The photoformation of a phthalide: a ketene intermediate traced by FSRS

The photo-isomerization of o-acetylbenzaldehyde (oABA) in acetonitrile was studied by femto-and nanosecond transient absorption spectroscopy. Spectroscopic signatures are assigned with the aid of TD-DFT, TD-CAM-DFT and DFT-MRCI computations. The isomerization yields a lactone, 3-methylphthalide (3MP), with a quantum yield of 0.3 (30%). As evidenced by femtosecond stimulated Raman spectroscopy (FSRS), the isomerization proceeds via a ketene intermediate. It is formed within similar to 2-3 ps after photoexcitation. Intersystem crossing (ISC) populating the triplet state of oABA seems to compete with the ketene formation. Experiments on the non-reactive meta-and para-derivatives, which undergo efficient ISC with time constants of 5 ps, support this statement. The triplet state of oABA also contributes to the ketene formation, presumably involving a biradical intermediate. The ketene exhibits a lifetime of 1.4 mu s and generates an additional intermediate in the cascade towards the lactone.