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

Abstract

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 (Ph₂I⁺PF₆⁻) 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 Ph₂I⁺PF₆⁻, the derived bimolecular rate coefficient for ET is k_ET = (1.8 ± 0.5) × 10¹⁰ M⁻¹ s⁻¹ in DCM, which is consistent with diffusion-limited kinetics. This ET occurs from the first excited singlet (S₁) state of N-EC, in competition with intersystem crossing to populate the triplet (T₁) 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) × 10¹⁰ M⁻¹ s⁻¹. Corresponding measurements for solutions of photoexcited 9-phenylcarbazole (9-PC) and Ph₂I⁺PF₆⁻ give k_ET = (5 ± 1) × 10⁹ 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 Me₂Ph₂I⁺PF₆⁻ and t-butyl-Ph₂I⁺PF₆⁻) respectively give k_ET = (1.2 ± 0.3) × 10¹⁰ M⁻¹ s⁻¹ and k_ET = (5.4 ± 1.5) × 10⁹ 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.