Vibrationally Resolved Absorption, Fluorescence, and Preresonance Raman Spectroscopy of Isolated Pyronin Y Cation at 5 K

Abstract

Exploring how charge-changing affects the photoluminescence of small organic dyes presents challenges. Here, helium tagging photodissociation (PD) action spectroscopy in the gas phase and dispersed laser-induced fluorescence (DF) spectroscopy in the solid Ne matrix are used to compare the intrinsic photophysical properties of pyronin Y cation [PY] + and its one electron-reduced neutral radical [PY] • at 5 K. Whereas the cation shows efficient visible photoluminescence, no emission from the neutral, in line with theoretical predictions, was detected. B3LYP/aug-cc-pVDZ calculations based on the TD-DFT/FCHT method allow for unambiguous assignment of recorded vibrationally resolved absorption and emission spectra. Surprisingly, our experimental sensitivity was high enough to also observe electronic preresonance Raman (ePR-Raman) spectra of [PY] + , with a significant efficiency factor (EF). These characteristics of the [PY] • /[PY] + pair suggest that appropriately functionalized derivatives may open new perspectives in the area of in vivo bioimagining microscopy and find applications in various sophisticated stimulated-Raman spectroscopies.

Acriflavine Is More than It Seems: Resolving Optical Properties of Multiple Isomeric Constituents at 5 K

“…the fluorescence excitation spectrum (λ max = 445 nm). This is as expected owing to the lower thermal energy of the ions in the He-TAG experiment (and therefore reduced population of vibrationally excited electronic ground state species). Interestingly though, while the two distinctive sets of absorption features in our PD spectrum are not resolved in their fluorescence excitation spectrum, Dobbie et al recorded a DF spectrum showing two partially resolved emission bands (discussed later) …”

“…As reported by us for other fluorophores, ,,, we have observed that both [ACF] + and [mpf] + can also undergo some charge changing during soft-landing deposition into neon, resulting in strongly fluorescent neutrals which emit significantly to the red of their charged precursors when excited in the UV. We will report details in a future study .…”

“…the combined effect of Stokes shifts and different experimental environments, is calculated to be 262 cm –1 (5.1 nm) and 196 cm –1 (3.7 nm), respectively. Comparing this blue shift with other matrix-isolated cationic dye systems reported by our group, , suggests that there is very little to almost no structural change upon excitation of each of the isomers to their respective higher electronic state (prior to fluorescence).…”

“…SI provides further details (including a measurement extending to 465 nm showing no measurable depletion above the noise limit beyond 443.1 nm). Note that two distinct sets of spectral features can be seen in Figure which we assign to a superposition of [ACF] + and [mpf] + absorptions–assuming to first order that (i) depletion cross sections correspond to absorption cross sections and (ii) He-atom induced spectral shifts are negligible. , Starting from the red, an intense and sharp absorption peak appears at 443.1 nm (22571 cm –1 ), followed by multiple peaks with weaker depletions. Subsequently, three significantly more intense absorptions positioned at 434.8 nm (23000 cm –1 ), 433.9 nm (23048 cm –1 ), and 433.0 nm (23094 cm –1 ) can be seen, followed by a weaker vibronic progression to shorter wavelength (Table S2 lists all the peak positions).…”

“…We have recently used mass-selective ESI deposition to soft-land samples of both Rhodamine B and Pyronin Y dye cations into a (codeposited) neon cryomatrix. Laser-induced visible region dispersed photoluminescence (PL) spectra of these matrix isolated species were compared to the absorption spectra of the corresponding cations in gas-phase as measured via helium tagging photodepletion spectroscopy (He-TAG PD).…”