Harunori Fujita

Excluded‐Volume Effects in Dilute Polymer Solutions. I. Equilibrium Properties

Data of the statistical radius 〈 S2 〉1 / 2 and the second virial coefficient A2 were obtained for sharp fractions of polychloroprene (PCP) in methyl ethyl ketone (MEK), n‐butyl acetate, and carbon tetrachloride at 25°C and in trans‐decalin at temperatures ranging from 1° to 50°C. MEK at 25°C and trans‐decalin at 2°C were found to be theta solvents for PCP. From these data two dimensionless quantities defined by αs2 = 〈 S2 〉 / 〈 S2〉0and Ψ = A2M1/2[4π3/2NA( 〈 S2 〉 / M3 / 2]−1 were calculated for all combinations of molecular weight M, solvent species, and temperature, where 〈 S2 〉0 is the value of 〈 S2 〉 under θ conditions and NA is Avogadros number. These two quantities should be functions of a single parameter z, usually known as the interaction parameter, which is proportional to M1/2. With the data for these quantities from the present experiments on PCP as well as from Berrys experiments on polystyrene, comparative tests of typical theories for the excluded‐volume effects on dilute polymer solutions ...

Exciton migration dynamics in a dendritic molecular aggregate

Abstract We investigate exciton migration dynamics in two types of dendritic molecular aggregate models, i.e., Bethe lattice, with mutually distinct intermolecular interaction between adjacent linear legs at the branching points. Each monomer molecule is assumed to be a dipole unit (a two-state model) coupled with each other by the dipole–dipole interaction. The generation of one exciton in the aggregates by the incident laser field and its subsequent dynamical behavior are investigated in a numerically exact manner. The two types of models show different exciton energy structures and distribution. It is also found that relaxation effects in the exciton states are essential for exciton migration from the periphery to the core.

Excluded‐Volume Effects in Dilute Polymer Solutions. II. Limiting Viscosity Number

Measurements of [η] were made for seven fractions of polychloroprene (PCP) in methyl ethyl ketone, n‐butyl acetate, and carbon tetrachloride at 25°C and for three fractions of PCP in trans‐decalin at a number of temperatures ranging from 0.05° to 65°C. On a log–log graph paper, the values of αη3( = [η]/[η]θ) calculated from these data formed a composite curve when plotted against αs3( = 〈S2〉3/2/〈S2〉03/2) obtained from light‐scattering experiments under the same conditions. This curve is characterized by a relatively marked upward curvature in the vicinity of the origin, followed by a linear portion of unit slope. Its initial tangent has a slope of 0.65, which is much smaller than 0.81 predicted theoretically by Kurata and Yamakawa. Plots of αη3 vs the interaction parameter z were prepared by evaluating z for each αs by use of the Yamakawa equation. They were fitted by a single curve having a slight downward curvature, suggesting that no drainage effect manifests itself in the viscosity behavior of the sys...

Some Topics Concerning the Radius of Gyration of Linear Polymer Molecules in Solution

Fixmans integral for the distribution function P(S2) obeyed by S2 of the unperturbed polymer chain was evaluated analytically, and its asymptotic forms valid for t ≪ 1 and t ≫ 1 were derived. Here S is the radius of gyration of the chain and t is the ratio of S2 to its statistical average 〈S2〉. The values of P(S2) calculated from the analytic solution agreed with Koyamas computer values over the entire range of t. The leading term of the asymptotic form for t ≪ 1 confirmed the previous result of Fixman. The asymptotic form for t ≫ 1 agreed not with Fixmans but with that of Forsman. The Flory–Fisk theory for the excluded volume effect in polymer chains was reformulated with the exact form being substituted for P(S2) in their basic equation for αs2. Here αs is the linear expansion factor of a polymer molecule defined in terms of 〈S2〉. The calculated curve for αs3 as a function of the interaction parameter z differed markedly from that derived by Flory and Fisk who had employed an approximate expression f...

Mechanism of exciton migration of dendritic molecular aggregate: a master equation approach including weak exciton-phonon coupling

Abstract A master equation approach including weak exciton–phonon coupling is applied to the exciton dynamics of a dendritic molecular aggregate modeled after a phenylacetylene dendrimer, D25, which exhibits an efficient light-harvesting property. The mechanism of exciton migration from the periphery to the core is studied by analyzing relaxation terms among the exciton states originating in weak exciton–phonon coupling. Partial overlaps of exciton distributions between neighboring exciton states are found to be important for realizing the unique migration behavior by stepwise transfer from the periphery to the core via multi-step exciton states.

Approximate Theory of the Expansion of a Linear Polymer Chain in Nonideal Solvent

A new rigorous expression is derived for the partition function of a linear polymer chain with its end‐to‐end distance fixed to a specified value R, subject to the condition that the interaction of a pair of chain segments is of a short‐range nature and characterized by the binary cluster integral β. Starting with this expression and estimating the total number of segmental collisions in such a chain by use of the approximation of uniform distribution of segments and of the ellipsoid approximation to the over‐all shape of the molecule, an approximate closed expression for α(z) is obtained, where α is the expansion factor of the molecule defined in terms of the statistical average of R2, and z is the familiar parameter characterizing the strength of excluded‐volume effect. It is shown that the well‐known α5 equation of Flory as well as the α3 equation of Kurata, Stockmayer, and Roig correspond to two extreme cases of the present theory. In the region of α3 up to about 3, the derived equation for α3 gives a...

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

We investigate the size-dependency of static polarizabilities (α) (in the chain-length direction) of three types of model oligomers (made of phenylene vinylenes), i.e., para-oligomers and meta-oligomers with fractal and nonfractal structures by the finite-field method using molecular orbital calculations. The fractal-structured meta-oligomers are considered to be models of dendron parts involved in Cayley-tree-type dendrimers. The chain-length dependence of α of para-oligomers is found to be much larger than that of nonfractal-structured meta-oligomers, while the fractal-structured meta-oligomers exhibit an attractive chain-length dependence of α; the α values of meta-structured oligomers are near to those of nonfractal-structured meta-oligomers in small chain-length region, while they are close to those of para-oligomers in sufficiently large chain-length region. Using the polarizability density analysis, the spatial contributions of π-electrons to α of nonfractal-structured meta-oligomers are found to b...

Third-order nonlinear optical properties of fractal- and nonfractal-structured oligomers modeled after dendron parts in Cayley-tree-type dendrimers

As an extension of our previous study on polarizabilities [J. Chem. Phys. 115, 1052 (2001)], we investigate the molecular third-order nonlinear optical properties, i.e., the second hyperpolarizabilities (γ), of three types of model oligomers (made of phenylene vinylenes), i.e., para-oligomers and meta-oligomers with fractal and nonfractal structures, which are considered to be models of dendron parts involved in Cayley-tree-type dendrimers. Similarly to the polarizability case, the chain-length dependence of γ of para-oligomers is found to be much larger than that of nonfractal meta-oligomers, while fractal meta-oligomers exhibit an intermediate chain-length dependency of γ between them. The spatial contributions of π-electrons to γ of these systems are elucidated using hyperpolarizability density analysis. It is found that the spatial contributions of π-electrons to γ of nonfractal meta-oligomers are much smaller than those of para-oligomers and are well decoupled at the meta-substituted benzene rings, w...

Exciton Migration Dynamics of D58-like Dendritic Molecular Aggregate

Abstract We report the exciton population dynamics of D58-like dendritic molecular-aggregate model involving chromophores at the position of acetylene units of a phenylacetylene dendrimer D58. It is found that the relaxation effect is essential for exciton migration from the periphery to the core.

Remarkable enhancement of two-photon absorption in cation molecules

We have calculated the imaginary parts of γ(-ω,ω,ω,-ω) (1mγ), which describe two-photon absorption (TPA), for several conjugated molecules with charged defects by the sum-over-state (SOS) approach using the semi-empirical MO method. The 1mγvalues are found to be remarkably enhanced by introduction of charged defects compared to those of similar-size neutral molecules. The main reasons can he ascribed to the decrease in the transition energy and the increase in the magnitude of transition moment between the first allowed excited state and the ground state. We propose novel systems with large TPA cross sections in connection with a contribution of symmetric resonance structures with invertible polarization (SKIP).