Crystal Structure of Dinaphtho[2,1,1′,2′]furan Picrate

Dinaphtho[2,1,1′,2′]furan (DNF, 1) is well characterized as a heteroatom helicene,1 i.e., an analogue of [5]helicene replacing one double bond with isoelectronic ether oxygen.2 Owing to the expanded π-system of DNF, DNF as an advanced material has been applied in various devices, such as field-effect transistors3 and light-emitting devices.4 One of the unique applications of DNF is as a precursor of Buchiwild-type monodentate optically active phosphine (MOP) ligands.5 Furthermore, Tsubaki synthesized the highly conjugated fan-shaped oligonaphthofurans using the DNF unit.6 In the beginning of the last century from the 1900s to the 1930s, the structure of DNF was confused with that of binapthyl 3.1,7 At that time, the two compounds were distinguished solely based on the formation of picrate, i.e., DNF 1 produces a red charge transfer complex with picric acid 2 (vide infra) and 3 is not formed. Because of the many applications of DNF currently, especially in electronic devices mentioned above, the structure of a DNF-based charge-transfer complex should be determined. In this study, we synthesized the DNF picrate and characterized it by spectroscopic studies. Bulk samples for spectroscopic and differential scanning calorimetry (DSC) studies were harvested from a hot EtOH solution (20 mL) of 1 (20 mg) and 2 (57 mg, Wako Pure Chemical Industries, Ltd.) to obtain red picrate. A single crystal suitable for X-ray diffraction analysis was obtained from a diluted EtOH solution evaporated slowly to give red plates. The following results were obtained: melting points (MPs): 161.4°C 2019 © The Japan Society for Analytical Chemistry