Figure 3.

UV−vis spectroscopic titration of pK_as of 1. (A) UV−vis spectra of 1 at pH 1.86 (–○–), pH 4.08 (−−−−), pH 8.07 (⎯⎯⎯), pH 11.90 (⎯△⎯) and pH 13.01 (⎯●⎯). [1] = 0.02 mM (B) The acidic pK_a of 1 was determined at 503 nm (⎯○⎯). The absorbance at 503 nm (A503) decreases between pH 2–6. The non−linear curve fit was performed using the equation, $A_{503}=\left\{\left({\in }_{503}^{AH}\left[{\mathsf{H}}^{+}\right]+{\in }_{503}^A{K}_a^1\right)/\left(\left[{\mathsf{H}}^{+}\right]+K_a^1\right)\right\}{\left[A\right]}_T.$ (C) The basic pK_a of 1 was determined at 428 nm (⎯○⎯) and 500 nm (⎯●⎯).Between pH 8 to 13, the absorbance 428 nm (A428) decreases but the absorbance at 500 nm (A500) increases. The non−linear curve fit was performed using the following equations, $A_{428}=\left\{\left({\in }_{428}^A\left[{\mathsf{H}}^{+}\right]+{\in }_{428}^{A-}{K}_a^2\right)/\left(\left[{\mathsf{H}}^{+}\right]+K_a^2\right)\right\}{\left[A\right]}_T$ and $A_{500}=\left\{\left({\in }_{500}^A\left[{\mathsf{H}}^{+}\right]+{\in }_{500}^{A-}{K}_a^2\right)/\left(\left[{\mathsf{H}}^{+}\right]+K_a^2\right)\right\}{\left[A\right]}_T$. AH is the monoprotonated, A is the neutral and A⁻ is the monoanion species of 1. (D) The pK_a assignment of 1.