Defining the Complete Chemical Space of π-Conjugated Hydrocarbons by Unit-Based Construction

π-Conjugated hydrocarbons are foundational molecular scaffolds in materials science. However, their design and discovery are limited by the lack of systematic frameworks for exploring their chemical space while preserving structural validity. We present a unit-based molecule generation framework tailored to π-conjugated hydrocarbons, named CARBOT. This framework comprises a minimal set of chemically rational unit transformations, including vinyl- and ethynyl-group addition and cyclization via C–C bond formation. CARBOT is provably sound and complete within the scope of neutral Kekulé hydrocarbons with carbons restricted to sp and sp2 hybridization, excluding allenes and carbenes. To accelerate access to aromatic frameworks, additional composite transformations are introduced. A depth-first search combined with a substructure-matching score enables rediscovery of a wide range of π-conjugated hydrocarbons within the defined space, including nonplanar nanographenes, highly strained carbon nanobelts, and Möbius hydrocarbons, and it can be scaled up to molecules as large as C200H100. The molecule generator is expected to provide a systematic foundation for the design and discovery of functional π-conjugated molecules.