Ando Naoto, Mitsui Masaaki, Nakajima Atsushi
Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan.
J Chem Phys. 2007 Dec 21;127(23):234305. doi: 10.1063/1.2805185.
The evolution of the electronic structure of molecular aggregates is investigated using anion photoelectron (PE) spectroscopy for anionic clusters of anthracene (Ac) and its alkyl derivatives: 1-methylanthracene (1MA), 2-methylanthracene (2MA), 9-methylanthracene (9MA), 9,10-dimethylanthracene (DMA), and 2-tert-butylanthracene (2TBA). For their monomer anions (n=1), electron affinities are confined to the range from 0.47 to 0.59 eV and are well reproduced by density functional theory calculations, showing the isoelectronic character of these molecules. For cluster anions (n=2-100) of Ac and 2MA, two types of isomers I and II coexist over a wide size range: isomers I and II-1 (4< or =n<30) or isomers I and II-2 (n> or = approximately 40 for Ac and n> or = approximately 55 for 2MA). However, for the other alkyl-substituted Ac cluster anions (i.e., 1MA, 9MA, DMA, and 2TBA), only isomer I is exclusively formed, and neither isomer II-1 nor II-2 is observed. The vertical detachment energies (VDEs) of isomer I in all the anionic clusters depend almost linearly on n(-1/3). In contrast, the VDEs of isomers II-1 (n> or =14) and II-2 (n=40-100), appeared only in Ac and 2MA cluster anions, remain constant with n and are approximately 0.5 eV lower than those of isomer I. The PE spectra revealed the characteristics of each isomer: isomer I possesses a monomeric anion core that is gradually embedded into the interior of the cluster with increasing n. On the other hand, isomers II-1 and II-2 possess a multimeric (perhaps tetrameric) anion core, but they differ in the number of layers from which they are made up; monolayer (isomer II-1) and multilayers (isomer II-2) of a two-dimensionally ordered, finite herringbone-type structure, in which electron attachment produces only little geometrical rearrangement. Moreover, the agreement of the constant VDEs of isomer II-2 with the bulk data demonstrates the largely localized nature of the electronic polarization around the excess charge in a crystal-like environment, where about 50 molecules provide a charge stabilization energy comparable to the bulk.
利用阴离子光电子（PE）光谱研究了蒽（Ac）及其烷基衍生物：1-甲基蒽（1MA）、2-甲基蒽（2MA）、9-甲基蒽（9MA）、9,10-二甲基蒽（DMA）和2-叔丁基蒽（2TBA）的阴离子簇分子聚集体的电子结构演变。对于它们的单体阴离子（n = 1），电子亲和能限制在0.47至0.59 eV范围内，并且通过密度泛函理论计算得到了很好的再现，表明了这些分子的等电子特性。对于Ac和2MA的簇阴离子（n = 2 - 100），两种异构体I和II在很宽的尺寸范围内共存：异构体I和II - 1（4≤n＜30）或异构体I和II - 2（对于Ac，n≥约40；对于2MA，n≥约55）。然而，对于其他烷基取代的Ac簇阴离子（即1MA、9MA、DMA和2TBA），仅专门形成异构体I，未观察到异构体II - 1和II - 2。所有阴离子簇中异构体I的垂直脱附能（VDE）几乎与n^(-1/3)呈线性关系。相反，仅在Ac和2MA簇阴离子中出现的异构体II - 1（n≥14）和II - 2（n = 40 - 100）的VDE随n保持恒定，并且比异构体I的VDE低约0.5 eV。PE光谱揭示了每种异构体的特征：异构体I具有一个单体阴离子核心，随着n的增加逐渐嵌入簇的内部。另一方面，异构体II - 1和II - 2具有多聚体（可能是四聚体）阴离子核心，但它们在组成它们的层数上有所不同；二维有序的有限人字形结构的单层（异构体II - 1）和多层（异构体II - 2），其中电子附着仅产生很少的几何重排。此外，异构体II - 2的恒定VDE与体相数据的一致性表明，在类似晶体的环境中，围绕多余电荷的电子极化在很大程度上是局部化的，其中约50个分子提供的电荷稳定能与体相相当。
