Effect of Solvent Polarity on the Spectral Characteristics of 5,10,15,20-Tetrakis(p-hydroxyphenyl)porphyrin

Abstract

The electronic absorption and vibrational spectra of deprotonated 5,10,15,20-tetrakis(p-hydroxyphenyl)porphyrin (THPP) are studied as a function of solvent polarity in H2O-DMF, H2O-acetone, H2O-methanol, and DMF-acetone mixtures. The maximum absorption wavelength (λmax) of the lowest energy electronic absorption band of deprotonated THPP shows an unusual solvatochromism-a bathochromic followed by a hypsochromic shift with reduced polarity. According to the correlation analysis, both specific interactions (H-bonds) and nonspecific interactions affect the spectral changes of this porphyrin. Furthermore, the solvent polarity scale ET(30) can explain both shifts very well. At higher polarity (ET(30) > 48), THPP exists as a hyperporphyrin. The ET(30) is linear with λmax and a decrease in solvent polarity is accompanied by a bathochromic shift of λmax. These results can be rationalized in terms of the cooperative effects of H-bonds and nonspecific interactions on the spectra of hyperporphyrin. At relatively low polarity (45.5 < ET(30) < 48), hyperporphyrin gradually becomes Na2P as ET(30) reaches the critical value of 45.5. The spectrum of the hyperporphyrin turns into the three-band spectrum of the metalloporphyrin, which is accompanied by a hypsochromic shift of λmax.