Hefei National Laboratory for Physical Science at Microscale, and Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, PR China.
J Phys Chem B. 2009 Nov 5;113(44):14581-7. doi: 10.1021/jp9061972.
We characterize the nature of the low-lying excited states in H-aggregated perylene bisimide (PBI) dyes in terms of the recently proposed long-range-corrected density functional theory (LRC-DFT) and the mixed intramolecular Frenkel exciton (FE) and the intermolecular charge-transfer exciton (CTE) model. The singlet vertical excitation energies of H-aggregated PBI dimers and multimers are theoretically evaluated by considering the aggregation details. It is found that the dimer absorption follows closely the spectra of PBI aggregates in solution and demonstrates strong mixing of two kinds of excitons. The experimentally observed low-energy absorption peak near 2.3 eV should not be a pure CT state but a mixed excitonic state. To model the electronic excitations of H-aggregated PBI multichromophoric oligomers, we adopt a mixed FE-CTE model Hamiltonian, whose microscopic parameters are determined from LRC-DFT/TD-LRC-DFT calculations performed on an individual chromophore or a chromophore pair. By involving four to five PBI chromophores, we have succeeded in reproducing the experimental trends of optical properties in PBI stacking, such as the large blue shift of the absorption band, the mixed FE-CTE nature of the low-energy absorption bands, and the large Stokes shift of emission spectra. The long-range correction on DFT exchange-correlation energy functionals is crucial to correctly evaluate the photophysical characteristics of pi-pi stacks.
我们从最近提出的长程校正密度泛函理论（LRC-DFT）和混合分子内 Frenkel 激子（FE）和分子间电荷转移激子（CTE）模型的角度，描述了 H-聚集的并五苯二酰亚胺（PBI）染料中低能激发态的性质。通过考虑聚集细节，理论上评估了 H-聚集 PBI 二聚体和多聚体的单重垂直激发能。结果发现，二聚体吸收与溶液中 PBI 聚集体的光谱密切吻合，并表现出两种激子的强烈混合。实验观察到的 2.3 eV 附近的低能吸收峰不应是纯 CT 态，而是混合激子态。为了模拟 H-聚集 PBI 多色寡聚物的电子激发，我们采用了混合 FE-CTE 模型哈密顿量，其微观参数是通过对单个发色团或发色团对进行 LRC-DFT/TD-LRC-DFT 计算确定的。通过涉及四个到五个 PBI 发色团，我们成功地再现了 PBI 堆积中光学性质的实验趋势，例如吸收带的大蓝移、低能吸收带的混合 FE-CTE 性质以及发射光谱的大斯托克斯位移。DFT 交换相关能量泛函的长程校正对于正确评估 pi-pi 堆积的光物理特性至关重要。
