Presentation on theme: "Energetics of Charge Separation in Medium Polarity Solvents Brian Albert 1, Juan Carlos Alicea 2, Andrew R. Cook 3, Kate Dorst 2, John R. Miller 3, Lori."— Presentation transcript:
Energetics of Charge Separation in Medium Polarity Solvents Brian Albert 1, Juan Carlos Alicea 2, Andrew R. Cook 3, Kate Dorst 2, John R. Miller 3, Lori Zaikowski 2 1 Columbia University, 2 Dowling College, 3 Brookhaven National Lab, Chemistry Department Energetics of charge separation in highly polar media containing electrolyte are accurately determined from the differences of readily measured redox potentials. However, energetics change in less polar media due to changes in solvation energies, and can be crudely estimated using the Born equation or computational chemistry techniques, but are not readily measured. In order to provide accurate energies for charge separation reactions in specific systems and calibrations for computational chemistry techniques, this project examined the reduction of quinones in the presence and absence of an electrolyte and as a function of solvent polarity. Reactions were studied via UV/VIS spectroscopy and conductivity. Introduction For moderately unfavorable reactions - AQ, EtAQ, and AN(CN) 2 - electrolyte reduces the energy of forming ions in acetonitrile. For highly unfavorable reactions – fluorenone and phenazine - and fav- orable reactions – BQ, ClBQ, and Me 4 BQ – electrolyte does not affect the energy of forming ions. Charge transfer complexes are observed, but only when no ions form. Decrease in conductivity at lower solvent polarity indicates that fewer free ions are formed. This may be due to fewer ions being formed and/or that ion pairs are favored. Future investigations include spectroscopic experiments on these molecules in less polar solvents such as THF and butyl ether, and expanding the conductivity experiments to include the molecules studied spectroscopically. Examination of energetics of electron transfer in medium and low polarity solvents for quinone- metallocene redox pairs should enable generalizations to be made about such energies for other molecules as a function of solvent polarity. Dr. Sean McIlroy for his advice and help around the lab. Staff of Office of Educational Programs and BNL Chemistry Dept. NSF Award #03-35799 and NSF Supplemental Funding for Faculty and Student Team JCA and LZ. DOE DE-AC02-98-CH10886 supported AC, KD, JM. Acknowledgments Eight quinones (Fig. 1) were titrated with cobaltocene Cocp 2 as a reducing agent in acetonitrile ( = 38) or THF ( = 7.6) with and without the electrolyte tetrabutylammonium tetrafluoro-borate (TBABF 4 ). 100 uM quinone solutions were titrated in a dry Argon atmosphere in 1 cm path length quartz cuvettes with Cocp 2 solutions of 3-10mM. Continuous wave spectra from 200-900 nm were recorded with an OceanOptics spectrometer. Anion peaks were identified by comparision with literature spectra of the anions obtained electrochemically. To calculate K 2 the ratio of free ions to ion pairs was determined by conductivity with a Scientifica 645 meter. While ion pairs and anions have similar absorption spectra, only free ions conduct. 20, 60, and 200 uM solutions of quinone in acetonitrile or THF were titrated with solutions of Cocp 2 and decamethylferrocene (FeCp* 2 ) Spectroscopic Results Spectroscopic Methods Conclusions Quinone anion absorbances shown as a function of cobaltocene to quinone ratio. MeCN solvent. Fluorenone and phenazine spectra indicated charge transfer complexes but no anion formation. BQ and ClBQ showed a decrease in anion absorbance after 1 equivalent. Quinone (A)Metallocene (D) Conductivity Results Dissociation of quinone-metallocene ion pairs was measured by continuous conductivity and comparison with spectra. With a decrease in solvent polarity, lower conductivity was observed than expected based on spectroscopic anion absorbance. Conductivity of BQ, F 4 BQ, and Cl 4 BQ titrated with Cocp 2 and Fecp* 2 in acetonitrile indicates complete ion dissociation. However, in THF the reactions yielded only a fraction of free ions. Molar conductivity of 167 uM (F 4 BQ) with 1 eq. Cocp 2 indicates: in THF: K 1 for formation of ion pairs = 4.26 x 10 4 (∆Gº = -26.4 kJ/mol) and K 2 for dissociation into free ions = 2.7 x 10 -7 (11 kJ/mol). Hence 75% of reactants form ion pairs, but only 3.28% separate into free ions. Energetics Electron acceptor (A) and electron donor (D) BQ + Cocp 2 BQ - + Cocp 2 + K 1 K 2 A + D (A -, D + ) A - + D + neutral molecules ion pair free ions Conductivity Methods MeCN BQ - a Measured in DMF with TBABF 4 b Prince, R.C., Gunner, M.R., and Dutton, P.L. 1982. Quinones of Value to Electron-Transfer Studies: Oxidation-Reduction Potentials of the First Reduction Step in an Aprotic Solvent. In Function of Quinones in Energy Conserving Systems (B.L. Trumpower, ed.) Academic Press, New York, 29-33. c Pedersen S. U.; Christensen T. B.; Thomasen T.; Daasbjerg K. 1998. J. Electroanal. Chem. 454, 123. d Calculated from Eº(Cocp 2 ) vs SCE = -900 mV. Bard AJ; Garcia E. 1993. Inorg. Chem. 32, 3528. e Peover, M.E. and Davies, J.D. 1964. Trans. Faraday Soc. 60, 476. Extinction coefficients determined from fits of anion absorbance data. E 1/2 (mV/SCE) a, b BQ = -400 F 4 BQ = +100 Cl 4 BQ = +140 Cocp 2 = -900 Fecp* 2 = -80 Benzoquinone 1 (BQ), chlorobenzoquinone 2 (ClBQ), tetramethylbenzoquinone 3 (Me 4 BQ), fluorenone 4, anthraquinone 5 (AQ), ethylanthraquinone 6 (EtAQ), dicyanoanthracene 7 An(CN) 2, phenazine 8.