Theory vs Experiment: Electrochemical and DFT-Studies of Substituted Thiophenes Mohammed Al-anber, Bianca Milde, Wasim Alhalasah, Heinrich Lang, and Rudolf Holze Technische Universität Chemnitz, Institut für Chemie, AG Elektrochemie, 09107 Chemnitz, Germany Introduction Thiophenes and their substituted relatives are well studied building blocks of intrinsically conducting polymers in electrochemistry and materials science. In inorganic chemistry, in particular in coordination chemistry, they are considered as connecting units bridging e.g. metal centers in organometallic compounds. Such oligoor polythiophene wires either end-capped or with in-chain transition metal chromophores are of interest for e.g. potential use in molecular electronics as active layers. Thiophene S 2 9 2,2’-Bithiophene S S 3 2,2’:5’,2’’-Terthiophene S 3-(2-Thienoyl)-1,1,1trifluoracetone 5-(1,3-Dioxo-4,4,4trifluorobutyl)-2,2’bithiophene 5-(1,3-Dioxo-4,4,4trifluorobutyl)-2,2’:5’,2terthiophene 5,5’-Bis(1,3-dioxo-4,4,4trifluorobutyl)-2,2’bithiophene 2-Chloro-5-(1,3-dioxo4,4,4-trifluorobutyl)thiophene 4 5 6 7 8 S CF3 O O S CF3 S S O S O S O O Cl S S O CF3 O O S F 3C 2,5-Dichloro-3-(1,3-dioxo4,4,4-trifluorobutyl)thiophene Cl 9 S O O CF3 O CF3 S O O CF3 S Br Cl 10 3-Bromo-2-nitrothiophene S 11 3,3’’-Dinitro-2,2’:5’,2-terthiophene 12 Acetyl acetone S NO2 NO2 O2N O H3C O 13 Hexafluoracetyl acetone F3C S O S O CH3 CF3 # 1 2 3 4 Ered /V - 5 6 7 8 9 10 11 12 13 - 1.68 - 1.68 - 1.2 - 1.30 - 1.55 - 1.43 - 1.34 - - 1.38 Experimental and Calculated Data on Compounds 1 - 13 Eoxid1/V E0/V Eoxid2/V Ei/eV Ea/eV HOMO/eV LUMO/eV ΔE/eV 1.32* 6.486 0.408 -6.486 -0.408 6.078 0.91 5.709 1.373 -5.709 -1.373 4.336 0.68 5.345 1.814 -5.345 -1.814 3.531 1.88* 7.208 2.502 -7.208 -2.502 4.706 - 0.81 1.095 1.49 6.031 2.685 -6.031 -2.685 3.346 0.93 5.763 2.788 -5.763 -2.788 2.975 1.64 6.688 3.430 -6.688 -3.430 3.258 - 0.90 1.7 7.245 2.733 -7.245 -2.733 4.512 7.390 2.735 -7.390 -2.735 4.655 1.7 7.593 3.292 -7.593 -3.292 4.301 1.45 6.426 3.563 -6.426 -3.563 2.863 1.230 -6.830 -1.230 5.600 1.068 3.143 -8.685 -3.143 5.542 400 380 3 360 8 340 Correlation of HOMO-LUMO energy differences with UVVis absorption maxima of compounds 1 - 3 and 5 - 9. 9 320 300 2 280 260 240 cpds. 1 - 3 lin. fit for cpds. 1 - 3 cpds. 5 - 9 lin. fit for cpds. 5 - 9 180 2 1 4 6 E / eV 2.0 4 1.8 10 8 7 1.6 5 11 Correlation of electrochemical oxidation potentials of compounds 1 - 3 and 4 - 10 with HOMO energies. E ox,FeC / V 1.4 1 1.2 cpds. 1 - 3 lin. fit for cpds. 1 - 3 cpds. 4 - 10 lin. fit for cpds. 4 - 10 1.0 6 2 3 0.6 -7.5 600 600 400 400 200 0 -200 I / A I / A 200 -400 4 8 9 (second cycle) -600 0 -800 -1000 -200 -1200 -400 -7.0 -6.5 -6.0 -5.5 EHOMO / eV But: -0.5 -1.0 7 8 Correlation of electrochemical reduction potentials of compounds 4 - 9 with LUMO energies. E ox,FeC / V 4 9 -1.5 6 5 -2.0 -2.5 -3.6 -3.4 -3.2 -3.0 -2.8 -2.6 -2.4 ELUMO / eV Conclusions and Outlook DFT calculations yield HOMO and LUMO energies in good agreement with experimental data obtained from UV-Vis spectroscopy and cyclic voltammetry (in case of LUMO energies and reduction potentials agreement is not yet satisfactory). The predictive capability is more general than the empirical Hammett-approach. Results and Discussion 4 5 6 274 266 285 212 247 210 - 5 0.8 Cyclic voltammetry (CV): One-compartment glass cell; platinum disc (1 mm2) working electrode; acetonitrile + 0.1 M tetra-n-butylammonium hexafluorophosphate (TBFP); all electrode potentials are converted to ferrocene/ferrocinium redox couple as reference point; scan rate of dE/dt = 200 mV s-1, 1 mM of the studied compound; T = 25 °C; argon 4.6. UV-Vis spectra: Gene-sys 6 spectrometer (Thermo Electron Corp.); 1 nm resolution, 1 mM in acetonitrile. DFT calculation of the three-parameter compound functional of Becke (B3LYP) with 6-31G(d) basis set was used to optimize the geometry as well as to calculate the ionization energies Ei and electron affinities Ea of neutral compounds. Ionization energies were computed as the energy differences between the neutral molecule and the respective radical cation (Koopman’s theorem) in which the radical cation has the same molecular geometry as the neutral molecule (the Frank-Condon state was assumed for the cations). Radical cations were treated as open shell systems (UB3LYP); Gaussian-98W software. 390 421 410 338 331 272 304 7 200 Experimental λmax2/nm 246; 193 281; 222 - 6 420 220 In conceivable applications both their electrochemical response and their electrooptical properies are of interest. We have synthesized a family of substituted thiophenes and studied their redox electrochemistry and UV-Vis spectroscopy. Optimization of molecular properties including tailoring for particular applications can be simplified substantially, when predictive tools from theoretical chemistry are employed. We have used density functional theory (DFT) to calculate electron affinities and ionization potentials of the selected molecules. λmax1/nm 216 303 383 - 440 max1 / nm 1 Acknowledgements -1400 -2 0 EFeC / V 2 -2 0 2 EFeC / V CVs of the platinum electrode in an electrolyte solution of 0.1 M TBFP in acetonitrile containing 1 mM of compounds as indicated, argon purged, dE/dt = 200 mV s-1. Stimulating discussions of substituent effects in organic chemistry with K. Banert and H.-J. Schäfer, generous donation of compounds 5 – 11 by Prof. S. Al-Taweel and financial support from the Fonds der Chemischen Industrie and the Deutsche Forschungsgemeinschaft (Graduiertenkolleg GRK 829/1) are gratefully appreciated.