Gas‐Phase Ion Fluorescence Spectroscopy of Tailor‐made Rhodamine Homo‐ and Heterodyads: Quenching of Electronic Communication by π‐Conjugated Linkers

Abstract

While many key photophysical features are understood for electronic communication between chromophores in neutral compounds, there is limited information on the effect of charges in practically relevant ionic chromo/fluorophores. Here we have chosen positively charged rhodamines and prepared a selection of homo‐ and heterodimers with alkyl or π‐conjugated, acetylenic bridges. Protonated molecules were transferred as isolated ions to gas phase where there is no solvent screening of charges, and fluorescence spectra were measured with a custom‐made ion‐trap setup. Our work reveals strong polarization of the π‐spacer (induced dipole/quadrupole) when it experiences the electric field from one/ two dyes. Hence, π‐spacers provide efficient shielding of charges by reducing the Coulomb interaction, whereas two dye cations polarize each other when connected by an alkyl. The screening influences the Förster resonance energy transfer efficiency that relies on the dipole–dipole interaction.

Gas‐phase Förster resonance energy transfer in mass‐selected and trapped ions

“…Clearly, the spectra become narrower and blueshift when the internal energy is reduced as reported in previous work (Greisch et al, 2013;Vogt et al, 2021). Emission band maxima for R575 + are at 504 ± 2 nm/495 ± 1 nm (room temperature/100 K) in good agreement with previously reported values (Forbes & Jockusch, 2011;Greisch et al, 2013;Petersen et al, 2020).…”

“…They count molecular ions where two (or three) dyes are covalently attached to a linker, either an alkyl (Dashtiev et al, 2005;V. Frankevich, Chagovets, et al, 2014;, π-spacers not in conjugation with the dyes (Dashtiev et al, 2005;Petersen et al, 2020;Zhao et al, 2021), peptides (Daly et al, 2014;Daly, Kulesza, Poussigue, et al, 2015;Kulesza et al, 2015Kulesza et al, , 2016Kulesza et al, , 2017, and even proteins Daly et al, 2017;R. Wu et al, 2022).…”

“…The fluorescence spectra of F1 2+ and F2 3+ are broader than that of R640 + monomers, which is likely associated with many contributing conformers, and the systems being rather flexible. Experiments were therefore done on more rigid FRET systems with π-conjugated acetylenic bridges ("rods"), not in conjugation with the dyes (Figure 15) (Petersen et al, 2020). The FRET pair was R575 + /RB + that has a larger spectral overlap than R575 + /R640 + .…”

“…FRET systems composed of R575 + and RB + linked by a π‐spacer (buta‐1,3‐diyne or diethynylbenzene). Reproduced from Petersen et al (2020), with permission from John Wiley and Sons. FRET, Förster Resonance Energy transfer.…”

“…It should be noted that the positive charge of the dye is evenly distributed over the dye but is for simplicity shown as a point charge in the center. Reproduced from Petersen et al (2020) and Zhao et al (2021), with permission from John Wiley and Sons. [Color figure can be viewed at wileyonlinelibrary.com]…”