Real-time observation of ultrafast electron injection at graphene-Zn porphyrin interfaces

We report on the ultrafast interfacial electron transfer (ET) between zinc(II) porphyrin (ZnTMPyP) and negatively charged graphene carboxylate (GC) using state-of-the-art femtosecond laser spectroscopy with broadband capabilities. The steady-state interaction between GC and ZnTMPyP results in a red-shifted absorption spectrum, providing a clear indication for the binding affinity between ZnTMPyP and GC via electrostatic and π-π stacking interactions. Ultrafast transient absorption (TA) spectra in the absence and presence of three different GC concentrations reveal (i) the ultrafast formation of singlet excited ZnTMPyP*, which partially relaxes into a long-lived triplet state, and (ii) ET from the singlet excited ZnTMPyP* to GC, forming ZnTMPyP˙(+) and GC˙(-), as indicated by a spectral feature at 650-750 nm, which is attributed to a ZnTMPyP radical cation resulting from the ET process.