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 tetraphenyl-thieno[3,4-d]pyridazine 1 and N-methylpyrrolo[3,4-d]pyridazine 2 with two embedded seven-membered rings. Owing to the incorporated heptagon, both of 1 and 2 exhibited Cs-symmetric saddle conformations in the solid state with mean plane deviation around 0.38 Å. π-Expanded thienopyridazine 1 showed a 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 the crystal. In addition to the Cs-symmetric saddle conformations found in the crystals, DFT calculation found C2-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 occur fast even at −60 °C as evidenced by 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.