Pyrene-cored [5]helicenes were prepared by a facile, efficient Wittig reaction and an intramolecular photocyclization reaction utilising 2,7-di-tert-butylpyrene-4-carbaldehyde and naphthalene/pyrene-based phosphorus ylides. Optical properties based on UV À vis absorption and fluorescence spectra were investigated. X-ray crystallography revealed that the pyrene-based [5]helicenes exhibited strong face-to-face p À p interactions and edge-to-face p À p interactions. HOMO and LUMO energies and molecular orbitals were also studied by density functional theory (DFT) calculations. This study has revealed that the torsion angle of the helical structure plays a role in determining the p À p interactions and the frontier molecular orbital energy levels. Thus, pyrene-based helicenes need to be considered when one designs new highly efficient organic light-emitting diodes and organic semiconductor materials.