Substrate influence on the interlayer electron–phonon couplings in fullerene films probed with doubly-resonant SFG spectroscopy

Abstract

We present doubly-resonant sum frequency generation (DR-SFG) spectra of fullerene thin films on metallic and dielectric substrates as a way to investigate the interplay between nuclear and electronic coupling at buried interfaces. Modal and substrate selectivity in the electronic enhancement of the C₆₀ vibrational signatures is demonstrated for excitation wavelengths spanning the visible range. While the SFG response of the totally symmetric A_g(2) mode of fullerene is distinctly coupled to the optically allowed electronic transition corresponding to the HOMO–LUMO+1 of C₆₀ (ca. 2.6 eV), the T_1u(4) vibrational mode appears to be coupled to a symmetry-forbidden HOMO–LUMO transition at lower energies (ca. 2.0 eV). For dielectric substrates, the DR-SFG intensity of the T_1u(4) mode shows lack of enhancement for upconversion wavelengths off-resonance with the optically-dark LUMO. However, the T_1u(4) mode shows a unique coupling to an intermediate state (∼2.4 eV) only for the fullerene films on the gold substrate. We attribute this coupling to unique interactions at the buried C₆₀/gold interface. These results demonstrate the occurrence of clear electron–phonon couplings at the C₆₀/substrate interfaces and shed light on the impact of these couplings on the optical response of electronically excited fullerene. This coupling may influence charge and energy transport in organic electronic devices mediated by vibrational motions. We also demonstrate a potential use of this added selectivity in chemical imaging.