Carbonyl Spectator Bonds as Sensitive Sensors for Charge Transfer Reactions on the Femtosecond Time Scale

The feasibility of using the vibrational Stark effect for the observation of charge transfer reactions on a short time scale is demonstrated. The photoinduced oxidation of ferrocenophanone induces a fast shift of the carbonyl stretching frequency which is observable by femtosecond time-resolved IR spectroscopy. The observed shift is in good agreement with the IR spectrum of chemically oxidized ferrocenophanone and with theoretical predictions based on vibrational Stark effect calculations. The time dependence of the signal mirrors the charge transfer dynamics on the fs to ps time scale, as observed by optical spectroscopy. This shows that the vibrational Stark effect provides access to observing charge transfer reactions in the IR on the fs time scale. Since the Stark effect is sensitive to changes of the electric field alone, the sensor bond does not need to be part of the molecular system under investigation, but may be a noninvolved “spectator” bond located in its immediate surroundings.