Primary Photophysical and Photochemical Processes in the Photoreduction of a Naphthoquinone methide: 2‐phenylnaphtho[1, 8‐ bc ]furan‐5‐one

The photochemical reactivity of the title compound 3 was studied in the context of a novel acid catalysed photocyclisation reaction recently discovered by Barton et al. [1b]. The following results were obtained by flash photolysis methods and steady state irradiations in the presence and absence of quenchers and sensitisers combined with luminescence measurements: The fluorescence of 3 in benzene is very weak and short‐lived (ϕ F1 ≤ 10 −4 , τ F1 ≤ 10 −9 s) due to efficient intersystem crossing (ϕ T = 0.8 ± 0.15). On the other hand, the conjugate acid 4 exhibits an intense green fluorescence (ϕ F1 = 0.9 7 ± 0.1) with a lifetime of (4. ± 1.) · 10 −9 s. Acidity constants of 4 in the ground and first excited triplet state were estimated in acetic/trifluoroacetic acid mixtures by comparison with Hammett indicators: p K (S 0 ) = −0.3, p K (T 1 ) = +1. The Förster cycle yields p K (S 1 ) = +6, however, adiabatic protonation of S 1 ( 3 ) to produce fluorescent 4 is not efficient. The lowest ππ * triplet state of 3 (E T = 45 ± 1 kcal/mol, 3 τ = 8 · 10 −4 s in dilute, degassed benzene solution) is an intermediate in the photoreduction of 3 by oxidisable substrates. Hydrogen abstraction from 2‐propanol by T 1 ( 3 ) yields the semiquinone methide radical 5 which dimerises to 6 . On readmission of air after irradiation, 6 is reoxidised to the starting material 3 . Predictions derived from PPP SCF CI calculations are in accord with the observed spectroscopic and photochemical properties of 3 .