The notion of (anti)aromaticity is a successful concept in chemistry to explain the structure and stability of polycyclic hydrocarbons. Cyclopentadienyl and fluorenyl cations are among the most studied classical antiaromatic systems. In this work, fluorenyl cations are investigated by high-resolution gas-phase infrared spectroscopy in helium droplets. Bare fluorenyl cations are generated in the gas phase by electrospray ionization. After mass-to-charge selection, ions are captured in ultracold helium nanodroplets and probed by infrared spectroscopy using a widely tunable free-electron laser in the 600–1700 cm–1 range. The highly resolved cryogenic infrared spectra confirm, in combination with DFT computations, that all cations are present in their singlet states
