Photoluminescence properties and excited state dynamics of monolayer perylene on graphite (0001)

The correlation between the photoluminescence properties and excited state dynamics of perylene $({\mathrm{C}}_{20}{\mathrm{H}}_{12})$ formed on a graphite (0001) substrate was investigated at the monolayer limit. Time-resolved two-photon photoemission spectroscopy was used to evaluate the lifetime of the excited state on the order of nanoseconds. On the molecular monolayer films, this unexpectedly long lifetime at the interface is significantly different from those typically observed for adsorption-induced electronic states, where ultrafast decays on the order of femtoseconds are dominant with electron/hole scattering. On the graphite (0001) surface, the standing molecular orientation of perylene indicates that the excited states are electronically decoupled from the substrate, which results in the suppression of the ultrafast nonradiative decay. As a result, orange light luminescence (610 nm) is observed with visible strength, which is ascribed to the deexcitation from the excited state of the dimeric molecular arrangement at the interface. Understanding the photoluminescence properties at the organic/electrode interface could be a key to the realization of organic optoelectronic thin-film devices with only a few molecular layers.