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

Abstract

Noncovalent nanohybrids between meso-(phydroxyphenyl)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 TPPH 2+ and their complexes with a molecular model of GO. It was demonstrated that both TPPH and TPPH 2+ molecules can be assembled onto the surface of GO, but it was clearly shown that the stronger interaction with GO occurs for TPPH 2+ . The stronger interaction in the acidic environment can be rationalized by the electrostatic attraction between positively charged TPPH 2+ and negatively charged GO, whereas the interaction between TPPH 4− 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 TPPH 2+ 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.

Graphene Oxide Functionalized with Cationic Porphyrins as Materials for the Photodegradation of Rhodamine B

“… 38 , 42 , 46 , 47 Theoretical calculations predict that only a 30° change in the orientation of the meso-substituents toward the porphyrin core can result in a bathochromic shift of the Soret band by as much as 30 nm. 48 Our previous calculations have shown that the interaction of ZnTPPH and TPPH with GO leads to a twisting of the side rings relative to the porphyrin core from about 60° to 45° and 39° for ZnTPPH and TPPH, respectively, 38 , 47 confirming the flattening of the porphyrin upon complexation. Moreover, the experimentally observed Soret band red shift of 16 nm for the ZnTPPH adsorbed on the GO surface was well reproduced in theory.…”

“… 38 The slightly smaller red shift observed for ZnTMPyP in comparison to TMPyP upon adsorption on GO ( Figure S11 ) can be attributed to less profound flattening of the ZnTMPyP as observed previously for the TPPH and ZnTPPH. 38 , 47 …”

“… 42 In general, it is observed that an electrostatic interaction between the cationic porphyrin and the negatively charged graphene material enhances quenching of the porphyrin emission, 29 , 42 , 51 in comparison to the neutral porphyrin where the interaction with the graphene-based material is attributed mainly to π–π stacking interactions. 21 , 47 , 51 , 52 …”

Functionalization of Graphene Oxide with Porphyrins: Synthetic Routes and Biological Applications

“…%) of the porphyrin onto GO. [157,159] CV The CV curves reveal that GO/porphyrin hybrids can act as good electrode materials for the electrocatalytic oxidation of other species, such as hydrazine or dopamine. The oxidation may occur at lower oxidation potential and higher oxidation peak current.…”

“…[158] More recently, Gacka et al have described the preparation of non-covalent hybrids between 5,10,15,20-tetrakis(4-hydroxyphenyl)porphyrin (TPPH) and GO sheets at different pH values (Scheme 11). [159] While TPPH at neutral pH does not possess any charge, the protonation of its inner imino N atoms increases its charge to + 2, and the deprotonation of the À OH groups in the substituents decreases its charge to À 4. At acidic pH, the interaction of TPPH 2 + with GO was found to be stronger and it was largely suppressed at basic pH.…”