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

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 H(2)O-DMF, H(2)O-acetone, H(2)O-methanol, and DMF-acetone mixtures. The maximum absorption wavelength (λ(max)) 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 E(T)(30) can explain both shifts very well. At higher polarity (E(T)(30) > 48), THPP exists as a hyperporphyrin. The E(T)(30) is linear with λ(max) and a decrease in solvent polarity is accompanied by a bathochromic shift of λ(max). 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 < E(T)(30) < 48), hyperporphyrin gradually becomes Na(2)P as E(T)(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 λ(max).