The photochemical ring-opening reaction of 1′,3′,3′-trimethylspiro-[2H-1-benzopyran-2,2′-indoline] has been followed by transient absorption spectroscopy, covering the range 370–650 nm with a temporal resolution of 0.42 ps. During the pump pulse, induced transient absorption is observed only at wavelengths shorter than 500 nm, with a maximum near 440 nm. The absorption at 440 nm decays within ≈5 ps, while a broad absorption band appears at 550 nm in the region characteristic of merocyanine absorption. The risetime of the absorption at 550 nm is approximately 1.3 ps. The reaction is described as a barrierless photoisomerisation on the excited singlet potential energy surface towards a nonplanar cisoid form, followed by internal conversion and rapid attainment of planarity.
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