Meta-Ortho Effect on the Excited State Pathways of Chloroanilines

Direct excitation of aromatic compounds grants access to high-energy intermediates that can be utilised in organic synthesis. Understanding and predicting the substituent effects at the excited state for aromatic molecules remains challenging for the synthetic photochemist. In this work, we present an experimental and computational investigation of the excited state of the isomeric chloroanilines, which promptly react by losing the chloride when the amino group is in para position, but are non-reactive and non-emissive in the meta and ortho isomers. XMS-CASPT2//CASSCF computations explain this apparent contradiction of the meta-ortho selectivity rule of Zimmerman, which originates from the substituent effects lowering to a different extent the barrier to populate the prefulvenic conical intersection that deactivates non-radiatively the singlet excited state of the chloroanilines. Computational chemistry allows to elucidate the observed selectivity in the photochemistry of chloroanilines. A meta-ortho effect of the substituents favours the population of the prefulvenic conical intersection which leads to rapid deactivation of the m- and o-isomers of chloroaniline, while the para derivative lives long enough to emit and populate the reactive triplet state which leads to C-Cl dissociation.+image