Photochemical and photophysical studies of bicyclo[4.3.0]non-1(6)-en-2-one

Article abstract of DOI:10.1021/ja00044a012

The properties of the lowest excited triplet state of bicyclo[4.3.0]non-1 (6)-en-2-one (BNEN) have been investigated by nanosecond transient absorption spectroscopy, photoacoustic calorimetry, measurements of ¹O₂ quantum yields, and photochemical studies in solution. Both the lifetime (1.4 μs) and the energy (74-76 kcal mol^-1) of the BNEN triplet reflect its exceptional conformational rigidity, which prohibits relaxation along the ³(π,π*) potential energy surface by twisting around the C=C bond. Fumaro- and maleonitrile quench the BNEN triplet at a diffusion-controlled rate by triplet energy transfer. This process leads to cis-trans isomerization of the alkenes. The BNEN triplet displays extensive self-quenching with k_sq = 2.6 × 10⁷ M^-1 s^-1. This process involves intermolecular H-abstraction, but formation of dimeric triplet 1,4-biradicals cannot be excluded. The product of the former is a radical (3) with a lifetime of 4.1 μs. Its transient absorption spectrum (λ_max 299 nm) is hypsochromically shifted with respect to that of the BNEN triplet (λ_max 311 nm). The solvent effects on the photophysical properties suggest that the T₁ state of BNEN is a (π,π*) state in acetonitrile and an (n,π*) state in benzene. The BNEN triplet undergoes neither [2 + 2] cycloaddition reactions with itself, cyclopentene, fumaronitrile, or maleonitrile nor the lumiketone rearrangement, but readily abstracts H-atoms from itself, cyclopentene, and 2-propanol.