1,3-Cyclopentanediyl Diradicals:  Substituent and Temperature Dependence of Triplet−Singlet Intersystem Crossing

The lifetimes of 33 1,3-diaryl-1,3-cyclopentanediyl triplet diradicals were determined by laser flash photolysis of the corresponding azoalkane precursors. The first-order decay rate constants range from 0.8 to 16.7 × 10 5 s - 1 in degassed solution at 294 K and exhibit a systematic, but nonlinear dependence on Brown's σ + substituent constants. Analysis of spin−orbit coupling in terms of the two-electrons-in-two-orbitals model indicates that the electronic effect on the rates of intersystem crossing operates by influencing the weight of ionic contributions in the lowest singlet state wave functions of the diradicals. Counter to intuition, but in agreement with the prediction by the model, push−pull substitution does not enhance the ionic contribution. Arrhenius parameters, E a = 2−6 kcal mol - 1 and A = 10 7 −10 10 s - 1, were determined from the temperature dependences of the decay rate constants. A sensitive statistical test is used to establish that the Arrhenius parameters exhibit enthalpy−entropy compensation, that is, an approximate isokinetic relationship.