Intrinsic optical properties of several rhodamine cations were probed by measuring their dispersed fluorescence spectra in vacuo. Three different rhodamine structures were investigated, each with four different chalcogen heteroatoms. Fluorescence band maxima were blue-shifted by between 0.15 and 0.20 eV (1200-1600 cm(-1)) relative to previous solution-phase measurements. Trends in emission wavelengths and fluorescence quantum yields previously measured in solution are generally reproduced in the gas phase, confirming the intrinsic nature of these effects. One important exception is gas-phase brightness of the Texas Red analogues, which is significantly higher than the other rhodamine structures studied, despite having similar fluorescence quantum yields in solution. These results expand the library of fluorophores for which gas-phase photophysical data is available, and will aid in the design of experiments utilizing gas-phase structural biology methods such as Forster resonance energy transfer.
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