Synthesis of 2,′:5′,2″-terpyridine and 2,2′:5′,2″:5″,2‴-quaterpyridine and their photocatalysis of the reduction of water

Abstract

2,2′:5′,2″-Terpyridine (OPy-3) and 2,2′:5′,2″:5″,2‴-quaterpyridine (OPy-4) are prepared by Ni°complex-catalysed polycondensation of 2,5-dichloropyridine in the presence of an excess of 2chloropyridine. These two compounds show more efficient photocatalysis of H₂ evolution by the reduction of water than p-terphenyl and p-quaterphenyl in the presence of triethylamine as an electron donor, and RuCl₃ or K₂PtCl₆ as a source of Ru^o or Pt^o colloids which function as an electron mediator. The primary photochemical processes of OPy-n(n= 3 or 4) and the successive reactions have been investigated by γ-radiolysis, pulse radiolysis and laser flash photolysis. The OPy-n molecules are converted to their anion radicals through reductive quenching of the excited states. A mechanism for the photocatalysis of hydrogen evolution is proposed in which the anion radicals supply electrons to protons on the metal colloids. The anion radicals undergo protonation in competition with electron transfer, resulting in the loss of photocatalytic activity.