Fig. 1. Steady-state absorption spectroscopy of TCPP and H ₂ TCPP ²⁺ . (a) Molecular structure of carboxylate-form of tetrakis(4-carboxyphenyl)-porphyrin and its fully protonated diacid (TCPP⁴⁻ and H₂TCPP²⁺). (b) UV/vis spectra of 1.26 μM TCPP⁴⁻ in DMF at different acid concentrations from blue to red (0, 4, 32, 64, 128, 256, 512, and 1024 mM MSA). The Soret band (S₀ to S₂ transition) is located at 23.9 × 10³ cm⁻¹ (419 nm) for TCPP and at 22.4 × 10³ cm⁻¹ (446 nm) for H₂TCPP²⁺. The region with the four Q-bands of the porphyrin (lower energy section of the plot) is zoomed in 10 times for visualization purposes. H₂TCPP²⁺ only shows two Q-bands for reasons discussed in the text. (c) Absorbance at 22.4 × 10³ cm⁻¹, a direct measure to the amount of H₂TCPP²⁺, as function of MSA concentration. We indicate three regions, neutral (I), intermediately protonated (II), and fully protonated (III).