Solvatochromism of triarylbilindiones: far-red-absorbing bilindiones formed in aprotic amides and amines

Abstract

Solvatochromism of triarylbilindiones was investigated to shed light on the mechanism and interactions involved in the phytochrome Pr–Pfr isomerisation. 1,19,21,24-Tetrahydro-5,10,15-tri(4-cyanophenyl)bilin-1,19-dione exhibited an electronic absorption maximum at 631 nm in toluene, while it exhibited the absorption maximum at 729 nm in N,N-dimethylformamide. A similar far-red-absorbing bilindione (Pfr) was formed in aprotic amides such as N,N-dimethylacetamide and 1-methylpyrrolidone, but was not formed in protic amides such as formamide and N-methylformamide. The Pfr was formed in the presence of a relatively high concentration (∼0.1 M) of amines such as butylamine, 1,2-ethanediamine and piperidine in methanol. The formation of Pfr in amides was inhibited by the addition of protic polar solvents, in particular hydrogen donor solvents such as 2,2,2-trifluoroethanol and methanol, while the formation of Pfr in the presence of amines was favourable in polar solvents. Tricyanophenyl bilindione tends to form Pfr than does tri(4-methoxyphenyl)bilindione. These observations suggest that the bilindiones act as hydrogen donors to form a hydrogen bond complex with amides or amines.