Deviation from Point Dipole Analysis for Exciton Quenching in Quaterthiophene-Terminated Self-Assembled Monolayers on Au(111)

A geometry-specific analysis of exciton quenching in the quaterthiophene (4T)-terminated alkanethiolate self-assembled monolayers (4TC n S-SAMs, where n is the number of methylene units; n = 3, 5, 6, 7, 8, 9, and 13) on Au(111) has been performed. In the previous studies we elucidated the n-dependent lifetime (τ) of the photoexcited 4T group in SAMs. In this study, using X-ray reflectivity (XRR) measurements, we evaluated the actual intralayer thickness of 4TC n S-SAMs on Au(111) and examined the quenching process as a function of distance (d) between the photoexcited moiety and the Au substrate. We confirmed that τ precisely follows the power law, i.e., τ ∝ d α , which is expected from the point dipole model analysis of the excitation energy transfer (ET) processes. Therefore, we attribute the dominant quenching mechanism to ET from the exciton state of 4T to the Au substrate rather than the quantum tunneling (QT) of excited electrons. However, the fitted parameter, α, is 4.28 ± 0.14 and thus deviates from the theoretical value on bulk-dumping models and previously measured values for admolecules on Au substrates, i.e., typically α ≈ 3. The origin of the deviation from the typical value is quantitatively discussed.