  We study the collective behavior of molecules placed in an infrared (IR)
microcavity, incorporating the local fluctuations, i.e., dynamical disorder.
The cooperative feature in vibrational polaritons is shown to be dynamically
eroded, due to intermolecule coherence. To further resolve such process, we
develop a two-dimensional infrared spectroscopy (2D-IR) for molecules
interacting with cavity modes. The cooperative feature in correspondence to the
spectroscopic signal is specified. The results reveal the dark states by the
cross peaks apart from the ones for polaritons, as a result of the breakdown of
cooperativity between molecules. We further show that the breakdown of
cooperativity profoundly connects to the localization of the vibrational
excitations whereas the polariton modes are extended wave over several
molecules. Besides, our work offers new physical insight for understanding the
recent 2D-IR experiments where the interaction between dark modes and bright
polaritons was evident.
