Direct Detection of an Acyclic Dimeric Intermediate in Charge Transfer-Induced [2π + 2π] Cycloaddition

Radical cation-mediated [2 + 2] cycloadditions have been known since the discovery of Wurster's salt. However, the acyclic radical cation dimer intermediate, which accounts for the experimentally observed cyclic head-to-head cyclobutane products, has been spectroscopically elusive. Herein, using broadband femtosecond transient absorption spectroscopy, we report the first experimental signature of an <i>n</i>-vinylcarbazole cation radical dimer, obtained on photoexcitation of the charge transfer complex between a dendritic-bridged tripyridiniumtriazine cation (dBTPT⁺³) and the substrate <i>n</i>-vinylcarbazole. Electronic structure calculations at the level of ωB97XD/6-311G(d,p) indicate that the ∼640 nm absorbing acyclic dimer intermediate prefers a head-to-head cis-conformation due to through-space radical-to-cation charge transfer interaction. Our work therefore shows that these weak Coulombic interactions drive the observed product selectivity.