10.1002/chem.201701412
Chemistry - A European Journal
COMMUNICATION
phosphorus center. In particular the methylphosphonium salt 5 is
a strong reversible two-electron acceptor comparable with, for in-
stance, p-benzoquinone or methylviologen which are reduced at
approximately –1.00 V under similar conditions.[26] Based on thor-
ough X-ray crystallographic analysis in conjunction with spectro-
scopic, electrochemical, and theoretical studies, we were not only
able to assess the capability of phosphorus to participate in π-
conjugation with the dicyanomethylene acceptor but also to en-
lighten its crucial role for the unique redox properties of these
compounds. Our findings suggest considerable potential of these
stable electron-deficient scaffolds towards the development of
functional n-type materials and redox active chromophores upon
further functionalization, both aspects that are currently being pur-
sued in our laboratories.
Acknowledgements
Figure 4. Kohn-Sham frontier molecular orbitals (FMOs; ωB97XD/def2-TZVP);
5 was calculated in the presence of the counterion.
This work was supported by the Deutsche Forschungsgemein-
schaft (DFG) as part of SFB 953 “Synthetic Carbon Allotropes”.
The “Solar Technologies Go Hybrid” (SolTech) initiative of the
Free State of Bavaria and the Graduate School Molecular Science
(GSMS) of FAU Erlangen-Nürnberg are gratefully acknowledged
for their generous support. P.O.D. thanks Mario Barbatti and Dan-
iele Fazzi for discussions and technical support.
The second reduction event becomes further facilitated upon
functionalization of the phosphorus center, reaching –1.18 V for
the phosphonium salt 5. Moreover, the potential difference be-
tween the two reductions is considerably narrowed when replac-
ing the nitrogen in S1 (530 mV) with the phosphorus in 2 (250 mV).
These observations clearly support the strong electronic and
structural impacts of the phosphorus moiety on the dicyanometh-
ylene-bridged π-system, resulting in significantly stabilized LUMO
energy levels and decreased electrostatic repulsion in the nega-
tively charged states.[25] The calculated adiabatic electron affini-
ties (EAs) nicely reflect the experimentally observed increase of
the electron-acceptor strengths when going from amine S1 to
phosphonium cation 5 (Table S6).[19]
Keywords: phosphorus heterocycles – electron acceptors –
reduction – push-pull – cyano
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1.49
1.74
1.30
1.53
1.26
1.44
S1 (N)
(P)
2
3
4
(P=S)
(P=O)
(PMe)
0.96
1.18
[4]
[5]
5
-2.5
-2.0
-1.5
-1.0
-0.5
Potential [V]
Figure 5. Cyclic voltammograms of compounds S1, 2–5 (scan rate
100 mV⋅s–1) and 5 (different scan rates 50-1600 mV⋅s–1) in CH2Cl2 with 0.1 M
nBu4NPF6, vs. Fc/Fc+).
In summary, through a concise synthetic strategy we realized
a series of novel organic electron acceptors based on a dicy-
anomethylene-bridged acridophosphine scaffold. In contrast to
the nitrogen-containing analogue, the organophosphorus com-
pounds readily underwent remarkably facilitated reversible two-
electron electrochemical reductions with the reduction potentials
that were efficiently modulated through the derivatization of the
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