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 pi 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 13C-NMR spectrum of this isomer was assigned with the aid of C-atom charge density data derived from AM1 calculations.