Broad Infrared Absorption of Doped Tetrathiafulvalene-Derivative Monolayers on Thiolate-Covered Au(111)

Abstract The vibronically coupled broad infrared (IR) absorption of self-assembled tetrathiafulvalene (TTF) derivative monolayers on functional alkanethiolate-covered Au(111) surfaces was studied by infrared reflection absorption spectroscopy (IRAS), atomic force microscopy (AFM), and time-dependent density functional theory (TD-DFT) calculations. The broad IR absorption band was caused by the presence of dopants during self-assembly, and was affected by the morphology of the resulting monolayer. Interestingly, the broad IR peak at 4700 cm–1 shifted to 6700 cm–1 as the temperature decreased from 296 to 90 K. These phenomena are unique to molecular monolayer films on substrates. By comparison of the IR data with the TD-DFT calculation results, we confirmed that the origin of the broad IR absorption originated from charge transfer (CT) excitation of dimer cations and revealed the absorption characteristics.