Noncovalent Porphyrin-Graphene Oxide Nanohybrids: The pH-Dependent Behavior

Noncovalent nanohybrids between meso-(p-hydroxyphenyl)porphyrin (TPPH) and graphene oxide (GO) sheets were studied as a function of pH. The overall charge of the TPPH molecule changes between negative (-4), neutral, and positive (+2) depending on the pH of the solution. Results of Fourier transform infrared spectroscopy, thermogravimetric analysis, and elemental analysis confirm successful noncovalent functionalization of GO sheets with TPPH. We applied a number of methods to probe the ground-state as well as the excited-state interaction between the components of the new material. The experimental results were additionally supported by theoretical calculations that included optimizations of the ground-state structures of TPPH and TPPH2+ and their complexes with a molecular model of GO. It was demonstrated that both TPPH and TPPH2+ molecules can be assembled onto the surface of GO, but it was clearly shown that the stronger interaction with GO occurs for TPPH2+. The stronger interaction in the acidic environment can be rationalized by the electrostatic attraction between positively charged TPPH2+ and negatively charged GO, whereas the interaction between TPPH4- and GO at basic pH was largely suppressed. Our comprehensive analysis of the emission quenching led to the conclusion that it was solely attributed to static quenching of the porphyrin by GO. Surprisingly, fluorescence was not detected for the nanohybrid, which indicates that a very fast deactivation process must take place. Ultrafast time-resolved transient absorption spectroscopy demonstrated that although the singlet excited-state lifetime of TPPH2+ adsorbed on the GO sheets was decreased in the presence of GO from 1.4 ns to 12 ps, no electron-transfer products were detected. It is highly plausible that electron transfer takes place and is followed by fast back electron transfer.