MOLECULAR AND ELECTRONIC-STRUCTURE OF PYRACYLENE

Pyracylene (cyclopent[fg] acenaphthylene, 1) is available in two steps from pyrene and, contrary to previous experience, can be crystallized, stored as a solid, and even sublimed without decomposition. The X-ray structure of 1 exhibits a pronounced alternation of bond lengths along the 12-π periphery. The absorption spectrum (with transition moment directions) and the photoelectron spectrum of 1 are reported and analyzed. Pyracylene is quite stable to irradiation both as a solid and in solution. Radiationless deactivation of the lowest excited singlet state is very rapid; fluorescence and intersystem crossing yields were below our limits of detection (øF < 3 ⨯ 10-4, øT < 2 ⨯ 10-2). The lowest triplet state, observed by flash photolysis with a sensitizer (λmax = 360 nm, ϵ > 104 M-1 cm-1; = 520 nm, t = 7500 M-1 cm-1), is also rather short-lived (τ = 4.6 μs). The triplet energy of 1 is bracketed in the range of 103 ± 20 kj mol-1 on the basis of energy-transfer experiments. A major component of flame soots, which had been tentatively attributed to 1 by GC-MS analysis, is shown not to be identical with 1. The heat of formation of 1 is calculated as ΔfH = 410 kj mol-1 with the MMP2 force field. © 1990, American Chemical Society. All rights reserved.