Photoelectrochemical reduction of chlorinated nitrobenzenes: heavy atom versus radical ion lifetime effects

The photoelectrochemical behaviour of several chlorinated nitrobenzenes has been investigated using steady state channel flow cell methods. It has been shown that, in acetonitrile+0.1 M TBAHFP, 4-chloronitrobenzene (p-CNB), 2-chloronitrobenzene (o-CNB), 2,4-dichloronitrobenzene (2,4-DCNB), 2,6-dichloronitrobenzene (2,6-DCNB) and 1,3,5-trichloro-2-nitrobenzene (TCNB) can undergo reversible one-electron reductions to form radical anions that are stable on the timescale of at least tens of seconds under dark conditions. When the electrode surface is irradiated with light of intensity 2 mW cm−2, the electrochemically formed radical anions can absorb light to form an excited state from which dechlorination is favoured. Reactivity orders of o-CNB>p-CNB>2,4-DCNB>2,6-DCNB>TCNB at 330 nm, and o-CNB>2,4-DCNB>2,6-DCNB>p-CNB≅TCNB at ∼470 nm have been determined. The relative order is rationalised in terms of radical anion lifetime effects and the role of heavy atom subsituents in reducing the excited state lifetime.