1. Dorothy Butler, Anthony Petty II, Dr. Tom Guarr*
Michigan State University Bioeconomy Institute, Holland, MI 49424
Introduction:
Bispyridinium compounds with extended π-systems were synthesized as potential organic materials
for molecular electronics. Extended pyridiniums are easily doped with electrons, this feature is essential
in applications for molecule-based electronics.1 Organic materials are easier to synthesize and have
easily tunable electronic properties. Also, organic materials are lower in manufacturing costs and better
for the environment than traditional inorganic materials.
Previous work in our lab has focused around the synthesis of phenylene-bridged π-extended
bispyridinium compounds for an organic electronics application. By varying the pyridinium nitrogen
substituent we have been able to tune the molecules electrochemical and photophysical properties. We
have varied the ortho substituents from phenyl to 4-tolyl in order to investigate the effect of electron
donation from the 2 and 6 positions.
Synthesis :
Synthesis of the bispyridinium was achieved by first synthesizing the pyrylium which was then reacted
with two equivalents of a primary amine to give the bispyridinium product.
Aldol Condensation:
One mole of terephthalaldehyde was reacted with two moles of p-methylacetophenone to form the 1,4-
phenylene bridged α-β unsaturated diketone.
Michael Addition:
2
The aldol product was then reacted with two additional equivalents of p-methylacetophenone to form
the 1,4-phenylene bridged tetraketone. Reaction took place under reflux to form the thermodynamic 1,4
Michael product, followed by an acid workup.
Pyrylium Formation:
The pyrylium is produced by reacting the 1,4-phenylene bridged tetraketone with triflic acid and
triphenyl methanol.
Bispyridinium Formation:
The final products are π-extended N-butyl and N-phenyl bispyridiniums that are generated by the
reaction between the pyrylium and two equivalents of a primary amine.
References:
1. V. Kolivoska. “Electron Dopable Molecular Wires Based on the Extended Viologens”. Phys. Chem.
Chem. Pys., 13, 11422-11429, 2011
2. W. Porter. “Synthesis and Characterization of a Highly Redcuing Neutral “Extended Viologen” and
the Isostructure Hydrocarbon 4,4’’’’ Di-n-octyl-p-quaterphenyl”. Journal of American Chemical
Society, 127, 16559-16566, 2005
KOH
KOtBu
2
2
2
Electrochemical Results:
+2e- +2e-
+2e- +2e-
Figure 1: Proposed reduction schemes for para-bispyridiniums
-6.5
-5.5
-4.5
-3.5
-2.5
-1.5
-0.5
0.5
1.5
2.5
3.5
-1900 -1700 -1500 -1300 -1100 -900 -700 -500 -300 -100
Current(uA)
Potential (mV)
Figure 2: Cyclic Voltammetry for N-Phenyl Bispyridinium. Two largely reversible, two
electron reductions occur at -681 and -1406 mV. Reductions are shifted slightly more negative in
comparison to the non-methylated phenylene bridged bispyridiniums. (0.2 M TEABF4 in MeCN
vs. Ag/AgCl)
Figure 3: Cyclic Voltammetry for N-Butyl Bispyridinium. Two largely reversible, two
electron reductions occur at -749 and -1841 mV. Reductions shifted slightly more negative in
comparison to the non-methylated phenylene bridged bispyridiniums. (0.2 M TEABF4 in MeCN
vs. Ag/AgCl)
-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
-2300 -2100 -1900 -1700 -1500 -1300 -1100 -900 -700 -500 -300 -100
Current(uA)
Potential (mV)
Abstract:
Para-phenylene bridged bispyridinium compounds with N-butyl and N-phenyl substituents were
successfully synthesized and electrochemically characterized. Cyclic voltammetry for both species
exhibited a multistep four electron reduction; this process is largely reversible under
anhydrous/anaerobic conditions. The first cathodic peak corresponds to an overall two electron
reduction which creates a neutral quinoidal structure, followed by a second two electron reduction that
results in a relatively stable dianionic species. 2 The introduction of methyl substituents on the molecular
periphery appears to have only minor effects on the electrochemical behavior. The target compounds
also display interesting photochemistry, and preliminary results from fluorescence studies will be
presented.
Conclusion:
Electrochemical and fluorescence properties of N-substituted para-phenylene bridged bispyridiunium
compounds have been studied. Both species synthesized were analyzed by cyclic voltammetry and
exhibited a multistep, four electron, largely reversible reduction. The first cathodic peak corresponds to
an overall two electron reduction which creates a neutral quinoidal species, followed by a second two
electron reduction that results in a relatively stable dianionic species. By varying the pyridinium nitrogen
substituent we were able to investigate the effects of electron donation toward redox processes.
Reductions shifted to more positive potentials as the electron withdrawing character of the nitrogen
substituent increases. The N-butyl compound had reduction potentials at -749 and -1841 mV, in
comparison, the N-phenyl an electron withdrawing group had potentials at -681 and -1406 mV. The
introduction of methyl substituents on the molecular periphery appears to have only minor effects on the
electrochemical behavior. The 2,6 phenyl substituted phenylene bridged bispyridiniums previously
synthesized and analyzed in our lab demonstrated relatively insignificant differences in electrochemical
behavior.
The fluorescence’s data for N-Phenyl was complicated by a decomposition pathway that led to several
emissive photoproducts. On the other hand, N-butyl compound displayed a broad emission band in the
blue range and a quantum yield of 0.39.
Acknowledgements:
Hope College Chemistry Department
Michigan State University Bioeconomy Institute Staff
This project was made possible in part by a generous grant from the Holland/Zeeland Community
Foundation. Financial support from the Michigan Strategic Fund through Lakeshore Advantage to
Michigan State University is also gratefully acknowledged.
Fluorescence:
0
20000
40000
60000
80000
100000
120000
140000
160000
340 390 440 490 540 590 640 690 740
Intensity
Wavelength (nm)
N-Butyl Bispyridinium Emission in Acetonitrile
Excitation @ 328 nm
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
350 400 450 500 550 600 650 700
Intensity
Wavelength (nm)
N-Phenyl Bispyridinium Emission in Acetonitrile
Excitation @ 334 nm
Figure 4: Emission Spectra of N-Phenyl and N-Butyl Bispyridiniums. Emission spectra were
taken in acetonitrile, maintaining the excitation wavelength at the absorbance peak wavelength of the
compound. The emissive intensity was measured across a range of wavelengths from approximately
350-750 nm.