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(-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 O-3(2) 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)=5.6x10(5)s(-1) and k(RISE)=5.9x10(5)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.