Self-assembly of highly luminescent bi-1,3,4-oxadiazole derivatives through electron donor-acceptor interactions in three-dimensional crystals, two-dimensional layers and mesophases

We report on the synthesis and self-assembly of a new series of highly luminescent bi-1,3,4-oxadiazole derivatives [2,2'-bis(4-alkoyphenyl)-bi-1,3,4-oxadiazole, BOXD-n, n 1, 3, 4, 5, 6, 7, 10, 16]. Fully conjugated conformations were demonstrated either by computer simulation or in a single-crystal state. Well-defined 3D donor-acceptor (DA) architectures with strong face-to-face and edge-to-edge donor acceptor interactions were observed in the single-crystal structure of BOXD-1. Highly oriented face-on two-dimensional DA layered structures due to edge-to-edge donor-acceptor interactions were observed on highly oriented pyrolytic graphite (HOPG) in BOXD-7 and BOXD-16. Nematic phase, smectic C phase with large tilted angle (theta approximate to 50 degrees) and relatively large transition enthalpic values and a highly ordered smectic X phase were demonstrated in BOXD-n (n 5, 6, 7, 8, 10, 16) through tailing the terminal chains and relatively large scale monodomains were prepared in the smectic X phase of BOXD-5 even without any surface treatment. Strong blue fluorescent emissions were observed in BOXD-6 either in cyclohexane (Phi(F) approximate to 92%) or in solid state (Phi(F) approximate to 57%). The donor-acceptor interactions between alkyoxyphenylene rings and 1,3,4-oxadiazole rings were thought to be the driving force for the molecules to self-assemble into a large angle tilted layered structure.