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Abstract
The direct trans→ cis photoisomerization of 4-nitro-4'-dimethylaminostilbene (NDMAS) in toluene solutions and the role of triplet states have been studied by standard techniques and laser flash photolysis, respectively. The quantum yields of trans→ cis photoisomerization (Φ t → c), of fluorescence (Φ f) and of triplet formation (Φ T) as well as the triplet lifetime (τ) have been measured as functions of temperature (−80 to 100°C). Different activation energies, obtained from Φ f and Φ t → c measurements, indicate the presence of two consecutive thermally activated steps. None of the hitherto published mechanisms for trans- cis photoisomerization account for this result. The rans→ cis photoisomerization pathway is tentatively assigned to the sequence ▪. The steps marked with Δ are thermally activated steps, A* is an excited trans state and p* is an excited twisted state which decays to the twisted ground state ( 1 p) which in turn deactivates to the cis and trans forms.
Published Version
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