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 P_r–P_fr 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 (P_fr) 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 P_fr 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 P_fr 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 P_fr in the presence of amines was favourable in polar solvents. Tricyanophenyl bilindione tends to form P_fr 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.

Keywords:

• bilindione
• solvatochromism
• amide
• hydrogen bond

Acknowledgements

This work was supported by ‘Creating Research Center for Advanced Molecular Biochemistry’, Strategic Development of Research Infrastructure for Private Universities, the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.