Conformational isomerism in squaraines: saturation transfer NMR studies on hydroxy squaraines

Abstract

The proton and 13 C NMR of azulene and dialkylaminohydroxyphenyl squaraines have been investigated. The four-membered ring carbon resonances occur in the 180–190 ppm range and are attributed to the cyclobutenediylium character of this structure. Some charge delocalization from the central ring to the pendent aromatic substituents was also observed, indicative of a substituent effect of the squaraine moiety on aminoaromatic systems equivalent to that of a formyl or acetyl group. Hydroxy squaraines 10 and 11 show the presence of two isomeric squaraine species in the proton NMR in deuterochloroform. Saturation transfer experiments confirm that these species are readily interconverted, and cis/trans isomerism [Formula: see text] is consistent with these observations. The free energy difference between the two isomers at 318 K is calculated to be 3.0 and 3.2 kJ/mol for hydroxysquaraines 10 and 11 respectively. Keywords: squaraines, saturation transfer, NMR.