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

• Published in: Physical chemistry chemical physics : PCCP 2015-01, Vol.17 (1), p.376-386
• Main Authors: Fröbel, Sascha, Buschhaus, Laura, Villnow, Torben, Weingart, Oliver, Gilch, Peter
• Format: Article
• Language: English
• Subjects: Absorption spectroscopy; Acetonitrile; Benzaldehydes - chemistry; Benzofurans - chemistry; Ethylenes - chemistry; Femtosecond; Formations; Isomerism; Isomerization; Ketenes; Ketones - chemistry; Lactones; Lactones - chemistry; Light; Physical chemistry; Quantum Theory; Spectrum Analysis, Raman
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c4cp03351e

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 ∼2-3 ps after photo-excitation. 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 μs and generates an additional intermediate in the cascade towards the lactone.