J. zyxwvutsrqpo Am. Lnem. zyxwvu aoc. IYYV, zyxwvutsr I I c, JXIL-JJU I Relative Electron Donor Strengths of Tetrathiafulvene Derivatives: Effects of Chemical Substitutions and the Molecular Environment from a Combined Photoelectron and Electrochemical Study Dennis L. Lichtenberger,*.+ Roy L. Johnston,+.* Klaus Hinkelmann,s Toshiyasu Suzuki,* and Fred Wudl*** Contribution from the Laboratory for Electron Spectroscopy and Surface Analysis, Department of Chemistry, University of Arizona, Tucson, Arizona 85721, and the institute for Polymers and Organic Solids and Departments of Chemistry and Physics, University of California, Santa Barbara, California 93106. Received August 11, 1989 Abstract: Interest in organic metals and superconductors has prompted studies of the effects of chemical substituents on the organic electron donor tetrathiafulvalene (TTF). Electron-donating substituents on TTF should lead to reduced ionization potentials and generally greater electron transfer in organic donor/acceptor compounds. However, the relative electron donor abilities may also be influenced by their molecular environment and intermolecular interactions. In order to address these questions, the valence ionization potentials of TTF and two derivatives, zyxwv bis(ethy1enedioxo)tetrathiafulvalene (BEDO-TTF) and bis(ethy1enedithio)tetrathiafulvalene (BEDT-TTF), have been measured in the gas phase by photoelectron spectroscopy and compared with oxidation potentials from solution electrochemical measurements in a variety of solvents. The order of decreasing first ionization potentials is BEDT-TTF zyxwvuts = TTF (6.7 eV) > BEDO-TTF (6.46 eV). However, the order of solution oxidation potentials is BEDT-TTF > BEDO-TTF > TTF. The solvent dependence of these oxidation potentials scales linearly with the cation solvation power of the solvent, expressed as the difference between the first and second oxidation potentials of BEDO-TTF. Extrapolation to the zero-solvation limit (Le., the gas phase) reproduces the order of ionization potentials determined by photoelectron spectroscopy. When predicting whether a derivative is a better electron donor than TTF, it is therefore necessary to consider the molecular environment of the donor molecule. The nature of solvation and/or interaction with the acceptor molecules may result in trends that are opposite to those based purely on ionization potentials or theoretical calculations on single isolated molecules. The past decade has witnessed the growth of the field of organic metals and, more recently, organic superconductors formed as solid compounds of electron donor and acceptor molecules.' Tetra- thiafulvalene (TTF) (la) and its derivatives have been the most TTF BE DO la lb BEDT IC widely investigated electron donors.] We have recently reported a derivative of TTF that has sp3 hybridized oxygen atoms directly attached to TTF.2 This compound, bis(ethy1enedioxo)tetra- thiafulvalene (BEDO-TTF, further abbreviated to BEDO) (lb), which is the oxygen analogue of the well-known bis(ethy1enedi- thio)tetrathiafulvalene (BEDT-TTF, or BEDT)3 (IC), was syn- thesized in the belief that ?r donation from the oxygen substituents into the TTF x system will lower the first ionization energy, thereby making the substituted TTF a better electron donor than TTF itself. In this way it is hoped to make donor/acceptor solids with superior conducting and superconducting properties.2 In order to evaluate the effect of derivatization on the ionization of TTF, we report here the gas-phase photoelectron (PE) spectrum of BEDO and BEDT, together with a remeasurement of the spectrum of 7TF. Better electron-donating capabilities are favored if the first ionization potential (IP) is lowered. Qualitatively, the ' University of Arizona. rSERC/NATO Postdoctoral Fellow 1988-1989. 1 University of California. substitution of the hydrogen atoms on the external carbons of TTF by oxygen atoms can have two effects. First, A donation into the high-lying occupied x molecular orbitals (MOs) of TTF should raise them, thereby lowering the IPS. However, substituting hydrogen with the more electronegative oxygen can also result in a stabilization of the 'ITF MOs by withdrawing electron density through the u framework. The relative strengths of these two effects will determine the direction of the shift of the IPS. In addition to qualitative MO arguments, the results of MO calcu- lations of the extended Hiickel and Fenske-Hall varieties are used to help understand differences in the PE spectra. Another factor capable of influencing the relative ease of electron donation is likely to be the environment of the molecules in the donor/acceptor compound. This is essentially a solid-state solvation effect and depends on crystal packing and charge localization. An indication of the importance of solvation effects can be obtained by comparing solution-phase electrochemical oxidation potentials with the ionization potentials determined in the gas phase. It has been found, for example, that the electro- chemical oxidation of BEDO to its radical cation in acetonitrile/O.l M (TBAP) F6 (tetrabutylammonium hexafluorophosphate) occurs at higher potentials than for TTF,2 in contrast (vide infra) to the results of photoelectron spectroscopy and molecular orbital cal- culations that are presented in this paper. In order to test the assumption that this discrepancy is due to solvation effects, we investigate the oxidation processes for TTF, BEDO, and BEDT in a variety of solvent/electrolyte systems. The oxidation potentials are determined and related to various solvent parameters. The (1) (a) Proceedings of the International Conference on Synthetic Metals ICSM '88; Shirakawa H., Yamabe, T., Eds.; Synth. Met. 1988, 27. (b) Proceedings of the International Conference on Synthetic Metals ICSM '86; Shirakawa, H.; Yamabe, T., Eds.; Synth. Met. 1987, 17-19. (c) Chen, W.; Cava, M. P.; Takassi, M. A.; Metzger, R. M. J. Am. Chem. Soc. 1988,110, 7903. (2) Suzuki, T.; Yamochi, H.; Srdanov, zyx G.; Hinkelmann, K.; Wudl, F. J. Am. Chem. Soc. 1989, 111, 3108. (3) Kobayashi, H.; Kobayashi, A.; Sasaki, Y.; Saito, G.; Inokuchi, H. Bull. Chem. Soc. Jpn. 1986, 59, 301.