Triangular Rhodamine Triads and Their Intrinsic Photophysics Revealed from Gas‐Phase Ion Fluorescence Experiments

Abstract

When ionic dyes are close together, the internal Coulomb interaction may affect their photophysics and the energy-transfer efficiency. To explore this, we have prepared triangular architectures of three rhodamines connected to a central triethynylbenzene unit (1,3,5-tris(buta-1,3-diyn-1-yl) benzene) based on acetylenic coupling reactions and measured fluorescence spectra of the isolated, triply charged ions in vacuo. We find from comparisons with previously reported monomer and dimer spectra that while polarization of the πsystem causes redshifted emission, the separation between the rhodamines is too large for a Stark shift. This picture is supported by electrostatic calculations on model systems composed of three linear and polarizable ionic dyes in D 3h configuration: The electric field that each dye experiences from the other two is too small to induce a dipole moment, both in the ground and the excited state. In the case of heterotrimers that contain either two rhodamine 575 (R575) and one R640 or one R575 and two R640, emission is almost purely from R640 although the polarization of the π-system expectedly diminishes the dipole-dipole interaction.

Cryogenic Ion Fluorescence Spectroscopy: FRET in Rhodamine Homodimers and Heterodimers

“…Based on this geometry, we calculated the shift in transition energy as a function of θ numerically by minimizing the potential energy before and after photoexcitation (Figure 3b). The underlying physics behind the calculations has been described in previous work for similar model systems [8,14, 15] . Briefly, the charge is taken to be in the center of the dye in the absence of an electric field, and the redshift in emission is ascribed to a larger polarizability in S 1 than in S 0 .…”

“…The underlying physics behind the calculations has been described in previous work for similar model systems. [8,14,15] Briefly, the charge is taken to be in the center of the dye in the absence of an electric field, and the redshift in emission is ascribed to a larger polarizability in S 1 than in S 0 . Hence in S 1 the two charges better avoid each other than in S 0 .…”

“…In previous work, we explored the fluorescence from homo‐ and heterodimers of rhodamine cations using our home‐built LUNA (LUminescence iNstrument in Aarhus) setup [8,11,14] . As a rhodamine dye is positively charged and has a high polarizability along its long axis, it is very sensitive to the nearby environment [6a,7a,8,14,15] . Indeed, we found that for a dimer, each dye senses the electric field from the other, which results in red‐shifted emission (Stark shift) compared to monomer dyes [14] .…”

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

“…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).…”

“…Hence the effect of shielding by the π-system seems less important than in the dyads discussed above, which could simply be due to the larger unfavorable interaction between three induced dipoles than between just two (Figure 16). However, reorientation factors should also be considered, and these are likely larger for triads than for dyads (Zhao et al, 2021). Finally, but not least, gas-phase tmFRET experiments were recently done by Zenobi and coworkers R.…”