Acid–base properties of the N3 ruthenium(II) solar cell sensitizer: a combined experimental and computational analysis

We report a combined spectro-photometric and computational investigation of the acid–base equilibria of the N3 solar cell sensitizer [Ru(dcbpyH2)2(NCS)2] (dcbpyH2 = 4,4′-dicarboxyl-2,2′ bipyridine) in aqueous/ethanol solutions. The absorption spectra of N3 recorded at various pH values were analyzed by Single Value Decomposition techniques, followed by Global Fitting procedures, allowing us to identify four separate acid–base equilibria and their corresponding ground state pKa values. DFT/TDDFT calculations were performed for the N3 dye in solution, investigating the possible relevant species obtained by sequential deprotonation of the four dye carboxylic groups. TDDFT excited state calculations provided UV-vis absorption spectra which nicely agree with the experimental spectral shapes at various pH values. The calculated pKa values are also in good agreement with experimental data, within <1 pKa unit. Based on the calculated energy differences a tentative assignment of the N3 deprotonation pathway is reported.