Stark realities.

Electric fields affect any process or transition that involves the movement of charge. Stark spectroscopy is a general term describing the study of spectral changes in the presence of electric fields, and it has proven to be a broadly useful approach for characterizing the change in dipole moment and polarizability for electronic and vibrational transitions. This article focuses primarily on the evolution of the approach and interconnected applications in diverse fields from our laboratory and prospects for the future. Our work began with studies of chromophores in photosynthetic reaction centers whose function is light-driven charge separation, so perturbations by an electric field were a natural approach. The same methods have been applied to many other biological and nonbiological chromophores. A common theme has been understanding the mechanism(s) of symmetry breaking in molecules or organized assemblies of high symmetry. Spectral shifts in organized systems due to mutations, conformational changes, and ligand binding can, in some cases, be interpreted as Stark shifts. In this case, Stark spectroscopy in a well-defined electric field provides a calibration of the probe transition's sensitivity to an electric field, and the Stark shifts of suitable probes can be used to measure the magnitude and direction of electric fields in proteins, nucleic acids, and membranes. Electric fields can also perturb the populations or reaction dynamics of processes where charge separation occurs. When detected by spectroscopic methods, we call these nonclassical Stark effects. Nonclassical Stark effects arise in the spectroscopy of intervalence charge transfer transitions and both ground- and excited-state electron transfer reactions. Because the movement of charge is ubiquitous in chemistry, biology, and materials science and because electric fields directly affect the energetics of charge-separated species, many phenomena can be viewed as generalizations of the Stark effect.