Tuning the Electronic Properties of Acetylenic Fluorenes by Phosphaalkene Incorporation

Article abstract of DOI:10.1002/chem.201503430

Versatile synthetic protocols for 2,7- and 3,6-diacetylenic fluorene-9-ylidene phosphanes (F9Ps) were developed. Protodesilylation of trimethylsilyl-protected acetylenic F9Ps affords terminal acetylenes that can be employed in Sonogashira and Glaser-type C-C coupling reactions to give thienyl-decorated and butadiyne-bridged fluorene-9-ylidene phosphanes, respectively. As evidenced by UV/Vis spectroscopy and cyclic voltammetry and corroborated by ab initio calculations, the presence of the P center in the F9Ps induces a significantly reduced HOMO-LUMO splitting that originates from stabilization of the LUMO levels. Variation of the acetylene substitution pattern is an additional tool to influence the optical and electronic properties. Whereas 3,6-disubstituted F9Ps have strong absorptions around 400nm, mainly due to π-π? transitions, 2,7-diacetylenic F9Ps exhibit longest-wavelength absorptions that have significant charge-transfer character with an onset around 520nm. Bringing together the goodies: Fluorenes, acetylenes, and phoshosphaalkenes were combined in a novel class of compounds: acetylenic fluorene-9-ylidene phosphanes (F9Ps). Experimental and computational studies show that the exocyclic P center stabilizes the LUMO of the F9Ps and leads to significantly reduced HOMO-LUMO splittings. Variation of the acetylene substitution pattern is an additional tool to alter the optical and electronic properties.

Full text of DOI:10.1002/chem.201503430

DOI: 10.1002/chem.201503430
Full Paper
&
Fluorenes
Tuning the Electronic Properties of Acetylenic Fluorenes by
Phosphaalkene Incorporation
[
a]
Yurii V. Svyaschenko, Andreas Orthaber,* and Sascha Ott*
Abstract: Versatile synthetic protocols for 2,7- and 3,6-diace-
tylenic fluorene-9-ylidene phosphanes (F9Ps) were devel-
oped. Protodesilylation of trimethylsilyl-protected acetylenic
F9Ps affords terminal acetylenes that can be employed in
Sonogashira and Glaser-type CÀC coupling reactions to give
thienyl-decorated and butadiyne-bridged fluorene-9-ylidene
phosphanes, respectively. As evidenced by UV/Vis spectros-
copy and cyclic voltammetry and corroborated by ab initio
calculations, the presence of the P center in the F9Ps indu-
ces a significantly reduced HOMO–LUMO splitting that origi-
nates from stabilization of the LUMO levels. Variation of the
acetylene substitution pattern is an additional tool to influ-
ence the optical and electronic properties. Whereas 3,6-dis-
ubstituted F9Ps have strong absorptions around 400 nm,
mainly due to p–p* transitions, 2,7-diacetylenic F9Ps exhibit
longest-wavelength absorptions that have significant
charge-transfer character with an onset around 520 nm.
Introduction
studies on diacetylenic 9-dicyanomethylene fluorenes and fluo-
renones with electron-rich carbazole substituents at the acety-
lenic termini have shown that variation of the substitution pat-
tern can also have an impact on the donor–acceptor proper-
Carbon-rich compounds that have an extended p-conjugated
system have been widely investigated as materials in electronic
[
1]
[12b,15b]
and photonic applications, and well defined one-, two-, and
three-dimensional molecular architectures have been prepared
ties (Figure 1a, b).
An alternative approach to conjugated materials with unpre-
cedented properties is heteroelement incorporation. The use
of main group elements as p-type dopants has recently led to
extraordinary examples of p-conjugated materials with small
[
2]
for such purposes. The effective conjugation path can be ex-
panded by locking aryl groups into coplanarity to decrease
their rotational freedom and to maximize orbital overlap. Fluo-
renes in which two phenyl groups of a biphenyl are tied to-
gether with a methylene bridge are a prominent example of
this strategy. As one of the most powerful building blocks in
[
16]
bandgaps
and materials with interesting bonding situa-
[
17]
tions. Especially the chemistry of organophosphorus p-con-
jugated materials has seen considerable development in
recent years, with the synthesis of novel P-based conjugated
[
3]
organic electronics, fluorenes have been incorporated into
[
4]
[5]
linear and cyclic oligomers (radialenes), blue-emitting co-
compounds and the elucidation of their optical and electro-
[
6]
[7]
[16d,18]
polymers, metal–organic hybrid polymers, self-assembled p
chemical properties.
Fascinating phosphaalkene architec-
[
8]
[9]
stacks, and purely organic polymers. In addition, fluorene
derivatives have recently been used as building blocks for con-
tures have been prepared and investigated, such as a benzene
[
19]
with multiple phosphaethene substituents, linear poly- and
[
10]
[20]
[21]
ductive polymers in organic thin-film transistor applications
oligomers, and radialenes with endo- and exotopic phos-
phaalkene units. Over the last decades, attempts have been
made to investigate the influence of phosphorus incorporation
into the exocyclic double bond of fluorene-9-ylidene phos-
[
11]
and photovoltaics.
Functionalization of fluorenes is often
achieved through coupling of halogenated precursors (mainly
[
12]
in 2,7- and 3,6-positions) or exploiting the acidity of the 9-H
[
13]
[22]
[23]
position. The latter approach is commonly used for the in-
troduction of solubilizing groups such as long alkyl chains that
phanes (F9Ps) and their allenic congeners. Whereas P in-
corporation generally has a profound effect on the properties
of the compounds, additional variations of the P-substituents
gives only minor alterations in the optoelectronic properties of
[14]
control aggregation in solution or the solid state. Changing
from a CH (or CR ) bridge in fluorenes to fluorenones (C=O) or
2
2
[
24]
9
-methylidene fluorenes (C=CR ) can lead to significant altera-
the heterofluorenes. Radical anions of F9P derivatives have
been prepared electrochemically and chemically, and their sta-
bility is rationalized by a resonance structure with a phosphinyl
2
[
12b,15]
tions of the optoelectronic properties.
Moreover, recent
[
25]
[
a] Dr. Y. V. Svyaschenko, Dr. A. Orthaber, Prof. Dr. S. Ott
radical and an anionic fulvene.
Department of Chemistry - Šngstrçm Laboratory
Uppsala University, Box 523, 75120 Uppsala (Sweden)
E-mail: Andreas.Orthaber@kemi.uu.se
Sascha.Ott@kemi.uu.se
In the present work, we studied the impact of heteroele-
ment incorporation, in the form of phosphaalkenes, on acety-
lene-extended fluorene frameworks. Moreover, the impact of
the rigid core was investigated and compared to C,C-diphenyl
(phospha)alkenes. Inspired by previous works, different acety-
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201503430.
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Figure 1. Examples of 3,6- (A) and 2,7-diacetylene substituted fluorenes (B) with carbazole donor termini (a, b). Studied F9Ps (c, d) and nonlocked C,C-diphenyl
phosphaalkene reference compounds (e).
lene substitution patterns were investigated to give insights
into the bonding situation and the effect of the carbon–phos-
phorus exchange on the frontier molecular orbitals. In this
work, we sought to combine phosphaalkenes, fluorenes, and
oligoacetylenes to afford a new class of planar p-conjugated
compounds with significantly smaller HOMO–LUMO gaps and
demonstrate their synthetic scope as building blocks for the
construction of future p-conjugated materials.
Results and Discussion
Synthesis
Several approaches can be envisaged for the synthesis of ace-
tylenic F9Ps, which mainly differ in the order in which the dif-
ferent substituents are introduced at the fluorene core. For
most of the compounds described herein, it turned out to be
more efficient to assemble the acetylenic fluorene framework
first, and then introduce the unsaturated phosphorus frag-
ment. Thus, acetylenic fluorenes 2a,b and 3a,b were first pre-
Scheme 1. i) RCꢀCH, 808C, CuI, [PdCl
ii) nBuLi, À788C, Mes*PCl , THF, 1 h; iii) DBU, 2 h; iv) K
v) 2-Iodothiophene, THF, MeOH, CuI, [PdCl (PPh ], RT, 38%.
2
(PPh
3
)
2
], toluene/iPr
2
NH, overnight;
, MeOH, THF, 30%;
pared by Sonogashira cross-coupling reactions from 3,6-dibro-
mofluorene (1a) and 2,7-dibromofluorene (1b; Scheme 1).
Whereas 1b is commercially available, 1a had to be prepared
in six steps from 9,10-phenanthrenequinone by a modified lit-
2
2
CO
3
2
3 2
)
[12c]
erature procedure (see the Supporting Information).
a strategy was exemplified by the deprotection of phosphaal-
The phosphane can easily be introduced on the diacetylenic
kene 7a with K CO and isolation of the terminal bis-acetylene
2
3
fluorenes 2,3 by treatment with nBuLi and addition of the re-
8a. The latter compound engages in Sonogashira cross-cou-
pling reactions, for example, with 2-iodothiophene to afford
9a (Scheme 1).
sulting Li salt to a solution of Mes*PCl (Mes*=2,4,6-tri-tert-bu-
2
tylphenyl) to give chlorophosphanes 4a,b and 5a,b, which
3
1
were characterized by P NMR spectroscopy and used without
further purification. Dehydrochlorination with the non-nucleo-
philic base 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) estab-
lished the P=C bond and completed the synthesis of 6,7 in
modest to good overall yields (16–84%).
In addition, phosphaalkenes with a dihalo-substituted fluo-
rene core as in 12 and monohalo-substituted fluorenes 15 and
16 were targeted. These compounds are appealing monomers
[
27]
for the synthesis of p-conjugated conductive materials, olig-
[
13]
omers, or macrocycles, since they could engage in other CÀ
C cross-coupling reactions such as Yamamoto or Suzuki cou-
pling. Dibromofluorenes 1a,b can be easily lithiated at À788C
to give the corresponding lithium salts, which react with
In the case of 7a,b, it is possible to protodesilylate the acet-
ylene termini, which can then be further elaborated by desira-
ble substituents to fine-tune the photophysical and electro-
[20e,f,26]
chemical properties of the F9Ps.
The feasibility of such
Mes*PCl with formation of chlorophosphanes 11 a,b. The reac-
2
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3
1
tion was monitored by P NMR spectroscopy and, after com-
pletion, the resulting intermediates 11 were treated with DBU,
as described above, to afford dibromofluorenes 12a,b in good
yields (Scheme 2). A constant low temperature (below À708C)
during lithiation plays a critical role in this reaction. If the tem-
perature is allowed to exceed À408C, formation of sizeable
amounts of presumably polymeric side products that compli-
cate purification is observed.
a mixture of E/Z isomers. In situ deprotection of the mixture of
isomers with K CO , followed by Glaser-type coupling, afforded
2
3
butadiyne-bridged dimeric 18 as a mixture of EE, EZ, and ZZ
isomers, with the ZZ isomer being the main product of about
31
50%, as determined by P NMR spectroscopy. Recrystallization
from pentane gave a single compound, which was assigned as
1
the ZZ isomer on the basis of the characteristic H NMR shift
(d=4.90 ppm) of H1, which lies directly under the Mes* frag-
[
29]
ment, and the expected dd coupling pattern (see Support-
ing Information).
To compare the effect of the different functional groups at
the fluorene core, that is, the phosphaalkene and the acety-
lenes at different positions, as well as the influence of the fluo-
rene itself, a number of different reference compounds were
[
22c]
synthesized. Fluorene 10,
which lacks the acetylene units,
was prepared from lithiated fluorene by following the typical
reaction sequence with dichlorophosphane followed by dehy-
drohalogenation with DBU (Scheme 2).
C,C-Diphenyl phosphaalkenes 23 and 24, which lack the cen-
tral bond of the fluorene core, were synthesized by a similar
approach. Chlorophosphanes 21,22 were generated by lithia-
tion of 19 and 20, followed by reaction with Mes*PCl in THF.
2
Surprisingly, dehydrohalogenation to give the targeted phos-
phaalkenes was found to be very sluggish, and even heating
the reaction mixture in the presence of DBU to reflux in THF
(
668C) for several hours did not convert the starting materials.
Under even harsher conditions, phosphaalkenes 23 and 24
could finally be obtained from their corresponding chlorophos-
phanes 21 and 22 in refluxing toluene (111 8C) after 2 d
(
Scheme 3).
Scheme 2. Synthesis of substituted fluorene-9-ylidene phosphanes. i) nBuLi,
À788C, Mes*PCl
v) RCꢀCH, 808C, CuI, [PdCl
dine, MeOH, Cu(OAc) , K CO
2
, THF, 1 h; ii) DBU, 2 h; iii) nBuLi, À788C, 5 min; iv) DBU, 2 h;
(PPh ], toluene/iPr NH, overnight, 75%; vi) pyri-
, 77%; vii) 1) Mes*PCl , THF, 1 h; 2) DBU, 2 h.
2
3
)
2
2
2
2
3
2
Asymmetric monobromofluorenes 15 and 16 were targeted
from dibromofluorenes though lithium/bromide exchange re-
actions, followed by protonation. Interestingly, treatment of
phosphaalkene-containing dibromofluorene 12 with nBuLi and
subsequent quenching with water did not afford any mono-
brominated fluorene, but resulted in the formation of a com-
plex mixture of polymeric materials that most likely stem from
[16c,28]
initial attack of BuLi at the P=C unit.
However, addition of
nBuLi to chlorophosphanes 11 a,b, followed by treatment with
DBU and subsequent aqueous workup affords monobromo-
fluorenes 15,16 as a mixture of E/Z isomers in a ratio of about
Scheme 3. Synthesis of acetylene-substituted C,C-diphenyl phosphaalkenes.
i) nBuLi, À788C, Mes*PCl , THF, 1 h; ii) DBU, toluene, reflux, 2 d.
2
1
:1 in good total yields (Scheme 2). Prolonged lithiation leads
to significantly lower yields and increased amounts of unsub-
stituted F9P 10. Attempts to separate E/Z isomers were not
successful, since the two isomers interconvert to restore the in-
itial equilibrium within 1 d under ambient conditions.
Fundamental to our study are possibilities to determine the
influence that the phosphorus center exerts on the overall p
system and, in order to do so, several carbon analogues were
prepared. Phenylacetylene-terminated compounds 26a,b were
prepared according to a literature procedure from fluorenones
The synthetic versatility of the monohalogenated precursors
was demonstrated by the preparation of dimeric compound
[
30]
25a,b which are easily accessible from dibromofluorenones
by coupling with phenylacetylene (see the Supporting Infor-
mation). Compounds 25a,b were treated with isopropylmag-
18. Starting from 15, Sonogashira coupling with trimethylsilyl-
acetylene (TMS-acetylene) gave monoacetylenic F9P 17 as
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nesium chloride, and, after aqueous workup, the resulting alco-
hols were heated to reflux in acetic acid in the presence of
about 0.15 equivalents of p-toluenesulfonic acid (p-TsOH) to
afford acetylene-substituted 9-isopropylidene fluorenes 26a,b
ment with twist angles of 39.00(8)8and 41.29(8)8 effectively dis-
rupts p conjugation throughout the system (Figure 2). It can
be assumed that a similar conformation persists also in solu-
tion, since it avoids steric clashes between the a protons of
the two phenyl rings that would occur if the phenyl rings were
coplanar.
(
Scheme 4).
2
,7-Diacetylenic F9P 7b also crystallizes in the monoclinic
space group P2 /n. The P=C bond length is slightly shorter
1
(
(
1.687(2) Š) with similar bending towards the Mes* group
106.2(1)8). Both phenyl rings of the fluorene and the P=C
moiety span an almost perfect plane with maximum deviations
of 0.063(3) Š and a twist angle of the phenyl rings of 1.0(1)8.
Similarly, we obtained single crystals of 6a as its methanol sol-
vate in the triclinic space group P 1¯ . The poorly defined metha-
Scheme 4. i) iPrMgCl, THF, HCl; ii) p-TsOH, AcOH, reflux, 2 d.
nol molecule around (1/2,1/2,1/2) was modeled with a position-
al disorder of the OH group. The phosphaalkene fragment
shows similar structural parameters as in 7b and 24 (P1ÀC1
1.693(2) Š and C30-P1-C1 103.25(11)8) and, as expected, high
planarity of the F9P core (max. deviation 0.060(2) Š with
a phenyl twist angle of 1.1(1)8). However, only one of the two
terminal phenyl rings is nearly coplanar with the central
moiety (7.4(1)8), whereas the other is twisted by 66.1(1)8.
X-ray crystallographic studies
The solid-state structures of representatives of each class of
phosphaalkene-containing fluorenes as well as C,C-diphenyl
phosphaalkenes were investigated by X-ray crystallography of
crystals that were obtained by evaporation of binary or ternary
solvent mixtures (CH Cl , MeOH, MeCN). The extended C,C-di-
2
2
phenyl phosphaalkene 24 crystallizes in the monoclinic space
group P2 /n. The P=C bond has a length of 1.699(2) Š, which
1
Spectroscopic and theoretical investigations
lies in the expected range, and exhibits typical bending with
a Mes*ÀP=C angle of 106.77(8)8. The deviation of the phenyl
rings from the least-squares plane spanned by the CÀP=C frag-
Intrigued by previous studies on donor–acceptor systems built
around fluorene cores, we were interested in the photophysi-
[31]
Figure 2. ORTEP plots of compounds 24, 6a, and 7b at 50% probability. Hydrogen atoms and solvent molecules are omitted for clarity. Structural and solu-
[32]
tion details are given in the Supporting Information.
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cal and electrochemical properties of the F9Ps in comparison
with their all-carbon congeners. The different behavior of 2,7-
versus 3,6-diacetylene-substituted fluorene-9-ylidene malononi-
trile derivatives has been discussed previously in terms of their
different acceptor parts, that is, the malonitrile or fluorene
[
12b]
unit.
UV/Vis spectroscopy
The influence of the exocyclic P center in the F9Ps on the elec-
tronic properties of the compounds becomes apparent on
comparison of the UV/Vis spectra with those of the corre-
sponding all-carbon-based dibenzofulvenes (Figure 3, top).
Whereas the carbon analogue 26a shows a broad absorption
around 350 nm with a low-intensity feature reaching into the
visible region, its P analogue 6a has a prominent absorption at
4
00 nm. The effect of heteroatom incorporation is even more
pronounced in the 2,7-disubstituted systems, for which the
onset of absorption in the visible region is 524 nm for 6b, as
opposed to 395 nm for 26b.
The UV/Vis spectroscopic data (Table 1) also allow evaluation
of the effect of the acetylene groups and their position around
the fluorene core. Not surprisingly, the non-acetylenic fluorene
10 exhibits the most blueshifted end absorption of the series,
while introduction of a TMS-acetylene moiety in the 3-position
of the fluorene, as in 17 (lmax =380 nm), leads to a redshift of
1
6
2 nm. Introduction of a second TMS-acetylene moiety in the
-position (7a) leads to an additional shift of 12 nm, and the
longest-wavelength absorption maximum is observed at
Figure 3. Top: UV/Vis absorption spectra of solutions of F9Ps 6a,b and their
3
92 nm.
carbon analogues 26a,b in CH
2 2
Cl . Bottom: comparison between the UV/Vis
spectra (CH Cl ) of F9P 6a and its C,C-diphenyl phosphaalkene analogue 23,
2
2
The position of the acetylene substituents at the fluorene
and of the monoacetylenic F9P 17 and the butadiyne-bridged dimeric 18.
core significantly alters the optical properties of the com-
pounds. The 2,7-disubstituted fluorene 6b has a strong ab-
sorption around 354 nm with a shoulder at 382 nm and a long
tail that extends far into the visible region (lonset =524 nm). In
contrast, the longest-wavelength absorption maximum in the
The strongest effect on the optical properties of the com-
pounds comes from the fluorene core itself, and is best illus-
trated by comparing fluorene 6a with the C,C-diphenyl phos-
phaalkene 23, which lacks the central C7ÀC8 bond. The
lowest-energy absorption maximum of the latter is observed
at 355 nm, that is, blueshifted by 45 nm compared to that of
the former (Figure 3, bottom). A comparison of the monoace-
tylenic phosphaalkene 17 with its dimeric form 18 clearly indi-
3
,6-disubstituted isomer 6a is significantly redshifted with
a broad maximum around 400 nm. It is clear from these results
that the position of the acetylene groups at the fluorene core
influences the degree to which they participate in the frontier
molecular orbitals, as is discussed below.
À1
Table 1. UV/Vis Spectroscopic and electrochemical data in CH
2
Cl
2
at 258C. Electrochemical data for 1 mm solutions (0.1m NBu
4
PF
6
) at n=100 mVs (all
+
/0
potentials vs. Fc ). Reversible peaks are given as their half-wave potentials [E1/2 =(Epa +Epc)/2)], and irreversible peaks as the corresponding cathodic (Ep,c
or anodic peak potentials (Ep,a).
)
À1
À1
Compound
l
max [nm] (e [m cm ])
Reduction E1/2 [V]
Oxidation Ep,a [V]
[
b]
6
6
7
7
1
1
2
2
2
a
b
a
b
0
7
3
6a
6b
400 (35500)
354(84000), 385 (22000), 471 (2100)
392 (7070)
389 (24000)
368 (18200)
380 (23200)
317 (57100), 355 (32000)
347 (22200), 361 (18000), 410 (5000)
352 (14 500)
À1.74
À1.81
À1.74
À1.81
À1.99
À1.85
1.04
1.07
1.10
1.15
1.06
[
a]
[a]
[b]
[b]
[b]
[b]
[
b]
[b]
[b]
1.11
0.95
[
a]
[b]
À2.30
[
a]
[a]
[b]
[b]
À2.36
1.01 1.21
[
b]
[b]
À2.41
0.94
[
a] Shoulder. [b] Irreversible.
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cates extended conjugation across the butadiyne unit, which
shifts the maximum absorption by more than 20 nm from 380
to 404 nm.
from a p–p* and minor contributions from an n–p* transition.
Interestingly, a charge-transfer transition similar to that ob-
#
served for 6b was observed neither experimentally nor by cal-
#
culations for 6a.
TDDFT calculations
Electrochemistry
The most prominent bands in the UV/Vis spectra of the stud-
ied phosphaalkenes and their carbon analogues were assigned
on the basis of time-dependent DFT (TDDFT) calculations. In all
phosphaalkenes, the Mes* group was replaced by the electron-
ically very similar 2,6-dimethylphenyl substituent (as indicated
by a hash sign prior to each compound number). All structures
were fully optimized at the B3LYP/6-311G* level of theory. Opti-
mized structures were investigated with TDDFT at the same
level of theory with consideration of ten states for singlets and
The redox properties of the F9Ps and their all-carbon conge-
ners were examined by cyclic voltammetry. All observed trends
are very similar for the TMS- and phenyl-terminated alkynes,
and hence only the latter are discussed in detail. In addition,
first-principles calculations were performed on the compounds
and the results correlated to the experimental findings. Togeth-
er, it emerged that an interplay between the different acety-
lene substitution patterns and heteroatom incorporation dic-
tates the electrochemical properties of the compounds.
#
triplets each. While the intense absorption in 6b at 400 nm is
All F9Ps show a reversible reduction between À1.7 and
a single transition, two electronic transitions that involve the
complete conjugated system contribute to the experimentally
observed absorption in 26b around 345 nm. In addition, there
+
/0
À2.0 V versus ferrocenium/ferrocene (Fc ), and the parent
F9P 10 exhibits the most negative reduction potential (E
=
1/2
#
À1.99 V) of the series (Figure 5), which is attributed to the for-
mation of the anionic fulvenoid with phosphinyl radical. The
CVs of the all-carbon reference compounds 26a,b and diphen-
yl phosphaalkene 23 show only nonreversible reductions that
can be observed at considerably more negative potentials
is a weaker absorption in 6b with a maximum of approxi-
mately 552 nm giving rise to its orange color (onset of 525 nm
in the experimental UV spectrum of 6b). This electronic transi-
tion has charge-transfer character, since the donor orbitals are
mainly acetylene-based, whereas the accepting orbitals are
predominantly located at the dibenzofulvenoid core of the
[
15b]
(
Supporting Information).
[
33]
molecule (Figure 4).
The 3,6-disubstituted systems show a clear redshift for the
maximum-wavelength absorptions compared to their 2,7-sub-
stituted congeners. Compound 26a exhibits a broad maximum
at around 350 nm with a clear shoulder extending into the visi-
#
ble region. In the case of 6a, we observed a similar situation
with a broad feature around 400 nm with major contributions
Figure 5. Cyclic voltammograms of compounds 6a,b and unsubstituted F9P
À1
+/0
1
2 2 4 6
0. (1 mm in CH Cl , 0.1m NBu PF , n=100 mVs , vs. Fc ).
Calculations on 6a,b and 26a,b, showed that the LUMOs are
an antibonding combination of the entire fluorene/dibenzoful-
vene core (including the exocyclic double bond) with partial
contributions from the phenylacetylene fragments (Figure 6),
irrespective of the position of the acetylene substituents. Incor-
poration of the heteroelement in 6b leads to significant stabili-
zation of the LUMO compared to that of its all-carbon conge-
Figure 4. Electron-density difference maps of selected transitions. Isosurfaces
[
34]
were created with GaussSum and plotted with GabeEdit at 99%. Dark iso-
surfaces correspond to electron depletion, and light gray isosurfaces indicate
#
higher electron densities. 6a transition 1 [498 nm, HOMOÀ1!LUMO
#
ner 26b ( 6b: À2.75 eV versus 26b: À2.20 eV), which explains
[
(
[
78%], HOMO!LUMO (17%)]; 26a transition 1 [405 nm, HOMO!LUMO
the cathodically shifted reduction potential for the latter com-
pound. A similar situation of LUMO energies and shapes is
found for the 3,6-substituted compounds 6a and 26a. Com-
#
#
96%)]; 6b transition 1 [552 nm, HOMO!LUMO (96%)]; 6b transition 4
380 nm, HOMO!LUMO+1 (98%)]; 26b transition 1 [430 nm, HOMO!
LUMO (90%)].
Chem. Eur. J. 2016, 22, 4247 – 4255
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g
Figure 7. Calculated orbital energies and HOMO–LUMO splittings E for
#
model systems 6a,b and 26a,b. Black and gray bars denote occupied and
empty orbitals, respectively. The secondary axis (gray diamonds) is the
HOMO–LUMO splitting. All values are given in electron volts.
#
Figure 6. Frontier molecular orbitals of F9Ps 6a,b and their C analogues
6a,b. Calculated at the B3LYP/6-311G* level of theory.
perimentally observed decrease of the HOMO–LUMO gaps (by
UV/Vis onsets and CV experiments) for the phosphaalkene de-
rivatives is caused by a significant stabilization of the LUMO
levels, whereas the HOMO levels are almost unaltered in the C
and the P compounds. Consequently, the calculated static
HOMO–LUMO splittings Eg for the fluorene-9-ylidene phos-
phanes are between 2.8 and 2.9 eV, whereas their carbon ana-
2
paring the LUMOs of 6a and 6b (Figure 6) suggests that the
,6 substitution pattern allows a larger contribution from the
3
acetylene units, which gives rise to increased delocalization
that can be even detected in the reduction potentials in the
respective CVs, which differ by 70 mV.
logues show E values between 3.4 and 3.6 eV.
g
In the anodic scan, the differences in the CVs of 6a,b and
2
6a,b are more subtle. DFT investigations showed that, in the
Conclusion
case of 2,7 substitution, the HOMO extends over the p systems
of the phenylacetylene moieties and the fluorene core, but ex-
cludes the pp orbitals of the exocyclic double bond. In the ab-
sence of any contribution from the exocyclic double bond, the
We have developed flexible synthetic procedures for diacety-
lenic F9Ps. The synthetic versatility of compounds with TMS-
acetylene moieties was demonstrated by protodesilylation re-
actions to afford the terminal acetylenes, which could then be
further elaborated by Sonogashira cross-coupling or Glaser-
type homocoupling reactions to afford bis-thienyl-functional-
ized compounds and a butadiyne-bridged dimer, respectively.
Most intriguing are the significant differences in the frontier
molecular orbitals depending on heteroelement substitution
and variation of the acetylene substitution patterns. The pres-
ence of the P center causes a significant decrease in the
HOMO–LUMO splitting, as evidenced by UV/Vis spectroscopy
and cyclic voltammetry, and corroborated by DFT calculations.
The fluorene-9-ylidene phosphanes presented herein thus
open new possibilities for highly planar systems that incorpo-
rate an exocyclic phosphaethene unit as new functional build-
ing block for low-bandgap materials.
#
energies of the Kohn–Sham orbitals are very similar for 6b
and 26b (À5.56 and À5.58 eV, respectively). In contrast, the
HOMO of 6a is significantly different to that of its carbon ana-
logue 26a, as the HOMO of the former is rather asymmetric
and mainly localized on the side of the molecule that is oppo-
site to the bulky P-substituent (Figure 6). As a consequence,
the extent of p conjugation is decreased. In contrast to the sit-
#
uation in 26a and the 2,7-disubstituted compounds 6b and
#
26b, the HOMO in 6a has contributions from the pp orbitals
of the exocyclic phosphaalkene. Notably, this stabilization
counterbalances the destabilization that is caused by the more
localized p conjugation, and the calculated orbital energies are
#
rather similar for both 6a (À5.74 eV) and 26a (À5.85 eV). This
finding is also corroborated by the very similar experimental
oxidation potentials (Table 1 and Supporting Information).
Comparing the phenyl-terminated 6a,b with the TMS-termi-
nated F9Ps 7a,b shows that the phenyl termini in the former
compounds do not participate in the LUMOs, as the experi-
mentally observed reduction potentials are literally identical. In
contrast, the oxidation potentials are significantly shifted to
milder potentials when the phenyl termini are present, which
indicates a sizable contribution of the phenyl-based p system
to the HOMOs of the compounds.
Variation of the substitution patterns around the fluorene
core leads to significant changes in optical and electronic
properties. Whereas 3,6-disubstituted compounds have strong
absorptions around 400 nm, mainly due to p–p* transitions,
2,7-diacetylenic F9Ps exhibit longest-wavelength absorptions
that have significant charge-transfer (CT) character with an
onset around 520 nm. Such transitions are highly desirable for
the construction of future donor–acceptor arrays based on the
fluorene core, as previously described for classical all-carbon
[
12b,35]
Figure 7 summarizes the results for the calculated model
fluorenes.
Further enhancement of the CT character is the
#
compounds 6a,b and their carbon analogues 26a,b. The ex-
subject of ongoing work.
Chem. Eur. J. 2016, 22, 4247 – 4255
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2
1
JCP =17 Hz, Ar), 137.5 (d, JCP =15 Hz, Ar), 134.2 (d, J =57 Hz, i-
Experimental Section
CP
Mes*), 132.1 (Ar), 132.0 (Ar), 130.4 (JCP =2 Hz, Ar), 130.2 (JCP =4 Hz,
Ar), 127.5 (Ar), 127.4 (Ar), 127.3 (Ar), 126.2 (JCP =7 Hz, Ar), 123.6
Synthetic procedures were carried out under an inert atmosphere
by using modified Schlenk techniques unless otherwise stated. Re-
agents were obtained from commercial suppliers and used as re-
(
1
Ar), 123.4 (Ar), 122.9 (m-Mes*), 122.6 (Ar), 122.5 (JCP =8 Hz, Ar),
22.4 (d, JCP =7 Hz, Ar), 122.1 (d, JCP =3 Hz, Ar) 120.5 (d, JCP =24 Hz,
ceived. Solvents were freshly distilled over sodium or CaH . NMR
2
Ar), 94.1 (JCP =2 Hz, CꢀC), 93.8 (J =3 Hz, CꢀC), 84.1 (J =2 Hz, Cꢀ
CP
CP
spectra were recorded with a JEOL Eclipse+ spectrometer operat-
ing at a proton frequency of 400 MHz or a Varian Unity Innova in-
strument operating at a proton frequency of 300 MHz. The spectra
C), 83.6 (JCP =2 Hz, CꢀC), 38.3 (tBu), 35.3 (tBu), 32.9 (J =7 Hz, tBu),
CP
31
3
1.7 (tBu); P NMR (161 MHz, CDCl ): d=268.1; HRMS (solution in
3
+
CHCl /ACN, AgTFA) calcd for C H PS Ag [M+Ag] : 761.1433;
3
43 41
2
1
were referenced internally to residual solvent peaks ( H and
found: 761.1429.
13
31
C NMR) or externally to 85% aqueous H PO ( P NMR).
3
4
{
Bis[4-(phenylethynyl)phenyl]methylene}(2,4,6-tri-tert-butyl-
[
3,6-Bis(phenylethynyl)-(9H-fluoren-9-ylidene)](2,4,6-tri-tert-
phenyl)phosphane (23)
butylphenyl)phosphane (6a)
A solution of nBuLi (2.5m in hexane, 0.52 mmol, 0.21 mL) was
added dropwise at À788C to a stirred solution of diphenylmethane
A solution of nBuLi (2.5m in hexane, 0.52 mmol, 0.21 mL) was
added dropwise at À788C to a stirred solution of substituted fluo-
rene 2a (183 mg, 0.5 mmol) in THF (3 mL) and the reaction mixture
was stirred for 0.5 h. This solution was slowly transferred via cannu-
la to a solution of Mes*PCl2 (0.5 mmol, 0.174 g) in THF (4 mL)
cooled to À788C and the reaction mixture was stirred for 1 h. After
this, a solution of DBU (1m in THF, 0.75 mmol, 0.75 mL) was added
and the reaction mixture was allowed to warm to room tempera-
ture. The reaction mixture was passed through a pad of silica and
all volatile substances were evaporated. The solid residue was puri-
fied by chromatography on silica with 10% toluene in hexane.
19 (184 mg, 0.5 mmol) in THF (3 mL), and the reaction mixture was
stirred for 0.5 h. The resulting solution was added dropwise via
cannula to a solution of Mes*PCl (0.5 mmol, 0.174 g) in THF (4 mL)
2
at À788C and the reaction mixture was stirred for 1 h. After this,
the solvent was removed in vacuo. The residue was dissolved in
dry toluene (10 mL) and a solution of DBU (1m in THF, 0.75 mmol,
0
4
.75 mL) was added. The reaction mixture was heated to reflux for
8 h. After cooling to room temperature the reaction mixture was
passed through a pad of silica and all volatile substances were
evaporated. The solid residue was purified by chromatography on
1
3
Yield: 144 mg, 45%. H NMR (300 MHz, CDCl ): d=8.25 (dd, J
=
3
HH
1
4
silica with 20% toluene in hexane. Yield: 48 mg, 15%. H NMR
8
7
5
.0 Hz, J =4.3 Hz, 1H), 7.83 (brs, 1H), 7.76 (s, 1H), 7.68–7.46 (m,
PH
3
4
(300 MHz, CDCl ): d=7.61–7.45 (m, 6H), 7.45–7.28 (m, 10H), 7.04
3
H), 7.46–7.30 (m, 6H), 6.91 (dd, JHH =8.2 Hz, JHH =1.2 Hz, 1H),
3
3
4
3
4
(d, JHH =8.4 Hz, 2H), 6.40 (dd, JHH =8.4 Hz, JPH =1.3 Hz, 2H), 1.50
.00 (dd, J =8.2 Hz, J =2.6 Hz, 1H), 1.48 (s, 9H), 1.45 (s, 18H);
HH
PH
13
1
1
3
1
(s, 18H), 1.38 (s, 9H); C NMR (75 MHz, CDCl
47 Hz, P=C), 154.9 (o-Mes*), 151.3 (p-Mes*), 145.7 (d, JCP =28 Hz,
Ar), 142.2 (d, JCP =17 Hz, Ar), 135.5 (d, JCP =62 Hz, i-Mes*), 131.9
(Ar), 131.81 (Ar), 131.76 (Ar), 130.6 (d, JCP =2 Hz, Ar), 129.4 (d, JCP =
3
): d=178.7 (d, JCP
=
C NMR (75 MHz, CDCl ): d=168.9 (d, J =44 Hz, P=C), 154.6 (o-
3
CP
2
2
Mes*), 152.1 (p-Mes*), 143.2 (d, JCP =27 Hz, Ar), 138.9 (d, JCP
=
2
1
2
11 Hz, Ar), 138.3 (d, J =17 Hz, Ar), 137.7 (d, JCP =14 Hz, Ar), 134.4
CP
3
1
(
d, J =57 Hz, i-Mes*), 131.9 (Ar), 131.8 (Ar), 130.8 (JCP =2 Hz, Ar),
CP
3
7
Hz, Ar), 129.0 (d, J =18.6 Hz, Ar), 128.61 (Ar), 128.58 (Ar), 128.52
CP
1
7
30.6 (JCP =4 Hz, Ar), 128.7 (Ar), 128.6 (Ar), 128.5 (Ar), 126.4 (JCP
Hz, Ar), 123.6 (Ar), 123.5 (Ar), 123.04 (m-Mes*), 122.98 (Ar), 122.9
=
(
(
Ar), 128.49 (Ar), 123.6 (Ar), 123.5 (Ar), 123.0 (d, JCP =4 Hz, Ar), 122.3
m-Mes*), 121.7 (d, JCP =3 Hz, Ar), 90.7 (CꢀC), 90.3 (CꢀC), 89.8 (Cꢀ
(
Ar), 122.5 (JCP =2 Hz, Ar), 120.7 (d, JCP =24 Hz, Ar), 91.0 (JCP =3 Hz,
C), 89.7 (CꢀC), 38.4 (tBu), 35.3 (tBu), 33.4 (d, J =7 Hz, tBu), 31.8
CP
CꢀC), 90.6 (br, CꢀC), 90.3 (J =3 Hz, CꢀC), 38.5 (tBu), 35.5 (tBu),
CP
31
3
1
(tBu); P NMR (121 MHz, CDCl ): d=251.6; HRMS (solution in
3
3
2
3.0 (J =7 Hz, tBu), 31.8 (tBu); P NMR (121 MHz, CDCl ): d=
CP 3
+
CHCl /MeOH, AgTFA) calcd for (C H P) Ag [2M+Ag] : 1393.5900;
3
47 47
2
67.3; HRMS [solution in CHCl /MeOH, silver trifluoroacetate
3
+
found: 1397.5894.
(
AgTFA)] calcd for (C H P) Ag [2M+Ag] : 1389.5586; found:
47 45 2
1
389.5584.
Acknowledgements
[3,6-Bis(thiophen-2-ylethynyl)-(9H-fluoren-9-ylidene)](2,4,6-
tri-tert-butylphenyl)phosphane (9a)
The Swedish Research Council, the Gçran Gustafsson Founda-
tion, COST action CM1302 SIPs, the Lars-Hiertas Memorial Fund
(FO2014-0024), and Uppsala University (U3MEC molecular elec-
tronics priority initiative) are greatly acknowledged for financial
support. The authors would like to thank Dr. A. I. Arkhypchuk
for fruitful discussions. Y.V.S. is grateful to the Swedish Institute
for a scholarship in the Visby Program (382/00386/2012). A.O.
acknowledges the Austrian Science Funds (FWF, Project J3193-
N17).
Compound 9a was synthesized in analogy to the literature proce-
[20f]
dure.
[Pd(PPh ) Cl ] (0.03 mmol, 22 mg), CuI (0.06 mmol, 12 mg),
3 2 2
and aqueous K CO (2m, 1.5 mL) were added successively to a de-
2
3
gassed solution of 7a (0.32 mmol, 0.2 g) and 2-iodothiophene
0.63 mmol, 0.13 g) in THF (20 mL), MeOH (10 mL), and Et N (5 mL)
(
3
under an argon atmosphere. The reaction was monitored by TLC
2% EtOAc in hexane) and quenched after completion by the addi-
(
tion of brine (10 mL) after approximately 20 h. The reaction mix-
ture was extracted with EtOAc (3”50 mL) and the combined or-
ganic phases were dried over MgSO and concentrated in vacuo.
4
The product was purified by column chromatography (silica, 5%
1
Keywords: alkynes
fluorenes · phospaalkenes
·
conjugation
·
electrochemistry
·
EtOAc in hexane). Yield: 0.08 g, 38%. H NMR (400 MHz, CDCl ): d=
3
3
4
8
7
.22 (dd, J =7.8 Hz, J =3.8 Hz, 1H), 7.78 (s, 1H), 7.71 (s, 1H),
HH PH
.57 (s, 2H), 7.47 (dd, J =8.0 Hz, J =1.1 Hz, 1H), 7.37–7.20 (m,
H), 7.02 (m, 2H), 6.87 (dd, J =8.2 Hz, J =1.2 Hz, 1H), 5.00 (d,
HH HH
JHH =7.9 Hz, 1H), 1.46 (s, 9H), 1.42 (s, 18H); C NMR (100 MHz,
CDCl ): d=168.5 (d, JCP =44 Hz, P=C), 154.4 (o-Mes*), 152.0 (p-
Mes*), 143.1 (d, J =27 Hz, Ar), 138.6 (d, JCP =10 Hz, Ar), 138.2 (d,
3
4
HH
PH
3
4
5
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Received: August 28, 2015
Published online on February 2, 2016
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4255
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