Raman and surface-enhanced Raman scattering of a series of size-separated polyynes

Solutions of hydrogen-capped polyynes were prepared by laser ablation of graphite powder in n-hexane and subjected to size separation by high-performance liquid chromatography. Solutions of size-selected polyynes CnH2 (n = 8–16) were investigated by normal Raman (NR) and surface-enhanced Raman scattering (SERS) spectroscopy. A main band appearing in the 2000–2200 cm−1 region of the NR spectra showed a systematic downward shift as the chain length increased. The observed NR bands were assigned to Raman-active CC stretching vibrational modes by comparison with calculations based on density functional theory. Raman bands observed in SERS spectra were very broad and located at frequencies lower than the NR bands. A systematic band shift with increasing chain length was also observed for one of the bands. This band was thus assigned to a counterpart of the strong band in the NR spectra. These results made it possible to assign the origins of previously reported SERS bands of mixed polyyne solutions.