the square law. After that, the upconverted efficiencies turn to decline due to the saturation effect. The rules between structures and upconversion efficiencies of pyridinium series compounds are as follows. (l)The upconversion efficiencies of dyes with the same anions increase with the increase of electron-donating abilities of the donors. (2) The anions have some influence on the upconversion efficiencies as well. The upconversion efficencies of dyes with Bph4" as anions are the lowest among ail dyes, which indicates that anions take part in the frontier molecular orbital to some extent and influence the intramolecular charge transfers.Upconverted efficiencies of dye DEASPI in DMF have been measured at different pump wavelengths. The pump wavelength of the highest upconverted efficiency is red-shift relative to the position of double absorption peak wavelength. Upconverted efficiencies of dye ASPI in solvents of DMF and benzyl alcohol have been measured at different pump wavelengths. The upconverted efficiency spectrum of ASPI in benzyl alcoholis red-shift relative to that in DMF.The relationships between structure and one- and two-photon properties of new type of boron compounds have been systematically studied in experiments. Main conclusions are drawn as follows. (1) For boron compounds with the same conjugated bridge and acceptor but different donors, the peak wavelengths of both the linear absorption spectra and one- and two-photon fluorescence spectra are red-shift with the increase of electron-donating abilities of the donors. This indicates that the energy differences between ground state and excited state of the compounds decrease with the increase of electron-donating abilities of the donors. (2) The peak wavelengths of linear absorption spectra and one- and two-photon fluorescence spectra of boron compounds with styryl-thiophene as the conjugated bridge are red-shift compared with those of compounds with stilbene as conjugated bridge. This indicates that the energy difference between ground state and excited state of the compounds with styryl-thiophene as the bridge is smaller than that of the compounds with stilbene as bridge. (3) At wavelength of 800 nm, two-photon absorption cross-sections of compounds with stilbene as conjugated bridge are commonly larger than those with styryl-thiophene as bridge. The two-photon absorption cross-sections of compounds with the same bridge usually decrease with the increase of electron-donating abilities of the donors.The one- and two-photon fluorescence spectra and one-photon absorption spectra of boron compounds in solvents with different polarities have been studied. For pull-push type of compounds, the relative shifts of absorption or emission spectra with the polarities of solvents result from the dipole-dipole interactions between dye molecules and solvent molecules. The one- and two-photon fluorescence spectra of boron compounds are red-shift with the increase of polarities of solvents, which indicates that the dipole moment of excited state is larger than that of ground state for the these compounds. Two-photon fluorescence spectra of symmetric molecules are red-shift with the increase of the polarities of solvents as well. This phenomenon can be explained by the interactions between higher-order mutipole moments of symmetric molecules and dipole moments of solvent molecules. It is intramolecular symmetric charge transfers induced by excitations that result...