Heptagon-embedded π-Expanded Thieno- and N-Methylpyrrolo-pyridazines with Substantial Out-of-plane Dipole Moment

Abstract

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.

π-Expansion of 2,3,6,7-Tetraazanaphthalene with Two Embedded Heptagons: Highly Twisted Structure and Lone-Pair/π* Interaction in the Crystal

“…The synthesis of 1 is shown in Scheme . The synthesis of N -methylpyrrolo­[3,4- d ]­pyridazine 6 was recently reported by us . The oxidative ring-opening reaction of 6 with Na 2 Cr 2 O 7 ·2H 2 O in acetic acid gave 1,4-diketone 7 .…”

“…Twisting of a π-electronic system leads to unique structural and electronic characteristics such as chiroptical properties , and large spin–orbit coupling . Various twisted compounds have been synthesized by introducing bulky substituents, − cross-linked chains, − and nonhexagonal rings. − The degree of twisting (ω twist ) of a molecule can be evaluated by the torsion angle per distance between a set of bonds at both ends of a molecule. Electronic perturbation induced by molecular twist depends on the degree of twisting, but highly twisted π-electronic systems with ω twist values of >10°/Å are quite limited.…”

“…Reported twisted molecules 1–6 ,,,, and their ω twist values experimentally determined by the set of bonds colored red. R 1 = 3,5-bis­(trifluoromethyl)­phenyl.…”

“…The molecular design was based on our previously reported doubly diphenylene-fused N -methylpyrrolo­[3,4- d ]­pyridazine 6 with two embedded heptagons. Density functional theory (DFT) calculations suggested a moderately twisted structure of 6 (ω twist = 10.8°/Å) as a metastable conformation . Upon expansion of the central unit from pyrrolopyridazine to tetraazanaphthalene, the twisted form became the most stable conformer of 1 .…”

“…In the solid, 1 exhibited an absorption tail (10% of the maximum) at 512 nm, which is red-shifted from that measured for a CHCl 3 solution (451 nm), suggesting effective interaction through T-shaped stacking. Compound 1 was practically non-emissive (quantum yield of <0.01) both in CHCl 3 and in the solid state, while compound 6 was fluorescent both in CHCl 3 (quantum yield of 0.17) and in the solid state (quantum yield of 0.063) . TD-DFT calculations showed the oscillator strengths of the lowest-energy transition of 6 and 1 were 0.2193 and 0.0008, respectively (Table S2).…”