Heptagon-Embedded π-Expanded Thieno- and N -Methylpyrrolo-Pyridazines with Substantial Out-of-Plane Dipole Moment.

Herein, we describe the synthesis and characterization of fully fused tetraphenylthieno[3,4- d ]pyridazine 1 and N -methylpyrrolo[3,4- d ]pyridazine 2 with two embedded seven-membered rings. Owing to the incorporated heptagon, 1 and 2 exhibited C s -symmetric saddle conformations in the solid state with mean plane deviation around 0.38 Å. π-Expanded thienopyridazine 1 showed a one-dimensional (1-D) columnar packing along the b axis with net dipole moment aligning perpendicular to the b axis in the polar crystal system Pc . On the other hand, 2 formed a partially π-stacked brick-work structure. In addition to the C s -symmetric saddle conformations found in the crystals, density functional theory (DFT) calculation found C 2 -symmetric twisted conformations of both 1 and 2 close in energy to the saddle conformations. The barrier of conformational interconversion was calculated to be 32 ( 1 ) and 31 kJ·mol -1 ( 2 ), and the interconversion occurs fast even at -60 °C as evidenced by variable-temperature (VT)-NMR studies. While 1 and 2 have moderately curved structures, optical and electrochemical studies revealed effective π-conjugation over the fused diphenylene units, which is also supported by DFT calculation. As the result of the intrinsic large dipole moment of thieno- and pyrrolo-pyridazines and the notably curved structure, 1 ( 2 ) has a substantial out-of-plane dipole moment of 2.0 (3.3) D in the saddle conformations.