With the aim to design fluorescent solids, a series of indeno[1′,2′:4,5]pyrido[2,1-a]isoindol-5-ones with various substituents was prepared. In these π-extended pentacyclic derivatives, the presence of a methyl group in the 7-position was found to have a critical influence on the fluorescence properties in the solid state. Crystal packing of the non-substituted derivatives shows strong π–π interactions causing quenching of the fluorescence. In contrast, by introducing a methyl substituent in the 7-position we obtained compounds with fluorescence quantum yield up to 32% in the solid state.