The absorption and fluorescence emission spectra of meso-tetra(aryl)porphyrin dications with weak and strong carboxylic acids: a comparative study

The stereoelectronic effects of meso substituents on the absorption and emission spectra of a series of para- or ortho-substituted meso-tetra(phenyl)porphyrins (H₂TPP, H₂T(2-Cl)PP, H₂T(2-Me)PP, H₂T(4-Cl)PP, H₂T(4-OMe)PP, H₂T(4-Me)PP) and their dications with CF₃COOH and HCOOH have been investigated. The f_Soret/f_Q(0,0) ratio was used as a criterion for the comparison of the degree of configuration interaction (CI) between the excited states. Reverse mirror image relationships were observed between the absorption and emission spectra of the porphyrins and their dication species. The much larger stokes shifts observed in the emission spectra of the dications compared to the free base porphyrins reveal major geometrical differences between the ground and excited states of the dications. With the exception of H₄T(4-OMe)PP, the fluorescence quantum yields (Φ_f) of the dications were much larger than those of the free base porphyrins. The natural radiative lifetimes (τ₁, τ_2, τ₃) of the dications estimated by the Forster, Strickler and Berg equations were generally much shorter than those of the free base porphyrins. Also, the radiative rate constant (k_r) and the non-radiative one (k_nr) of the dications were significantly larger than those of the free base porphyrins. The splitting of the bands in the absorption and emission spectra of the free base porphyrins was clearly observed, but little or no splitting was observed in the case of the spectra of the dications.