Relation between pressure shift and electric-field shift of single-molecule lines in a polymer glass

The pressure shifts and the electric-field shifts of individual chromophores in an amporphous matrix are--due to strong disorder--subject to broad distributions. By means of single-molecule spectroscopy we measured both the pressure and the electric-field shift of about 800 tetra-tert-butylterrylene molecules in polyisobutylene. We found a significant correlation of 0.52 (Kendall's correlation coefficient) between the two observables. Analytical calculations and Monte Carlo simulations based on a model by Laird and Skinner predict a nonzero, yet, distinctly smaller correlation. The Monte Carlo simulations showed that the usual assumptions of a spherical shape and isotropic polarizability of the chromophores in glassy systems is an oversimplification of the complex nanoscopic structure and cannot reproduce our experimental results. By taking the molecular anisotropy into account, we obtain agreement of the simulated and the measured correlation between pressure shift and electric-field shift parameter.