The Bicyclo[1.1.1]pentane Framework—an Excellent Relay for π/σ Conjugation

Photoelectron spectra of the bicyclo[1.1.1]pentanes 1 and 2 reveal a high degree of splitting (0.7 eV) between the π-MOs of the ethynyl residues and the 4p orbitals of the bromine atoms, respectively. This is presumably due to strong interaction between the π systems and the σ system of the bicyclopentane framework. A similarly strong interaction has been demonstrated between the π and σ systems in 3. These observations are of considerable interest from the standpoint of the bicyclo[1.1.1]pentyl moiety serving as a potential subunit in the construction of oligomers and for promoting energy transfer over long molecular distances.