The Molecular Design Investigations On Two-photon Absorption Materials

Recently. two-photon absorption of organic and organometallic materials has received more and more attention due to the continuous improvement of the laser technique. Materials with large two-photon absorption response can be applied in the fields of chemistry, physics and biology. Two-photon absorption as one of nonlinear optical responses also plays a significant role in the national defense (such as laser protection). Design and synthesis of materials with large two- photon absorption cross section is the important basis of its development. In this thesis, in order to design and select materials with large two-photon absorption response, we have performed systematic theoretical research of various organic and organometallic materials. The quantum chemical methods-density functional theory and AM1 have been applied to optimize the molecular equilibrium geometries. On the basis of the optimized structures. one- and two-photon absorption properties are obtained by ZINDO program combined with SOS equation. We have designed some unknown compounds using the same method so as to provide a theoretical basis of synthesizing novel materials with large two-photon absorption cross section. The following is the main results:1. The electronic structures, one-photon absorption and two-photon absorption properties of a series of ferrocene-based chromophores with TCF type acceptors (TCF=2-dicyanomethylene-3-cyano-4-methyl-2,5-dihydrofuran) have been studied. The results have revealed that the one-photon absorption and two-photon absorption of ferrocenyl derivative are affected by the strength of acceptor, especiallyÏ€-bridge conjugation length. Two-photon absorption cross section increases with increasing the strength of acceptor and theÏ€-bridge conjugation length. The TCF type acceptor with phenyl group can lead to larger two-photon absorption cross section. Quadrupole molecules have the largest two-photon absorption cross sections (2000 GM - 3000 GM) which are about 4 times as that of the corresponding dipolar molecules, indicating larger interactions between the top and bottom branches. During the two-photon excitations of ferrocenyl derivatives, the charges transfer from central metal to ligands.2. The electronic structures, the one-photon and two-photon absorption properties of two series of the olefin-linked paracyclophane (pCp) oligomers have been studied. Relationship between the two-photon absorption cross sections and the molecular chain length is obtained. The maximum two-photon absorption cross section increases in proportion to N^a (N denotes the number of the repeat units) and the values of a depend on different molecular structures. The olefin-linked pCp oligomers, which have good transparency and large two-photon absorption cross sections, are promising candidates for two-photon absorption materials.3. The electronic structures, one-photon and two-photon absorption properties of the meso-meso singly, the meso-βdoubly and the meso-mesoβ-ββ-βtriply linked Zn^â…¡-porphyrin dimers and trimers have been comparatively studied. The results show that on going from the dimers to the trimers, due to the increased conjugation chain, the maximum two-photon absorption positions are red-shifted and the maximum two-photon absorption cross sections produce a remarkable enhancement. The calculated maximum two-photon absorption cross sections of the studied molecules are in the range of 25.4-616.3×10^-48 cm⁴s/ photon. Compared with the meso-meso singly linked Zn^â…¡-porphyrin derivatives, the meso-p doubly and the meso-mesoβ-ββ-βtriply linked Zn^â…¡-porphyrin derivatives have larger two-photon absorption cross sections at 700-1000 nm.4. Electronic structures, one-photon and two-photon absorption properties of Zn(â…¡)-porphyrin compound (Z_h), meso-meso singly-linked Zn(â…¡)-diporphyrin compound (Z2_H) and Zinc(â…¡) meso, meso-coupled porphyrin dimer (Z2_h-G) via intermolecular hydrogen bonding interactions have been comparatively studied. The results show that formation of hydrogen bondings between carboxyl groups and cyclic urea reduces the dihedral angle between the adjacent porphyrin planes in Z2_H-G, leading to increased electronic interactions between the two porphyrin planes and increased couplings between electronic states. Electronic spectrum of Z_h can be described with the four-orbital model, while the descriptions of the electronic spectra of Z2_H and Z2_h-G need eight frontier orbitals. Theoretical calculations reveal that two-photon absorption cross sections increase on going from Z_h to Z2_h to Z2_h-G due to increase of the couplings between electronic states through transition dipole moments.5. The two-photon absorption properties of sumanene and triphenylene derivatives have been comparatively studied. The results indicate that sumanene and triphenylene derivatives have relatively large two-photon absorption cross sections and the high transparency. And the two-photon absorption cross section increases as strength of donor/acceptor and conjugation length increase. In comparison with classical octupolar compounds, there exists stronger electron coupling between the branches of sumanene and triphenylene derivatives. Compared with triphenylene derivatives, the bowl-shaped structure of sumanene segment has an important effect on two-photon absorption of sumanene derivatives. It blocks electron transitions on the interior of sumanene segment and from the peripheric groups to molecular center.6. The two-photon absorption properties of a new octupolar compoundsumanene 30 derivative and relative molecules have been comparatively studied. The results indicate that the TPA cross sections of the octupolar compounds increase with the electron-accepting ability of the center. Sumanene 30 is a stronger electron-acceptor than 1,3,5-tri(nitro)benzene.7. The equilibrium geometries and one- and two-photon absorption properties of a series of tetra-paracyclophane derivatives have been determined. The results show that the tetra-paracyclophane derivatives have large tow-photon absorption cross sections, which are about two times larger than that of the corresponding linear counterparts. The introduction of donor/ acceptor groups has improved the two-photon absorption cross sections of paracyclophane derivatives. In particular, the increased molecular chain plays a crucial role in increasing two-photon absorption cross section. The calculated results also reveal that the dimers of D-Ï€-A (D and A denote donor and acceptor, respectively) chromophore have relatively large two-photon absorption cross sections among studied compounds.8. The equilibrium geometries, electronic structures, one- and two-photon absorption properties of a series of octupolar complexes with the Cu(â… ), Zn(â…¡) and Al(â…¢) as coordinate centers and the bis-cinnamaldimine as ligands have been studied. Compared with the dipolar metal complexes, all the octupolar metal complexes (including tetrahedral and octahedral complexes) have relatively large two-photon absorption cross sections, indicating that building octupolar metal complex is an effective route to design of promising two-photon absorption material. Lewis acidity of metal center and molecular symmetry are two important factors for enhancement of two-photon absorption cross section of metal complex. Due to the stronger Lewis acidity of Zn(â…¡) than Cu(â… ) as well as Al(â…¢) than Zn(â…¡), the tetrahedral Zn(â…¡) complex exhibits a TPA cross section larger than that of the tetrahedral Cu(â… ) complex, the maximum TPA position of the octahedral Al(â…¢) complex is red-shifted relative to the octahedral Zn(â…¡) complex, and at the same time, the octahedral Al(â…¢) complex has a large two-photon absorption cross section. Compared with the tetrahedral complexes, the two-photon absorption cross sections of the octahedral complexes are enhanced due to the increased number of ligands.9. The molecular equilibrium structures, electronic structures, one- and two-photon absorption properties of C_2v (Zn(â…¡), Fe(â…¡) and Cu(â… )) dipolar, D_2d (Zn(â…¡) and Cu(â… )) and D₃ (Zn(â…¡)) octupolar metal complexes featuring different functionalized bipyridyl ligands have been studied. The calculated results show that one- and two-photon absorption properties of metal complexes are strongly influenced by the nature of the ligands (donor endgroups andÏ€linkers) and metal ions as well as by the symmetry of the complexes. Increase of the length of theÏ€-conjugated backbone and the Lewis acidity of the metal ions, and the increase of ligand-to-metal ratio result in a substantial enhancement of the two-photon absorption cross sections of metal complexes. Substitution of C=N and N=N for C=C plays an important role in altering the maximum two-photon absorption wavelengths and the maximum two-photon absorption cross sections of metal complexes. Of them, the C=N substituted metal complexes have relatively large two-photon absorption cross sections. Replacing styryl with thienylvinyl makes one-photon absorption wavelength red-shift, at the same time leads to great decrease of the maximum two-photon absorption cross sections of metal complexes. The possible reason is discussed. In the range of 500 nm - 1250 nm, octupolar metal complexes exhibit the intense two-photon absorptions, therefore are promising candidates for two-photon absorption materials.10. The electronic structures, one- and two-photon absorption properties of triphenylamine (boron, aluminum)-cored dendritic compounds have been studied. The dentritic structure- two-photon absorption property relationship has been discussed. These compounds exhibit large two-photon absorption cross sections and good transparency. They are promising two-photon absorption materials for optical power limiting.