Size-dependency of polarizabilities of fractal- and nonfractal-structured oligomers modeled after dendron parts in Cayley-tree-type dendrimers

Quantum Master Equation Approach to the Second Hyperpolarizability of Nanostar Dendritic Systems

We investigate the dynamic second hyperpolarizability (gamma) of nanostar dendritic systems using the quantum master equation approach. In the nanostar dendritic systems composed of three-state monomers, the multistep exciton states are obtained by the dipole-dipole interactions, and the directional energy transport, i.e., exciton migration, from the periphery to the core is predicted to occur by the relaxation between exciton states originating in the exciton-phonon coupling. The effects of the intermolcecular interaction and the exciton migration, i.e., exciton relaxation, on the gamma in the third-harmonic generation (THG) are examined in the three-photon off- and on- resonance regions using the two-exciton model. Furthermore, the method for analysis of spatial contributions of excitons to gamma is presented by partitioning the total gamma into the one- and two-exciton contributions. It turns out that the exciton relaxation between exciton states causes significant broadening of the spectra of gamma and their mutual overlap as well as the relative increase of two-exciton contributions in the nanostar dendritic system.

Theoretical studies on nonlinear optical properties of formaldehyde oligomers byab initio and density functional theory methods

The first and second hyperpolarizability beta and gamma are obtained for formaldehyde oligomers (H2CO)n (n = 1-7) using computational methods. We have used the finite field (FF) approach and hyperpolarizability density analysis (HDA) to predict the microscopic first and second nonlinear hyperpolarizability of the formaldehyde oligomers. The spatial contributions of electrons to the hyperpolarizability by using plots of HDA are presented. It has been found from the numerical stability checking of the hyperpolarizability calculations that the calculated values by FF method are more stable than those by HDA approach. The values of beta are zero when n is even as the molecule possesses centrosymmetry, and when n is odd, the differences among beta values are not clear. The gamma values are increased with increase in n.

Exciton migration dynamics in a dendritic molecule: Quantum master equation approach usingab initiomolecular orbital configuration interaction method

We investigate the exciton migration dynamics in a dendritic molecular model composed of pi-conjugation linear-leg units (acetylenes and diacetylene) and a benzene ring (branching point) using the quantum master equation approach with the ab initio molecular orbital (MO) configuration interaction (CI) method. The efficient migration of exciton from short-length linear legs (acetylenes) to long-length linear leg (diacetylene) via a benzene ring is observed. As predicted in previous studies, the exciton (electron and hole) distributions are relatively well localized in each generation segmented by the meta-branching point (meta-substituted benzene ring) though the electron and hole distributions are delocalized and are somewhat spatially different from each other within each generation. It is found that the excitons localized in the generation composed of short linear legs occupy in higher-lying exciton states, while those in the generation composed of long linear legs do in lower-lying ones. These features suggest the decoupling of pi-conjugation at the meta-branching point. On the other hand, the relaxation effect between exciton states is found to be caused by the exciton-phonon coupling, in which the existence of common configurations (electron-hole pairs) in CI wave functions between adjacent exciton states (having primary distributions on short and long linear-leg regions, respectively) is important for the relaxation between their exciton states. This feature indicates the importance of partial penetration of pi-conjugation through the meta-substituted benzene ring in excited states for such exciton migration.

Theoretical study on second hyperpolarizabilities of phenylacetylene dendrimer: toward an understanding of structure-property relation in NLO responses of fractal antenna dendrimers

This contribution explores the relation between molecular second hyperpolarizabilities (gamma) and molecular architecture in phenylacetylene dendrimers using the semiempirical molecular orbital method, that is, INDO/S method. The orientationally averaged gamma of a large-size phenylacetylene dendrimer, which is composed of 24 units of phenylacetylenes and is referred to as D25, is found to be about 50 times as large as that of the diphenylacetylene monomer. In contrast, the gamma(s)() value of D25 is found to be about 6 times as small as that of the para-substituted phenylacetylene oligomer (L25) composed of 24 units of phenylacetylenes. To investigate the structure-property relation in gamma for D25 and L25, we examine the spatial contributions of electrons to gamma values using gamma density analysis. The present analysis reveals that the dominant contributions of electrons to gamma of D25 are localized in the linear-leg regions parallel to the applied electric field and the contributions are also well segmented at the meta-connected points (benzene rings), while the spatial distribution of the gamma density of L25 is extended over the whole region of the chain, and the dominant contribution stems from the both-end regions. It is found for D25 that the magnitude of contributions to gamma in the internal region is more enhanced than that in the outer region. We further found that the magnitudes of contributions in internal linear-leg regions of D25 are somewhat larger than those of the same-size isolated linear-leg molecules. This suggests that the slightly remaining pi-conjugations via the meta-branching points still enhance the contributions to gamma localized in the linear-leg regions. These features of spatial contributions to gamma of D25 are found to originate in the fractal architecture, in which pi-conjugation lengths in the linear-leg region increase on going from the periphery to the core. Finally, fractal antenna dendrimers are expected to be promising novel nonlinear optical (NLO) substances with a controllability of the magnitude and spatial contribution of the third-order NLO properties.

Theoretical studies of magnetic interactions in Mn(II)(hfac)(2)[di(4-pyridyl)phenylcarbene] and Cu(II)(hfac)(2)[di(4-pyridyl)phenylcarbene]

Bis(hexafluoroacetylacetonato(hfac))manganese(II) coordinated with di(4-pyridyl)phenylcarbene, Mn(II)(hfac)(2)[di(4-pyridyl)phenylcarbene] (1a) and its copper analogue Cu(II)(hfac)(2)[di(4-pyridyl)phenylcarbene] (2a) have attracted great interest from the viewpoint of photoinduced magnetism. The complexes 1a and 2a are regarded as the new d-pi-p conjugated systems containing transition metal ion and carbene as spin sources. The magnetic measurements demonstrated antiferromagnetic and ferromagnetic effective exchange interactions for 1a and 2a, respectively. Here, we have performed UHF and UHF plus DFT hybrid calculations (UB3LYP) to elucidate the nature of the through-bond effective exchange interaction between Mn(II) (or Cu(II)) ion and triplet carbene sites in 1a (or 2a) and their model complexes. The natural orbital analysis of the UHF and UB3LYP solutions and CASCI calculations for the simplest models of 1a and 2a are performed to elucidate relative contributions of spin polarization (SP) and spin delocalization (SD) (or superexchange (SE)) interactions for determination of the sign of J(ab) values. Mn(II) carbene complex 1a shows an antiferromagnetic interaction because of the pi-type antiferromagnetic SE effect and the pi-type SP effect, while the positive J(ab) value for Cu(II) carbene complex 2a can be explained by the fact that ferromagnetic SE and SP interactions due to orbital orthogonality are more effective than the sigma-type antiferromagnetic SE interaction. The ligand coordination effects of both 4-pyridylcarbene and hfac play crucial roles for determination of the J(ab) values, but the ligand coordination effect of hfac is more important for the active control of charge or spin density distributions than that of 4-pyridylcarbene. The spin alignment mechanisms of 1a and 2a are indeed consistent with SE plus SP rule, which is confirmed with the shape and symmetry of natural orbitals, together with charge and spin density distributions.