Large aromatic molecules such as large acenes and polycyclic aromatic hydrocarbons are regarded as promising materials for organic electronic devices. However, it is normally difficult to synthesize such large aromatic molecules because of the low solubility and stability. On the other hands, we have developed thermal and photochemical precursor methods to overcome these problems. Bicyclo[2.2.2]octadiene(BCOD)-fused acenes can be converted to the corresponding acenes by the thermally induced retro-Diels-Alder reaction, while α-diketone-type precursors can be converted simply by photoirradiation. Specifically, irradiation of a-diketone-type precursors at the n-π* absorption leads to the release of two molecules of CO, and the corresponding acenes can be prepared quantitatively in solutions or in films. Importantly, these precursor molecules are often more soluble and stable than the corresponding one.

Here we report the synthesis of thermally convertible pentacene and photochemically convertible heptacene precursor molecules. Namely, BCOD moieties were introduced in a pentacene aromatic core as thermally removable leaving groups, while α-diketone moieties were introduced in a heptacene aromatic core as photochemically removable leaving groups. In addition, we also report a combined scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM) study to observe the conversion of precursor molecules into the corresponding acenes on a pristine Au(111) substrate under ultrahigh vacuum (UHV) conditions. For example, the STM and nc-AFM images clearly displays that the annealing of pentacene precursor molecule deposited Au(111) substrates at 390 K result in the one-dimensional (1D) Au-directed organometallic nanostructures. Further annealing at 475 K produced the 1D pentacene organometallic nanostructures.

Our result indicates that the precursor method and on-surface synthesis would be a good combination for making novel structure.