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

1984; Volume: 88; Issue: 12 Linguagem: Inglês

10.1002/bbpc.198400050

0005-9021

Helmut Görner, Dietrich Schulte‐Frohlinde,

Spectroscopy and Quantum Chemical Studies

Abstract 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 ( 3 t*) and 3 p* and allows the estimation of k p and the triplet equilibrium constant K 5 = k 5 /k −5 = [ 3 p*]/[ 3 t*]. K 5 shows a trend to decrease with increasing solvent polarity. For NS this corresponds to a shift of the 3 t* ⇄ 3 p* equilibrium from — 75% of the 3 p* configuration in n‐pentane to ˜25% in methanol. This shift is explained by the greater stabilization of the planar 3 t* configuration by polar solvents in comparison to that of the less polar twisted 3 p* configuration. The solvent dependent change in k p is less pronounced, giving average values of 3.0 · 10 7 and 1.7 · 10 7 s −1 for NS and NMS, respectively.