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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.

We seek a better understanding of the factors that influence electronic delocalization in open-shell, conjugated -electron systems. Unsaturated organic carbenes serve as model systems in which these factors can be probed in detail. Propynylidene (I),1-8 the simplest acetylenic carbene, can be viewed as the prototypical example of a conjugated polyyne chain containing a "defect" in the form of the carbene center. We are interested in determining whether this defect remains localized in the chain, propagates through the chain, or delocalizes over a substantial portion of the chain. If the defect remains localized, bond-shift isomerism is possible in both unsymmetrically-substituted propynylidenes (e.g., phenylpropynylidenes 2 and 3)118-10 and symmetrically-substituted carbenes consisting of two or more alkyne units (e.g., diphenylpentadiynylidenes 4 and 5).1•11 The chemical rearrangements of acetylenic carbenes are fundamentally interesting in their own right1-11 and have recently been exploited in synthetic chemistry.12 In addition, these carbenes serve as simple models for (i) defects in other conjugated systems, such as conducting polymers, and (ii) reactive intermediates involved in formation of new allotropes of carbon. In this study, we describe our preliminary findings concerning the structure and rearrangements of 1,3-diphenylpropynylidene (7a,b). Our experiments reveal a type of isomerism which, by virtue of the symmetric disubstitution, cannot be attributed to bond-shift isomerism. This observation raises questions concerning recent reports of bond-shift isomerism in acetylenic carbenes.9 *•11 Photolysis (X > 571 nm) of 1,3-diphenyldiazopropyne (6),13 matrix-isolated in argon at 10 K,14 gives rise to a ca. 4:1 mixture of two species, as monitored by ESR, IR, and UV/vis spectros-

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