Yang Chou-Hsun, Wang Chun-I, Wang Yi-Siang, Hsu Chao-Ping
Institute of Chemistry, Academia Sinica, 128 Section 2 Academia Road, Nankang, Taipei 115, Taiwan.
National Center for Theoretical Sciences, 1, Section 4, Roosevelt Road, Taipei 106, Taiwan.
J Chem Theory Comput. 2024 Aug 14;20(16):6981-91. doi: 10.1021/acs.jctc.4c00742.
Many charge-transporting molecular systems function as ordered or disordered arrays of solid state materials composed of nonpolar (or weakly polar) molecules. Due to low dielectric constants for nonpolar systems, it is common to ignore the effects of outer-shell reorganization energy (λ). However, ignoring λ has not been properly supported and it can severely impact predictions and insights derived. Here, we estimate λ by two means: from experimental ultraviolet photoelectron spectra, in which vibronic progression in these spectra can be fitted with the widths of peaks determining the low-frequency component in reorganization energy, regarded to be closely associated with λ, and from molecular dynamic (MD) simulation of nonpolar molecules, in which disorder or fluctuation statistics for energies of charged molecules are calculated. An upper bound for λ was obtained as 505 and 549 meV for crystalline anthracene (140 K) and pentacene (50 K), respectively, by fitting of experimental data, and 212 and 170 meV, respectively, from MD simulations. These values are comparable to the inner-sphere reorganization energy (λ) arising from intramolecular vibration. With corresponding spectral density functions calculated, we found that λ is influenced both by low- and high-frequency dynamics, in which the former arises from constrained translational and rotational motions of surrounding molecules. In an amorphous state, about half of the λ's obtained are from high-frequency components, which is quite different from the conventional polar solvation. Moreover, crystalline systems exhibit super-Ohmic spectral density, whereas amorphous systems are sub-Ohmic.
许多电荷传输分子系统作为由非极性（或弱极性）分子组成的固态材料的有序或无序阵列发挥作用。由于非极性系统的介电常数较低，通常会忽略外壳重组能（λ）的影响。然而，忽略λ并未得到充分支持，并且会严重影响所得到的预测和见解。在这里，我们通过两种方法估计λ：从实验紫外光电子能谱，其中这些光谱中的振动进展可以用决定重组能中低频成分的峰宽来拟合，该低频成分被认为与λ密切相关；以及从非极性分子的分子动力学（MD）模拟，其中计算带电分子能量的无序或涨落统计。通过对实验数据的拟合，对于结晶蒽（140 K）和并五苯（50 K），λ的上限分别为505和549毫电子伏特，从MD模拟中得到的值分别为212和170毫电子伏特。这些值与分子内振动产生的内球重组能（λ）相当。通过计算相应的光谱密度函数，我们发现λ受低频和高频动力学的影响，其中前者源于周围分子受限的平移和旋转运动。在非晶态下，所得到的λ约有一半来自高频成分，这与传统的极性溶剂化有很大不同。此外，晶体系统表现出超欧姆光谱密度，而非晶系统是亚欧姆的。
