Cyclic [5]Paraphenyleneacetylene: Synthesis, Properties, and Formation of a Ring-in-Ring Complex Showing a Considerably Large Association Constant and Entropy Effect

In the ring: The title compound ([5]CPPA), the smallest belt‐shaped conjugated system known to date, forms a considerably stable ring‐in‐ring complex with [8]CPPA (see picture). [8]CPPA⊃[5]CPPA has a smaller contact area than [9]CPPA⊃[6]CPPA but its large Ka and ΔH values reveal that the interaction results from the electrostatic attractive force rather than the dispersion force.