Optically Gated Dissociation of a Heptazinyl Radical Liberates H through a Reactive πσ* State.

Using trianisole heptazine (TAHz) as a monomeric analogue for carbon nitride, we performed ultrafast pump-photolysis-probe transient absorption (TA) spectroscopy on the intermediate TAHzH heptazinyl radical produced from an excited state PCET reaction with 4-methoxyphenol (MeOPhOH). Our results demonstrate an optically gated photolysis that releases H and regenerates ground state TAHz. The TAHzH radical signature at 520 nm had a lifetime of 7.0 ps, and its photodissociation by the photolysis pulse is clearly demonstrated by the ground state bleach recovery of the closed-shell neutral TAHz. This behavior has been previously predicted as evidence of a dissociative πσ* state. For the first time, we experimentally demonstrate photolysis of the TAHzH heptazinyl radical through a repulsive πσ* state. This is a critical feature of the proposed reaction mechanisms involving water oxidation and CO reduction.