Analysis of the Pathways for Cis ⇄ Trans Photoisomerization of 4‐Nitro‐,4,4′‐Dinitro‐, and 4‐Nitro‐4′‐methoxystilbene in Solution. XII1

Abstract

AbstractQuantum yields for the direct trans → cis (Φl→c) and cis → trans (Φc→l,) photoisomerization of 4‐nitro‐,4,4′‐dinitro‐, and 4‐nitro‐4′‐methoxystilbene (NS, DNS, and NMS, respectively) were measured in solution at room temperature as a function of the concentration of quenchers (ferrocene, azulene, and oxygen). The results are in full agreement with a previously proposed reaction scheme for NMS, involving a triplet state as an intermediate in the trans → cis photoisomerization and both triplet and excited singlet states in the cis → trans photoisomerization. The branching ratios for decay of twisted configurations of the first excited singlet (1p*) and the lowest triplet (3p*) into the ground states and the yields for intersystem crossing from the first excited trans and cis singlet configurations to the lowest triplet state (Φlisc and Φcisc, respectively) were determined. Φlisc and Φcisc are large (0.45–0.86) for NS and DNS and significantly smaller for NMS in polar solvents. Since the trans triplet state (3t*) is in thermal equilibrium with 3p* it is this 3t* ⇄ 3p* equilibrium which is accessible for the influence of triplet quenchers and solvent properties in the cis ⇄ trans photoisomerization. At room temperature oxygen quenches mainly 3p* while at lower temperatures (between 0 and — 140°C) also3t* is quenched. The yields for the conversion 1t* → 1p* and for formation of 4a,4b‐dihydrophenanthrene (DHP) are small. From the kinetic analysis no indication was found for involvement of excited states of DHP in the cis → trans photoisomerization of 4‐nitrostilbenes.