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Two types of donor(D)–acceptor(A) calix[4]arenes have been theoretically studied using DFT//B3LYP/6-31G(d) method and ZINDO/CISD method. The calculations show that the substitution of CC by the conjugation bridge CC and NN plays an important part in altering one-photon absorption (OPA) and two-photon absorption (TPA) properties. The maximum OPA wavelengths of all studied compounds are less than 400 nm, which means high transparency. The geometry of the calixarenes strongly influences the TPA properties of the studied compounds. In addition, the nitro derivatives have a wider TPA response range than other non-nitro derivatives. The tetrasubstituted calix[4]arenes (type B calixarenes) have a larger TPA cross-section values than the bisubstituted calix[4]arenes (type A calixarenes). Copyright © 2009 John Wiley & Sons, Ltd.

Two series of porphyrin-thiophene chromophores were theoretically studied that exhibit large two-photon absorption cross-section in the visible region. The density functional theory (DFT) associated with ZINDO/SDCI methods has been used to calculate the molecular geometries, electronic structures, one-photon and two-photon absorption properties. The results show that the number of thiophene units affects the properties of one-photon absorption (OPA) and two-photon absorption (TPA). Porphyrin-thiophene chromophores featuring two or three thiophene units have wide TPA response ranges; they can be applied to many nonlinear optical areas, such as optical limiting. Intervening ethynyl unit is beneficial to extend the conjugated pathway, and increase the two-photon absorption cross-section. At the same time, the OPA and TPA wavelengths are bathochromically shifted. From viewpoint of the high transparency and large nonlinear optical response, porphyrin-thiophene chromophores will be promising TPA materials.

The electronic structures, one-photon absorption (OPA) and two-photon absorption (TPA) properties of the azulenylporphyrins and azulene-fused porphyrins have been comparatively studied by using DFT/B3LYP/6-31G(d) and the ZINDO/SDCI method. With the number of azulenyl groups increasing, the OPA wavelengths of all molecules are red-shifted in 400{600 nm and the two-photon absorption cross section is gradually enlarged. The azulene-fused structures facilitate an expanding conjugated area and increasing TPA cross section. The origin of TPA properties of studied compounds is studied with a two-level model. In summary, the azulene-fused porphyrins exhibit strong two-photon absorption.

Two series of bis(styryl)benzene derivatives (BSBD), namely the single-BSBD and the double-BSBD, were investigated. The equilibrium geometries and electronic structures were obtained by using the density functional theory B3LYP and 6-31G basis set. In succession, the one- and two-photon absorption properties of all the molecules were studied theoretically with a ZINDO-SOS (sum-over-states) method in detail. It can be seen that the double-BSBDs have larger two-photon absorption (TPA) cross sections in the visible-IR range than the corresponding single-BSBDs, demonstrating that increasing the molecular dimension is a very effective method to enhance the values of the TPA cross sections. On the other hand, it can be also noticed that the values of the TPA cross sections are correlative with the ability of donating (accepting) electrons of the terminal substituent groups R [N(CH3)2, CH3, H and CF3] in these molecules. That is, the intramolecular charge transfer is also a factor for the enhancement of the TPA efficiency. To sum up, the idea of increasing the molecular dimension to enhance the TPA cross section value is a helpful direction to explore better TPA materials for practical applications. And the double-BSBD molecules are promising TPA materials for the further investigation from the standpoint of the high transparency and the larger TPA cross sections.

The equilibrium geometries, electronic structures, one- and two-photon absorption (TPA) properties of a series of octupolar complexes with the Cu(I), Zn(II) and Al(III) as coordinate centers and the bis-cinnamaldimine as ligands have been studied using the B3LYP/6-31G(d) and ZINDO-SOS methods. Compared with the dipolar metal complexes, all the octupolar metal complexes (including tetrahedral and octahedral complexes) have relatively large TPA cross-sections, indicating that building octupolar metal complex is an effective route to design of promising TPA material. Lewis acidity of metal center and molecular symmetry are two important factors for enhancement of TPA cross-section of metal complex. Due to the stronger Lewis acidity of Zn(II) than Cu(I) as well as Al(III) than Zn(II), the tetrahedral Zn(II) complex exhibits a TPA cross-section larger than that of the tetrahedral Cu(I) complex, the maximum TPA position of the octahedral Al(III) complex is red-shifted relative to the octahedral Zn(II) complex, and at the same time, the octahedral Al(III) complex has a large TPA cross-section. Compared with the tetrahedral complexes, the TPA cross-sections of the octahedral complexes are enhanced due to the increased number of ligands.

On the basis of the equilibrium geometric structures obtained at the B3LYP/6-31G level, the one-photon absorption (OPA) and two-photon absorption (TPA) properties of donor/acceptor-functionalized tetrakis(phenylethynyl)benzenes (TPEBs) and bis(dehydroben-zoannuleno) benzenes (bisDBAs) have been determined by using ZINDO-SOS method. The influence of the molecular structures on OPAs and TPAs have been discussed. Among the donor/acceptor substituted regioisomers, the ortho- and meta-substituted compounds have the largest two-photon responses, mainly due to the existence of the larger dipole moment differences between the ground states and the intermediate states. The diacetylene bridges in bisDBAs result in the red-shift of OPAs and TPAs and decrease of TPA cross-sections. From the point of view of transparency/nonlinearity, the TPEBs and bisDBAs are promising candidates for TPA materials.

The electronic structures, one-photon absorption (OPA) and two-photon absorption (TPA) properties of the meso – meso singly, the meso -β doubly and the meso – meso β–β β–β triply linked Zn II -porphyrin dimers and trimers have been comparatively studied by using the hybrid B3LYP functional combined with the 6-31G basis set and ZINDO-SOS methods. The results show that on going from the dimers to the trimers, due to the increased conjugation chain, the maximum TPA positions are red-shifted and the maximum TPA cross sections produce a remarkable enhancement. The calculated maximum TPA cross sections of the studied molecules are in the range of 25.4–616.3 × 10 −48 cm 4 s/photon. Compared with the meso – meso singly linked Zn II -porphyrin derivatives, the meso -β doubly and the meso – meso β–β β–β triply linked Zn II -porphyrin derivatives have larger TPA cross sections at 700–1000 nm. They are promising candidates for TPA materials.

Two series of dimethylfluorene core derivatives, which have different conjugation length or different peripheral substituted groups, are theoretically investigated in the paper. Using the density functional theory B3LYP with 6-31G basis set, we have obtained the equilibrium geometries and electronic structures for all the studied molecules. On the basis of the geometrical and electronic structures, the one- and two-photon absorptions properties are studied theoretically with ZINDO-SOS method in detail. The computational results are in good agreement with the available experimental values. According to our calculations, from the series I molecule to the series II counterparts, the maximum one-photon absorption (OPA) intensities and the maximum two-photon absorption (TPA) cross sections values δmax are obviously increased as the conjugation length increases. And for both series the maximum OPA wavelength λ max ( 1 ) and δmax values are increased with the stronger electron-donating or electron-accepting capability. This demonstrates that the conjugation length and the different substituent groups play important roles in the OPA and the TPA cross section.

Using density-functional theory and ZINDO-SOS method, we carry out a comparative study of the two-photon absorption (TPA) properties of a new octupolar compound—sumanene 3O derivative and relative molecules. The results indicate that the TPA cross sections of the octupolar compounds increase with the electron-accepting ability of the center. Sumanene 3O is a stronger electron-acceptor than 1,3,5-tri(nitro)benzene. Compared with the conventional octupolar compounds, the sumanene 3O derivative, which has the largest TPA cross section among the studied molecules here, is a more promising TPA material.

The electronic structures and the one- and two-photon absorption (TPA) properties of two series of the olefin-linked paracyclophane (pCp) oligomers have been studied using AM1 and ZINDO-SOS methods. The relationship between the TPA cross sections and the molecular chain length is obtained. The maximum TPA cross section increases in proportion to Nα (N denotes the number of repeat units) and the values of α depend on different molecular structures. The olefin-linked pCp oligomers, which have good transparency and large TPA cross sections, are promising candidates for TPA materials.Key words: two-photon properties, paracyclophane oligomer, AM1, ZINDO-SOS.

On the basis of the equilibrium geometric structures obtained by using the hybrid B3LYP functional combined with the 6-31G basis set, the electronic structures, one- and two-photon properties of triphenylamine (boron, aluminum)-cored dendritic compounds have been studied by using ZINDO/SDCI and sum-over-states (SOS) formula. The dentritic structure–TPA property relationship has been discussed. These compounds exhibit large TPA cross-sections and good transparency. They are promising TPA materials for optical power limiting.

The equilibrium geometries and one- and two-photon absorption (TPA) properties of a series of tetra-paracyclophane derivatives have been determined by using the hybrid B3LYP combined with 6-31G basis set and ZINDO–SOS methods. The results show that the tetra-paracyclophane derivatives have large TPA cross-sections, which are about two times larger than that of the corresponding linear counterparts. The introduction of donor/acceptor groups has improved the TPA cross sections of paracyclophane derivatives. In particular, the increased molecular chain plays a crucial role in increasing TPA cross section, according to the three-state model, due to the larger transition dipole moment between the ground state and intermediate state. The calculated results also reveal that the dimers of D–π–A (D and A denote donor and acceptor, respectively) chromophore have relatively large TPA cross sections among studied compounds.

A series of heterocycle-based molecules using π-deficient (pyridinium) and π-excessive (pyrrole) heteroaromatic rings as acceptor and donor, respectively, have been designed and their two-photon absorption properties have been investigated theoretically by using the AM1 and ZINDO methods. On the basis of correct geometries and UV-VIS spectra, the positions and strengths of two-photon absorption of these molecules were reported. Our calculated results reveal that molecule F, which has the largest two-photon cross-section (2457.29×10−50 cm4 s photon−1) among studied molecules, is a more promising two-photon absorption material.

The molecular equilibrium structures, electronic structures, and one- and two-photon absorption (TPA) properties of C2v (Zn(II), Fe(II) and Cu(I)) dipolar and D2d (Zn(II) and Cu(I)) and D3 (Zn(II)) octupolar metal complexes featuring different functionalized bipyridyl ligands have been studied by the ZINDO-SOS method. The calculated results show that one- and two-photon absorption properties of metal complexes are strongly influenced by the nature of the ligands (donor end groups and pi linkers) and metal ions as well as by the symmetry of the complexes. The length of the pi-conjugated backbone, the Lewis acidity of the metal ions, and the increase of ligand-to-metal ratio result in a substantial enhancement of the TPA cross sections of metal complexes. Substitution of C=N and N=N for C=C plays an important role in altering the maximum TPA wavelengths and the maximum TPA cross sections of metal complexes. Of them, the C=N substituted metal complexes have relatively large TPA cross sections. Replacing styryl with thienylvinyl makes the one-photon absorption wavelength red shift and at the same time leads to a great decrease of the maximum TPA cross sections of metal complexes. The possible reason is discussed. In the range 500-1250 nm, octupolar metal complexes exhibit intense TPAs and therefore are promising candidates for TPA materials.

The electronic structures, one-photon absorption (OPA), and two-photon absorption (TPA) 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 by using the ZINDO-SOS method. The results have revealed that OPA and TPA of ferrocenyl derivatives are affected by the strength of the acceptor, especially the pi-bridge conjugation length. The TPA cross section increases with increasing acceptor strength and pi-bridge conjugation length. The TCF-type acceptor with a phenyl group can lead to a larger TPA cross section. Quadrupole molecules have the largest TPA cross sections (2000-3000 GM), which are about 4 times that of the corresponding dipolar molecules, indicating larger interactions between the top and bottom branches. Finally, the origins of the two-photon excitations for ferrocenyl derivatives are analyzed. The calculations show that ferrocenyl derivatives with TCF-type acceptors (especially quadrupole molecules) are promising candidates for TPA materials.