Space filling shapes the interaction networks in mixed pyrrole-benzene trimers and tetramers.

The structural preferences of molecular assemblies are governed by a delicate balance between strong directional forces and diffuse dispersion contacts. Mixed trimers of pyrrole (Py) and benzene (Bz) provide an ideal benchmark to probe this interplay: the robust N-H⋯π interaction anchoring the Py-Bz dimer competes with the drive toward compact, dispersion-stabilized arrangements in larger clusters. Here, we report the first high-resolution structural characterization of the Py-(Bz)2 and (Py)2-Bz trimers and the (Py)2-(Bz)2 tetramer using chirped-pulse Fourier transform microwave spectroscopy combined with dispersion-corrected DFT calculations and intensity-based cross-correlation analysis. The results show that while N-H⋯π and C-H⋯π interactions serve as primary anchors, the overall geometries are dictated by space-filling principles that maximize dispersion contacts. These findings establish pyrrole-benzene hetero clusters as a rigorous benchmark for theoretical methods and provide fundamental insight into the forces guiding aromatic aggregation and self-assembly in complex molecular systems.