Fluorescence decay of quasimonolayered porphyrins near a metal surface separated by short-chain alkanethiols

We investigated the spectral feature and fluorescence lifetime of quasimonolayered porphyrins at very short distance to metal substrates (1-2.5 nm) through fine-tuning the length of alkanethiols. The ordered self-assembled monolayers of alkanethiols on Au(111) act as a uniform electronic decoupling layer and suppress the interface quenching via charge transfer. However, the fluorescence quenching via nonradiative energy transfer to the metal still prevails in the porphyrin-alkanethiol-metal sandwich structures. The decay rates are found to follow a 1/d³ dependency on spacer thickness, which suggests that the classical electromagnetic theory appears still valid at distance down to 1 nm through volume damping.