The aggregation of phenylacetylene in the gas phase was investigated by selectively recording the IR spectra of the clusters consisting of up to six monomer units, which perhaps are the largest clusters of substituted benzene ever reported. Analysis of the IR spectra with aid of a B97-D3/aug-cc-pVDZ level calculations reveals the formation of anti-parallel π-stacked structure for dimer and structure incorporating exclusively aromatic C–H···π interactions. A hitherto unknown assembly of one-dimensional array of anti-parallel π-stacked units emerge. The aggregation behaviour of phenylacetylene in the gas phase is fundamentally different from benzene, phenol and aniline vis-a-vis their crystal structures. The structures of the three known polymorphic crystal structures can be reconciled by the formation of supramolecular synthon with acetylenic C–H···π interactions, which is preferred over energetically favored aromatic C–H···π interactions. Furthermore, the small (phenylacetylene)_n [n = 3-6] clusters incorporating aromatic C–H···π interactions can be envisaged as liquid-like aggregates which under varied conditions lead formation of lead to formation of multiple polymorphs during in-situ cryo-crystallization.

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