π-Stacked and unstacked aggregate formation of 3,3′-diethylthiatricarbocyanine iodide, a near-infrared dye

Abstract

The absorbance and fluorescence spectra of 3,3′-diethylthiatricarbocyanine iodide (DTCI), a near-infrared dye, were measured in methanol and water at different concentrations. DTCI exhibited distinctive monomer and excimer (dimer) fluorescence emission bands on photo excitation in the two solvents. The monomer emission band at around 600–700 nm is unprecedented, and the dimeric nature of the typical emission band at around 750–850 nm has not been recognized in solution. In the ground state, DTCI is proposed to form π-stacked and unstacked aggregates in methanol and water, respectively, at higher concentrations. The proposed unstacked aggregates in water may be of high molar mass leading to a short relaxation time in NMR analysis, which did not show clear signals.