Electron Attachment to Dye-Sensitized Solar Cell Components: Cyanoacetic Acid

On delicate balance between formation and decay of tetracyanoethylene molecular anion triggered by resonance electron attachment

Low-energy (0-15 eV) resonance electron interaction with isolated tetracyanoethylene (TCNE) molecules is studied in vacuo by means of dissociative electron attachment (DEA) spectroscopy. Despite this molecule being relatively small, the long-lived molecular anions TCNE- are formed not only at thermal electron energy via a vibrational Feshbach resonance mechanism but also via shape resonances with the occupation of the π4* and π5* molecular orbitals by an incident electron. Dissociative decays of TCNE- are mostly observed at incident electron energy above the π7* temporary anion state predicted to lie at 1.69 eV by means of B3LYP/6-31G(d) calculations combined with the empirical scaling procedure. Electron attachment to the π6* orbital (predicted at 0.85 eV) leads to the generation of long-lived TCNE- species, which can decay via two competing processes: extra electron detachment, which appears in hundreds of microseconds, or elimination of two cyano groups to form the [TCNE - 2(CN)]- negative fragment on a tens of microsecond timescale. The latter is accompanied by the generation of a highly toxic cyanogen molecule as a neutral counterpart. Since the electron transfer to the acceptor molecule TCNE plays a key role in the formation of single-molecule magnets, the present data are of importance to understand the long-term behavior and likely harmful effects produced by cyanide-based prospective materials.

Microsecond dynamics of molecular negative ions formed by low-energy electron attachment to fluorinated tetracyanoquinodimethane

Low-energy (0-15 eV) electron interactions with gas-phase 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) molecules are studied under single collision conditions using dissociative electron attachment spectroscopy. The experimental findings are supported by density functional theory calculations of the virtual orbital energies and energetics of the dissociative decays. Long-lived molecular negative ions F4-TCNQ^- are detected in a wide electron energy range (0-3 eV) with electron detachment times in the range of milliseconds. Although plenty of decay channels are observed, their intensities are found to be very small (two to four orders of magnitude relative to the F4-TCNQ^- signal). These findings prove that the structure of this strong electron-accepting molecule bearing an excess electron is robust in its electronic ground state, even when highly (up to 6 eV) vibrationally excited. As many as nine metastable fragment anions formed slowly (in the 16-23 µs range) are found in the negative ion mass spectrum of F4-TCNQ, as never observed before in compounds possessing high electron-accepting ability. The present results shed some light on microsecond dynamics of isolated F4-TCNQ molecules under conditions of excess negative charge, which are important for understanding the functionality of nanoscale devices containing this molecule as a structural element.

Resonance electron attachment to tetracyanoquinodimethane

Resonance interaction of low energy (0-14 eV) electrons with gas-phase 7,7,8,8-tetracyanoquinodimethane (TCNQ) was investigated using dissociative electron attachment (DEA) spectroscopy. Spectral features associated with formation of long-lived TCNQ molecular negative ions are detected at incident electron energies of 0.3, 1.3, and 3.0 eV. A variety of negative fragments is observed around 4 eV, and slow (microseconds) dissociative decay channels are detected at about 3 eV, in competition with simple re-emission of the captured electron. The average electron detachment time from the TCNQ(-) negative ions formed at 3 eV was evaluated to be 250 μs. The experimental findings are interpreted with the support of density functional theory (DFT) calculations of the empty orbital energies, scaled with an empirical equation, and by comparison with earlier electron transmission spectroscopy (ETS) data. A possible mechanism for the unusual formation of long-lived molecular anions above zero energy (up to 3 eV) is briefly discussed. The present results on the interactions between electrons and isolated TCNQ molecules could give more insight into processes observed in TCNQ adsorbates under conditions of excess negative charge. In particular, electron-stimulated surface reactions are hypothesized, likely occurring when condensed TCNQ molecules are exposed to electron beam irradiation.

Comparative interface metrics for metal-free monolayer-based dye-sensitized solar cells

The first quantitative comparison between self-assembled monolayers of homologous carboxylate- and phosphonate-terminated organic dyes that are of use in dye-sensitized solar cells (DSSCs) is reported. (Cyanovinyl)phosphonate-terminated oligothiophenes and (cyanovinyl)carboxylate-terminated oligothiophenes were synthesized on TiO(2) thin film electrodes. Structurally analogous organics were compared for the effect of the anchoring groups on photochemical properties in solution as measured by UV/vis spectroscopy and for reactivity with the electrode surface. Monolayers were grown on the TiO(2) electrodes either by "tethering by aggregation and growth" (T-BAG) or by solution dipping. Surface roughness and homogeneity, elemental composition, and thickness of the monolayers were evaluated by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and ellipsometry. Molecular loadings for each monolayer on TiO(2) were quantified by quartz crystal microgravimetry (QCM), and the stability of bonding between each class of dyes and the TiO(2) was evaluated by measuring desorption, also by QCM; the carboxylates underwent significant dissociation in aqueous media but the phosphonates did not. DSSCs were prepared from each congener and from simple oligothiophene phosphonates to determine the effect of the cyanovinyl group on device behavior; all DSSCs were studied under irradiation from a AM 1.5G solar light source; the effect of cyanovinyl group termination was comparable to that of adding a thiophene moiety, and the DSSC using a self-assembled monolayer of (sexithiophene)phosphonate (6TP) had total power conversion efficiency (η) of ca. 5%.