ABSTRACT

A cluster produced in the gas phase is a nite nanoscale aggregate consisting of two to several thousands of atoms or molecules with a diameter of several nanometers or less. Because of the intermediate nature of the cluster between individual molecules and those of condensed matter, the cluster o en exhibits unique size-speci c characteristics that are rather di erent from those of their corresponding bulk materials. Also, gaseous clusters enable us to explore the gradual size evolution of the structural, electronic, optical, thermodynamic, and chemical properties from an isolated molecule to condensed matter systems under conditions that are completely free of chemical and physical impurities including environmental factors. e number of atoms or molecules making up the cluster (i.e., cluster size or n) can be apparently de ned using mass spectrometer, allowing us to probe the transition of such properties in a stepwise fashion. Among the class of clusters, “molecular clusters,” where constituents aggregate via weak van der Waals forces, have been regarded as an ideal model system to provide informative, molecular-level description of many physical and chemical processes occurring in bulk and interfacial environments. Most importantly, molecular clusters can provide a direct insight into intermolecular van der Waals interactions, which play an important role in determining the structures and properties of molecular assemblies in chemistry, biology, and so -material science. Over the last two decades, therefore, an enormous amount of experimental and theoretical studies

pertinent to molecular clusters have been performed, and many excellent review articles on the study of molecular clusters have already been published in the last 20 years.1-23 Since this chapter focuses only on the recent topics of molecular clusters consisting of π-conjugated organic molecules, the reader is recommended to read those review articles for an in-depth understanding of various aspects of molecular clusters.