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.