Photophysics of isolated Rose Bengal anions

Dye molecules based on the xanthene moiety 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 the rose bengal doubly deprotonated dianion [RB – 2H]2–, deprotonated monoanion [RB – H]−, and doubly deprotonated radical anion [RB – 2H]•– is investigated using photodetachment, photoelectron, and dispersed fluorescence action spectroscopies, and tandem ion mobility spectrometry (IMS) coupled with laser excitation. 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 that of 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 individually 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-IMS-photo-IMS strategies, providing distinct but overlapping photodissociation action spectra in the visible spectral range. Gas-phase fluorescence was not detected from photoexcited [RB – 2H]2– due to rapid electron ejection. However, both [RB – H]− and [RB – 2H]•– show a weak fluorescence signal. The [RB – H]− action spectra show a large Stokes shift of ∼1700 cm–1, while the [RB – 2H]•– action spectra show no appreciable Stokes shift. This difference is explained by considering geometries of the ground and fluorescing states.