1,3-cyclopentanediyl diradicals: Substituent and temperature dependence oftriplet-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 precur sors. The first-order decay rate constants range from 0.8 to 16.7 x 10(5) s (-1) in degassed solution at 294 K and exhibit a systematic, but nonlinear dependence on Brown's sigma(+) substituent constants. Analysis of spin-orbi t coupling in terms of the two-electrons-in-two-orbitals model indicates th at the electronic effect on the rates of intersystem crossing operates by i nfluencing the weight of ionic contributions in die lowest singlet state wa ve functions of the diradicals. Counter to intuition, but in agreement with the prediction by the model, push-pull substitution does not enhance the i onic 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 de cay rate constants. A sensitive statistical test is used to establish that the Arrhenius parameters exhibit enthalpy-entropy compensation, that is, an approximate isokinetic relationship.