Exciton dynamics in nanostar dendritic systems using a quantum master equation approach: core monomer effects and possibility of energy transport control

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

“…The present nanostar dendritic aggregate model is composed of two types of three-state monomers: the first type is the monomer in dendron parts, i.e., leg regions, with excitation energies ( E 21 = 35 000 cm -1 and E 31 = 37 000 cm -1 ) and transition moments (μ 21 = 15 D and μ 32 = 20 D), while the second type is the monomer in core region with excitation energies ( E 21 = 28 500 cm -1 and E 31 = 30 500 cm -1 ) and transition moments (μ 21 = 15 D and μ 32 = 20 D). The lower-lying excitaed states in the core monomer as compared to those of other monomers in dendoron parts mimic the energy acceptor group in real nanostar dendrimers. , The feature that the intermolecular distance (15 a.u.) between neighboring monomers through branching points is larger than that (15 au) in the linear-leg region corresponds to the well decoupling of π-conjugation at the meta-substituted benzene rings in real phenylacetylene dendrimers. − …”

“…Excitation energy transport is one of the essential processes in photosynthesis in green plants on earth and also finds an important application in photonics and biology. − In particular, the electron-transfer and energy-transfer processes in π-conjugated oligomers and polymers have attracted much attention in view of organics-based materials including nonlinear optical properties and semiconducting properties and have been actively investigated, for example, by the groups of Brédas and Marder. − The efficient and controllable energy transport is known to be one of the fascinating properties of dendritic systems with ordered fractal-like architecture, which exhibits a directed, multistep energy transport of absorbed light. There have been lots of studies on the efficient light-harvesting properties of phenylacetylene dendrimers and dendritic aggregates with a fractal-antenna (Cayley-tree) structure. − It has been found that the efficient energy transport is carried out by the multistep exciton migration from the periphery to the core, and is related to two peculiar structural features: (I) increase in the lengths of linear-legs involved in each generation as going from the periphery to the core and (II) meta-branching points (meta-substituted benzene rings). These features are predicted to provide multistep exciton states, in which the exciton distribution is spatially well localized in each generation and the exciton energy decreases as going from the periphery to the core region. − In our previous studies using the dipole-coupled dendritic aggregate models, − we have also pointed out the necessity of the relaxation effect between exciton states, originating in exciton−phonon coupling, for such efficient exciton migration in addition to multistep exciton states.…”

Theoretical Study on Exciton Dynamics in Dendritic Systems: Exciton Recurrence and Migration

“…On the basis of such an exciton state structure, several theoretical studies have been performed that have elucidated the mechanism of the exciton migration process in dendritic systems [ 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 ]. Using the Frenkel exciton model [ 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 ] and molecular orbital (MO) based exciton model [ 31 , 32 , 33 , 34 , 35 , 36 ], the relationships between the mechanism of energy migration process and tree-like architecture have been clarified: the coupling between exciton and nuclear vibrational states (phonon bath) are essential for the irreversible and directional energy migration in dendritic systems [ 19 , 20 , 21 ]. Our previous studies have elucidated that the weak exciton-phonon coupling causes the relaxation between exciton states, i.e., energy migration from the periphery to the core, using a dipole-coupled dendritic aggregate model [ 21 , 23 , 24 , 25 ] and ab initio MO based exciton model […”

“…Using the Frenkel exciton model [ 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 ] and molecular orbital (MO) based exciton model [ 31 , 32 , 33 , 34 , 35 , 36 ], the relationships between the mechanism of energy migration process and tree-like architecture have been clarified: the coupling between exciton and nuclear vibrational states (phonon bath) are essential for the irreversible and directional energy migration in dendritic systems [ 19 , 20 , 21 ]. Our previous studies have elucidated that the weak exciton-phonon coupling causes the relaxation between exciton states, i.e., energy migration from the periphery to the core, using a dipole-coupled dendritic aggregate model [ 21 , 23 , 24 , 25 ] and ab initio MO based exciton model [ 33 ]. In conclusion, the efficient multistep relaxation (incoherent energy migration) between exciton states turn out to require partial overlaps of spatial exciton distributions between neighboring exciton states, which respectively possess exciton distributions in adjacent generations linked with meta -branching points.…”

“…Namely, the structural features (I) and (II) are found to satisfy these conditions. On the basis of this structure-property relationship, we have investigated the dependence of exciton migration on the variation in excitation energy of core monomer using the aggregate model of “nanostar” composed of phenylacetylenes, and have indicated the possibility of control of energy transfer rate by tuning the energy gap between the core monomer and dendrons (linear legs) in the first generation [ 25 , 36 ].…”