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.