Influence of the solvent-porphyrin interaction on the UV–Vis absorption of free base imidazol cationic porphyrin

• The (tetra(1,3-dimethylimidazolium-2-yl) porphyrin tetraiodide) was studied.
• The study involved experimental measurements and Quantum chemical calculations.
• Experimental and theoretical data agree very well.
• Evidences of solvent and porphyrin interaction on UV–Vis band are shown.
• H-type porphyrin aggregate shields the porphyrin-solvent interaction.

In the present work, we investigate the UV–Vis absorption spectrum of a free base cationic porphyrin derivative (5,10,15,20-tetrakis (1,3-dimethylimidazolium-2-yl) porphyrin tetraiodide) (H2TDMImP) in polar solvents. Our results show that, for the present sample, the less energetic transitions belonging to Soret band are favored when compared to other ionic porphyrins and that the Q-band presents a rhodo type structure. Quantum chemical calculations suggest that such spectral feature is caused by the strong interaction between the cationic outlying group, present in this porphyrin, and solvent molecules. Increasing the porphyrin concentration, the spectral changes suggest that H-type porphyrin aggregates are formed and that such aggregate structures protect the outlying groups of porphyrin against interactions with solvent molecules.