Gas-Phase Ion Spectroscopy of Congo Red Dianions and Their Complexes with Betaine

Congo Red (CR) is an azo dye that is negatively charged in aqueous solutions. Here we report on the intrinsic electronic properties of CR dianions from mass spectroscopy experiments on bare dianions and their complexes with betaine (B). As betaine is a zwitterion, it possesses a large dipole moment and is a good reporter on the sensitivity of CR to microenvironmental changes. Photoexcitation of CR^2- in the visible region resulted in several fragment ions after absorption of at least three photons, with major fragmentation routes due to breakage of one or both C-NN bonds, one azo linkage, and/or the bonds to sulfite. Their yields as a function of excitation wavelength reveal a broad absorption in the visible region with the lowest-energy band located at ∼500 nm. Features are observed with a spacing of ∼1500 cm^-1. One photon was sufficient to dissociate CR^2-·B, and its action spectrum was almost identical to those of CR^2- in accordance with previous findings that a symmetric ion is essentially unaffected by changes in its microenvironment. Electron detachment occurs in the UV with threshold energy of 3.6 ± 0.1 eV for CR^2- and 3.81 ± 0.06 eV for CR^2-·B. Attempts to measure fluorescence from photoexcited CR^2- were unsuccessful.