Vibrational exciton nano-spectroscopy: Probing molecular order and disorder on the nanoscale
Journal Article
Overview
abstract
Transition dipole coupling between localized molecular vibrations represents a fundamental mechanism of energy delocalization in molecular solids. These collective states, referred to as vibrational excitons, give rise to distinct vibrational frequency shifts and mode splitting encoding information about molecular orientation, packing, conformation, and structural disorder. Originally developed to describe ideal crystalline materials, vibrational exciton spectroscopy and modeling have since evolved into a versatile tool applicable to a broad range of chemical systems, including molecular crystals, self-assembled monolayers (SAMs), molecular liquids, and biomolecular assemblies. However, diffraction-limited vibrational Raman and IR spectroscopy spatially averages over macroscopic ensembles of molecules, making it challenging to resolve inhomogeneities on nanometer length scales, which dictate the overall response of many molecular solids. Recently extended to the nanoscale, vibrational coupling nano-crystallography (VCNC) combines IR scattering-type scanning near-field optical microscopy (IR s-SNOM) with vibrational exciton theory to spatially resolve local molecular order and domain structure with nanometer resolution. In this review, we describe recent advances in near-field spectroscopy of vibrational excitons to image local molecular order in crystals, SAMs, and biological systems. Finally, we conclude with a perspective for VCNC to study excited-state dynamics, energy transport, collective quantum phenomena, and nanoscale structural evolution in complex molecular systems.
