Haverkort Frank, Stradomska Anna, Knoester Jasper
Zernike Institute for Advanced Materials, University of Groningen , Nijenborgh 4, 9747 AG Groningen, The Netherlands.
J Phys Chem B. 2014 Jul 24;118(29):8877-90. doi: 10.1021/jp5049277. Epub 2014 Jul 15.
Using first-principles simulations, we investigated the initial steps of the self-aggregation of the dye pseudoisocyanine (PIC) in water. First, we performed molecular dynamics (MD) simulations of the self-aggregation process, in which pile-of-coins oligomers ranging from dimers to stacks of about 20 molecules formed. The oligomer structures were found to be very flexible, with the dimers entering a weakly coupled state and then returning to a stable π-π stacked conformation on a nanosecond time scale. The structural information from the MD simulations was combined with quantum chemical calculations to generate a time-dependent Frenkel exciton Hamiltonian for monomers, dimers, and trimers, which included vibronic coupling. This Hamiltonian, in turn, was used to calculate the absorption spectra for these systems. The simulated dimer spectrum compared well to experiment, validating the face-to-face stacked dimer arrangement found in our MD simulations. Comparison of the simulated trimer spectrum to experiment suggested that oligomers larger than the dimer cannot be abundant at the onset of J-aggregation. Finally, the conformation of the PIC J-aggregate was investigated by testing the stability of several possible conformations in our MD simulations; none of the tested structures was found to be stable.
我们使用第一性原理模拟研究了染料假异氰酸（PIC）在水中自聚集的初始步骤。首先，我们对自聚集过程进行了分子动力学（MD）模拟，在此过程中形成了从二聚体到约20个分子堆叠的硬币状低聚物。发现低聚物结构非常灵活，二聚体进入弱耦合状态，然后在纳秒时间尺度上回到稳定的π-π堆积构象。将MD模拟得到的结构信息与量子化学计算相结合，生成了包含振动耦合的单体、二聚体和三聚体的含时Frenkel激子哈密顿量。反过来，该哈密顿量用于计算这些系统的吸收光谱。模拟的二聚体光谱与实验结果吻合良好，验证了我们MD模拟中发现的面对面堆叠二聚体排列。模拟的三聚体光谱与实验的比较表明，在J-聚集开始时，大于二聚体的低聚物不可能大量存在。最后，通过在我们的MD模拟中测试几种可能构象的稳定性来研究PIC J-聚集体的构象；未发现测试的结构是稳定的。
