Yellow and red polymorphic co-crystals of phenyl-substituted pyrazinacene and naphthalene via π-hole⋯π interactions

Abstract

The phenyl-substituted dicyanoazanaphthalene (1, C₂₀H₁₀N₆) is a yellow compound that forms both yellow and red crystals upon co-crystallization with naphthalene (2). These crystals represent a rare example of the polymorphism of 1·2, forming two stable states, elucidating the role of donor–acceptor and π-hole⋯π interactions in the co-crystallization process. The yellow columnar crystal features an alternating arrangement of 1 and 2, while the red plate crystal exhibits paired 1 and 2 units arranged in a herringbone motif. Although no significant differences in molecular structures were observed, the red crystal, at 160 K, exhibited a higher density (D_c = 1.332 g cm⁻³) and planarity of the pyrazinacene framework compared to the yellow crystal (D_c = 1.294 g cm⁻³), which correlates with its distinct optical and luminescent behaviors.