Large aromatic molecules such as polyacenes 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 hand, we have developed thermal and photochemical precursor methods to overcome these problems. Briefly, bicyclo [2.2.2] octadiene-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. Importantly, these precursor molecules are often more soluble and stable than the corresponding post conversion compounds, indicating that this methodology enables us to prepare solution-processed organic electronic devices. This paper highlights the design and synthesis of large aromatic compounds and demonstrates a new way for multicomponent layer-by-layer preparation of organic semiconducting films through thermal and photochemical precursor methods.