Understanding the vibrational solvatochromism of the carbonyl stretch in dilute PCBM solutions

PCBM, a fullerene derivative, is widely used as an electron transport material in electronic devices, such as organic solar cells. It is known that the electron transfer efficiency in amorphous PCBM films is greatly affected by the underlying nanoscale disorder, which gives rise to the broad IR spectra of the carbonyl stretch of PCBM in these systems. In order to understand the heterogeneity in PCBM-containing systems and its manifestation in the IR spectra, we develop an efficient mixed quantum/classical model for the spectral simulations, which are based on classical molecular dynamics simulations and extensive quantum calculations. With this spectroscopic model we reproduce the unusual vibrational solvatochromism in dilute PCBM solutions, and the origin of the solvatochromism is discussed. Some approximations for spectral simulations are evaluated with the goal of employing them in the modeling of nonlinear vibrational spectra.