The selective detection of molecules with less energy consumption depends critically on novel sensing concepts and the emergence of new sensor materials. Excitons and dipole moments are two strongly correlated states that have shown coupled electronic interactions on the nanoscale, and they are highly sensitive to changes in their surroundings. Here, we present exciton-dipole coupling in the two-dimensional (2D) assembly of molecular rubrene excitonic crystals to selectively detect molecules. The presence of molecules with a dipole moment transforms excitons into charge transfer, resulting in a pronounced conductivity change of freestanding rubrene nanosheets. The exciton-dipole coupling exhibits highly efficient molecular selectivity, as it offers an unambiguous electronic fingerprint for the detection of molecules - in contrast to common sensing schemes relying on the quantification of intensity changes and optical peak shifts.