Photophysics of Isolated Rose Bengal Anions.

Dye molecules based on the xanthene unit are widely used as fluorescent probes in bioimaging and technological applications due to their large absorption cross-section for visible light and high fluorescence quantum yield. These applications require a clear understanding of the dye's inherent photophysics and the effect of a condensed-phase environment. Here, the gas-phase photophysics of Rose Bengal doubly deprotonated dianion [RB-2H]2-, deprotonated monoanion [RB-H]- and doubly deprotonated radical anion [RB-2H].-, are investigated using photodetachment, photoelectron, tandem ion mobility spectrometry (IMS) coupled with laser excitation, and dispersed fluorescence action spectroscopies. For [RB-2H]2-, photodetachment action spectroscopy reveals a clear band in the visible (450-580 nm) with vibronic structure. Electron affinity and repulsive Coulomb barrier (RCB) properties of the dianion are characterized using frequency-resolved photoelectron spectroscopy, revealing a decreased RCB compared with fluorescein dianions due to electron delocalization over halogen atoms. Monoanions [RB-H]- and [RB-2H].- differ in nominal mass by 1 Da, but are difficult to study using action spectroscopies that isolate target ions using low-resolution mass spectrometry. This work shows that the two monoanions are readily distinguished and probed using the IMS-photo-IMS and photo-IMSphoto-IMS strategies, providing distinct but overlapping photodissociation action spectra in the visible spectral range. Gas-phase fluorescence was not detected from [RB-2H]2- due to rapid electron ejection. However, both [RB-H]- and [RB-2H].- show weak fluorescence signal. The [RB-H]- action spectra show a large Stokes shift of ~1,700 cm-1, while the [RB-2H].- action spectra show no significant Stokes shift. This difference is explained by considering geometries of the ground and fluorescing states.