Photoinduced energy and charge transfer in layered porphyrin-gold nanoparticle thin films

In thin films of porphyrin (H₂P) and gold nanoparticles (AuNPs), photoexcitation of porphyrins leads to energy and charge transfer to the gold nanoparticles. Alternating layers of porphyrins and octanethiol protected gold nanoparticles (d_core ∼3 nm) were deposited on solid substrates via the Langmuir-Schäfer method, forming bilayer films denoted as H ₂P/AuNP. Photoinduced electron transfer from the gold nanoparticle layer to the porphyrin layer was observed as a distinct photovoltage response of the H₂P/AuNP film when studied via the time-resolved Maxwell displacement charge (TRMDC) method. Time-resolved fluorescence and absorption measurements of the H₂P/AuNP film demonstrated a significant reduction of the lifetime of the excited singlet state of porphyrin caused by the gold nanoparticles. Transients of the charge transfer reaction were not observed in the time-resolved absorption measurements, which indicates that the quantum yield of the charge transfer is low in the H₂P/AuNP film. Energy transfer from the excited singlet state of porphyrin to the gold nanoparticles is the main deactivation path of excited porphyrins in the H ₂P/AuNP film. The critical distance of the energy transfer was estimated to be 6.4 nm, based on the dependence of fluorescence quenching on the distance between the porphyrin and gold nanoparticle layers.