Proximity-Induced Fluorescence Quenching in Rhodamine Systems <i>In Vacuo</i>: Effect of Charges and Aromatic Moieties

FRET is a valuable technique for exploring conformations of macromolecules in solution and in the gas phase. Donor fluorescence quenching is often identified from shortened excited-state lifetimes. However, when dyes are incorporated into proteins, the local microenvironment can affect the photophysics and energy transfer. To examine the effect of nearby charges and aromatic moieties on lifetimes, we investigated different cationic rhodamine-575 model systems <i>in vacuo</i>. In homodimers, the internal Coulomb repulsion induces a distance-dependent increase in lifetime, ranging from 5.90 ns (single dye) to 6.78 ns (shortest interdye linker), which we attribute to reduced oscillator strengths as corroborated by TD-DFT calculations. Our findings highlight that excited-state lifetimes are not necessarily directly correlated with fluorescence quantum yields, contrasting typical intuition. We discuss different quenching mechanisms and compare with results obtained in solution, as the opposite effect is observed in methanol, where excited-state lifetimes decrease upon bringing the dyes closer together. In the case of homotrimers in the gas phase, lifetimes systematically decrease with the number of nonprotonated (neutral) dyes. This suggests enhanced nonradiative decay rates driven by strong interactions between dyes and nearby aromatic moieties.