Fluctuation-induced acceleration of inter-ligand exciton transfer in bis(dipyrrinato)Zn(II) complex

Abstract

Exciton transfer dynamics between chromophores depends on excitonic coupling, which is governed by relative orientation between the chromophores. While the excitonic coupling is treated as a static parameter in many cases, structural dynamics can introduce time-dependence on the excitonic coupling. However, influence of the dynamics of excitonic coupling on the exciton transfer has been scarcely understood. In the present study, exciton transfer under dynamical fluctuation in excitonic coupling was investigated via combined use of non-adiabatic molecular dynamics simulations, exciton density analysis, and a simple two-state model, for inter-ligand exciton transfer in bis(dipyrrinato)Zn(II) as the example case. The reaction coordinate for the exciton transfer was obtained a posteriori via regression analysis where the target and explanatory variables are diabatic energy gaps and atomic displacements, respectively. The results suggest that dynamical angular fluctuation between the two dipyrrinato ligands incidentally increase the excitonic coupling, accelerating the exciton transfer between the ligands.

Figures (6)

• Figure 1: (a) Struture of Zn(dp)2. The white, gray, blue, and silver spheres represent H, C, N, and Zn atoms, respectively. (b) HOMO (lower) and LUMO (upper) of dp monomer anion. The orbitals have nonbonding, bonding, and antibonding characters with respect to the bonds highlighted by gray, blue, and red dashed circles, respectively. Visualized using VESTA softwareMomma2011 (isolevel=0.03).
• Figure 2: (a) Temporal changes in electronic state populations. Orange, blue, and green lines indicate the populations of electronic states classified as LE (Not Transferred), LE (Transferred), and CT. Dashed black line indicates the fitting curve according to eq. \ref{['fitting']}. (b) Temporal changes of $Q^{\rm ex}_{\rm dp1}$ and $Q^{\rm ex}_{\rm dp2}$ for two example trajectories that represent "adiabatic" exciton transfer pathway and (c) that represent "non-adiabatic" exciton transfer pathway.
• Figure 3: Schematic illustration of "adiabatic" and "non-adiabatic" exciton transfer pathways on adiabatic potential energy surfaces (PESs). The red and blue colors of PESs indicate that the exciton is dominantly localized on dp1 and on dp2, respectively.
• Figure 4: (a) Absolute values of excitonic coupling ($|V|$) versus displacement of the dihedral angle between the two dps ($|\phi-90^\circ|$) at the moments of "adiabatic" exciton transfer events. (b) Histograms of the dihedral angle between the two dps ($\phi$) for all snapshots (gray) and for snapshots at "adiabatic" exciton transfer events (blue). (c) Comparison of the diabatic energy gaps obtained from the "monomer" TD-DFT calculations ($\Delta E$), and those predicted from the linear regression model ($\Delta E_{\rm Pred}$). (d) Atomic displacement vector in the direction of $\bar{\bf Q}$ (green arrows). White, gray, blue, and silver spheres represent H, C, N, and Zn atoms, respectively.
• Figure 5: (a) VACFs for the dihedral angle ($\phi$) and the reaction coordinate ($Q$). (b) Power spectra of VACFs. (c) Temporal changes of $\phi$ and $Q$ for all NA-MD trajectories. The points where "adiabatic" and "non-adiabatic" exciton transfer events occurred are indicated by blue and red cross marks, respectively. (d) Distribution of $\phi$ sampled from the NA-MD trajectories (green). (e) Distribution of $Q$ sampled from the NA-MD trajectories and at "adiabatic" exciton transfer (blue).