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

Abstract

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 (C60) and graphene carboxylate (GC) are investigated using femtosecond (fs) pump-probe spectroscopy with broadband capabilities. The strong photoluminescence (PL) quenching of the porphyrin indicates electron and/or energy transfer from the photoexcited porphyrin to the nanocarbon materials. More interestingly, the Stern-Volmer plots of PL quenching shows linear and nonlinear patterns upon increasing the concentration of GC or C60 in the porphyrin solution, respectively, clearly indicating static and a combination of static and dynamic quenching at the interfaces with these nanocarbon materials. Using femtosecond transient absorption (TA) spectroscopy, ultrafast electron transfer from a singlet-excited porphyrin to the nanocarbon materials is clearly identified by the fast ground state bleach recovery and the formation of cation radical species. Furthermore, a fs-TA study revealed that both porphyrins show very long-lived ground state bleach (GSB) and excited state absorption (ESA), which can be attributed to the triplet-state formation. This work provides new physical insights into the electron transfer process and its driving force in donor-accepter systems that include nanocarbon materials.