Xiao-Juan Liu

Theoretical studies of the spectra and two-photon absorption cross sections for porphyrin and carbaporphyrins

Abstract The geometric and electronic structures of porphyrin and a series of carbaporphyrins have been theoretically studied using the time-dependent density functional theory (TDDFT). The two-photon absorption cross sections (TPACS) of these molecules are computed using the ZINDO-SOS formula. The calculated results indicate that when the N atom is substituted by the C atom, the molecular center is enlarged and the absorptions are red-shifted and that porphyrin, carbaporphyrin, opp -dicarbaporphyrin and adj -dicarbaporphyrin show TPACS in small absorption area, while tetracarbaporphyrin has fairly large two-photon absorptions in comparatively larger area, which may lead it to many practical applications.

The studies on the two-photon absorption cross-sections and the second hyperpolarizabilities of multi-branched stilbene derivatives

Abstract Using density functional theory and ZINDO method, the two-photon absorption (TPA) cross-sections ( δ ) and the second hyperpolarizabilities ( γ ) of a series of stilbene derivatives with multi-branched structures have been theoretically investigated. On the basis of correct geometry and UV–VIS spectrum, the position and strength of the TPA are reported. It is found that, the multi-branched structure increases the TPA cross-section and the second hyperpolarizability in comparison to the singly branched counterpart. The δ value increases as the donor strength, the acceptor strength of the center in multi-branched structures, and the conjugation length increase.

One- and two-photon absorption of three-coordinate compounds with different centers (B,Al,N) and a 2,2′-dipyridylnitrogen functional group

A series of three-coordinate octupolar compounds with varied centers (boron, aluminum, and nitrogen), which exhibit very large effective two-photon absorption cross sections have been theoretically studied. The ground state geometries and electronic structures are obtained using the density functional theory with the B3LYP functional and 6-31G(d) basis set, and the results are comparable to the available experimental determinations. Based on the correct geometrical and electronic structures, the one- and two-photon absorptions are predicted by the ZINDO-SOS method. Among these compounds, the boron (B) and aluminum (Al) centers act as acceptors, while the nitrogen center acts as donor according to the net charge changes during the excitation. It is found that (i) the compounds with boron and aluminum centers show two large two-photon absorption peaks, while the molecule with nitrogen center show only one two-photon absorption peak; (ii) the cross sections of the molecules with B or Al as centers are larger than that of the molecule with nitrogen as center; furthermore, the two-photon absorption cross section of the molecule with Al center is larger than that of the molecule with B center, from this point of view, our theoretical prediction provides for the experiment a good new candidate with large two-photon absorption cross section for further research; (iii) lengthening the conjugation bridge by inserting a benzene ring on the organoborane compounds (forming the investigated molecule B-2) enhances the two-photon absorption cross section, and keeping good transparency at the same time.

Comparative studies of the spectra and the second-order nonlinear polarizabilities for donor–acceptor ensembles between Zn-porphyrin/fullerene [60] and Zn-porphyrin/naphthalenediimide

Abstract A series of donor–acceptor ensembles with fullerene [60] and naphthalenediimide (NIm) as acceptors and Zn-porphyrin (ZnP) as donors have been theoretically studied. The donors and acceptors are separated by different rigid bridges (–CC–, –CONH– and –NHCO–). Their equilibrium geometries and UV–Visible spectra were calculated by means of INDO/2 and the INDO/CI methods. The calculated results indicated that the charge transfer from the donor—Zn-porphyrin to the acceptor takes place for the ensembles with fullerene [60] as acceptors while for the molecules with NIm as acceptors, charge transfer is weak. On the basis of correct electronic spectra, calculations of the nonlinear second-order optical polarizabilites β ijk and β μ values were carried out using the INDO/CI method combined with a Sum-Over-States (SOS) expression. We conclude that the bridge of –NHCO– leads to comparatively large nonlinear second-order polarizability than the other two bridges; and the 3D fullerene [60] acceptor is superior to the 2D NIm acceptor due to its symmetrical shape, large size, and properties of its π-electron system. We illustrate the phenomena by means of the comparison of the frontier orbitals and net charge in the ground and main excited states.

One- and two-photon absorption properties of novel multi-branched molecules

One-photon absorption (OPA) and two-photon absorption (TPA) properties of a class of novel multi-branched molecules have been investigated. These molecules exhibit strong one- and two-photon absorption and have comparatively large TPA cross sections. The one-photon absorption of compounds containing two different stilbene (or a stilbene and an azobenzene) derivatives offers an unprecedented combination of that of individual parts. On the basis of correct geometries, optimized by using the AM1 method, the TPA cross sections of molecules were examined. The peak TPA cross section values (δmax) calculated with the ZINDO-SOS method are in the range 11.35–41.18×10−48 cm4·s photon−1. The δmax value increases as the donor strength increases and the π-center has an important effect on the magnitude of the δmax value. Compared with individual parts, the TPA cross sections of combinations of two different stilbene (or a stilbene and an azobenzene) derivatives exhibit a significant enhancement. Thus, another design strategy to maximize the TPA cross section is established.

Theoretical investigation of two-photon absorption properties for a series of bifluorene molecules

Abstract The two-photon absorption (TPA) cross-sections of a series of bifluorene molecules with different substituents (constructing three types of structures D–π–D, A–π–A and D–π–A) were calculated using ZINDO/SOS program. The results showed that the A–π–A structure with strong substituent-nitro had the largest TPA cross-section for these molecules. To verify the results, we analyzed the charge quantity of the ground and the main excited states as well as the frontier orbitals of the investigated molecules. The equilibrium geometries were obtained with AM1 method, and using ZINDO-SOS to calculate the third-order nonlinearities of the molecules, then gained the TPA cross-sections. We found that for the compounds with bifluorene as π center, the donor and acceptor strength are the important factor for the enhancement of the TPA properties, and compared with molecules with fluorene as π center, the large TPA cross-sections of bifluorene are caused by coupling effects between the two monomers.