¹H NMR Spectral Evidence for a Specific Host−Guest Complexation Induced Charge Localization in Squaraine Dyes

Experimental support is provided for the charge localization in squaraines, a class of fundamentally and technologically important organic dyes, by ¹H NMR analysis through a host−guest complexation approach. Specific binding of Ca²⁺ ions to the squaraine 2 with a podand sidearm resulted in a charge-localized structure 2a with dramatic shifts and resolution of the proton signals when compared to those of 2.