The primary processes in the photoisomerization of spiro[3-methyl-8-methoxy-6-nitro-2H(1)-benzopyran-2,2′,3′,3′-dimethylpiperidine] (Me-SP) in toluene were studied from 273 to 323 K by means of nanosecond laser photolysis using the third harmonic (353 nm) of a neodymium-glass laser as the excitation light (pulses with a full width at half-maximum of 6 ns). The results indicate that benzopyran ring opening in Me-SP occurs in both the excited singlet state and the lowest triplet state. Photoisomerization in Me-SP does not lead to an all-trans isomer because of steric hindrance due to the two methyl groups in the 3′ position and the one methyl group in the 3 position, but instead leads to a cisoid isomer X characterized by a lifetime of about 3 s at 297 K and an absorption centred at 440 nm. X is formed exclusively from the excited singlet state of Me-SP. Two triplet species, one of which was formed from the other, were detected. Their lifetimes were 25 ns and 2.2 μs respectively at 297 K. The first triplet T was assigned to the lowest triplet state of Me-SP and the second triplet3Y to the lowest triplet state of a species (different from X) formed just after Cz.sbnd;O bond rupture. The triplet3Y relaxes towards the ground state of Me-SP. The activation energy of the T →3Y process is about 5 kcal mol-1. © 1985.
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