The structure and 13C-NMR of an indolenium squaraine dye

Six conformational isomers of bis(1-isopropyl-2,3,3-trimethylindolen-2-ylidene)squaraine were studied using MM+ molecular mechanical and AM1 quantum chemical methods. The calculated energies and structures indicate that the planarity of the π system is of great importance. These results were confirmed by NMR spectroscopy. The structure of the aforementioned dye was shown to be the trans-planar isomer. The 13 C-NMR spectrum of this isomer was assigned with the aid of C-atom charge density data derived from AM1 calculations.