Reversible and steady photogeneration of 4,4′-bipyridinium radical cations via the excitation of ion-pair charge-transfer complexes between 4,4′-bipyridinium and tetrakis-[3,5-bis(trifluoromethyl)phenyl]borate in organic solutions

Abstract

The excitation of an ion-pair charge-transfer (CT) band (λ_ex > 365 nm) of 4,4′-bipyridinium salts with tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB^–), in degassed organic solutions, results in the accumulation of 4,4′-bipyridinium radical cations which disappear reversibly with a lifetime of ca. 1 h at 20 °C. The photogeneration and thermal decay of 4,4′-bipyridinium radical cations were shown to be highly reversible by both u.v.-visible and e.s.r. spectroscopies. The radical formation efficiency was almost constant over a wide timescale of the CT complex formation apart from high value in the early stages. These results suggest that the extraordinarily bulky and stable TFPB^– anion works as a reversible electron donor in the reversible and steady photoreduction of 4,4′-bipyridinium ion via the excitation of an ion-pair CT band.