Electron Attachment to Dye-Sensitized Solar Cell Components: Rhodanine and Rhodanine-3-acetic Acid

The energies of electron attachment associated with temporary occupation of pi* and sigma* virtual orbitals of the pentaheterocyclic rhodanine molecule are measured in the gas phase with electron transmission spectroscopy. The corresponding orbital energies of the neutral molecule, supplied by B3LYP/6-31G(d) calculations and scaled using an empirically calibrated linear equation, are compared with the experimental vertical attachment energies. The same computational procedure is applied to rhodanine-3-acetic acid, proposed as a possible component of dye-sensitized solar cells. In addition, the (positive) vertical and adiabatic electron affinities are evaluated at the B3LYP/6-31-I-G(d) level. The calculations also indicate a thermodynamic tendency toward dissociation Of the ring H2C-S bond of the molecular anion. Dissociative electron attachment spectroscopy is used to measure the total anion current, as a function of the incident electron energy, and detect with a mass filter the negative fragments generated through the dissociative decay channels of the molecular anion in the 0-4 eV energy range. In rhodanine, only two intense negative fragments are observed due to loss of a hydrogen atom or a neutral ketene (H2C=C=O) molecule from the molecular anion. In the -CH2COOH derivative, in addition to these two dissociative channels, a great number of negative fragments are observed, mainly at zero electron energy, which often imply the occurrence of multiple bond cleavage and complex atomic rearrangements. These results cast serious doubts upon the stability of dyes containing rhodanine-3-acetic acid under conditions of excess negative charge.