Functionalization of Semiconductor Surfaces

CHAPTER 7

Covalent Binding of Polycyclic Aromatic Hydrocarbon Systems

KIAN SOON YONG AND GUO–QIN XU

7.1 INTRODUCTION

Polycyclic aromatic hydrocarbons (PAHs) are a class of compounds that are made up of fused aromatic rings and comprise of only carbon and hydrogen. Some examples of PAHs are shown in Fig. 7.1, which consist of the acene members, perylene, coronene, dibenzo[a, j]coronene, and acenaphthylene. The acenes are made up of linearly fused benzene rings, of which naphthalene, anthracene, tetracene, and pentacene are the most commonly studied molecules. Due to the sharing of π-orbitals between adjacent rings in PAHs, each ring segment possesses a different degree of aromaticity, according to Clar's sextet concept [1]. For instance, only one ring within naphthalene and anthracene can be assigned three double bonds without duplication and be considered as an aromatic sextet. The sextets are represented by the circles in Fig. 7.2, which are mobile within the molecules via the movements of two π-electrons as symbolized by the arrows. The highly conjugated π-electron system within the PAHs also makes them highly sought after as the active semiconducting materials in organic electronics [2]. Specifically, pentacene has aroused great attention due to its remarkable field-effect mobility that is comparable to that of amorphous silicon. Examples of organic electronic devices that incorporate pentacene are field-effect transistors [3,4] and solar cells [5]. Besides pentacene, tetracene ...