Molecular Orientation and Optical Properties of 3,3′-Diethylthiatricarbocyanine Iodide Adsorbed to Gold Surfaces: Consequences for Surface-Enhanced Resonance Raman Spectroscopy

3,3′-Diethylthiatricarbocyanine iodide (DTTC) is an important near-infrared Raman reporter and fluorescence marker showing high potential for biomedical imaging. It is commonly assumed that the dye molecules retain their monomeric optical properties when adsorbed to gold surfaces, and surface-enhanced Raman spectroscopy (SERS) experiments are typically designed to take optimal advantage of the monomer resonance enhancement using 785 nm light. To test this assumption, we investigated the optical properties of DTTC iodide thin films on transparent gold substrates using polarized and angle-dependent UV–vis spectroscopy. The surface absorption spectra demonstrate that DTTC molecules form strongly coupled dimers on gold surfaces, with blue-shifted absorption bands where the transition dipole moment is aligned parallel to the metal surface. These results clarify inconsistencies encountered in recent SERS experiments and suggest ways to enhance signal quality in DTTC-based biomedical imaging.