The Polymorphic Structures of a Squarylium Dye. The Monoclinic (Green) and Triclinic (Violet) Forms of 2,4 bis(2-hydroxy 4-diethylaminophenyl)-1,3-cyclobutadienediylium 1,3-diolate

Abstract

The title compound, a candidate for solar photovoltaic applications, crystallizes in at least two polymorphic structures. Triclinic form: a = 11.911 (4), b = 7.401 (9), c = 6.501 (7) Å, α = 92.78 (7), β = 111.9 (4), γ = 98.08 (6)°, = .046 for 955 reflections with F ₀ > 2σ(F ₀). Monoclinic form: a = 15.72 (4), b = 7.283 (3), c = 9.591 (3) Å, β = 106.11 (8)°, P2₁/a, Z = 2, R = .084, R_w = .077 for 804 reflections F ₀ > 2σ(F ₀). In both structures the geometric features of the essentially planar molecule are compatible with significant contributions from classical low energy resonance structures. Cocrystallization of the two modifications as well as two morphologies for the monoclinic form can be rationalized on the basis of competition between a small number of intermolecular interactions, which are discussed.