Dibenzophosphapentaphenes: Exploiting P chemistry for gap fine-tuning and coordination-driven assembly of planar polycyclic aromatic hydrocarbons

Article abstract of DOI:10.1021/ja300171y

A synthetic route to planar P-modified polycylic aromatic hydrocarbons (PAHs) is described. The presence of a reactive σ³, λ³-P moiety within the sp²-carbon scaffold allows the preparation of a new family of PAHs displaying tunable optical and redox properties. Their frontier molecular orbitals (MOs) are derived from the corresponding phosphole MOs and show extended conjugation with the entire π framework. The coordination ability of the P center allows the coordination-driven assembly of two molecular PAHs onto a Au^I ion.

Full text of DOI:10.1021/ja300171y

Communication
pubs.acs.org/JACS
Dibenzophosphapentaphenes: Exploiting P Chemistry for Gap Fine-
Tuning and Coordination-Driven Assembly of Planar Polycyclic
Aromatic Hydrocarbons
Pierre-Antoine Bouit,† Aude Escande,† Rozsa Szucs,‡ Denes Szieberth,‡ Christophe Lescop,†
́
̋
́
Laszlo Nyulaszi,‡ Muriel Hissler,*,† and Regis Reau*,†
́
́
́
́
́
†Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS - Universite
France
‡Department of Inorganic Chemistry, Budapest University of Technology and Economics, H-1521 Budapest, Hungary
́
de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex,
S
* Supporting Information
the reactive phosphorus heterocycle. Only photocyclization
appeared to be a suitable synthetic method,9 and the fused
ABSTRACT: A synthetic route to planar P-modified
biphenyl[b,d]thioxophosphole 110 (Scheme 1) was finally
polycylic aromatic hydrocarbons (PAHs) is described. The
presence of a reactive σ3,λ3-P moiety within the sp2-carbon
Scheme 1. Synthesis of σ3,λ3-Dibenzophosphapentaphene 5
scaffold allows the preparation of a new family of PAHs
displaying tunable optical and redox properties. Their
frontier molecular orbitals (MOs) are derived from the
corresponding phosphole MOs and show extended
conjugation with the entire π framework. The coordination
ability of the P center allows the coordination-driven
assembly of two molecular PAHs onto a AuI ion.
olycyclic aromatic hydrocarbons (PAHs), such as
benzocoronenes or nanographenes, are of great potential
P
in molecular electronics for the development of efficient
optoelectronic devices (solar cells, field-effect transistors, etc.).1
The possibility of performing molecular engineering of PAHs
using the power of organic chemistry is a key toward these
applications. Indeed, HOMO−LUMO gap and supramolecular
assembly of PAHs can be controlled through modification of
the size of the π-conjugated system or/and by the proper
choice of lateral aliphatic substituents.2 An alternative appealing
strategy involves the incorporation of heteroatoms (N,3 O,4 S,5
B6) within the conjugated sp2-carbon backbones of PAHs.
Surprisingly, no fully planarized P-containing PAHs have been
described to date,7 although σ3,λ3-P derivatives have proven to
be powerful platforms for molecular engineering of π-
conjugated scaffolds. For example, both the HOMO−LUMO
gap and supramolecular organization of phosphole-modified π
systems can be readily controlled using the versatile reactivity
(nucleophilicity, coordination ability) of the σ3,λ3-P centers.8
Herein we report that this approach based on P chemistry can
be extended to planar P-modified PAHs, with the synthesis of
dibenzophosphapentaphenes. Of particular interest, exploiting
the reactivity of the P center allows straightforward fine-tuning
of the gap in this novel family of PAH derivatives. Furthermore,
these compounds can be coordinated to metal ions, offering an
unprecedented way of assembling PAHs using coordination
chemistry.
designed as a key precursor following numerous unsuccessful
tries. For example, the choice of the PS function was crucial,
since this P moiety is less prone to react with in situ-generated
protic acid than the corresponding PO function.11 Derivative
1 (31P NMR: +58.7 ppm) was obtained using the classic
Fagan−Nugent heterole route12 followed by sulfurization.
Photocyclization of 1 over 20 h using the Katz-modified
method13 afforded a mixture of the monocyclized derivative 2
and the fully cyclized target dibenzophosphapentaphene 3
(Scheme 1). Increasing the reaction time did not improve the
yield of 3. Both compounds are air-stable and soluble in
common organic solvents (THF, CH2Cl2) and were isolated in
pure form (yields: 2, 50%; 3, 20%) after purification by
chromatography. The two products were fully characterized by
NMR spectroscopy and high-resolution mass spectrometry
(HRMS). Their 31P chemical shifts are typical for thioxophosp-
hole derivatives (31P NMR: 2, +51.6 ppm; 3, +46.0 ppm), and
the simplicity of the 1H NMR spectrum of 3 is consistent with a
highly symmetric skeleton [see the Supporting Information
(SI)].
Definitive proof for the proposed structure was provided by
an X-ray crystallographic study performed on single crystals of
3 (Figure 1). The sp2-carbon framework is planar (maximum
deviation from the mean sp2-C plane, 0.15 Å), and the P atom
The synthesis of P-containing PAHs using classical
approaches (Scholl reaction, thermolysis, etc.)1a with penta-
phenylphosphole derivatives failed because of the presence of
Received: January 6, 2012
Published: April 4, 2012
© 2012 American Chemical Society
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Communication
Scheme 2. Chemical Derivatization of σ3,λ3-
Dibenzophosphapentaphene 5
Figure 1. X-ray crystallographic structure of 3: (a) view of the discrete
dimers observed in the packing structure; (b) view of 3 perpendicular
to the sp2-C plane.
Table 1. Photophysical and Electrochemical Data
lies within the sp2-C plane (Figure 1b). The C−C bond lengths
range from 1.35 to 1.49 Å, as observed in prototype
benzocoronene derivatives,14 and the valence angles around
the sp2-C atoms vary from 114.3 to 125.5°. The σ4,λ5-
phosphorus atom has a pyramidal shape (Figure 1b) with the
usual valence angles, and the C−C and C−P bond lengths
within the phosphole ring are classic.8a This solid-state
structure shows that 3 combines the characteristic structural
features of PAH and phosphole derivatives. In the solid state, 3
forms discrete head-to-tail π dimers (Figure 1a) with an
intermolecular distance of 3.45 Å,15 similar to that observed in
other benzocoronenes or in graphite.14
Having constructed the P-modified PAH backbone, the next
step was to desulfurize16 the phosphole ring to obtain a
functionalizable σ3,λ3-P-dibenzophosphapentaphene. Treat-
ment of thioxophosphole 3 with methyl triflate afforded
phospholium ion 4 (31P NMR: +50.1 ppm), which was isolated
as a purple solid in 60% yield (Scheme 1). Derivative 4, which
is the first isolated phospholium ion bearing a P−S bond, is
remarkably air- and moisture-stable in solution and in the solid
state. The solid-state structure of this cationic dibenzophos-
phapentaphene (Figure S12 in the SI) is superimposable on
that of its neutral precursor 3, showing that chemical
modifications of the P moiety do not alter the planar sp2-
carbon backbone. Phospholium 4 was then converted into its
σ3,λ3-analogue 5 by treatment with the nucleophilic phosphane
P(NMe2)3 (Scheme 1). σ3,λ3-Dibenzophosphapentaphene 5
was isolated in 50% yield as an air-stable yellow powder that is
moderately soluble in THF or CH2Cl2. Its 31P NMR chemical
shift (−2.9 ppm) is a usual value for phosphole derivatives,8a
and the proposed structure is consistent with its NMR and MS
data (see the SI).
a
a
c
absorption
fluorescence
redox potentials (V)
b
compd λabs (nm)
log ε
λem (nm)
ϕF
Eo1x
Er1ed
d
3
4
5
6
7
8
514
569
472
524
554
508
4.04
3.95
4.34
4.11
4.00
544
669
489
549
599
537
0.21
0.03
0.80
0.52
0.19
0.08
0.71
1.01
0.44
0.77
0.97
0.75
−1.70
−1.04
<−2.10
−1.71
−1.31
−1.67
d
d
d
4.14
a
b
In CH2Cl2 (10−5 M). Measured relative to fluorescein (NaOH, 0.1
c
M), ϕref = 0.9. Measured in CH2Cl2 containing Bu4N+PF6− (0.2 M) at
a scan rate of 100 mV s−1. Potentials in V vs ferrocene/ferrocenium.
d
Reversible process
The P center in compound 5 retains versatile reactivity, as
illustrated by its transformations into oxidized phospholes
(thiooxophosphole 3, oxophosphole 6), phospholiums 4 and 7,
and neutral AuI complex 8 in almost quantitative yields
(Scheme 2). These compounds exhibit the expected multi-
nuclear NMR and MS data (see the SI). Overall, a family of
neutral and cationic dibenzophosphapentaphenes derivatives
3−8 was prepared (Scheme 2). This nicely illustrates that
introducing a reactive σ3,λ3-P center within the PAH framework
allows molecular engineering to be performed readily.
To evaluate the impact of the P modifications on the
electronic properties of these PAHs, the optical and electro-
chemical properties of dibenzophosphapentaphenes 3−8 were
investigated in CH2Cl2. The absorption spectrum of 5 consists
of a large, structured band in the visible range (λmax = 472 nm;
Table 1) that displays the typical PAH-like hyperfine structure
(Figure 2a).1a Neutral σ4-P derivatives 3, 6, and 8 (Scheme 2)
Figure 2. (a) UV−vis absorption and TD-DFT-simulated spectra
(vertical lines). (b) Normalized emission spectra of 3 (blue), 4
(purple), 5 (red), and 7 (green) in CH2Cl2 (10−5 M). (c, d) Kohn−
Sham MOs of 5: (c) HOMO; (d) LUMO.
exhibit red-shifted absorption spectra with similar shapes and
absorption maxima (Table 1, Figure 2a). A larger bathochromic
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Communication
shift is observed with P−alkyl and P−S phospholium salts 7
and 4 (Table 1). Remarkably, the absorption spectra of the
dibenzophosphapentaphenes 3−8, which have the same
polycyclic aromatic C skeleton but different P moieties
(Scheme 2), vary over a wide range covering almost the entire
visible spectrum (Figure 2a). All of these dibenzophospha-
pentaphenes are fluorescent in solution, with a gradual red shift
of λem in the series 5/3/7/4 (Table 1, Figure 2b). The Stokes
shifts are reasonably small for these rigid structures (5, Δν =
̅
737 cm−1; 3, Δν = 1072 cm−1; 7, Δν = 1355 cm−1) and the
̅
̅
emission bands are structured, suggesting a small rearrange-
ment of these molecules upon photoexcitation. Therefore, their
quantum yields are relatively high (>20%; Table 1). In contrast,
derivative 4 presents a larger Stokes shift (Δν = 2875 cm−1)
̅
accompanied by a decrease in the fluorescence quantum yield
(3%).
Figure 3. X-ray crystallographic structure of AuI-complex 9 and a view
of the infinite columns along the crystallographic b axis.
Analysis of the cyclic voltammetry data showed that the
chemical modifications performed on the P center of 5 lead to a
gradual increase in the oxidation and reduction potentials in the
series 5/3/7/4 (Table 1). The variation is even more
pronounced for the reduction potential, revealing that
phospholium salts 4 and 7 have rather high electron affinities.
It is noteworthy that 3, 6, 7, and 8 show reversible reduction
waves, indicating sufficient stability in their reduced state under
the measurement conditions (Table 1). The evolution of the
redox potentials within the 5/3/7/4 series is consistent with
the decrease in the optical gap (Figure 2a).
coordinated to the metal ion (Figure 3).23 The P−Au−P
fragment is linear (P−Au−P, 180.0°), and the two highly planar
sp2-C skeletons of 5 exhibit an anti conformation with respect
to the P−Au−P moiety (Figure 3). Interestingly, the packing of
9 is deeply modified relative to those of derivatives 3 and 4.
Intermolecular π−π interactions between coordinated dibenzo-
phosphapentaphenes take place (π−π distances, 3.50 Å),
affording infinite columns of π-stacked complexes 9 (Figure
3).15 In view of the rich coordination chemistry of phosphole
derivatives,8a the fact that 5 behaves as a classic two-electron P
donor toward metal centers opens an avenue for coordination-
driven assembly of phosphorus-modified PAHs.
To gain more insight into the electronic properties of these
novel P-modified PAHs, time-dependent density functional
theory (TD-DFT) calculations at the B3LYP/6-31+G* level of
theory17 were performed.18 This theoretical study showed that
the long-wavelength UV−vis absorption of PAHs 3−7 mainly
results from the HOMO−LUMO transition.19 These frontier
molecular orbitals are similar throughout the series 5−7 (5,
Figure 2c,d; 3−7, Figure S14). These π MOs are highly
delocalized on the sp2-carbon skeletons with a significant
contribution of the respective phosphole ring orbitals (Figure
S15).20 The HOMO, which has a nodal plane on the
phosphorus atom, is influenced by the inductive effects of the
P substituents. This explains why the shape of these orbitals is
unaltered upon modification of the P moiety (Figure S14). The
LUMO is subjected to the negative hyperconjugative
interaction between the σ*PSubstituent MO and the MOs of the
sp2-carbon framework. This results in stabilization of this empty
level for σ4-P-containing neutral and cationic systems and also
increases the antiaromatic character of these heteroles.8h,21 This
influence of the P substituent can be nicely seen through the
increasing weight of the P atom in the LUMO (Figure S14),
especially for cationic systems 4 and 7, which exhibit the
highest reduction potentials (Table 1). It is interesting to note
that the effect of P substitution on the electronic properties and
frontier MOs of these P-modified PAHs follows the same trend
as for the parent phosphole.20 Overall, these experimental and
theoretical data show that local chemical modification of the P
moiety of dibenzophosphapentaphenes is a powerful means of
fine-tuning the gap of these novel planar PAH derivatives.
The reactive P center in 5 also offers an unprecedented way
of organizing PAHs using coordination chemistry, as illustrated
by the assembly of two σ3,λ3-dibenzophosphapentaphenes 5 on
a AuI ion (Figure 3). Complex 8 (Scheme 2) was reacted with
AgOTf22 and free ligand 5, affording the new complex 9 (31P
NMR: +40.8 ppm). An X-ray crystallographic study confirmed
the unique structure of complex 9, in which two PAHs are
In conclusion, a synthetic route to the first planar σ3,λ3-P-
modified PAH is described. The presence of a reactive P moiety
within the sp2-C skeleton allows the facile synthesis of a new
family of PAHs with tunable optical and electrochemical
properties using a single precursor. This molecular engineering
of PAHs based on P chemistry and their assembly using
coordination chemistry shows the potential of modifying planar
π-extended frameworks using organophosphorus fragments.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures; NMR data; X-ray crystallographic
data and CIF files for 3, 4, and 9; computational procedures
and data; and HOMO and LUMO plots and energies for 3−7
and the related 1-methylphospholes. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
muriel.hissler@univ-rennes1.fr; regis.reau@univ-rennes1.fr
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Minister
̀
e de la Recherche et
de l′Enseignement Super
Recherche Scientifique (CNRS), the Institut Universitaire de
́
ieur, the Centre National de la
France (IUF), and the Region Bretagne. COST CM0802
́
(Phoscinet) and TAMOP-4.2.2/B-10/1-2010-0009 are also
acknowledged.
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(17) A systematic study showed that TD-DFT with the B3LYP
functional provides good results for vertical excitation energies of
organic molecules. See: Jacquemin, D.; Wathelet, V.; Perpete, E. A.;
Adamo, C. J. Chem. Theory Comput. 2009, 5, 2420.
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