Intermolecular vibrational coupling plays an important role in the photophysical behavior of molecular solids, yet direct spectroscopic access to these interactions remains challenging because of their short length scales (nm to sub-nm) and low energies (meV). Here, we use molecular vibrations as intrinsic labels of the local molecular environment to directly probe vibrational wavefunction delocalization—the formation of vibrational excitons—with precision infrared nanospectroscopy and imaging. We establish near-field spectroscopy of vibrational excitons as a form of optical crystallography highly sensitive to molecular order and disorder.
Beyond serving as a nanoscale probe of molecular order, vibrational excitons can also support collective behavior through the formation of delocalized coherent states. Cooperative emission in the form of superradiance is well established for dipole-coupled electronic excitons at visible wavelengths, but analogous cooperativity has not been demonstrated for transition-dipole-coupled vibrations in molecular solids in the mid-infrared. This is largely because molecular vibrations are predominantly nonradiative, with near-zero photon quantum yield, making conventional observation of superradiant emission inaccessible. By directly measuring the optical near-field of the coherent vibrational state, we observe an enhanced collective vibrational polarization. These results suggest that, in ordered molecular solids, vibrational mode hybridization can produce collective macroscopic dipole states and cooperative optical behavior, opening opportunities to investigate related many-body phenomena under ambient conditions even in systems with negligible radiative quantum yield.