Laser Flash Photolysis and Quantum Yields of the Direct Photochemical Cis‐Trans Isomerization of 4‐Nitro‐4′‐methoxystilbene in Cyclohexane Solutions in Presence of Quenchers. Model for the Photoisomerization of Nitrostilbenes

Abstract

AbstractQuantum yield (ϕ) measurements of the direct photochemical cisΦtrans isomerization of 4‐nitro‐4′‐methoxystilbene (NMS) using cyclohexane as solvent have been performed at room temperature as a function of ferrocene and of oxygen concentration. The results are interpreted using a model which is based on the assumption of nine states and all possible transitions between these states. Contrary to unsubstituted stilbene, NMS does not show the tr1 → p1 transition (tr1 and p1 are the trans and the twisted configurations of the first excited singlet state). The trans → cis isomerization occurs by rotation in the triplet state (from the trans (tr3) to the twisted (p3) configuration) followed by intersystem crossing to the twisted ground state (p0) and rotation in this singlet ground state to the cis ground state. The tr3 state is in equilibrium with the p1 state. ‐ The cis → trans isomerization occurs by rotation in the excited singlet state from the cis (c1) to the twisted (p1) configuration. From p1 a branching occurs to p3 (30%) and to the twisted ground state (70%). This branching is responsible for the prompt population of the triplet intermediate from the cis form as observed in laser flash photolysis. The p3 also decays to the twisted ground state from which the trans ground state is reached after rotation. In the cis → trans route all rotations occur in singlet states despite the fact that an intermediate triplet state is populated to a certain extent. Laser flash photolysis results are kinetically in agreement with this model.