Fig. 2.

(a) ¹H NMR spectrum of the 3d·TFA salt (top) in DMSO-d6 compared to the ¹H NMR of 3d in its neutral state (bottom). When TFA is added to 3d while in solution the ¹H NMR spectrum does not resolve, suggesting the presence of a kinetically stable aggregate. (b) ¹H NMR titration of 3d with Bu₄NCl in CD₃CN. The ¹H NMR titration shows resolved peaks prior to addition of Bu₄NCl, but upon addition of ca. one equivalent Bu₄NCl the sample forms insoluble aggregates. Upon addition of excess Bu₄NCl the aggregation appears to dissipate and the ¹H NMR spectra resolve.