Controllable Charge-Transfer Mechanism at Push-Pull Porphyrin/Nanocarbon Interfaces

Push-pull porphyrins are made of an electron donor (D), an electron acceptor (A), and a conjugated bridge connecting the D and A units. The tunability of their highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap can modulate their inter- and intramolecular charge-transfer (CT) and charge-separation (CS) characteristics and their reaction mechanism. Here, ultrafast charge transfer at the interfaces between 5,15-donor-acceptor push-pull porphyrins (Por-tBu and Por-OC8) and nanocarbon materials in the form of fullerene (C-60) and graphene carboxylate (GC) are investigated using steady-state and pump-probe spectroscopic techniques. The strong photoluminescence (PL) quenching of the porphyrin indicates an electron transfer from the photoexcited porphyrin to the nanocarbon materials. The results of steady-state and time-resolved experiments reveal that a static and both static and dynamic electron transfer are dominant in the presence of GC and C-60, respectively. This work provides new physical insights into the electron-transfer process and its driving force in donor-acceptor systems that include nanocarbon materials.