Bending Diamondoids: Strained [3]Cyclo(4,9-diphenyldiamantylene)

A strained macrocycle containing both aromatic biphenylene and aliphatic diamantylene units is presented. [3]Cyclo(4,9-diphenyldiamantylene) 1, synthesized via gold-mediated macrocyclization, is a cycloparaphenylene (CPP)-type macrocycle featuring a rigid sp3 hybridized cage in its backbone. The single-crystal X-ray structure of 1 reveals a triangular shape that is nearly flattened along the stiff diamantane units. Nevertheless, the diamantylene subunit undergoes bending and exhibits compressed C–C bonds on the inner diameter compared to stretched C–C bonds in the outer diameter. As a consequence, the connecting biphenylene units show larger bending compared to CPPs of similar size to maintain the overall radial curvature. This is reflected in the UV/Vis absorption and fluorescence spectra of the macrocycle, with the absorption maximum being red-shifted by 0.83 eV compared to non-strained biphenyl.