Substitution effect of hydroxyl group on photophysical properties of tetraphenylporphyrin (H₂TPP) and germanium(IV) tetraphenylporphyrin dichloride (Ge(IV)TPPCl₂)

Photophysical properties of tetraphenylporphyrin (H₂TPP), tetra(p-hydroxyphenyl)porphyrin ((p-OH)₄TPPH₂), germanium(IV) tetraphenylporphyrin dichloride (GeTPPCl₂), and germanium(IV) tetra(p-hydroxyphenyl)porphyrin dichloride (Ge(p-OH)₄TPPCl₂) have been investigated to elucidate the effects of para OH substituents on the free-base and the metal-incorporated porphyrin π systems by steady-state and time-resolved spectroscopic methods. The observed photophysical properties were correlated with various molecular aspects, such as extended π-conjugation, nonplanar structural distortion, and central heavy atoms. Changes to steady-state absorption and fluorescence spectral characteristics of the OH-substituted porphyrin derivatives were attributed to the enhanced π-conjugation between phenyl ring and porphyrin π-electron system in the ground and the excited states. Such π-conjugation effects of the OH-substituted derivatives are greater when Ge(IV) is introduced into the core. The enhanced π-electron interaction of the OH-substituted derivatives in the excited state shortened the fluorescence lifetimes but increased the fluorescence quantum yields since the radiative rate was enhanced more than the fluorescence decay rate increased. The radiative decay rate of the OH-substituted Ge(IV) porphyrin is even greater due to more intensive π-conjugation of the metalloporphyrin in the excited state. The metal incorporation into the core also enhanced the nonradiative decay rates due to increased spin-orbit coupling and, consequently, reduced the fluorescence lifetimes and quantum yields. However, the spin-orbit coupling effect is not as large as expected. On the other hand, the OH group substitution enhanced only the nonradiative decay rate of the Ge(IV) metalloporphyrin and did not increase that of the free base porphyrin. This is ascribed to much more intensive π-conjugation of the Ge(IV) metalloporphyrin in the excited state which may induce excited state charge transfer and structural distortion.