Triplet 1,3-Diphenylpropynylidene (Ph−C−C−C−Ph)

Abstract

Photolysis (lambda>571 nm) of 1,3-diphenyldiazopropyne (9) affords triplet 1,3-diphenylpropynylidene (3), as characterized by IR, UV/vis, and EPR spectroscopy in low-temperature matrices. Two conformational isomers of triplet 3 are spectroscopically distinguishable. The initially formed, non-relaxed conformer is believed to reflect the geometry of the diazo precursor, as enforced by the rigid matrix. Annealing the matrix permits the structure to relax to the equilibrium D2d geometry. The highly symmetric equilibrium structure of 3 is best envisioned as a 1,3-allenic diradical. Density functional theory calculations suggest that the equilibrium structure does not exhibit a bond-localized structure that would be characteristic of an acetylenic carbene. Chemical trapping with O2, however, affords products that are familiar as carbene trapping products: carbonyl oxide 10, ketone 11, and dioxirane 12. Irradiation (lambda>261 nm) of triplet 1,3-diphenylpropynylidene (3) results in cyclization to singlet diphenylcyclopropenylidene (6), a process that is photochemically reversible at lambda=232 nm. Diphenyl-1,2-propadienylidene (7) was not observed under any irradiation conditions.