Gas-phase action and fluorescence spectroscopy of mass-selected fluorescein monoanions and two derivatives†
Gaseous fluorescein monoanions are weakly fluorescent; they display a broad fluorescence spectrum and a large Stokes shift. This contrasts with the situation in aqueous solution. One explanation of the intriguing behavior in vacuo is based on internal proton transfer from the pendant carboxyphenyl group to one of the xanthene oxygens in the excited state; another that rotation of the carboxyphenyl group relative to the xanthene leads to a partial charge transfer from one chromophore (xanthene) to the other (carboxyphenyl) when the π orbitals start to overlap. To shed light on the mechanism at play, we synthesized two fluorescein derivatives where the carboxylic acid group is replaced with either an ester or a tertiary amide functionality and explored their gas-phase ion fluorescence using the home-built LUminescence iNstrument in Aarhus (LUNA) setup. Results on the fluorescein methyl ester that has no acidic proton clearly disprove the former explanation: The spectrum remains broad, and the band center (at 605 nm) is shifted even more to the red than that of fluorescein (590 nm). Experiments on the other variant that contains a piperidino amide are also in favor of the second explanation as here the piperidino already causes the dihedral angle between the planes defining the xanthene and the benzene ring to be less than 90° in the ground state (i.e., 63°), according to density functional theory calculations. As a result of the closer similarity between the ground-state and excited-state structures, the fluorescence spectrum is narrower than those of the other two ions, and the band maximum is further to the blue (575 nm). In accordance with a more delocalized ground state of the amide derivative, action spectra associated with photoinduced dissociation recorded at another setup show that the absorption-band maximum for the amide derivative is redshifted compared to that of fluorescein (538 nm vs. 525 nm).