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The molecular self-assembly behaviour of 2,2'-Bis-(4-hexyloxyphenyl)-bi-1,3,4-oxadiazole (BOXD-6) in solution, on surfaces and in bulk crystals, and its photo-physical properties were studied via a combination of experimental techniques and theoretical calculations. It is found that BOXD-6 molecules self-assemble into both H-and J-aggregates at moderate concentration (~10 -4 M) and then transit to exclusive J-aggregates at higher concentration (~10 -3 M) in tetrahydrofuran. In H-aggregation (α polymorph), BOXD-6 adopts a linear conformation and forms a one-dimensional layered structure; in Jaggregation (β polymorph), it adopts a Z-shaped conformation and form a more ordered two-dimensional layered structure. A π-stacking structure is observed in both cases, and adjacent molecules in the J-aggregation show larger displacement along the molecular long axis direction than that in H-aggregation. Although J-aggregates are almost the only component in concentrated solutions (10 -3 M), both H-and J-aggregates can be obtained if concentrated solution is transformed onto substrates through a simple drop-casting method. Such a phase transition during film formation can be easily avoided by adding water as precipitator; a film with pure J-aggregates is then obtained. In order to get more information on molecular self-assembly, intermolecular interaction potential energy surfaces (PES) were evaluated via theoretical calculations at the DFT level (M062x/6-31G**). The PES not only confirm the molecular stacking structures found in crystals but also predict some other likely structures, which will be the target of future experiments Please do not adjust margins Please do not adjust margins molecular self-assembly on substrates. In order to get deeper understanding of the molecular self-assembly, the intermolecular interaction potential energy surfaces were also evaluated through theoretical calculations at the Density Functional Theory (DFT) level.In H-aggregation, BOXD-6 adopts a linear conformation and forms a one-dimensional layered structure; in J-aggregation, it adopts a Z-shaped conformation and form a more ordered two-dimensional layered structure.

One of the energy-minimum structures predicted by the intermolecular interaction potential energy surface computed using the M062x/6-31G** method closely resembled the crystal packing.

The title mol­ecule, C24H14N4O2, lies on an inversion centre and the asymmetric unit containg one half-mol­ecule. The naphthalene ring systems are twisted slightly with respect to the oxadiazole rings, making a dihedral angle of 1.36 (6)°. These mol­ecules are π-stacked along the crystallographic a axis, with an inter­planar distance of 3.337 (1) Å. Adjacent mol­ecules are slipped from the ‘ideal’ cofacial π-stack in both the long and short mol­ecular axis (the long mol­ecular axis is defined as the line through the naphthalene C atom in the 6-position and the mol­ecular center, the short mol­ecular axis is in the mol­ecular plane perpendicular to it). The slip distance along the long mol­ecular axis (S 1) is 7.064 (1) Å, nearly a two-ring-length displacement. The side slip (S 2, along the short mol­ecular axis) is 1.159 (8) Å.

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