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

•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.

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.

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