Transient absorption spectroscopy of the electron transfer step in the photochemically activated polymerizations of N-ethylcarbazole and 9-phenylcarbazole

The polymerization of photoexcited N-ethylcarbazole (N-EC) in the presence of an electron acceptor begins with an electron transfer (ET) step to generate a radical cation of N-EC (N-EC˙(+)). Here, the production of N-EC˙(+) is studied on picosecond to nanosecond timescales after N-EC photoexcitation at a wavelength λ(ex) = 345 nm using transient electronic and vibrational absorption spectroscopy. The kinetics and mechanisms of ET to diphenyliodonium hexafluorophosphate (Ph(2)I(+)PF(6)(−)) or para-alkylated variants are examined in dichloromethane (DCM) and acetonitrile (ACN) solutions. The generation of N-EC˙(+) is well described by a diffusional kinetic model based on Smoluchowski theory: with Ph(2)I(+)PF(6)(−), the derived bimolecular rate coefficient for ET is k(ET) = (1.8 ± 0.5) × 10(10) M(−1) s(−1) in DCM, which is consistent with diffusion-limited kinetics. This ET occurs from the first excited singlet (S(1)) state of N-EC, in competition with intersystem crossing to populate the triplet (T(1)) state, from which ET may also arise. A faster component of the ET reaction suggests pre-formation of a ground-state complex between N-EC and the electron acceptor. In ACN, the contribution from pre-reaction complexes is smaller, and the derived ET rate coefficient is k(ET) = (1.0 ± 0.3) × 10(10) M(−1) s(−1). Corresponding measurements for solutions of photoexcited 9-phenylcarbazole (9-PC) and Ph(2)I(+)PF(6)(−) give k(ET) = (5 ± 1) × 10(9) M(−1) s(−1) in DCM. Structural modifications of the electron acceptor to increase its steric bulk reduce the magnitude of k(ET): methyl and t-butyl additions to the para positions of the phenyl rings (para Me(2)Ph(2)I(+)PF(6)(−) and t-butyl-Ph(2)I(+)PF(6)(−)) respectively give k(ET) = (1.2 ± 0.3) × 10(10) M(−1) s(−1) and k(ET) = (5.4 ± 1.5) × 10(9) M(−1) s(−1) for reaction with photoexcited N-EC in DCM. These reductions in k(ET) are attributed to slower rates of diffusion or to steric constraints in the ET reaction.