Two-dimensional 1H NMR spectroscopy of 1,1′-diethyl-2,2′-cyanine iodide in water and methanol

1,1′-Diethyl-2,2′-cyanine iodide is a well studied molecule because of its ability to form special type of molecular aggregates (J-aggregates) in water solutions and films. However, not many experimental details on the micro-molecular structure of the dye are available. We applied proton 2D NMR spectroscopy (COSY, NOESY, and TOCSY) to investigate the complex aromatic regions of the 1H NMR spectra of the dye in deuterated methanol and in deuterated water in monomeric and J-aggregated form. Signals from all six aromatic protons of the dye in monomeric form were identified. It was found that in monomeric form, the difference in chemical shifts for the two protons nearest to nitrogen in water compared to those in methanol is significantly bigger than for the other aromatic protons. NMR spectra of PIC in water in J-aggregated form contained a series of new proton’s peaks absent in the spectra of monomers indicating on significant structural changes of PIC upon J-aggregation. Possible explanations of the nature of structural changes and the origin of new peaks are discussed.