Bigger and Brighter Fluorenes: Facile π‐Expansion, Brilliant Emission and Sensing of Nitroaromatics

π‐Expanded butterfly‐like 2D fluorenes and 3D spirobifluorenes 1 – 5 were synthesized via a DDQ‐mediated oxidative cyclization strategy with a high regioselectivity. Through structural modification via π‐expansion, it was possible to achieve near‐ultraviolet absorption, bright‐blue emission, very high near‐unity fluorescence quantum yields in solution as well as in film states, and deep‐lying HOMO energy levels with excellent thermal stabilities. Furthermore, these electron‐rich compounds displayed a notable behavior towards sensing of nitroaromatic explosives, such as picric acid, up to a detection limit of 0.2 ppb.