A. Kellmann, L. Lindqvist, F. Tfibel
1986
Citations
0
Influential Citations
17
Citations
Journal
Journal of Photochemistry
Abstract
Abstract The primary processes in the photoisomerization of spiro[8-methoxy-6-nitro-2H(1)-benzopyran-2,2′–3′,3′-dimethylpiperidine] (H-SP) in toluene were studied from 271 to 324 K by means of nanosecond laser photolysis using the third harmonic (353 nm) of an ND–glass laser as the excitation light (pulses with a full width at half-maximum of 6 ns). The all-trans isomer, the photomerocyanine (lifetime, about 3 s at 297 K), is formed almost entirely from the lowest triplet T of H-SP by a stepwise process which occurs adiabatically on the lowest triplet surface. This process involves the sequential formation of three isomeric triplets one from the other: T, 3Z and 3A with lifetimes of 20 ns, 110 ns and 4 μs respectively at 297 K. 3Z and 3A are the triplets of open isomers with cisoid and transoid configurations respectively. 3A decays to a ground state isomer of transoid geometry, which is converted into the more stable all-trans isomer (photomerocyanine). The activation energies of the T→3Z and 3Z→3A processes are about 5 kcal mol−1 and about 6 kcal mol−1 respectively. The mechanism of photoisomerization for H-SP is compared with that previously established for the closely related system spiro[3-methyl-8-methoxy-6-nitro-2H(1)-benzopyran-2,2′–3′,3″-dimethylpiperidine].