Organic super-reducing photocatalysts generate solvated electrons via two consecutive photon induced processes

Photocatalysts with extremely strong reducing potential are often thought to operate through a consecutive photoinduced electron transfer (ConPeT) mechanism, where a first photon generates the radical anion of the photocatalyst via electron transfer and a second photon excites the radical anion into a super-reducing agent. Among them, 4CzIPN, (2,4,5,6-tetrakis(9H-carbazol-9-yl) isophthalonitrile) and the analogous 4DPAIPN (2,4,5,6-tetrakis(diphenylamino)isophthalonitrile) are supposed to operate following this principle, but the knowledge of the photophysical properties of the photogenerated radical anions is still very limited. An in-depth spectroscopic and computational study of their radical anions demonstrates that the excited states of 4CzIPN(center dot)(-) and 4DPAIPN(center dot)(-) are not behaving as super-reducing agents: they are very short lived (ca. 20 ps), not emissive and not quenched by common organic substrates. Most importantly, longer lived solvated electrons are generated upon excitation of these radical anions in acetonitrile and we propose that it is the solvated electron the species responsible for the exceptional reducing capability of this photocatalytic system.