Electron-Transfer Reactions Triggered by Uncharged or Cationic Photosensitizer: Methodology for Generation of o-Quinodimethane and Analysis of Back Electron-Transfer Process.

Photoinduced electron transfer (PET) followed by back electron transfer (BET) reactions of 1,2-bis(α-styryl)benzenes 1, generates o-quinodimethane derivative 2 via the corresponding radical cation 2^⋅+. This process serves as a facile method to form o-quinodimethane intermediates. The intermediacy of 2^⋅+ and 2 were confirmed by using various spectroscopic methods and trapping reactions with ³O₂ and dienophiles. Kinetic analysis using nanosecond time-resolved absorption showed that 2^⋅+ was transformed to 2 via a BET process, in which the decay of 2^⋅+ and rise of 2 have almost the same rate constants of k_DECAY=5.6×10⁵ s⁻¹ and k_RISE=5.9×10⁵ s⁻¹, respectively. The net BET rate constant evaluated by using Marcus theory is much faster than these experimental values, owing to the reduction based on diffusion. [ABSTRACT FROM AUTHOR]