A (nm) Figure 1. Electronic absorption spectra of 3 in EPA a t 77 K prior to irradiation (dotted lines), transients generated upon irradiation of 3 with 254-nm light (solid lines),I0 and 6Ia3l3 (dashed lines): (a) R = H; (b) R = CO,CH,. Small peaks at 285 and 300 nin in the spectrum of 6a are due to p-xylylene.ia366-nm light, the developed absorption was efficiently bleached and the absorption spectrum of 3a was largely restored. This behavior was reminiscent of [4]paracyciophane (6)la and suggested that the intermediate photochemically reverted to 3a. Irradiation of 3b in an EPA glass with 254-nm light at 77 K also led to a species which showed A, , , at 291, 345, and 415 (sh) nm in the UV-vis spectrum (Figure 1 b)I0,l2 and thermal and photochemical behavior similar to that observed above. In Figure 1 are also shown the absorption spectra of 6a and 6b which we recently reported.Ia The striking similarity in shape of the presently observed spectra to those of the corresponding 6 in the long wavelength region (>265 nm)13 indicates that the transient species produced here would certainly be 1. In 1, the a bond system in the four carbon bridge and that of the extremely bent benzene ring are nearly orthogonal to each other, and there could be little interaction between them.14 If the species generated had the structure of 2, the observed spectra would be much different from those of (12) Molar absorptivity at 291 nm was estimated to be ca. 9000. (13) In the short wavelength region (<265 nm), 1 exhibited peaks which would be due to the diene chromophore in the bridge. Since molar absorptivity of 6 has not yet been determined, the absorption intensity of the spectrum of 6 was adjusted to that of 1 in Figure 1.(14) When the ?r electron systems of formally conjugated chromophores are at right angles to each other and do not interact, the observed spectrum will appear to be a summation of the spectra of the separate chromophore^.'^ 0002-7863/89/1511-0369$01.50/0 6a and 6b. In 2, the a bonds overlap each other only weakly, and such a poorly conjugated system tends to exhibit a broad, we8.k absorption s p e c t r~m . '~ Cyclooctatetraene is a notable example,16 and the a bond system of 2 may be viewed as that of cyclooctatetraene perturbed by the etheno bridge double bond.Thus, on the basis of the regioselective addition of cyclopentadiene to give 4 and 5 and the transient absorption spectra remarkably similar in shape to those of the corresponding 6, it may be concluded that the irradiation of 3 leads to a species which is best represented by the structure 1. This conclusion, however, does not rule out the possibility that the structure 2 represents a local minimum on the potential energy surface of bicyclo-[4.2.2]decapentaene and may even be more stable than 1, though the present photoreaction of 3 provided no evidence suggesting that such was the case. If 1 were less stable than 2, 1 would be separated from 2 by a sizable energy barrier (>6 kcal/mol) since the absorption spectra of la and l b showed no sign of decay with...