Deprotonation yields, pKa, and aci-nitro decay rates in some substituted o-nitrobenzaldehydes

In this paper we report the deprotonation yields, the pK(a), and decay kinetics of the aci-nitro intermediates of some substituted 2-nitrobenzaldehydes that can be used as photoactivatable caged proton compounds. The decay of the aci-nitro absorbance for 2-nitrobenzaldehyde occurs within a few nanoseconds from photoexcitation. Addition of electron donating methoxy substituents at positions 4 and 5 leads to lower deprotonation yields, higher pK(a), and slower decays of the aci-nitro intermediates. On the contrary, the decay rate is accelerated by the introduction of an electron-withdrawing Cl atom at position 4 in the phenyl ring, with little influence on the deprotonation yield and pK(a) of the aci-nitro intermediate.