STUDY OF THE TRANS->CIS PHOTOISOMERIZATION OF 4-NITRO-4'-DIMETHYLAMINOSTILBENE IN TOLUENE SOLUTIONS

Article abstract of DOI:10.1016/0022-2860(82)85255-1

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 deg 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 trans->cis photoisomerization pathway is tentatively assigned to the sequence 1t*->ΔA*->Δp*->1p->(1-β)1c.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 (1p) which in turn deactivates to the cis and trans forms.