Structural studies of squaraines containing 2,3-dihydro-1H-perimidine terminal groups

• Published in: Dyes and pigments 2015-07, Vol.118, p.58-63
• Main Authors: Mistol, Jürgen, Ernst, Steffen, Keil, Dietmar, Hennig, Lothar
• Format: Article
• Language: English
• Subjects: 2,3-dihydro-1H-perimidine; 2D NMR-spectroscopy; 2nd order derivative UV–Vis spectroscopy; Squaraine; Squarylium dyes; Syn-anti isomerization
• ISSN: 0143-7208; 1873-3743
• DOI: 10.1016/j.dyepig.2015.02.019

Squaraines containing 2,3-dihydro-1H-perimidine terminal groups have been known since 1993. In the literature two structure types have been described; those involving bonding of the four-membered ring to C-4 and those involving bonding to C-6 of the dihydroperimidine unit. It was found by extensive application of one dimensional and two-dimensional nuclear magnetic resonance spectroscopic techniques, that the 2,3-dihydro-1H-perimidine terminal groups are linked at their 4-position with the four-membered cyclic ring, and not at their 6-position. It was also found by nuclear magnetic resonance spectroscopy that hindered rotation about the partial double bond between the 2,3-dihydro-1H-perimidine terminal groups and the central four membered cyclic ring, leads to a syn-anti isomerization of the dihydroperimidine derived squaraine dyes. This feature is confirmed by the detection of two isomers using 2nd order derivative ultraviolet–visible absorption spectroscopy. [Display omitted] •Squaraines containing 2,3-dihydro-1H-perimidine groups have been synthesized.•The linking of the perimidine unit at the four membered ring has been studied.•Syn-anti isomerization of the perimidine derived squaraines has been investigated.