Laser Study of the Triplet State of 4‐Nitrostilbenes in Solution; Estimation of the Equilibrium Constant (3t* ⇄ 3p*) and the Rate Constant for Intersystem Crossing (3p* → 1p). XIII1

Decay of the lowest triplet state of 4‐nitro‐, 4,4′‐dinitro‐, and 4‐nitro‐4′‐methoxystilbene (NS, DNS, and NMS, respectively) was studied by nanosecond laser photolysis in solution at room temperature. The triplet decays predominantly via intersystem crossing at the twisted configuration followed by internal rotation into the cis or trans ground states. Rate constants for triplet decay were determined in various solvents as a function of the concentration of ferrocene, azulene, and oxygen. Quenching by azulene and ferrocene leads only to trans isomers whereas quenching by oxygen does not markedly shift the photostationary state are room temperature. The results support the assumption of an equilibrium between the planar trans configuration (3t*) and 3p* and allows the estimation of kp and the triplet equilibrium constant K5 = k5/k−5 = [3p*]/[3t*]. K5 shows a trend to decrease with increasing solvent polarity. For NS this corresponds to a shift of the 3t* ⇄ 3p* equilibrium from — 75% of the 3p* configuration in n‐pentane to ˜25% in methanol. This shift is explained by the greater stabilization of the planar 3t* configuration by polar solvents in comparison to that of the less polar twisted 3p* configuration. The solvent dependent change in kp is less pronounced, giving average values of 3.0 · 107 and 1.7 · 107 s−1 for NS and NMS, respectively.