Infrared multiple photon dissociation action spectroscopy and computational studies of mass-selected gas-phase fluorescein and 2',7'-dichlorofluorescein ions

Fluorescein (FL) and its derivative 2',7'-dichlorofluoroescein (DCF) are well-known fluorescent dyes used in many biological and biochemical applications. Although extensive studies have been carried out to investigate their chemical and photophysical properties in different solvent media, little is known about their intrinsic behaviors in the gas phase. Here, infrared multiple photon dissociation (IRMPD) action spectra are reported for the three charged prototropic forms of FL and DCF and compared with computed IR spectra from electronic structure calculations. In each case, the measured spectra show good agreement with the calculated spectra of the lowest energy computed conformer. Moreover, the major bands of the monoanion IRMPD spectra show striking similarities to those of the dianions and are quite different from those of the cations. These experimental results clearly indicate that the gaseous monoanions are predominantly deprotonated on the xanthene chromophore, rather than the benzoate deprotonation site favored in solution. Investigations such as this, which provide a better understanding of intrinsic properties of ionic dyes, forms a baseline from which to elucidate solvent effects and will aid the rational design of dyes possessing desirable fluorescence properties.