Dendritic Molecular Aggregates Research Articles

Quantum dynamics of exciton recurrence motion in dendritic molecular aggregates

The quantum master equation approach involving weak exciton–phonon coupling has been applied to the investigation of the exciton recurrence motion in molecular aggregate models with dendritic structures. Each monomer in the models is assumed to be a dipole unit (a two-state monomer) coupled with each other by the dipole–dipole interaction. Two types of the dendritic molecular aggregates, which have mutually different interactions through the branching points, are examined. A generation of one-exciton states by an electric field and its subsequent exciton dynamics with and without relaxation terms have been investigated. It is found that the configuration of each dipole unit significantly affects the coherent motions of the exciton population. We also suggest the possibility of controlling the coherent exciton motions by tuning the frequency of an external electric field.

EXCITON MIGRATION IN DENDRITIC AGGREGATE SYSTEMS USING THE QUANTUM MASTER EQUATION APPROACH INVOLVING WEAK EXCITON-PHONON COUPLING

The quantum master equation approach involving a weak exciton-phonon coupling is applied to the exciton migration dynamics of dendritic molecular aggregates modeled after a phenylacetylene dendrimer, D25, which exhibits an efficient light-harvesting property. The mechanism of efficient 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 step-by-step transfer from the periphery to the core via multi-step exciton states. The same calculation method is applied to the exciton dynamics of a larger dendritic aggregate model, D127, modeled after the largest synthesized phenylacetylene dendrimer D127 in order to examine the dependencies of the exciton migration on the strength of the intermolecular interaction and the temperature of phonon bath. In the case of the relatively weak dipole-dipole coupling, exciton is not observed to migrate efficiently from the periphery to the core, while the largest exciton population is found to remain in the intermediate generations. This is ascribed to the fact that the thermal excitation to the higher exciton states significantly contributes to the exciton distribution in the equilibrium state when the weak intermolecular interaction reduces the energy difference between exciton states. This indicates that the intermolecular interaction is important not only for the overlaps of the exciton distributions between the exciton states, but also the relation between the exciton energy differences and thermal excitations, which spoil the distinct concentration of exciton distribution in the core generation. In the case of a low temperature, even if the intermolecular interaction is weak, the exciton population in the core region is found to be the largest in all the generations. This suggests that exciton tends to efficiently migrate from the periphery to the core when the temperature is sufficiently low. Implications of these theoretical results are discussed in relation to design of magneto-optical materials and other technological applications.

Theoretical study on the structural dependency of the exciton migration of a denritic molecular aggregate

We investigate an exciton migration of a dendritic molecular aggregate D25 involving two species of chromophores in order to elucidate the structural dependency of the light-harvesting property. We perform the exciton dynamics of D25 after the irradiation of an electric field using the master equation approach involving a weak exciton-phonon coupling. It is found that the feature of exciton migration of D25 is closely related to not only the energy differences between exciton states but also the spatial structure of a molecular aggregate.

One‐ and two‐exciton migration dynamics of a dendritic molecular aggregate

AbstractThe two‐exciton dynamics in a dendritic molecular aggregate composed of monomers coupled with each other by the dipole–dipole interaction is performed to investigate the effect of two‐exciton states on the exciton migration using the master equation approach. Although the spatial unique feature of exciton migration from the periphery to the core in the one‐exciton model is also observed in the two‐exciton model, about 36% of such exciton migration is found to be carried out by the relaxation among two‐exciton states in the present model. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003

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

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.

Theoretical Study on the Polarizabilities of Two-Dimensionally-Grown Dendritic Molecular Aggregates: The Artichitecture- and Size-Dependency

We investigated the architecture- and size-dependences of polarizabilities of various molecular aggregates. As examples, we considered several sizes of dendritic (fractal dimensional), linear (one-dimensional) and square-lattice (two-dimensional) aggregates.

Theoretical Study on Near-Resonant Third-Order Nonlinear Optical Properties (γ) of Dendritic Molecular Aggregates: Intermolecular-Interaction and Relaxation Effects on γ

Second hyperplarizability (γ) of dendritic molecular aggregate (D25) is calculated using a coupled-dipole model in the Markoff approximation. At the near resonant region, intermolecular-interaction and relaxation effects on γ are found to remarkably change the γ value.

Exciton Migration Dynamics of D58-like Dendritic Molecular Aggregate

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.

Theoretical study on the off-resonant polarizabilities of linear, square-lattice and dendritic molecular aggregates

We investigate the intermolecular-interaction effects on the off-resonant polarizabilities (a) of fractal-dimensional (dendritic), one-dimensional (linear) and two-dimensional (square-lattice) aggregate models in order to elucidate the structure-property relation in a. As a fractal-dimensional system, we consider a D25-like dendritic molecular-aggregate model, which involves chromophores at the position of acetylene units of a phenylacetylene dendrimer D25. It is found for a dendritic aggregate that intermolecular-interaction effects more enhance the contribution to a in the core region than that in the periphery region.

Theoretical study on the near-resonant second hyperpolarizability ( γ) of a dendritic molecular aggregate: the spatial contribution of intermolecular-interaction and relaxation to γ

Second hyperpolarizability (γ) of a dendritic molecular aggregate (D25) in the resonant region is calculated using a two-exciton coupled-dipole model. Using a visualization method of exciton generation to y, there is found to be a significant difference in the spatial contribution between intermolecular-interaction and relaxation effects on γ, which is dramatically changed compared to the case of the dendritic aggregate without intermolecular-interaction.

Third-order nonlinear optical properties of dendritic molecular aggregates: Effects of fractal architecture

We investigate the microscopic third-order nonlinear optical properties, i.e., the second hyperpolarizabilities (γ), of two different sizes of molecular aggregates with a dendritic, i.e., Bethe-lattice, structure. One possesses a nonfractal structure, while the other has a fractal structure. The aggregate is treated in a two-exciton model composed of two-state monomers coupled to each other by the dipole–dipole interaction. The off-resonant γ of the aggregates are calculated by the numerical Liouville approach, including relaxation effects. The total γ value is partitioned into the contribution of virtual exciton generation, and its spatial contribution to γ is analyzed in relation to the virtual excitation processes in the perturbation theory. It is found that the intermolecular-interaction effect enhances both one- and two-exciton-generation contributions, while the relaxation effect reduces those, although the one- and two-exciton-generation contributions have mutually opposite signs. From the comparison of spatial contributions to γ between the nonfractal and fractal aggregates, an enhancement of the contribution to γ from the periphery to the core is observed in the fractal structure, while such a feature is not observed in the nonfractal structure. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001

Exciton migration dynamics in a dendritic molecular aggregate

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.

Exciton Migration Pathways in Dendritic Molecular Aggregates

The energies and transition moments of exciton states for dendritic molecular aggregates are calculated using a model Hamiltonian involving dipole-dipole interaction. Exciton pathways are found to be remarkably influenced by the constituent monomer units.