Electronic Stark Effect in Isolated Ion Pairs

Stark spectral shifts of a molecular probe are commonly used to estimate the local electric field in condensed media. The very large fields reported, typically in the 0.1–10 GV m–1 range, are, however, difficult to reproduce in a controlled manner, limiting the calibration of these molecular probes to ranges below 0.1 GV m–1. In this context, we investigated gas-phase, isolated, molecular ion pairs, where a phenyl ring is immersed in the electric field produced by the nearby ionic groups. The intensity of the electric field is chemically tuned in the 1 GV m–1 range by changing the nature of the cations, and the phenyl ring response is monitored by UV spectroscopy. A quadratic Stark effect is observed, demonstrating the possibility to characterize molecular probes in a solvent-free environment and in the very large field range they typically meet in condensed media such as biological environments.