Laser-induced- and dispersed-fluorescence studies of rhodamine 590 and 640 ions formed by electrospray ionization: observation of fluorescence from highly-excited vibrational levels of S1 states.

Abstract

The gas-phase fluorescence excitation and emission properties of rhodamine 590 (R590) and rhodamine 640 (R640) cations were studied using a novel instrument that allows extension of optical excitation to the UV region. Ions were generated by an electrospray ion source, and desolvated ions were accumulated in a room-temperature quadrupole ion-trap. In addition to the well-known S1-S0 transition observed for both dyes, two ultraviolet (UV) bands (that have not been reported previously) were observed for R590. When compared with transitions observed for the dyes in methanol solutions, the S1-S0 transitions in the gas phase are shifted to significantly higher energies. These shifts suggest that the S1 states for both dyes are more stabilized by favorable solvent interactions than the S0 states. The UV bands observed for R590 were assigned to the transitions involving the higher-energy S3 and Sn states. The DF spectra obtained upon UV excitation are similar to those recorded for S1 excitation; however, the emission after UV excitation is shifted slightly to the red region. These observations are consistent with the emission from high-vibrational levels of the S1 state formed upon fast internal conversion from S3 or Sn. In vacuo, the absence of collisions extends the lifetimes of vibrationally-excited S1 states, allowing fluorescence from these states to be observed. A similar DF spectrum was also observed for the UV excitation of R640, suggesting that fluorescence from vibrationally hot S1 states is a general phenomenon for ions in the gas phase.