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

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 C vibrational signatures is demonstrated for excitation wavelengths spanning the visible range. While the SFG response of the totally symmetric A(2) mode of fullerene is distinctly coupled to the optically allowed electronic transition corresponding to the HOMO-LUMO+1 of C (ca. 2.6 eV), the T(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(4) mode shows lack of enhancement for upconversion wavelengths off-resonance with the optically-dark LUMO. However, the T(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 C/gold interface. These results demonstrate the occurrence of clear electron-phonon couplings at the C/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.