Mohammed Al-anber, Bianca Milde, Wasim Alhalasah, Heinrich Lang, and Rudolf Holze

Technische Universität Chemnitz, Institut für Chemie, AG Elektrochemie, 09107 Chemnitz, Germany 

Thiophenes and their substi-tuted relatives are well studied building blocks of intrinsically conducting polymers in elec-trochemistry and materials sci-ence. In inorganic chemistry, in particular in coordination che-mistry, they are considered as connecting units bridging e.g. metal centers in organome-tallic compounds. Such oligo- or 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.

Introduction

Cyclic voltammetry (CV): One-compartment glass cell; platinum disc (1 mm2) working electrode; acetonitrile + 0.1 M tetra-n-butyl ammonium hexafluorophosphate (TBFP); all electrode po-tentials 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.

Theory vs Experiment:Electrochemical and DFT-Studies of Substituted Thiophenes

Experimental

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.

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 satis-factory). The predictive capability is more general than the empirical Hammett-approach.

Correlation of electro-chemical oxidation poten-tials of compounds 1 - 3 and 4 - 10 with HOMO en-ergies.

Correlation of HOMO-LUMO energy differences with UV-Vis absorption maxima of compounds 1 - 3 and 5 - 9.

Conclusions and Outlook

1 T h i o p h e n e

2 2 , 2 ’ - B i t h i o p h e n e 9

3 2 , 2 ’ : 5 ’ , 2 ’ ’ - T e r t h i o p h e n e

4 3 - ( 2 - T h i e n o y l ) - 1 , 1 , 1 -t r i f l u o r a c e t o n e

5 5 - ( 1 , 3 - D i o x o - 4 , 4 , 4 -t r i f l u o r o b u t y l ) - 2 , 2 ’ -b i t h i o p h e n e

6 5 - ( 1 , 3 - D i o x o - 4 , 4 , 4 -t r i f l u o r o b u t y l ) - 2 , 2 ’ : 5 ’ , 2 -t e r t h i o p h e n e

7 5 , 5 ’ - B i s ( 1 , 3 - d i o x o - 4 , 4 , 4 -t r i f l u o r o b u t y l ) - 2 , 2 ’ -b i t h i o p h e n e

8 2 - C h l o r o - 5 - ( 1 , 3 - d i o x o -4 , 4 , 4 - t r i f l u o r o b u t y l ) -t h i o p h e n e

9 2 , 5 - D i c h l o r o - 3 - ( 1 , 3 - d i o x o -4 , 4 , 4 - t r i f l u o r o b u t y l ) t h i o -p h e n e

1 0 3 - B r o m o - 2 - n i t r o t h i o p h e n e

1 1 3 , 3 ’ ’ - D i n i t r o - 2 , 2 ’ : 5 ’ , 2 - t e r -t h i o p h e n e

1 2 A c e t y l a c e t o n e

1 3 H e x a f l u o r a c e t y l a c e t o n e

S

S S

S S S

S

O

C F 3

O

S S

O

C F 3

O

S S

O

C F 3

O

S

S S

O

C F 3

OO O

F 3 C

S

O

C F 3

O

C l

SC l

O

C F 3

O

C l

S N O 2

B r

S S S

O 2 NN O 2

O

C H 3

O

H 3 CO

C F 3

O

F 3 C

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 appli-cations can be simplified substantially, when predictive tools from theoretical chemistry are employed. We have used density functional theory (DFT) to calculate electron affinities and ioni-zation potentials of the selected molecules.

Results and Discussion

Experimental and Calculated Data on Compounds 1 - 13 # Ered

/V Eoxid1/V E0/V Eoxid2/V Ei/eV Ea/eV HOMO/eV LUMO/eV ΔE/eV λmax1/nm λmax2/nm 1 - - - 1.32* 6.486 0.408 -6.486 -0.408 6.078 216 - 2 - - - 0.91 5.709 1.373 -5.709 -1.373 4.336 303 246; 193 3 - - - 0.68 5.345 1.814 -5.345 -1.814 3.531 383 281; 222 4

- 1.38 - 0.81 -1.095

1.88* 7.208 2.502 -7.208 -2.502 4.706 - -

5 - 1.68 - - 1.49 6.031 2.685 -6.031 -2.685 3.346 390 274 6 - 1.68 - - 0.93 5.763 2.788 -5.763 -2.788 2.975 421 266 7 - 1.2 1.64 6.688 3.430 -6.688 -3.430 3.258 410 285 8 - 1.30 - 0.90 1.7 7.245 2.733 -7.245 -2.733 4.512 338 212 9 - 1.55 - - - 7.390 2.735 -7.390 -2.735 4.655 331 247 10 - 1.43 1.7 7.593 3.292 -7.593 -3.292 4.301 - - 11 - 1.34 1.45 6.426 3.563 -6.426 -3.563 2.863 - - 12 - - - - - 1.230 -6.830 -1.230 5.600 272 210 13 - 1.068 - - - 3.143 -8.685 -3.143 5.542 304 -

-2 0 2-400

-200

0

200

400

600

4 5 6

I / A

EFeC

/ V

-2 0 2-1400

-1200

-1000

-800

-600

-400

-200

0

200

400

600

4 8 9 (second cycle)

I / A

EFeC

/ VStimulating 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.

Acknowledgements

Correlation of electro-chemical reduction poten-tials of compounds 4 - 9 with LUMO energies.

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