Femtochemistry of selected di-substituted benzenes

• Overview of our work on photoreactive di-substituted benzenes.
• Femtosecond transient absorption, -fluorescence, -infrared and -Raman spectroscopy.
• Ultrafast hydrogen transfer and intersystem crossing.
• Singlet and triplet state reactivity.
• Mechanisms of photolabile protecting groups.

Ortho-di-substitution of benzenes provides the opportunity to study interactions between diverse functional groups in a defined environment. The substituents are forced to distances which allow chemical reactions to occur. In this feature article, we focus on photoreactive compounds containing nitro and carbonyl groups. The complex electronic structure of these molecules results in a rich photochemistry on the femto- and picosecond timescale. A combination of femtosecond spectroscopic techniques and quantum chemical calculations shows that all compounds studied undergo hydrogen transfers via both, singlet and triplet channels. In several cases these transfers are the starting point for further rearrangements. The experiments enabled us to elucidate the mechanisms of prominent photoreactions as the ones of ortho-nitrobenzaldehyde or of nitrobenzene derived photolabile protecting groups.

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