Decay of the Exciton in Quaterthiophene-Terminated Alkanethiolate Self-Assembled Monolayers on Au(111)

The dynamics of the photoexcited states in the quaterthiophene-terminated alkanethiolate self-assembled monolayer (SAM) on Au(111) are investigated using time-resolved two-photon photoemission (2PPE) spectroscopy. For SAMs with various alkylene (−(CH₂)_n−) chain lengths, a common stand-up configuration of the quaterthiophene (4T) group is confirmed by infrared reflection absorption spectroscopy (IRAS); the −(CH₂)_n– length controls the distance between 4T and Au. Although the binding energies of the occupied and unoccupied electronic levels of the 4T moiety are similar for all SAMs, the decay dynamics of the photoexcited electrons at 4T strongly depends on the −(CH₂)_n– length; the lifetime changes from 0.2 to 3.9 ps by changing n from 3 to 13. The n dependent decay is explained by the excitation transfer from 4T to the Au substrate.