Cationic Porphyrin‐Graphene Oxide Hybrid: Donor‐Acceptor Composite for Efficient Photoinduced Electron Transfer

Non‐covalent nanohybrids composed of cationic 5,10,15,20‐tetra(4‐trimethylammoniophenyl)porphyrin tetra(p‐toluenesulfonate) (TMAP) and the graphene oxide sheets were prepared under two pH values (6.2 vs. 1.8). The TMAP molecule was positively charged, regardless of the pH value during preparation. However, protonation of the imino nitrogens increased the overall charge of the porphyrin molecule from +4 to +6 (TMAP4+ and TMAP6+). It was found that at acidic pH, interaction of TMAP6+ with GO was largely suppressed. On the other hand, results of FTIR, Raman spectroscopy, thermogravimetric analysis, atomic force microscopy (AFM) and elemental analysis confirmed effective non‐covalent functionalization of graphene oxide with cationic porphyrin at pH 6.2. The TMAP4+‐GO hybrids exhibited well defined structure with a monolayer of TMAP4+ on the GO sheets as confirmed by AFM. Formation of the ground‐state TMAP4+‐GO complex in solution was monitored by the red‐shift of the porphyrin Soret absorption band. This ground‐state interaction between TMAP4+ and GO is responsible for the static quenching of the porphyrin emission. Fluorescence was not detected for the nanohybrid which indicated that a very fast deactivation process had to take place. Ultrafast time‐resolved transient absorption spectroscopy clearly demonstrated the occurrence of electron transfer from the photoexcited TMAP4+ singlet state to GO sheets, as proven by the formation of a porphyrin radical cation. Porphyrin‐GO complex: Herein, the pH‐dependent behaviour of non‐covalent cationic porphyrin–graphene oxide nanohybrids is investigated. Electrostatic attraction facilitates the formation of stable complex. Efficient electron transfer process occurs from the photoexcited singlet state of the porphyrin to GO.