Excess Electron Behaviors in Oligoacene Gaint Molecular Aggregates

Organic molecular clusters, consisting of 2-1000 molecules, are finite aggregates, providing a microscopic model to investigate the chemical and physical properties of molecular assemblies in organic nanomaterials or at interface. A unique nature of organic molecular clusters is originated from weak intermolecular van der Waals interactions, which results in narrow electronic bandwidths, and electron (or hole) charge carriers in organic molecular aggregates are often strongly localized on individual molecules, where molecular ions are formed. Such a molecular ion is instantaneously stabilized by the electronic polarization of its neutral neighbors, which significantly affects charge-transport energy levels in organic molecular aggregates. Since the number of constituent molecules can be tuned accurately in combination with mass spectrometry, size-selective investigations on a broad size range of organic molecular clusters can provide a deep understanding on the evolution of the electronic and geometric properties. In particular, oligoacenes such as tetracene and pentacene are classical examples for the study of charge carrier localization and transport. We have recently realized an efficient formation of large oligoacene molecular nanoclusters up to more than 200 constituent molecules; naphthalene (Nph), anthracene (Ac), tetracene (Tc), and so on. By adding a single excess electron, the corresponding nanocluster anions were produced in the gasphase, and the size-selective properties could be revealed by photoelectron spectroscopy. In particular, two types of anion states are shown to coexist in nanometer-scale cluster anions of oligoacene of Nph, Ac, and Tc. The photoelectron spectra of size-selected cluster anions containing 2 to 100 molecules revealed that rigid “crystal-like” cluster anions emerge, greater than ~2 nanometers in size, and coexist with the “disordered” cluster anion in which the surrounding neutral molecules are reorganizing around the charge core.