Multi-Stage Redox Systems Based on Dicationic P-Containing Polycyclic Aromatic Hydrocarbons

Article abstract of DOI:10.1002/chem.202001213

We report the straightforward synthesis of unprecedented electron-acceptors based on dicationic P-containing PAHs (Polycyclic Aromatic Hydrocarbons) based on copper mediated radical approach. In these systems, two phosphoniums are connected through various PAHs backbones. The impact of π-extension on both the optical and redox properties is investigated using a joint experimental (UV/Vis absorption, fluorescence and cyclic voltammetry) and theoretical approach (TD-DFT calculations). Finally, (spectro)-electrochemical studies prove that these compounds possess three redox states and EPR studies confirm the in situ formation of an organic radical delocalized on the PAH backbone.

Full text of DOI:10.1002/chem.202001213

A Journal of
Accepted Article
Title: Multi-stage redox systems based on Dicationic P-containing
Polycyclic Aromatic Hydrocarbons
Authors: Thomas Delouche, Antoine Vacher, Elsa Caytan, Thierry
Roisnel, Boris Le Guennic, Denis Jacquemin, Muriel Hissler,
and Pierre-Antoine Bouit
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202001213
Link to VoR: http://dx.doi.org/10.1002/chem.202001213
Supported by

10.1002/chem.202001213
Chemistry - A European Journal
COMMUNICATION
Multi-stage redox systems based on Dicationic P-containing
Polycyclic Aromatic Hydrocarbons
Thomas Delouche,[a] Antoine Vacher,[a] Elsa Caytan,[a] Thierry Roisnel,[a] Boris Le Guennic,[a] Denis
Jacquemin,[b]* Muriel Hissler,[a] and Pierre-Antoine Bouit[a]*
[a]
[b]
T. Delouche, Dr A. Vacher, Dr E. Caytan, Dr T. Roisnel, Dr B. Le Guennic, Prof. M. Hissler, Dr P. A. Bouit
Univ Rennes, CNRS, ISCR - UMR 6226
F-35000 Rennes, France
E-mail: pierre-antoine.bouit@univ-rennes1.fr
Prof. D. Jacquemin
CEISAM UMR CNRS 6230
2, rue de la Houssinière, University of Nantes, Nantes, France.
E-mail: denis.jacquemin@univ-nantes.fr
Supporting information for this article is given via a link at the end of the document.
Abstract: In this communication, we report the straightforward
synthesis of unprecedented electron-acceptors based on dicationic P-
containing PAHs (Polycyclic Aromatic Hydrocarbons) based on
copper mediated radical approach. In these systems, two
phosphoniums are connected through various PAHs backbones. The
impact of -extension on both the optical and redox properties is
The impact of -extension on both optical and redox properties is
investigated using a joint experimental/theoretical approach.
Finally, (spectro)-electrochemical studies prove that these
compounds possess three redox states and EPR studies confirms
the in situ formation of radicals demonstrating that these novel
acceptors behave as “viologen-like” redox systems.[4,6e]
investigated using
a
joint experimental (UV-Vis absorption,
fluorescence and cyclic voltammetry) and theoretical approach (DFT
calculations). Finally, (spectro)-electrochemical studies prove that
these compounds possess three redox states and EPR studies
confirms the in situ formation of an organic radical delocalized on the
PAH backbone.
Organic multi-stage redox systems are key component in many
technological fields ranging from organic electronics (solar cells,
batteries…) to spintronics.[1] Among the strategies used to design
such compounds, one approach consists in linking two cationic
heteroatoms through a -conjugated backbone featuring an even
number of sp2 C-atoms (Scheme 1).[2,3] This led to the preparation
of “Weitz type” derivatives which display three redox states
Scheme 1. General structure of multi-stage redox systems[3] and their
corresponding phosphorus containing analogs
including
a
stable radical cation intermediate.[2,3] Methyl-
Viologens (MV) are probably the most emblematic electron
acceptors of this family.[4,5] Although Hünig et al. mentioned in
1978 the applicability of this general strategy to systems in which
phosphorus is the unique heteroatom,[3] such derivatives have not
been described yet to the best of our knowledge, despite the wide
literature on organophosphorus based -systems and switches.[6-
8] Due to the specificities of P compared to the others heteroatoms
in terms of electronegativity, valence, coordination number, it
appeared interesting to us to study the impact of its insertion on
the optical/redox properties.[9]
To prepare the targeted bisphosphoniums, we rely on a copper-
mediated radical approach.[11] This method was recently used to
prepare -conjugated phosphoniums.[12] In our hands, the double
phosphacyclization of bisphosphine 1 cleanly afforded 1,8-
bisphosphapyrenium 2[OTf]2 (87% yield, Scheme 2), as an air
and moisture stable derivatives characterized by a single peak in
31P NMR ( = + 1.3 ppm). This compound was fully characterized
by multinuclear NMR, high-resolution mass spectrometry and X-
ray diffraction (vide infra). In addition, 2[OTf]2 is soluble in
classical organic solvents. To illustrate the versatility of this
To tackle this challenge, we here take advantage of the possibility
to convert a cationic 4,4-P+ into a neutral 4,5-P (Scheme 1) to
afford multi-stage redox systems based on organophosphorus
derivatives.[10] Indeed we report the general and straightforward
synthesis of an unprecedented family of dicationic P-containing
Polycyclic Aromatic Hydrocarbons (PAHs) 2-6[OTf]2 where two
4,4 P+ are connected through various -backbones (Scheme 2).
molecular
engineering
strategy,
various
polyaromatic
diphosphines were converted into their dicationic analogs 3-
6[OTf]2 in good yields (Scheme 2). All the precursors were
synthesized through lithiation of the corresponding dibromo-
derivatives
which
were
then
reacted
with
chlorodiphenylphosphine (see ESI). This allowed us to prepare a
novel family of dicationic P-containing PAHs featuring variable -
1
This article is protected by copyright. All rights reserved.

10.1002/chem.202001213
Chemistry - A European Journal
COMMUNICATION
cores. They all display two cationic 4,4 -P centers connected
through a bridge featuring an even number of sp2 C-atoms.
The spectroscopic properties of 2-6[OTf]2 are investigated in
diluted CH2Cl2 solutions (c = 5.10-6 mol.L-1, Fig. 2 and Table S3).
2-5[OTf]2 display structured absorption bands in the same window
with abs spanning from 417 to 432 nm. This absorption can be
attributed to a delocalized -* transition according to TD-DFT
calculations (Fig 2 and Fig. S13). While the size of the polycyclic
scaffold and the relative position of the P-atoms weakly affect the
maximum absorption wavelength, the vibronic progression and
the extinction coefficient are more dependent on the structure of
the molecule. In contrast, 6[OTf]2 displays a marked bathochromic
shift compared to 2[OTf]2 ( = 102 nm). These results illustrate
that the maximal wavelength of absorption of such compounds
cannot be easily rationalized by comparing the -extension. In this
case, it is determined by a complex interplay between the -
extension, the relative P-positions and the -curvature. [14] The
qualitative evolutions in the 2-6[OTf]2 series are reasonably
reproduced by TD-DFT (Table S3). Interestingly, the P centers
play a significant role in the transition in 2-5[OTf]2, whereas they
do not seem to be involved in 6[OTf]2 (Fig. S13). This is most likely
related to the non-planar character of the -core of 6[OTf]2 ,[15] and
is in line with the specific NICS(0) value noted for the anti-
aromatic P-rings (Fig. S12). More generally, the NICS(0)
calculations in this series nicely follow the Clar’s sextet rule for
polycyclic aromatic compounds (Fig. S12).
Scheme 2. Synthetic access to 2[OTf]2 and chemical structure of compounds
3-62+ (TfO- anions have been omitted for clarity)
60000
2[OTf]2
3[OTf]2
4[OTf]2
The structures of derivatives 2-6[OTf]2 were all confirmed by
single-crystal X-ray diffraction (Fig. 1, Table S1-2). In all
compounds, the 4,4-P atom shows a tetrahedral shape with
usual valence angles and C-P bond lengths. In 2-5[OTf]2, the
polyaromatic scaffold featuring the P-heterocycles is mainly
planar (maximum deviation from the mean C-sp2 plane, 0.32 Å),
and the lateral phenyl rings lie perpendicularly to the main plane.
The case of bisphosphaperylenium 6[OTf]2 is more peculiar as a
significant -curvature is observed (maximum deviation from the
mean C-sp2 plane, 0.78 Å, Fig. S1) due to steric hindrance
between H and Ph at the bay position.[13] This structural feature of
the solid state pertains in solution according to DFT calculations
(Fig. S17-19). Finally, no intermolecular contact could be
observed in the packing, which most likely results from the
presence of bulky tetrahedral P-atoms, lateral exocyclic phenyl
rings, and counterions.
40000
20000
0
5[OTf]2
6[OTf]2
325
425
525
 (nm)
1
0
400
600
800
1000
 (nm)
2
3
4
P
P
P
5
6
Figure 1. X-ray crystallographic structure of 2-62+ (H atoms, counterions and
solvent omitted for clarity).
Figure 2. UV-Vis absorption (up) and normalized emission spectra (middle) of
2-6[OTf]2 in CH2Cl2 (c = 5.10-6 M) Electron density difference computed for 22+
(down). The blue and red regions respectively indicate decrease and increase
of density upon photon absorption (contour threshold: 8 10-4).
2
This article is protected by copyright. All rights reserved.

10.1002/chem.202001213
Chemistry - A European Journal
COMMUNICATION
All derivatives display fluorescence in solution. In this case, the
emission wavelengths seem more affected by the molecular
structure, as a 41 nm shift is observed between 2[OTf]2 and
5[OTf]2. Again, a strong red-shift is observed for 6[OTf]2 (2-6
= 79 nm). All compounds display moderate photoluminescence
quantum yields PLQY (4 % <  < 19 %) with the lower value
unsurprisingly obtained for the red-emitting 6[OTf]2. Interestingly,
2-6[OTf]2 also display fluorescence in the solid-state (Fig S2 and
Table S3). It should be noted that 2-3[OTf]2 are blue-shifted in the
solid-state compared to the diluted solution while 4-6[OTf]2 are
mainly unchanged. This behaviour remains rather surprising as
no clear intermolecular interactions are observed in the X-ray
structures. The PLQY in solid state are higher than in solution
(11 %<  < 39 %) with higher values for 2[OTf]2 (39%) and 3[OTf]2
(37%). This overall increase in PLQY is attributed to the restriction
of rotations of the 8 lateral phenyl rotors that are present in each
molecule, and to the presence of tetrahedral P-atom which
prevent detrimental -stacking, as confirmed by the
crystallographic study.[16]
2[OTf]2
3[OTf]2
4[OTf]2
5[OTf]2
6[OTf]2
E (V vs Fc+/FC)
Figure 3. Cyclic voltammograms of 2-6[OTf]2 (c = 5.10-4 M) recorded in DCM
The electrochemical behavior of 2-6[OTf]2 was investigated by
cyclic voltammetry in dichloromethane solution. As anticipated by
the molecular design, while no oxidation waves are observed in
these conditions, all compounds display two distinctively
separated reduction waves at low potential (e.g., Ered1(2[OTf]2) =
-0.84 V vs Fc+/Fc and Ered2(2[OTf]2) = -1.23 V vs Fc+/Fc, Fig 3
and Table S3), suggesting the successive formation of a radical
cation and a neutral specie. While the first reduction wave is fully
reversible in all cases, the second reduction is quasi reversible for
2[OTf]2 and 4[OTf]2 (and reversible for the others). The CV traces
are not altered by repeated cycles (Fig. S4). However the
increase of the scan rates increases the reversibility of the waves
(Fig. S5), thus highlighting the role of electron transfer rate on the
stability of the reduced species. Interestingly, 2[OTf]2 displays
reduction potentials in the same range than MV[OTf]2 recorded in
the same conditions (Fig. S3). This clearly validates our design
strategy to prepare “viologene-like“ multi-stage redox systems.
The effect of the -core in the 2-6[OTf]2 series is rather
predominant with 6[OTf]2 having the less cathodic potential
(Ered1(6[OTf]2) = -0.64 V vs Fc+/Fc) and 5[OTf]2 the more
cathodic (Ered1(5[OTf]2) = -1.10 V vs Fc+/Fc) (Fig. 3). This trend
was already observed by Würthner et al. with comparable boron-
doped PAHs and confirms that the highest reduction potential in
these series do not correspond to the largest -surface.[14]
However, the presence of the charged P atoms renders the
dicationic P-containing PAHs considerably easier to reduce than
their neutral B-counterparts (Ered1(2[OTf]2) = -0.84 V vs Fc+/Fc
while its B-analog displays a reduction potential Ered1 = -1.46 V
vs Fc+/Fc). Thereby, this electrochemical study highlights the
good electron accepting properties of 2-6[OTf]2 and demonstrates
that three redox states can be easily reached.
(Bu4NPF6 (0.2 M), 200 mVs−1, potentials vs Fc+/Fc).
To gain more insights into the reduced forms of these derivatives,
spectroelectrochemical and EPR measurements were performed
on 2[OTf]2, as representative compound. As illustrated Fig. 4a,
each redox state of 2[OTf]2 features characteristic UV-vis
transitions. The absorption of the electrochemically generated
radical cation 2+· is characterized by the appearance of new
bands in the visible and NIR regions (Fig. S7), characteristic of a
highly delocalized organic radical. These new transitions
subsequently disappeared upon reduction to the neutral
derivative (Fig. 4a and Fig. S8). Qualitatively, the evolution of the
spectra upon successive reductions is reproduced by TD-DFT
(Fig S14). The absorption spectra of 22+ was further recovered
upon restoring the initial potential (Fig. S9), illustrating the
switching ability of this derivative. EPR was measured after
chemical reduction of 22+ with Zn powder (Fig. 4b). This
unambiguously confirmed the organic radical nature of the 2+·
with g-factor values of 2.0032. The hyperfine structure of the
experimental EPR spectrum is well reproduced (Fig. S10) taking
into account the coupling of the unpaired electron with the two
equivalent phosphorus atoms and the two sets of equivalent
hydrogen atoms. This demonstrates that the radical is fully
delocalized on the bisphosphapyrenic scaffold. The computed
spin density in the radical cation 2+· (Fig. 4c) also fully supports
this observation. The equalization of the C-C bond lengths in the
phosphacycle (evaluated by DFT, Table S5) are consistent with
the switching between 4-P+ cyclic phosphoniums for 22+ and 5-
phosphinine for 2. Accordingly, the aromaticity in the P-
heterocycle increases upon reduction (NICS(0)(22+)= +4.6;
NICS(0)(2)= -2.6, Fig. S16).[17] Altogether, these results illustrate
that 22+ possesses the characteristics for a new generation of
electro-responsive multi-stage organic system based on
organophosphorus derivatives.
3
This article is protected by copyright. All rights reserved.

10.1002/chem.202001213
Chemistry - A European Journal
COMMUNICATION
Keywords: Polycyclic Aromatic Hydrocarbons •
Organophosphorus • Redox systems • Luminescence • Radical
a)
[1]
[2]
T. Nishinaga, (Eds), Organic Redox Systems: Synthesis, Properties, and
Applications, Wiley-VCH, Weinheim, 2015.
Hünig S., Berneth H. (1980) Two step reversible redox systems of the
Weitz type. In: Organic Chemistry. Topics in Current Chemistry, Vol 92.
Springer, Berlin, Heidelberg.
[3]
[4]
[5]
K. Deuchert, S. Hünig, Angew. Chem. Int. Ed. 1978, 17, 875-886.
L. Striepe, T. Baumgartner, Chem. Eur. J. 2017, 23, 16924-16940.
(a) K. Madasamy, D. Velayutham, V. Suryanarayanan, M. Kathiresan,
K.-C. Ho, J. Mater. Chem. C 2019, 7, 4622-4637. (b) L. Fang, M. A. Olson,
D. Benítez, E. Tkatchouk, W. A. Goddard III, J. F. Stoddart, Chem. Soc.
Rev. 2010, 39, 17-29.
[6]
(a) T. Baumgartner, Acc. Chem. Res. 2014, 47, 1613-1622. (b) D. Joly,
P. A. Bouit, M. Hissler, J. Mater. Chem. C 2016, 4, 3686-3698. (c) A. I.
Arkhypchuk, E. Mijangos, R. Lomoth, S. Ott, Chem. Eur. J. 2014, 20,
16083-16087. (d) J. C.-H. Chan, W. H. Lam, H.-L. Wong, W.-T. Wong, V.
W.-W. Yam, Angew. Chem. Int. Ed. 2013, 52, 11504-11508. (e) M. Stolar,
J. Borau-Garcia, M. Toonen, T. Baumgartner, J. Am. Chem. Soc. 2015,
137, 3366-3371. (f) T. Baumgartner, R. Réau, Chem. Rev. 2006, 106,
4681-4727.
b)
c)
3460
3480
3500
[7]
Various derivatives fulfil the structural requirements of Scheme 1, but do
not show chemical/redox stability allowing the multi-stage redox
processing even if radical cations could be isolated: (a) F. Murakami, S.
Sasaki, M. Yoshifuji, Angew. Chem. Int. Ed. 2002, 41, 2574-2576. (b) F.
Biaso, T. Cantat, N. Mézailles, L. Ricard, P. Le Floch, M. Geoffroy,
Angew. Chem. Int. Ed. 2006, 45, 7036-7039. (c) P. Federmann, H. K.
Wagner, P. W. Antoni, J.-M. Mörsdorf, J. L. Pérez Lustres, H. Wadepohl,
M. Motzkus, J. Ballmann, Org. Lett. 2019, 21, 2033-2038. (d) P.
Hindenberg, F. Rominger, C. Romero-Nieto, Chem. Eur. J. 2019, 25,
13146-13151.
B(G)
Figure 4. (a) UV/Vis/NIR absorption spectra measured during the
electrochemical reduction of 22+ (c = 5.10-4 M) in a solution of Bu4NPF6 (0.2 M)
in DCM. (b) EPR spectrum of the chemically generated 2+· in DCM. (c) Spin
density in 2+· (contour threshold: 2.10-3).
In conclusion, a straightforward synthetic route to dicationic P-
containing PAH 2-6[OTf]2 is described. The absorption and
emission signatures of these compounds are changed with the -
conjugated segment, especially for the -curved 6[OTf]2. In
addition, their particular structures make that 2-6[OTf]2 possess
fluorescence both in solution and in the solid-state. Furthermore,
2-6[OTf]2 are good electron acceptors with two “viologen-like”
easily accessible reduction states. 22+ was used to study the three
redox states by a combined experimental (spectrolectrochemical
and EPR) and theoretical study. In particular, EPR
unambiguously proved the formation of the organic radical 2+·.
This first communication highlights the great potential of these
novel organophosphorus derivatives for further “viologene-like”
switching applications, taking advantage of the fluorescence of 2-
6[OTf]2 in solution and in the solid-state.[18] In addition, the
simplicity of the synthetic approach paves the way toward the
preparation of a virtually unlimited panel of redox-active P-
derivatives based on (twisted) acenes, helicenes, nanographenes
etc[9,15,19]
[8]
The compound 7[OTf]2 described by Anslyn et al. (S. Nieto, P. Metola, V.
M. Lynch, E. V. Anslyn, Organometallics 2008, 27, 3608-3610) was
recently studied for its redox properties but only the first reduction was
described (H. Cheng, X. Wang, L. Chang, Y. Chen, L. Chu, Z. Zuo,
Science Bulletin 2019, 64, 1896-1901. The second reduction is here
described in Fig. S6.
[9]
M. Stępień, E. Gońka, M. Żyła, N. Sprutta, Chem. Rev. 2016, 117, 3479–
3716.
[10]  represents the coordination number and  the valence of the P-atom,
see : Multiple Bonds and Low Coordination in Phosphorus Chemistry; M.
Regitz, O. Scherer (Eds.) Thieme: Stuttgart, 1990.
[11] Q. Ge, J. Zong, B. Li, B. Wang, Org. Lett. 2017, 19, 6670-6673.
[12] (a) A. Belyaev, Y.-T. Chen, S.-H. Su, Y.-J. Tseng, A. J. Karttunen, S. P.
Tunik, P.-T. Chou, I. O. Koshevoy, Chem. Commun. 2017, 53, 10954-
10957. (b) A. Belyaev, Y.-T. Chen, Z.-Y. Liu, P. Hindenberg, C.-H. Wu,
P.-T. Chou, C. Romero-Nieto, I. O. Koshevoy, Chem. Eur. J. 2019, 25,
6332-6341. (c) L. Qiu, W. Hu, D. Wu, Z. Duan, F. Mathey, Org. Lett. 2018,
20, 7821-7824.
[13] M. A. Majewski, M. Stępień, Angew. Chem. Int. Ed. 2019, 58, 86-116.
[14] This trend was already observed by Würthner et al. in their study of
comparable boron-doped PAHs: J. M. Farrell, C. Mützel, D. Bialas, M.
Rudolf, K. Menekse, A.-M. Krause, M. Stolte, F. Würthner, J. Am. Chem.
Soc. 2019, 141, 9096-9104.
Acknowledgements
[15] A. Bedi, O. Gidron, Acc. Chem. Res. 2019, 52, 2482-2490.
[16] Y. Hong, J. W. Y. Lam, B. Z. Tang, Chem. Soc. Rev. 2011, 40, 5361-
5388.
This work is supported by the Ministère de la Recherche et de
l’Enseignement Supérieur, the CNRS, the Région Bretagne, the
French National Research Agency (ANR Heterographene ANR-
16-CE05-0003-01). Y. Molard and G. Taupier (Scanmat-UMS
2001) are thanked for PLQY measurements. This work used the
computational resources of the CCIPL supercomputing center
installed in Nantes. T. Guizouarn (ISCR) is thanked for assistance
with the EPR measurements. M. Cordier and V. Dorcet (CDIFX,
ISCR) are thanked for assistance with X-ray diffraction.
[17] (a) N. Hashimoto, R. Umano, Y. Ochi, K. Shimahara, J. Nakamura, S.
Mori, H. Ohta, Y. Watanabe, M. Hayashi, J. Am. Chem. Soc. 2018, 140,
2046-2049. (b) Z. Benkö, L. Nyulaszi, Top. Heterocycl. Chem. 2009, 19,
27-83.
[18] Viologen derivatives are only luminescent under specific conditions: M.
Freitag, L. Gundlach, P. Piotrowiak, E. Galoppini, J. Am. Chem. Soc.
2012, 134, 3358-3366.
[19] (a) K. Dhbaibi, L. Favereau, J. Crassous, Chem. Rev. 2019, 119, 8846-
8953. (b) T. Y. Gopalakrishna, W. Zeng, X. Lu, J. Wu, Chem. Commun.
2018, 54, 2186-2199. (c) P.-A. Bouit, A. Escande, R. Szűcs, D. Szieberth,
4
This article is protected by copyright. All rights reserved.

10.1002/chem.202001213
Chemistry - A European Journal
COMMUNICATION
C. Lescop, L. Nyulászi, M. Hissler, R. Réau, J. Am. Chem. Soc. 2012,
134, 6524-6527.
5
This article is protected by copyright. All rights reserved.

10.1002/chem.202001213
Chemistry - A European Journal
COMMUNICATION
Entry for the Table of Contents
The straightforward synthesis of unprecedented electron-acceptors based on dicationic P-containing PAHs (Polycyclic Aromatic
Hydrocarbons) is reported. The impact of -extension on both the optical and redox properties is investigated using a joint experimental
and theoretical approach. (spectro)-electrochemical studies prove that these compounds possess three redox states and EPR studies
confirms the in situ formation of an organic radical delocalized on the PAH backbone..
Institute and/or researcher Twitter usernames: ((optional))
6
This article is protected by copyright. All rights reserved.