Redox potentials, photophysical properties, pK_as, and tautomer equilibrium constants for R-SA−.

R	E app(R-SAOH−/R-SA˙O) (V vs. NHE)	λ max, R-HSA (nm), εR-HSA (M−1 s−1)	λ max, R-SA− (nm), εR-SA− (M−1 s−1)	pKa	K EQ,Taut
OH	0.56a	335, 4200	320, 4100	16.9	0.03
OMe	0.79	333, 4200	318, 4100	16.6	0.04
Me	0.97	314, 3800	304, 3600	16.9	0.07
H	1.10	304, 3700	295, 3400	16.7b	0.10
F	1.11	314, 4400	305, 4100	15.8	0.07
Cl	1.16	316, 3500	307, 3200	15.6	0.11
Acetyl	1.30	305, 3100	292, 13 200	15.4	1.2

^aOxidation of OH-SA− was quasi-reversible. The value reported is the E_1/2.

^bAligns with literature value of 16.7.⁵⁰