Photochemistry of 3,6-Didehydropyridazine Biradical─An Untraceable Benzyne Analogue.

We report matrix isolation infrared spectroscopic studies to characterize 3,6-didehydropyridazine , a heterocyclic analogue of benzyne, combined with computations. In this regard, we have utilized 3,6-diiodopyridazine as a photolytic precursor. The experiments toward the generation of the biradical are carried out in argon and nitrogen matrices at 4 K. Instead of the elusive biradical, we have observed a ring-opening product maleonitrile upon irradiation at 254 nm. In contrast, prolonged irradiation at 254 nm leads only to isomerization, forming fumaronitrile . The mechanistic aspects of ring-opening, product selectivity, and photoisomerization steps have been investigated in detail using high-level ab initio computations. These studies have found that 3,6-didehydropyridazine is an untraceable intermediate, and the ring-opening step leading to maleonitrile is barrierless. In addition, we have proposed the involvement of the S (π-π*) state via conical intersection in the photoisomerization of maleonitrile.