Vibrational coupling infrared nano-crystallography*

Low energy intermolecular interactions define many of the functional properties of molecular systems. Coupling between molecular vibrations, in particular, has been shown to influence energy and charge transfer in molecular materials. However, probing these interactions from the nano- to molecular length scales and on few meV energy scales on which they arise has remained challenging. Here, using femtosecond broadband infrared vibrational optical nano-spectroscopy, we resolve the coupling between molecular vibrations in disordered polycrystals of representative molecular electronic materials. We use the spectral changes arising from vibrational coupling as a molecular ruler to measure the size of crystalline domains within the polycrystals. We provide a theoretical framework for infrared nano-crystallography to quantify the vibrational coupling, setting benchmarks for future studies. This technique of vibrational coupling nano-crystallography can be applied to the study of molecular disorder and its relationship with charge and energy transfer on few-nanometer length scales in molecular electronic, photonic, optoelectronic, and biological systems.