Excited state proton transfer to and from carbon: studies of enhanced excited state basicity of biphenyl derivatives and carbon acidity of dibenzosuberenes

Excited state proton transfer (ESPT) to and from carbon atoms of organic molecules are rare chemical events and only a handful of examples are known. In this work, we report additional examples of excited (singlet) state carbon acids (proton transfer from carbon to solvent water) displayed by several dibenzosuberene derivatives, the first system in which excited state carbon acid behavior was explicitly demonstrated. Substituent effects, quantum yields for exchange, and rate constants for quenching by water (acting as the base for the ESPT) are reported. For examples of ESPT to carbon, we report the photoprotonation in water and acid of two simple biphenyl derivatives. Unlike what is observed in ground state protonation, protonation of the excited state occurs exclusively on the ring not containing the activating substituent (hydroxy or methoxy), as demonstrated by deuterium incorporation experiments. Laser flash photolysis (LFP) data are consistent with formation of transient cyclohexadienyl cations obtained via protonation of the benzene ring. These results are consistent with the substantial charge transfer inherent in excited state biphenyls, making the ring not containing the electron donating activating substituent the more basic site.