(2,1-a)-Indenofluorene derivatives: Syntheses, X-ray structures, optical and electrochemical properties

Article abstract of DOI:10.1002/chem.201001830

Two novel fluorophores based on the (2,1-a)-indenofluorenyl backbone, dispiro[fluorene-9,11'-indeno[2,1-a]fluorene-12',9''-fluorene], (2,1-a)-DSF-IF and 11,12-dihydroindeno[2,1-a]fluorene (2,1-a)-IF have been prepared through original and efficient synthe

Full text of DOI:10.1002/chem.201001830

DOI: 10.1002/chem.201001830
ACHTUNGTRENNUNG(2,1-a)-Indenofluorene Derivatives: Syntheses, X-ray Structures, Optical and
Electrochemical Properties
Damien Thirion, Cyril Poriel,* Joꢀlle Rault-Berthelot, Frꢁdꢁric Barriꢂre, and
Olivier Jeannin[a]
Abstract: Two novel fluorophores
based on the (2,1-a)-indenofluorenyl
tures, the structural, optical and elec-
trochemical properties of these new
blue/violet emitters have been studied
in detail by a combined experimental
and theoretical approach. The proper-
ties of the (2,1-a)-DSF-IF and (2,1-a)-
IF are also compared to those of their
corresponding positional isomers based
on the (1,2-b)-indenofluorenyl back-
bone and those of related dispirofluor-
ene heteroacenes.
backbone,
indeno[2,1-a]fluorene-12’,9’’-fluorene],
(2,1-a)-DSF-IF and 11,12-
dihydroindeno[2,1-a]fluorene (2,1-a)-IF
dispiro[fluorene-9,11’-
ACHTUNGTRENNUNG
Keywords: blue/violet fluorescence ·
fluorescence
·
indenofluorene
·
ACHTUNGTRENNUNG
intramolecular cyclization · spiro
have been prepared through original
and efficient synthetic approaches.
After consideration of synthetic fea-
compounds
relationship
· structure–properties
Introduction
paved the way to the development of such molecules in the
field of organic electronics and the (1,2-b)-indenofluorenyl
core is nowadays a useful and a widely studied building
block not only for OLED applications[1,2,16–27] but also for
two-photon absorption[28] or OFET applications.[7,29–32] On
the contrary, the positional isomer of (1,2-b)-IF, namely
(2,1-a)-indenofluorene (2,1-a)-IF, has been very seldom
studied and has only been investigated for potential elec-
tronics applications very recently (Scheme 1).[33–36] An im-
portant feature of the two positional isomers is related to
their distinct geometric profiles. The methylene bridges are
Recent developments in the design of efficient materials for
organic electronics have shown that fused phenylene oligo-
mers are highly promising building blocks for such applica-
tions.[1,2] For example, Mꢀllenꢁs[3–6] and Markꢁs[7–9] groups
have developed highly efficient materials based on ladder
oligo- or polyphenylenes for various applications including
organic light emitting devices (OLED), organic field effect
transistors (OFET) and photovoltaic cells. Notably, rigid
and planar molecular structure of fluorene has turned this
molecule as one of the most promising material for blue
OLED applications.[1,2,10] The rigidity of the fluorene core
arising from the methylene bridging unit at the 2,2’-position
of the biphenyl core has led to the development of numer-
ous other bridged oligophenylene materials.[1,2] Of particular
interest was the structurally related (1,2-b)-indenofluorene
(1,2-b)-IF (Scheme 1), which possess some unique properties
due to the longer planar p-terphenyl backbone: for example,
increase of the quantum yield. Thus, the pioneering works
of Scherfꢁs and Mꢀllenꢁs groups on (1,2-b)-IF[11–15] have
[a] D. Thirion, Dr. C. Poriel, Dr. J. Rault-Berthelot, Dr. F. Barriꢂre,
Dr. O. Jeannin
Universitꢃ de Rennes 1, UMR CNRS 6226
“Sciences Chimiques de Rennes” Bat 10C
Campus de Beaulieu, 35042 Rennes cedex (France)
E-mail: cyril.poriel@univ-rennes1.fr
Scheme 1. ACTHNUTRGNEUNG
(1,2-b)-DSF-IF, (2,1-a)-DSF-IF, DSF-DIT[37] and their building
blocks (1,2-b)-IF, (2,1-a)-IF, DIT.[37] DIT: 10,11-dihydrodiindeno[1,2-
b:2’,1’-d]thiophene. DSF-DIT: dispiro[fluorene-9,10’-diindeno[1,2-b:2’,1’-
d]thiophene-11’,9’’-fluorene].
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001830.
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FULL PAPER
on opposite sides or on the
same side of the terphenyl core
in (1,2-b)-IF or (2,1-a)-IF, re-
spectively. As a consequence,
dispirofluorene derivatives (1,2-
b)-DSF-IF or (2,1-a)-DSF-IF
also possess their fluorene moi-
eties on opposite side or on the
same side, respectively, of the
terphenyl
backbone
(Scheme 1). Similar face-to-face
arrangement of the two fluo-
Scheme 2. Synthesis of (2,1-a)-DSF(R4)-IF and (1,2-b)-DSF(R4)-IF.
rene moieties found in (2,1-a)-
DSF-IF has been recently reported by Ohe and co-workers
with dispiro compounds, containing heterocycles such as
thiophene (DSF-DIT, Scheme 1).[37]
We thus started to investigate a new route towards (2,1-a)-
DSF-IF, which presents the advantage of being also suitable
for the synthesis of its parent hydrocarbon (2,1-a)-IF
(Scheme 3). This new approach is based on a coupling reac-
tion between the diketone 1 and a 2-halogenobiphenyl. The
key feature in this approach is to build up the (2,1-a)-inden-
ofluorenyl core prior to the final cyclization step in order to
avoid isomers formation.
The different synthetic approaches investigated in this
work for the synthesis of (2,1-a)-IF and (2,1-a)-DSF-IF are
presented in Scheme 3. We first consider access to (2,1-a)-IF
following the synthetic strategy reported by Covion Organic
Semiconductors (Scheme 3, route 1).[33]
As the synthesis of new materials with specific properties
is strongly sought worldwide for electronics applica-
tions,[1,2,38–40] it remains highly important to develop efficient
routes towards new fluorophores, such as (2,1-a)-indeno-
fluorene derivatives, in order to fully explore their potential
for organic electronic applications. This paper focuses on
new approaches towards the synthesis of fluorophores based
on the (2,1-a)-indenofluorenyl backbone namely dispiro[-
fluorene-9,11’-indeno
ACHTUNGTRENNUNG
DSF-IF and 11,12-dihydroindenoAHCNUTGTRENNUNG
After consideration of synthetic details, the structural char-
acterization, optical and electrochemical properties of these
new blue/violet fluorophores are reported in detail by a
combined experimental and theoretical approach. The prop-
erties of (2,1-a)-DSF-IF and (2,1-a)-IF are compared to
those of their corresponding (1,2-b)-indenofluorenyl posi-
tional isomers and those of the diindenothiophene deriva-
tives (DIT, DSF-DIT) recently reported by Ohe
(Scheme 1).[37] Such structures–properties relationships are
of great interest for the future design of efficient materials
for organic electronics.
Route 1 started with a cycloaddition reaction between
1,4-diphenyl-1,3-butadiene and dimethyl-but-2-ynedioate to
afford 2 with 85% yield. Dicarboxylate 2 was then dehydro-
genated in the presence of Pd/C to build up the terphenyl
backbone with the two methylcarboxylate groups in place.
Dimethylcarboxylate 3 was then reduced in its correspond-
ing diol 4 with diisobutyl aluminium hydride in dichlorome-
thane. The final step towards (2,1-a)-IF was the electrophilic
intramolecular cyclization reaction. Similar reactions have
been described for related molecules, either in concentrated
sulphuric acid at RT or 808C,[26,42] in acetic acid/hydrochloric
acid at 1008C[40,43] or in the presence of Lewis acids (TiCl4
or BF3·OEt2).[44–47] However, acidic treatment of 4 in these
different conditions failed to produce (2,1-a)-IF. In poly-
phosphoric acid at 1808C[33] we finally managed to isolate
(2,1-a)-IF with a low yield (less than 8%). This low yield
has been ascribed to intermolecular side oligomerization re-
actions. High dilution conditions and screening for solvent
and acid types (Brønsted vs Lewis acids) did not significant-
ly improve the yield of the reaction. After intensive scout-
ing, the best cyclization procedure found was to slowly add
a solution of diol 4 in 1,2-dichlorobenzene into polyphos-
phoric acid stirred at 1408C, giving (2,1-a)-IF with 27%
yield. This route offers a rapid access to (2,1-a)-IF but the
poor efficiency of the last cyclization step strongly hinders
the overall yield of the whole approach (18%).
Results and Discussion
Synthesis
Recently our group has developed an original synthetic ap-
proach towards spirofluorene–indenofluorene derivatives
based on a double lithium–halogen exchange reaction be-
tween 2,2’’-diiodo-[1,1’;4’,1’’]-terphenyl and tailored 9-fluore-
nones (see Scheme 2).[35,41] This synthesis involves in the last
step an intramolecular bicyclization reaction of a difluore-
nol. From a statistical point of view, this bicyclization may
occur on either side of the terphenyl backbone, leading to
the formation of two positional isomers possessing different
geometry: (2,1-a)- and (1,2-b)-DSF(R4)-IF (Scheme 2).
However, when R=H, the positional isomer with the face-
to-face fluorene moieties (2,1-a)-DSF-IF is only obtained
with a very low yield (<10%) and is difficult to separate by
column chromatography from its isomer (1,2-b)-DSF-IF.[41]
In the light of recent results for the synthesis of (1,2-b)-in-
denofluorenes,[22,31,46,48] ladder phenylenes,[3,42,46,49] isotrux-
enes[50–52] or for other fused aromatic molecules,[53,54] the syn-
thetic approach has been modified. In this context diketone
1[36,55] was the main target as this molecule would allow a
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13647

C. Poriel et al.
Scheme 3. Synthesis of (2,1-a)-IF and (2,1-a)-DSF-IF. PPA = Polyphosphoric acid.
straightforward access not only to (2,1-a)-IF through a
Wolf–Kishner reduction but also to (2,1-a)-DSF-IF through
a coupling reaction with 2-bromobiphenyl (Scheme 3). Thus,
the direct intramolecular cyclization of diester 3 was at-
already described for similar fused phenylene deriva-
tives.[26,49] The synthesis of (2,1-a)-DSF-IF was then carried
out through a two-step procedure. The lithium–bromine ex-
change of 2-bromobiphenyl with n-butyllithium at low tem-
perature, followed by addition of diketone 1 afforded diol 6
with 41% yield. It should be noted that several groups have
also reported low to moderate yields for similar coupling re-
actions.[22,51,52] Diol 6 was then involved in an intramolecular
cyclization reaction in the presence of boron trifluoride
etherate to provide the expected (2,1-a)-DSF-IF with an
overall yield of 38% over the two steps. Thus, the synthetic
strategy followed in route 2 (Scheme 3) provides (2,1-a)-IF
and (2,1-a)-DSF-IF with overall yields of 21 and 11%, re-
spectively. Route 2 is efficient and straightforward but still
suffers from the moderate yield of the intramolecular cycli-
zation reaction leading to key diketone 1. It should be
stressed that both (2,1-a)-IF and (2,1-a)-DSF-IF exhibit
good solubility in common organic solvent such as CH2Cl2
and THF, which is not the case for their corresponding (1,2-
b)-positional isomers (1,2-b)-IF and (1,2-b)-DSF-IF.[27] This
is of great importance for further solution-processable elec-
tronic applications. Thus, the present findings suggest that
diketone 1 has great potential as key structure to prepare
various oligomers with the (2,1-a)-indenofluorenyl back-
bone.
tempted. However, acidic treatment (concentrated
[26,48]
H2SO4
or various Lewis acids at RT) of diester 3 or of
dicarboxylic acid 5 only led to traces of 1. Even the Lewis
acid-promoted intramolecular Friedel–Crafts acylation
(TiCl4) of the acid dichloride prepared from 5 (oxalyl chlo-
ride in DMF at RT) known to be highly efficient for such
cyclization reaction,[46,53,56] did not give 1. We note, however,
that these different cyclization attempts at room tempera-
ture have led to the unexpected formation of the corre-
sponding phthalic anhydride derivative, namely 4,7-diphen-
yl-2-benzofuran-1,3-dione, through an intramolecular side
reaction (see Supporting Information). By increasing the
temperature to 808C the intramolecular cyclization of 3 or 5
in concentrated H2SO4 gave diketone 1 with a very low
yield (ca. 8%). Yang et al. have recently reported new
access to isotruxenone derivatives using similar intramolecu-
lar cyclization reaction and have highlighted the dramatic
effect of the temperature on the yield of the reaction.[50] Fol-
lowing the Yangꢁs conditions (5 min at 1408C in concentrat-
ed H2SO4) the cyclization of either 3 or 5 (Scheme 3, route
2) has finally led to the formation of the expected diketone
1 though with low to moderate yields (12 and 38%, respec-
tively). It is important to note that under similar conditions,
that is, 5 min at 1408C in H2SO4, the phthalic anhydride has
also led to the formation of 1 (yield: ꢀ15–30%, see Sup-
porting Information). It should be noted that diketone 1 has
also been obtained by the oxidation of the two methylene
bridges of (2,1-a)-IF in the presence of chromium oxide in
acetic anhydride with 75% yield (Scheme 3, route 1).
The molecular structure of (2,1-a)-IF and (2,1-a)-DSF-IF
1
were determined by H and 13C NMR spectroscopy and by
mass spectrometry. Moreover, the molecular arrangement of
(2,1-a)-DSF-IF were also confirmed by HMBC NMR ex-
periments (see Figure in Supporting Information). Indeed,
we have recently shown that spiro carbons are convenient
probes to detect neighboring hydrogen atoms through long-
range shift correlations.[34]
With key diketone 1 in hand, (2,1-a)-IF was finally ob-
tained in basic media through a Wolf–Kishner reduction as
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ACHTUNGTRENNUNG(2,1-a)-Indenofluorene Derivatives
FULL PAPER
Physicochemical properties
(2,1-a)-IF (Figure 1, bottom) and has been ascribed to p-
stacking interactions in (2,1-a)-DSF-IF, induced by the face-
to-face arrangement of the fluorene units. Indeed, in (2,1-a)-
IF the methylene carbons are nearly within the (2,1-a)-in-
denofluorenyl plane (defined through the central phenyl
ring 2). The distance between the (2,1-a)-indenofluorenyl
plane (defined through the central phenyl ring 2) and the
two methylene carbons have been evaluated around 0.03 ꢅ.
In addition, (2,1-a)-IF presents a flat structure, 10.8 ꢅ long,
with only two small distortions on both sides (dihedral
angles between the plane of the central phenyl ring 2 and
those of the side rings 1 and 3 are about 1.9 and 2.18, respec-
tively). All these observations lead us to conclude that in
the crystal structure of (2,1-a)-DSF-IF p-stacking interac-
tions occur between face-to-face fluorene units. The magni-
tude of the p-stacking interactions can be viewed as
“medium to weak” if they exhibit rather long centroid–cent-
roid distances (>4.0 ꢅ) together with large slip angles
(>308) and vertical displacements (d>2.0 ꢅ). In contrast,
“strong” p-stackings show rather short centroid–centroid
contacts (<3.8 ꢅ), small slip angles (<258) and vertical dis-
placements (<1.5 ꢅ), which translate into a sizable overlap
of the aromatic planes.[61–63] In (2,1-a)-DSF-IF the ring–cent-
roid/ring–centroid distances between two face-to-face
phenyl rings were estimated at about 3.65 and 3.81 ꢅ (see
Supporting Information). The vertical displacements and
ring slippages between two face-to-face phenyl rings span a
narrow range of 1.27 to 1.7 ꢅ for the vertical displacements
and of 20.4 to 26.58 for the ring slippage angles (see Sup-
porting Information). Such values of ring–centroid/ring–
centroid distances, vertical displacements and ring slippage
angles in (2,1-a)-DSF-IF may thus indicate “rather strong”
p-stacking interactions in the solid state between the two
face-to-face fluorene units.[61–63]
Structural properties: Single crystals of (2,1-a)-DSF-IF and
(2,1-a)-IF were obtained by slow diffusion of hexane in
CDCl3 solution and analyzed by X-ray diffraction in order
to elucidate their molecular structure (Figure 1).
Figure 1. Views of the molecular structure of (2,1-a)-DSF-IF (top) and
(2,1-a)-IF (bottom) from single crystal X-Ray diffraction data (hydrogen
atoms have been omitted for clarity).
ACHTUNGTRENNUNG(2,1-a)-DSF-IF is a dispiro molecule with a linear suprafa-
cial geometry (Figure 1, top).[57] The (2,1-a)-indenofluorenyl
core has a maximum length of 10.8, 0.3 ꢅ shorter than the
(1,2-b)-indenofluorenyl core.[27] Moreover, the (2,1-a)-inden-
ofluorenyl core presents two distortions on both sides as
previously observed for similar structures.[27,34,37] The dihe-
dral angles between the plane of the central phenyl ring 2
and those of the side rings 1 and 3 have been measured to
about 3.1 and 6.98, respectively (see phenyl rings labelling in
Figure 1). The distance between the two spiro carbons is
3.43 ꢅ, very close to the distance measured for its congener
(2,1-a)-DSF
N
Optical properties: The absorption and emission spectra of
(2,1-a)-IF and (2,1-a)-DSF-IF were measured in THF and
compared to their respective isomers (1,2-b)-IF and (1,2-b)-
DSF-IF, Table 1. The synthesis and the physico-chemical
properties of (1,2-b)-IF and (1,2-b)-DSF-IF have been re-
ported previously.[27] The absorption spectrum of (2,1-a)-IF
presents well-defined bands with two maxima at 307 and
322 nm (Figure 2, left). The absorption maximum of (2,1-a)-
IF is blueshifted by 12 nm compared to that of (1,2-b)-IF
and hence the (2,1-a)-IF band gap (3.76 eV) determined
from the edge of the UV/Vis absorption spectrum, is slightly
larger than that determined for (1,2-b)-IF (3.61 eV). This
band gap contraction suggests a better delocalization of p-
electrons in (1,2-b)-IF compared with (2,1-a)-IF. To the best
of our knowledge, similar examples of band gap contractions
between methylene-bridged p-terphenyl isomers are absent
from the literature. Band gap contractions, however, have
been previously reported in the literature between structur-
ally related p-terphenyl derivatives bridged by two ethene
sured for (2,1-a)-IF (3.35 ꢅ, Figure 1, bottom). This distance
between the two spiro carbons is between that measured for
p-stacked polyfluorenes (around 2.7 ꢅ) described by Ra-
thore and co-workers,[58] and those recently reported by Ohe
and co-workers for similar dispirofluorene–heteroacenes de-
rivatives with face-to-face fluorene moieties (>3.7 ꢅ), as for
example DSF-DIT[37] (see Scheme 1, bottom right; it should
also be mentioned that an analogue to DSF-DIT incorporat-
ing a thiophene ring[59] has been recently reported). In these
dispirofluorene–heteroacenes[37] the fluorene units display
an eclipsed conformation. Oppositely, in the p-stacked poly-
fluorenes described by Rathore[58] and those described by
Nakano[60] the fluorene units are not arranged in a perfect
face-to-face manner but are slightly staggered. Within these
polyfluorenes, through-space electronic interactions between
the p-stacked fluorenes have been shown.[58,60] In (2,1-a)-
DSF-IF, the fluorene units are also staggered since one of
the spiro carbon points slightly above the (2,1-a)-indeno-
fluorene plane (defined through the central phenyl ring 2)
while the other spiro carbon points below this plane. This
structural feature is not found in the crystal structure of
linkers, that is, picene and dibenzoACTHNUTRGNEUNG
[1,2;5,6]anthracene.[64]
The absorption spectrum of the dispiro derivative (2,1-a)-
DSF-IF, also presents well-defined bands with two maxima
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13649

C. Poriel et al.
Table 1. Optical data for (1,2-b)-IF, (2,1-a)-IF, (1,2-b)-DSF-IF and (2,1-a)-DSF-IF.
[c]
labs [nm]
lex [nm]
lem [nm]
Fliq
lemꢁlabs [nm]
DEopt [eV][d]
Ref.
[27]
G
289, 302, 319, 328, 334
307, 315, 322
298, 310, 328, 336, 345
295, 311, 323, 339
334
308
310
311
339, 347, 356
326, 343, 360
348, 355, 366, 388
345, 363, 380, 400
61
60
62
60
5
4
3
6
3.61
3.76
3.51
3.57
N
this work
this work
this work
G
ACHTUNGTRENNUNG
[a] In decalin. [b] In THF. [c] The relative quantum yield was measured with reference to quinine sulphate in 1n H2SO4 (F=0.546).[26] [d] Calculated
from the edge of the absorption spectrum using DEopt =hc/l.
The emission spectrum of
(2,1-a)-IF presents two main
bands at 326 and 343 nm and a
shoulder at 360 nm (Figure 2,
right), hypsochromically shifted
by 13 nm compared to its posi-
tional isomer (1,2-b)-IF. This
hypsochromic shift is similar to
that recorded by UV/Vis spec-
troscopy (see above). Because
of the rigid and planar structure
Figure 2. Left: Absorption spectra of (2,1-a)-DSF-IF (10ꢁ6 m in THF, g) and (2,1-a)-IF (10ꢁ6 m in THF, c).
of (2,1-a)-IF, the Stokes shift is
very small (4 nm) highlighting
Right: Emission spectra of (2,1-a)-DSF-IF (10ꢁ6 m in THF, g) and (2,1-a)-IF (10ꢁ6 m in THF, c).
very little rearrangements in
the excited state. Moreover
at 323 and 339 nm, redshifted by 16–17 nm from those of
(2,1-a)-IF (Figure 2, left). Thus, the incorporation of spiro-
fluorenes within the (2,1-a)-indenofluorenyl framework
leads to a bathochromic shift of the absorption maximum
and hence to a contraction of the band gap from 3.76 eV for
(2,1-a)-IF to 3.57 eV for (2,1-a)-DSF-IF. Ohe and co-work-
ers have recently highlighted a similar red shift within a
closely related series of DIT derivatives including some with
face-to-face fluorene units (Scheme 1, right).[37] Indeed, the
absorption spectrum of DSF-DIT presents a maximum
wavelength (lmax =373 nm) redshifted by 19 nm compared
(2,1-a)-IF is highly fluorescent with a quantum yield of
about 60% (compared to quinine sulphate) that is similar to
the quantum yield of (1,2-b)-IF.[26,27] The emission spectrum
of (2,1-a)-DSF-IF also exhibits two main bands at 345 and
363 nm, a smaller one at 380 nm and a shoulder at 400 nm
(Figure 2, right), blue shifted by around 19–20 nm compared
to (2,1-a)-IF. A similar shift of 18 nm has also been high-
lighted in fluorescence spectroscopy by Ohe and co-workers
between DSF-DIT and DIT.[37] As mentioned above this
shift is larger than that observed between the emission
maxima of (1,2-b)-IF and (1,2-b)-DSF-IF and may be attrib-
uted to the different electronic effect of the two cofacial flu-
orenes, found in (2,1-a)-DSF-IF, on the indenofluorenyl core
compared to the two non-directly interacting fluorene moi-
eties, found in (1,2-b)-DSF-IF. Finally, with a high quantum
yield (60%) and a very small Stokes shift (6 nm), (2,1-a)-
DSF-IF possess similar properties to those presented above
for its central constituting core (2,1-a)-IF.
Despite the absence of a typical excimer emission on the
fluorescence spectrum of (2,1-a)-DSF-IF (i.e., no large
Stokes shift, a low quantum yield and a lack of fine vibronic
structure)[35] the calculated optimized geometries in the
ground state and the first excited singlet state show an inter-
esting structural change (Figure 3). In the ground state
(Figure 3, left), the optimized geometry of (2,1-a)-DSF-IF
shows a typical up–down arrangement of the spiro carbon
atoms yielding a staggered conformation of the face-to-face
fluorene groups. Indeed, the calculated spiro–spiro torsion
angle is 9.98 in the optimized geometry, a value consistent
with that measured in the crystal structure (12.18) and with
other examples of this type of molecules.[34,35] On the other
hand, the optimized geometry of (2,1-a)-DSF-IF in the first
to that of its central constitutive building block, DIT (lmax
=
354 nm). On the contrary, the introduction of the two fluo-
rene units through the spiro linkages within the (1,2-b)-in-
denofluorenyl core leads to a smaller red shift of the absorp-
tion bands (ꢀ10–11 nm) and hence to a weaker contraction
of the band gap (3.61 eV for (1,2-b)-IF vs 3.51 eV for (1,2-
b)-DSF-IF, see Table 1). The amplitude of the red shift mea-
sured in the (1,2-b)-series is comparable to the red shift
measured between fluorene and 9,9’-spirobifluorene
(8 nm).[27] Other similar red shift between spiro and non-
spiro derivatives have been also reported in the literature
and notably assigned to spiroconjugation.[49,65,66] Conse-
quently, the larger red shift observed in the absorption spec-
tra between (2,1-a)-IF and (2,1-a)-DSF-IF (16–17 nm), simi-
lar to that observed by Ohe and co-workers (19 nm), may
be assigned to the specific face-to-face arrangement of the
two spirofluorene units. Indeed, the electronic influence on
the indenofluorene core of a cofacial fluorene p-dimer as
found in (2,1-a)-DSF-IF, should be different than that of
two non-directly interacting fluorene units as found in (1,2-
b)-DSF-IF.
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ACHTUNGTRENNUNG(2,1-a)-Indenofluorene Derivatives
FULL PAPER
Table 2. Electrochemical data and HOMO–LUMO levels for (1,2-b)-IF, (2,1-a)-IF, (1,2-b)-DSF-IF and (2,1-a)-DSF-IF.
Eox [V]/SCE[a]
Ered [V]/SCE[b]
HOMO [eV][c]
LUMO [eV][d]
LUMO [eV][e]
Ref.
[67]
G
1.31 (1e), 2.01 (>1e)
1.31 (1e), 1.98 (>1e)
1.47 (1e), 1.95 (>1e)
1.36 (1e), 1.69 (1e), 1.99 (>1e)
ꢁ2.55
ꢁ5.61
ꢁ5.62
ꢁ5.76
ꢁ5.64
ꢁ1.99
ꢁ1.94
ꢁ2.17
ꢁ2.03
ꢁ2.00
ꢁ1.86
ꢁ2.25
ꢁ2.07
G
ꢁ2.6
this work
[27]
G
ꢁ2.32
G
ꢁ2.48, ꢁ3.00
this work
[a] Obtained from CVs recorded in CH2Cl2/ACHTUGNTERNN[UGN NBu4]ACHTUGNTERNNUG[PF6] 0.2m. [b] Obtained from CVs recorded in DMF/ACHTNUEGRTG[NNUN NBu4]ACTHNUGTREN[UNGN PF6] 0.1m. [c] Calculated from the onset
oxidation potential. [d] Calculated from the onset reduction potential. [e] Calculated from the HOMO energy level and the edge of optical band gap.
singlet excited state (Figure 3, right), shows that the indeno-
fluorene core is now close to planarity with a calculated
spiro–spiro torsion angle of 2.38, and yield as a consequence,
an eclipsed arrangement of the face-to-face fluorene groups.
This significant structural change between the ground state
and the first excited singlet state of (2,1-a)-DSF-IF is consis-
tent with the larger Stokes shift observed for (2,1-a)-DSF-IF
compared with that of (1,2-b)-DSF-IF (6 vs 3 nm, Table 1).
Indeed, little structural change is found between the ground
state and the first excited singlet state of the (1,2-b)-DSF-IF
isomer (see Supporting Information). The geometry change
in (2,1-a)-DSF-IF is likely to be at the origin of the spectac-
ular excimer emission demonstrated previously for derived
molecules bearing aryl groups at the 2,7-positions of the
face-to-face fluorene moieties.[35,41]
redox wave with a maximum at ꢁ2.6 V (see CV in Support-
ing Information).
Figure 4. Cyclic voltammetry at 100 mVsꢁ1 in CH2Cl /
2 ACHTNUGTRNNEUG[NBu4]CAHTUNGTRENN[UGN PF6] 0.2m.
A)–B): (2,1-a)-IF 10ꢁ3 m, 1 cycle between 0.2 and 2.2 V (A), 10 cycles be-
tween 0.2 and 2.05 V (B). Inset in A): 1 cycle between 0.2 and 1.4 V. C):
Modified electrode (oxidation of (2,1-a)-IF 10ꢁ3 m at 2.0 V for 180 s) in-
vestigated in a (2,1-a)-IF free solution, 3 cycles between 0.4 and 2.4 V.
Figure 3. Optimized geometry of (2,1-a)-DSF-IF in the ground state (left,
spiro–spiro torsion angle 9.98) and in the first singlet excited state (right,
spiro–spiro torsion angle 2.38) showing the change in the relative ar-
rangement of the face-to-face fluorene groups.
For (2,1-a)-DSF-IF the first oxidation is also reversible
and monoelectronic. It occurs at E1 =1.36 V (inset Fig-
ure 5A, Table 2) and has been ascribed to the oxidation of
the indenofluorenyl core, as in the case of (1,2-b)-DSF-IF
(E1 =1.47 V).[27] This assignment is consistent with the calcu-
lated indenofluorene character of the HOMO (Figure 6,
bottom). This wave is anodically shifted by 70 mV compared
to that of (2,1-a)-IF. This shift is smaller than that observed
in the (1,2-b) series: 160 mV between (1,2-b)-DSF-IF and
(1,2-b)-IF. Again, and as already highlighted in the optical
properties section (see above), the structural arrangement
of the fluorene units (face-to-face or not) has a crucial
effect on the electronic properties of the indenofluorenyl
core. One can thus assume that the electron-withdrawing
effect of the cofacial fluorene p-dimer found in (2,1-a)-DSF-
IF, is weaker than the non-cofacial fluorenes found in (1,2-
b)-DSF-IF. To the best of our knowledge similar examples
appear to be very rare in the literature. House and co-work-
ers have nevertheless reported diphenylanthracene deriva-
Electrochemical properties: Electrochemistry of (2,1-a)-IF
and (2,1-a)-DSF-IF were investigated by cyclic voltammetry
(CV) and differential pulse voltammetry (DPV, see Support-
ing Information). Anodic and cathodic investigations were
conducted in dichloromethane and dimethylformamide
(DMF), respectively, and redox potentials were referenced
to the saturated calomel electrode (SCE). Energy levels
values have been calculated from the electrochemical meas-
urements, Table 2.
The first oxidation of (2,1-a)-IF is reversible (see inset of
Figure 4A) and occurs at a potential (E1 =1.31 V) identical
to that reported for (1,2-b)-IF.[67] This first oxidation is fol-
lowed at higher potentials by an irreversible process, about
twice in amplitude, with a maximum recorded at a potential
(E2 =1.98 V) almost identical to that reported for its posi-
tional isomer (1,2-b)-IF (E2 =2.01 eV).[67] Reduction of (2,1-
a)-IF (in DMF/ACHTUNGTRENNUNG[NBu4]ACHTUGNTREN[NUGN PF6] 0.1m) presents a quasi-reversible
Chem. Eur. J. 2010, 16, 13646 – 13658
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A second oxidation process (isoelectronic to the first) and
a third oxidation process are observed for (2,1-a)-DSF-IF at
1.69 and 1.99 V (see DPV in Supporting Information). The
second wave is partially reversible and is accompanied along
recurrent sweeps by a weak polymerization process (see
Supporting Information). Reduction of (2,1-a)-DSF-IF (in
DMF-[NBu4]ACHTUNRGTNEUNG[PF6] 0.1m) presents two successive redox
waves with maxima at ꢁ2.48 and ꢁ3.00 V, respectively, the
first being twice in intensity compared to the second (see
CV and DPV in Supporting Information).
From the redox and optical data (Tables 1 and 2), we
could estimate the lowest unoccupied molecular orbital
(LUMO) energy level and the highest occupied molecular
orbital (HOMO) energy level.[69,70] The onset oxidation po-
tentials of (2,1-a)-IF and (1,2-b)-IF (1.22 and 1.21 V, respec-
tively) are almost identical and lead to HOMO level energy
values around ꢁ5.6 eV for the two compounds (Table 2).
The LUMO levels of (2,1-a)-IF and (1,2-b)-IF, calculated as
follows: ELUMO (eV)= DEopt + EHOMO are thus lying at
ꢁ1.86 and ꢁ2 eV, respectively. The LUMO energy levels of
(2,1-a)-IF and (2,1-a)-DSF-IF have been also calculated
(ꢁ1.94 and ꢁ2.03 eV, respectively) from the onset reduction
potentials (ꢁ2.46 and ꢁ2.37 V, respectively, vs SCE) and are
in accordance with the LUMO calculated from the optical
band gap (Table 2). Such a trend in the LUMO energy
levels has been previously observed in the dibenzo-
Figure 5. Cyclic voltammetry at 100 mVs ꢁ1in CH2Cl /
2 ACHTNUGTRNNEUG[NBu4]CAHTUNGTRENN[UGN PF6] 0.2m.
A)–B): (2,1-a)-DSF-IF 2ꢆ10ꢁ3 m, 1 cycle between 0.2 and 2.25 V (A),
cycles 1, 5 and 10 between 0.2 and 2.25 V (B). Inset in A): cycles 1 and 3
between 0.25 and 1.55 V. C): Modified electrode (oxidation of (2,1-a)-
DSF-IF at 2.05 V for 30 s) investigated in a (2,1-a)-DSF-IF free solution,
3 cycles between 0.25 and 2.0 V.
tives where the phenyl rings can either be in a close face-to-
face arrangement or not.[68] Thus, the oxidation potential of
the anthracenyl core seemed to be influenced by the differ-
ent electronic effects caused by the different arrangements
of the phenyl rings. Indeed, when the phenyl rings are p-
stacked the oxidation potential of the anthracenyl core is
lower (1.30 V/SCE) than when phenyl rings are not p-
stacked (1.34 V/SCE).[68]
AHCTUNGTREG[NNNU 1,2;5,6]anthracene/picene series in which the LUMO level
of picene is slightly higher than the LUMO level of diben-
zoanthracene.[64]
Oppositely to (2,1-a)-IF and (1,2-b)-IF which possess
identical HOMO level energy values (see above), (2,1-a)-
DSF-IF presents a HOMO level lying at about ꢁ5.64 eV,
slightly above that reported for its isomer (1,2-b)-DSF-IF
(ꢁ5.76 eV, Table 2).[27] Theoretical calculations may shed
light on this difference between the spiro and the non-spiro
series. In the case of (1,2-b)-DSF-IF, theoretical calculations
point to an indenofluorenyl-based primary oxidation as
shown by the calculated nature of the HOMO level
(Figure 6, bottom left).[27] However, the HOMO of (2,1-a)-
DSF-IF presents a mixed character with major coefficients
found on the (2,1-a)-indenofluorenyl core but with neverthe-
less a significant contribution of the fluorene units (Figure 6,
bottom right). Such a difference in the character of the
HOMO level in the di-spiro series has been assigned to the
presence in (2,1-a)-DSF-IF of two face-to-face fluorene
units. This feature may hence explain the different HOMO
level energy values between (2,1-a)-DSF-IF and (1,2-b)-
DSF-IF. Thus, the character of the HOMO of (2,1-a)-DSF-
IF significantly extends into the spiro-linked fluorene units
(oppositely to (1,2-b)-DSF-IF), leading to increased conju-
gation. The increased delocalization is hence consistent with
the larger red shift observed in UV/Vis spectroscopy (see
above) between the (2,1-a) series, (2,1-a)-IF vs (2,1-a)-DSF-
IF, and the (1,2-b) series, (1,2-b)-IF vs (1,2-b)-DSF-IF.
The LUMO levels of (2,1-a)-DSF-IF and (1,2-b)-DSF-IF
do not follow the trend observed for the HOMO levels. The
LUMO energy level of (2,1-a)-DSF-IF is indeed lying at
Figure 6. Gas-phase DFT calculations (B3LYP/6-31G*) for (1,2-b)-DSF-
IF, (1,2-b)-IF, (2,1-a)-IF and (2,1-a)-DSF-IF (from left to right). The cal-
culated frontier molecular orbitals (after geometry optimization with
DFT) are shown with a cut-off of 0.04 [ebohrꢁ3 1/2. The HOMO/LUMO
]
energy levels have been estimated by electrochemistry and UV/Vis spec-
troscopy (see Table 2).
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ACHTUNGTRENNUNG(2,1-a)-Indenofluorene Derivatives
FULL PAPER
ꢁ2.07 eV (above the LUMO of its positional isomer (1,2-b)-
DSF-IF, ꢁ2.25 eV, Table 2). This trend in the LUMO energy
values is consistent with that of (2,1-a)-IF and (1,2-b)-IF
(LUMOs respectively lying at ꢁ1.86 and ꢁ2.00 eV, see
above). Accordingly, we note that for both (2,1-a)-DSF-IF
and (1,2-b)-DSF-IF theoretical calculations show a LUMO
almost exclusively centred on the indenofluorene core
(Figure 6) as opposed to the mixed character observed for
the HOMO level of (2,1-a)-DSF-IF (Figure 6, bottom right).
Similar differences between the electronic distributions of
the HOMO and the LUMO levels have been also highlight-
ed by Hintschich and co-workers[71] for a polyspirobifluorene
derivative.
stacked aryl moieties, have been previously reported for
dibenzobicycloACTHNUTRGNEUNG
[4.4.1]undecane[72] and for different p-systems
including among others naphthalene units bridged by bi-
phenyl linkages,[73] various oligofluorene derivatives[58,60] or
porphyrins dimers.[74] We also note that the oxidation of a
fluorene p-dimer reported by Rathore and co-workers
occurs at about 1.42 V/SCE.[58] The more anodic value found
for (2,1-a)-DSF-IF (1.69 V/SCE) may be simply accounted
for by the presence of a positive charge borne by the (2,1-
a)-indenofluorenyl core in [(2,1-a)-DSF-IF]+
C
.
Anodic polymerization: Recurrent scans involving the (2,1-
a)-IF two oxidation waves lead to a regular electropolymeri-
zation process (Figure 4B). The polymerization is evidenced
i) by the appearance and growth of new redox waves at po-
tentials lower than E1 and between E1 and E2 (see arrows in
Figure 4B), ii) by the gradual shift of the onset oxidation po-
tential and iii) by the coverage of the electrode with an in-
soluble thin film of poly-(2,1-a)-IF. Such electrodeposition
process has been previously observed for (1,2-b)-IF oxida-
tion.[67] However, the yield of polymerization estimated by
the measurement of the new waves intensity at each cycle
(Figure 4B) appears largely lower for (2,1-a)-IF compared
to (1,2-b)-IF.[67] Polymerization of (2,1-a)-IF is also observed
along oxidation at fixed potential. For example, a deposit
prepared by oxidation at 2 V for 180 s, removed from the
electrolyte, rinsed in dichloromethane and studied in a new
“monomer free” solution, presents the profile shown in Fig-
ure 4C. The CV of poly-(2,1-a)-IF exhibits two reversible
waves with maxima at 1.38 and 1.92 V and with an onset oxi-
dation potential at 0.67 V. This onset oxidation potential is
shifted by 550 mV towards less positive value compared to
that of its corresponding monomer (2,1-a)-IF (1.22 V). It is
important to note that the onset potential shift varies with
the thickness of the deposit and with the method used for
the electrodeposition process (recurrent sweeps along cyclic
voltammetry or chronoamperometry). This onset potential
difference between poly-(2,1-a)-IF and (2,1-a)-IF is consis-
tent with an extended p-conjugation in poly-(2,1-a)-IF as
previously observed for other electrogenerated p-conjugated
polymers.[25,75] In addition, poly-(2,1-a)-IF appears stable in a
wide potential range, that is, between 0.67 and 2.4 V. Poly-
(2,1-a)-IF is hence highly more stable than structurally relat-
ed electrogenerated polymer such as poly-(1,2-b)-indeno-
fuorene, poly-(1,2-b)-IF[67] or poly-fluorene, poly(F).[76]
Indeed, poly-(1,2-b)-IF and poly(F) present stable p-doping
processes between 0.75 and 1.76 V and between 0.97 and
1.5 V, respectively. Poly-(2,1-a)-IF also possess interesting
electrochromic properties, switching from yellow in its neu-
tral state to green and blue in different oxidation states at
higher potential values. These electrochromic properties
clearly sign different p-doping levels of the polymer. Given
the importance of electrochromic properties in material sci-
ence, the present polymers might be of interest.[77]
Regarding the subsequent oxidations, (1,2-b)-DSF-IF and
(2,1-a)-DSF-IF display significantly different behaviors.
Indeed, in (1,2-b)-DSF-IF the first oxidation (1.47 V,
Table 2) is followed by a multiple electron process at 1.95 V,
which may be assigned to the oxidation of the fluorenyl
units (as pointed out by the calculated nature of the SOMO
of [(1,2-b)-DSF-IF]+ Figure 7, left) and also most certainly
C
Figure 7. Calculated frontier molecular orbital SOMO (after geometry
optimization with DFT-B3LYP/6-31G*), shown with a cut-off of 0.04
1/2
+
[ebohrꢁ3
]
of [(1,2-b)-DSF-IF]+ (left) and [(2,1-a)-DSF-IF] (right).
C
C
to the second oxidation of the indenofluorenyl core. Note
that the second oxidation in (1,2-b)-IF occurs at 2.0 V,[67]
whereas the second oxidation of 9,9’-spirobifluorene occurs
at 1.86 V.[27] Oppositely to (1,2-b)-DSF-IF, (2,1-a)-DSF-IF is
oxidized along three reversible one-electron processes at
1.36, 1.69 and 1.99 V. The second reversible and monoelec-
tronic oxidation of (2,1-a)-DSF-IF (1.69 V) is not found
either for (2,1-a)-IF or (1,2-b)-DSF-IF. The redox potential
of the second reversible and monoelectronic oxidation of
(2,1-a)-DSF-IF wave is lower than the redox potential of
both the second oxidation of (2,1-a)-IF (1.98 V, Table 2) and
of (1,2-b)-DSF-IF (1.95 V, Table 2).[27] Such a difference in
the CVs of (1,2-b)-DSF-IF and (2,1-a)-DSF-IF may be ra-
tionalized by the nature of the SOMO involved in the
second monoelectronic oxidation (1.69 V) of [(2,1-a)-DSF-
IF]+ . Indeed, theoretical calculations indicate that the
C
SOMO of [(2,1-a)-DSF-IF]+ has mainly a fluorene charac-
C
ter (Figure 7, right). As it is known that the oxidation of p-
stacked systems is more facile than their non-stacked ana-
logues,[58,72,73] it is reasonable to assign the second oxidation
of (2,1-a)-DSF-IF to the face-to-face fluorene dimer. Analo-
gous observations highlighting interactions between two
AHCTUNGTREG(NNNU 2,1-a)-DSF-IF may be also polymerized by anodic oxida-
tion. Indeed, the electroplolymerization process is observed
at the third oxidation wave recorded at 2 V (Figure 5B)
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13653

C. Poriel et al.
whereas a very weak polymerization process is observed
along recurrent sweeps involving only the two first oxidation
processes (see Figure in Supporting Information). The (2,1-
a)-DSF-IF polymerization yield roughly evaluated by the
measurement of the new waves intensity at each cycle ap-
pears higher than that of (2,1-a)-IF, indicating that the poly-
merization occurs both through the indenofluorenyl and the
fluorenyl units. However the polymerization yield of (2,1-a)-
DSF-IF is lower than that of (1,2-b)-DSF-IF[27] clearly indi-
cating that the geometry of (2,1-a)-DSF-IF is less favourable
to the carbon-carbon coupling of the monomer units. As ex-
posed above the electrochemical behavior of poly-(2,1-a)-
DSF-IF varies with the method used for the deposition and
with the thickness of the deposit. For example, a poly-(2,1-
a)-DSF-IF deposit synthesized at fixed potential (2.05 V for
30 s) exhibits a reversible oxidation wave (Figure 5C) with a
maximum at 1.44 V. The onset potential of poly-(2,1-a)-
DSF-IF is recorded at 0.92 V, shifted by 280 mV towards
less positive values compared to its monomer (2,1-a)-DSF-
IF (1.2 V). Such a result is in accordance with extended p-
conjugation in the polymer. In addition, poly-(2,1-a)-DSF-IF
is stable in a wide potential range, that is, between 0.92 and
2 V, although slightly less stable compared to poly-(2,1-a)-
IF, which presents an electrochemical window of stability
between 0.67 and 2.4 V (see above). Poly-(2,1-a)-DSF-IF
also possesses electrochromic properties switching from
transparent to green and blue when increasing its p-doping
level. These electrochromic properties may be classically as-
signed to the p-doping level of the polymer but in part also
to the high flexibility of the fluorene arrangement in the
neutral, cation and dication states as suggested by theoreti-
cal calculations (Figure 8). Indeed, we note that geometry
discussed before, in the first singlet excited state (see
Figure 3).
Conclusion
In summary, we have developed new synthetic strategies
toward original fluorophores based on the rarely studied
(2,1-a)-indenofluorenyl core, namely (2,1-a)-DSF-IF and
(2,1-a)-IF. These molecules have been prepared by expedi-
ent syntheses through a key diketone intermediate. We be-
lieve that this intermediate might be of great interest for the
elaboration of other spiro or non-spiro derivatives based on
the (2,1-a)-indenofluorenyl core. The structural, optical and
electrochemical properties of (2,1-a)-DSF-IF and (2,1-a)-IF
have been studied in detail by a combined experimental and
theoretical approach. These properties have been compared
to their corresponding positional isomers, namely (1,2-b)-
DSF-IF and (1,2-b)-IF and also compared to structurally re-
lated diindenothiophene derivatives recently reported by
Ohe and co-workers.[37] As the (2,1-a)-indenofluorenyl core
is almost absent from the literature but seems to be promis-
ing for organic electronic applications, the present finding
may pave the way to the development of this rarely studied
class of molecules. The comparison of the di-spiro mole-
cules, (2,1-a)-DSF-IF and (1,2-b)-DSF-IF, with their respec-
tive central cores, (2,1-a)-IF and (1,2-b)-IF, have stressed
that the electronic properties of the present dispiro mole-
cules are mainly governed by the central indenofluorene
backbone but might be tuned by the structural arrangement
of the two p-systems linked through the spirocarbons. Intro-
duction of the (2,1-a)-indenofluorenyl core in organic elec-
tronic devices and elucidation of the excimer emission
found with aryl substituted (2,1-a)-DSF-IFs[35] are currently
underway in our laboratory.
Experimental Section
Synthesis: Commercially available reagents and solvents were used with-
out further purification other than those detailed below. Dichlorome-
thane was distilled from P2O5 drying agent Sicapent (Merck). 1,2-Dichlor-
obenzene was distilled from CaCl2 drying agent. THF was distilled from
sodium/benzophenone and toluene was distilled from sodium prior to
use. Light petroleum refers to the fraction with b.p. 40–608C. Reactions
were stirred magnetically, unless otherwise indicated. Analytical thin
layer chromatography was carried out using aluminium backed plates
coated with Merck Kieselgel 60 GF254 and visualized under UV light (at
254 and/or 360 nm). Chromatography was carried out using silica 60A
CC 40–63 mm (SDS). 1H and 13C NMR spectra were recorded using
Bruker 300 MHz instruments (1H frequency, corresponding 13C frequency
is 75 MHz); chemical shifts were recorded in ppm and J values in Hz.
The residual signals for the NMR solvents are: CDCl3; 7.26 ppm for the
proton and 77.00 ppm for the carbon, CD2Cl2; 5.32 ppm for the proton
and 53.80 ppm for the carbon, [D6]DMSO; 2.50 ppm for the proton and
39.52 ppm for the carbon. The following abbreviations have been used
for the NMR assignment: s for singlet, d for doublet, t for triplet, m for
multiplet and br for broad. High-resolution mass spectra were recorded
at the Centre Rꢃgional de Mesures Physiques de L’Ouest (Rennes) on a
Bruker MicrO-Tof-Q2 or on a Micromass MS/MS ZABspec Tof (EBE
Figure 8. Optimized geometries of (2,1-a)-DSF-IF in three incremental
oxidation states (0/1+/2+) from left to right, spiro–spiro torsion angle re-
spectively 9.9, 4.8 and 08.
optimizations of (2,1-a)-DSF-IF in the neutral, cation and
dication forms yield an interesting trend for the respective
arrangement of its cofacial fluorene units. As (2,1-a)-DSF-
IF gets oxidized, the fluorene moieties move towards an
eclipsed conformation, while the concurrent decrease of the
up-down tilt of the spiro carbon atoms brings the indeno-
fluorene core to planarity in the dication, Figure 8. Given
the importance of the fluorene arrangement for optical
properties of phenyl substituted derivatives,[35,41] the present
result may be significant in that it points to a large flexibility
of the fluorene arrangement upon gradual oxidation, and, as
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ACHTUNGTRENNUNG(2,1-a)-Indenofluorene Derivatives
FULL PAPER
Tof geometry) and reported as m/z. Elemental analyses were recorded at
the Service de Microanalyse-CNRS (Gif sur Yvette). Names of chemicals
have been generated with the naming service of ACD-I lab, which deter-
mines the chemical name according to systematic application of the no-
menclature rules agreed upon by the International Union of Pure and
Applied Chemistry and International Union of Biochemistry and Molec-
ular Biology.
Route 2: Compound 3 (200 mg, 0.577 mmol) or 5 (200 mg, 0.628 mmol)
was added to a solution of concentrated H2SO4 (30 mL) at 1408C and
stirred for 5 min. After cooling to room temperature the mixture was
poured into water (300 mL) and extracted with i) ethyl acetate and ii) di-
chloromethane. The combined organic layers were dried (MgSO4), the
solvent was removed in vacuo and the residue purified by column chro-
matography on silica eluting with light petroleum/ethyl acetate 7:3. Com-
pound 1 was obtained as an orange solid (19.5 mg, 12% starting from 3
or 67.3 mg, 38% starting from 5). M.p. (hexane) 2458C; 1H NMR
(300 MHz, CD2Cl2): d=7.68 (s, 2H; ArH), 7.67 (d, J=7.2 Hz, 2H; ArH),
7.60–7.50 (m, 4H; ArH), 7.36 ppm (td, J=7.5, 0.9 Hz, 2H; ArH);
13C NMR (75 MHz, CD2Cl2): d=190.8 (CO), 145.6 (C), 144.0 (C), 135.2
(C), 134.0 (C), 132.6 (CH), 129.9 (CH), 125.4 (CH), 124.8 (CH),
120.6 ppm (CH); IR: n=1721 (C=O), 1604, 1469, 1425, 1267, 1187,
Dimethyl 3,6-diphenylcyclohexa-1,4-diene-1,2-dicarboxylate (2): 1,4-Di-
phenyl-1,3-butadiene (12.5 g, 60.7 mmol) was dissolved in dry toluene
(60 mL) under argon. Dimethyl but-2-ynedioate (8.2 mL, 66.7 mmol) was
then added and the solution was stirred at reflux for 20 h. After evapora-
tion to dryness, the crude yellow solid was washed with i) 2-propanol and
ii) hexane. The title compound was obtained as a colorless solid (18.0 g,
85%). M.p. (hexane) 958C (lit. 988C);[78] 1H NMR (300 MHz, CDCl3):
d=7.40–7.26 (m, 10H; ArH), 5.79 (s, 2H; CH=), 4.48 (s, 2H; CH),
3.56 ppm (s, 6H; Me); 13C NMR (75 MHz, CDCl3): d=167.9 (COOMe),
141.2 (C), 135.6 (C), 128.7 (CH), 128.3 (CH), 127.1 (CH), 126.0 (CH),
52.0 (Me), 44.0 ppm (CH).
1084 cmꢁ1
;
UV/Vis (THF): lmax =389, 372, 290 nm; HRMS (ESI+,
MeOH): m/z: calcd for C20H10O2Na: 305.0579 [M+Na]+; found:
305.0581.
11,12-Di(biphenyl-2-yl)-11,12-dihydroindenoACTHNUTRGNE[UNG 2,1-a]fluorene-11,12-diol
(6): 2-Bromobiphenyl (0.58 mL, 3.40 mmol) was dissolved in dry and de-
gassed THF (20 mL) under argon. The mixture was cooled to ꢁ788C and
stirred at this temperature for 10 min. A solution of nBuLi (1.6m in
hexane, 2.13 mL, 3.41 mmol) was added dropwise in 4 min. The resulting
yellow solution was stirred for a further 50 min and ketone 1 (120 mg,
0.42 mmol), dissolved in dry and degassed THF (60 mL), was added
dropwise in 4 min via a cannula. The reaction was allowed to stir over-
night (from ꢁ788C to room temperature) and the resulting mixture was
poured into a saturated solution of ammonium chloride (50 mL) and ex-
tracted with dichloromethane and ethyl acetate. The combined extracts
were dried (MgSO4), the solvent removed in vacuo and the residue puri-
fied by column chromatography on silica eluting with light petroleum/
ethyl acetate 8:2. The resulting solid was washed with hexane to obtain 6
as a colorless solid (102 mg, 41%). M.p. (hexane) >3008C; 1H NMR
(300 MHz, CDCl3): d=8.00 (d, J=7.5 Hz, 2H; ArH), 7.67 (t, J=7.5 Hz,
2H; ArH), 7.43 (t, J=7.5 Hz, 2H; ArH), 7.25 (t, J=7.5 Hz, 2H;
ArH),7.18–7.14 (m, 4H; ArH), 7.06 (d, J=7.5 Hz, 2H; ArH), 6.94 (d, J=
7.5 Hz, 2H; ArH), 6.85 (s, 2H; ArH), 6.80 (t, J=7.5 Hz, 2H; ArH),
6.60–6.48 (m, 4H; ArH), 6.37 (d, J=7.5 Hz, 2H; ArH), 5.66 (d, J=
7.5 Hz, 2H; ArH), 1.88 ppm (s, 2H; OH); 13C NMR (75 MHz, CDCl3):
d=149.2 (C), 144.9 (C), 141.4 (C), 140.7 (C), 140.5 (C), 140.3 (C), 139.4
(C), 132.2 (CH), 129.0 (CH), 128.7 (CH), 128.6 (CH), 128.2 (CH), 127.8
(CH), 127.7 (CH), 126.7 (CH), 126.5 (CH), 125.9 (CH), 125.2 (CH),
124.0 (CH), 120.3 (CH), 120.0 (CH), 82.5 ppm (COH); IR: n=3527
(OH), 1469, 1428, 1167, 1027 cmꢁ1; HRMS (ESI+, MeOH/CH2Cl2 99:1):
m/z: calcd for C44H30O2Na: 613.2144 [M+Na]+, found: 613.2139.
Dimethyl 1,1’:4’,1’’-terphenyl-2’,3’-dicarboxylate (3): Compound 2 (10.0 g,
28.7 mmol) and palladium on carbon (5% Pd, 3.0 g, 33.8 mmol) were dis-
solved in dry toluene (100 mL) under argon and the solution was stirred
at reflux for 21 h. The hot mixture was filtered on Celite 545 and washed
with hot toluene. After evaporation to dryness, the solid was washed with
hexane. The title compound was obtained as a colorless solid (9.1 g,
91%). M.p. (hexane) 1618C; 1H NMR (300 MHz, CD2Cl2): d=7.54 (s,
2H; ArH), 7.46–7.35 (m, 10H; ArH), 3.60 ppm (s, 6H; Me); 13C NMR
(75 MHz, CD2Cl2): d=168.9 (COOMe), 140.2 (C), 140.1 (C), 132.4 (C),
132.1 (CH), 128.8 (CH), 128.6 (CH), 128.2 (CH), 52.7 ppm (Me); IR: n=
1741 (C=O), 1719 (C=O), 1437, 1311, 1231, 1157, 1070 cmꢁ1; HRMS
(ESI+, MeOH/CH2Cl2 99:1): m/z: calcd for C22H18O4K: 385.0842
[M+K]+; found: 385.0841.
1,1’:4’,1’’-Terphenyl-2’,3’-diyldimethanol (4): Compound
3
(3.9 g,
11.2 mmol) was dissolved in dry CH2Cl2 (60 mL) under argon. The solu-
tion was carefully added, in four portions, to a solution of diisobutylalu-
minium hydride (1m in hexane) (56.0 mL, 56.0 mmol) over 40 min. The
resulting mixture is stirred for 1.5 h, poured into a saturated solution of
ammonium chloride (40 mL) and extracted with dichloromethane. The
combined organic layers were dried (MgSO4) and the solvent removed in
vacuo. Recrystallization from hexane afforded 4 as a colorless solid
(2.9 g, 88%). M.p. (hexane) 1248C; 1H NMR (300 MHz, CD2Cl2): d=
7.49–7.38 (m, 10H; ArH), 7.35 (s, 2H; ArH), 4.73 (d, J=5.1 Hz, 4H;
CH2), 3.23 ppm (t, 2H; J=5.1 Hz, OH); 13C NMR (75 MHz, CD2Cl2):
d=142.9 (C), 141.5 (C), 138.3 (C), 130.2 (CH), 129.8 (CH), 128.5 (CH),
127.6 (CH), 60.8 ppm (CH2); IR: n=3370 (OH), 3299 (OH), 1744, 1466,
1396, 1336, 1205 cmꢁ1; HRMS (ESI+, MeOH/CH2Cl2 99:1): m/z: calcd
for C20H18O2Na: 313.1205 [M+Na]+; found: 313.1203.
11,12-DihydroindenoACTHNUTRGNEU[GN 2,1-a]fluorene ((2,1-a)-IF)
Route 1: Polyphosphoric acid (6 g) was stirred at 1408C for 5 min. Diol 6
(200 mg, 0.70 mmol) dissolved in 1,2-dichlorobenzene (30 mL) was then
added to the polyphosphoric acid over 10 min and the solution was
stirred for a further 5 min. After cooling to room temperature, the mix-
ture was poured into a 10% aqueous NaOH solution (30 mL) and ex-
tracted with dichloromethane. The combined extracts were dried
(MgSO4), the solvent removed in vacuo and the residue purified by
column chromatography on silica eluting with light petroleum/dichloro-
methane (9:1). The resulting solid was washed with hexane to obtain
(2,1-a)-IF as a colorless solid (47 mg, 27%).
1,1’:4’,1’’-Terphenyl-2’,3’-dicarboxylic acid (5): Compound
3 (1.4 g,
4.0 mmol) was dissolved in a mixture of ethanol (200 mL) and water
(15 mL). NaOH (6.5 g, 16.0 mmol) was added and the solution was
stirred at 908C for 16 h. After cooling to room temperature, water
(50 mL) was added, and concentrated HCl was added until pH 1. A col-
orless precipitate was obtained, filtered off and washed several times
with water. The title compound was obtained as a colorless solid (1.22 g,
95%). M.p. (hexane) >3008C; 1H NMR (300 MHz, [D6]DMSO): d=
13.14 (br, 2H; OH), 7.52 (s, 2H; ArH), 7.50–7.39 ppm (m, 10H; ArH);
13C NMR (75 MHz, [D6]DMSO): d=168.9 (COOH), 139.7 (C), 138.2
(C), 132.8 (C), 131.0 (CH), 128.5 (CH), 128.2 (CH), 127.7 ppm (CH); IR:
n=3371 (OH), 3231 (OH), 1743 (C=O), 1700 (C=O), 1452, 1418, 1214,
1186, 1157, 1068 cmꢁ1; HRMS (ESIꢁ, MeOH/H2O 95:5): m/z: calcd for
C20H13O4: 317.0819 [MꢁH]ꢁ; found: 317.0815.
Route 2: Compound 1 (30 mg, 0.11 mmol), hydrazine hydrate (200 mL,
3.30 mmol) and potassium hydroxide (150 mg, 2.67 mmol) were dissolved
in diethylene glycol (10 mL) and the solution was stirred at 1808C for
24 h. After cooling to room temperature, the mixture was poured into a
solution of concentrated HCl at 08C. A colorless precipitate was ob-
tained, filtered off and washed several times with water. (2,1-a)-IF was
IndenoACHTUNGTRENNUNG[2,1-a]fluorene-11,12-dione (1)
obtained as
a colorless solid (21 mg, 75%). M.p. (hexane) 2708C;
Route 1: (2,1-a)-IF (180 mg, 0.71 mmol) and chromium trioxide (495 mg,
4.95 mmol) were dissolved in acetic anhydride (60 mL) under argon. The
solution was stirred for 20 h and poured into a solution of 10% HCl at
08C and extracted with dichloromethane. The combined extracts were
dried (MgSO4), the solvent removed in vacuo and the residue purified by
column chromatography on silica eluting with light petroleum/ethyl ace-
tate 7:3. Compound 1 was obtained as an orange solid (150 mg, 75%).
1H NMR (300 MHz, CD2Cl2): d=7.82 (s, 2H; ArH), 7.81 (d, J=7.2 Hz,
2H; ArH), 7.59 (d, J=7.2 Hz, 2H; ArH), 7.38 (t, J=7.2 Hz, 2H; ArH),
7.30 (t, J=7.2 Hz, 2H; ArH), 3.95 ppm (s, 4H; CH2); 13C NMR (75 MHz,
CD2Cl2): d=143.6 (C), 142.3 (C), 141.1 (C), 139.9 (C), 127.2 (CH), 126.9
(CH), 125.5 (CH), 120.2 (CH), 119.0 (CH), 35.8 ppm (CH2); IR: n=1737,
1457, 1427, 1372, 1103 cmꢁ1; UV/Vis (THF): lmax =322, 315, 307 nm;
Chem. Eur. J. 2010, 16, 13646 – 13658
ꢄ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
13655

C. Poriel et al.
HRMS (ESI+, MeOH/CH2Cl2 90:10): m/z: calcd for C20H14: 254.1096
[M]+, found: 254.1096. elemental analysis calcd (%) for C20H14 C 94.45,
H 5.55; found C 94.34, H 5.54.
Computational details: Full geometry optimization with Density func-
tional theory (DFT)[79,80] and Time-Dependent Density Functional
Theory (TD-DFT) calculations were performed with the hybrid Becke-3
parameter exchange[81–83] functional and the Lee–Yang–Parr non-local
correlation functional[84] (B3LYP) implemented in the Gaussian 09 (Revi-
sion A.02) program suite[85] using the 6-31G* basis set[86] and the default
convergence criterion implemented in the program. The figures were
generated with MOLEKEL 4.3.[87]
Dispiro[fluorene-9,11’-indeno
G
((2,1-a)-
DSF-IF): Difluorenol 6 (80 mg, 0.14 mmol) was dissolved in dichlorome-
thane (60 mL) and stirred for 15 min at reflux. Boron trifluoride etherate
(48% BF3) (100 mL) was added and the resulting mixture was stirred 2 h
at reflux. After evaporation to dryness, the crude mixture was purified by
column chromatography on silica gel, eluting with light petroleum/ethyl
acetate 8:2 and washed with hexane. The title compound was obtained as
a colorless solid (72 mg, 93%). M.p. (cyclohexane) 2348C; 1H NMR
(300 MHz, CD2Cl2): d=8.08 (s, 2H; ArH), 7.79 (d, J=7.5 Hz, 2H; ArH),
7.25 (d, J=7.5 Hz, 4H; ArH), 7.20 (td, J=7.5 Hz, J=0.9 Hz, 2H; ArH),
7.01 (td, J=7.5 Hz, J=0.9 Hz, 4H; ArH), 6.83 (td, J=7.5 Hz, J=0.9 Hz,
2H; ArH), 6.59 (td, J=7.5 Hz, J=0.9 Hz, 4H; ArH), 6.15 (d, J=7.5 Hz,
4H; ArH), 5.97 ppm (d, J=7.5 Hz, 2H; ArH); 13C NMR (75 MHz,
CD2Cl2): d=151.2 (C), 146.1 (C), 143.9 (C), 143.7 (C), 141.1 (C), 140.2
(C), 127.8 (CH), 127.4 (CH), 127.3 (CH), 127.2 (CH), 123.3 (CH), 122.6
(CH), 120.5 (CH), 120.4 (CH), 119.9 (CH), 66.3 ppm (Cspiro); IR: n=
1635, 1446, 1071, 942 cmꢁ1; UV/Vis (THF): lmax =339, 323, 311, 295 nm;
HRMS (ESI+, CH2Cl2): m/z: calcd for C44H26: 554.2035 [M]+, found:
554.2034.
X-ray determination: Crystal was picked up with a cryoloop and then
frozen at 100 K under a stream of dry N2 on a APEX II Brucker AXS
diffractometer for X-ray data collection (MoKa radiation, l=0.71073 ꢅ).
Structure was solved by direct methods (SIR97)[88] and refined
(SHELXL-97)[89] by full-matrix least-squares methods as implemented in
the WinGX software package.[90] An empirical absorption correction was
applied. Hydrogen atoms were introduced at calculated positions (riding
model) included in structure factor calculation but not refined.
CCDC 782136, 782137 contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif
AHCTUNGTRENNUNG
Spectroscopic studies: UV-visible spectra were recorded in solution using
a UV-Visible-NIR spectrophotometer CARY 5000-Varian or SHIMAD-
ZU UV-1605 spectrophotometer. The optical band gap was calculated
from the absorption edge of the UV-vis absorption spectrum using the
formula DEopt [eV]=hc/l, l being the absorption edge [m]. With h=
6.6ꢆ10ꢁ34Jsꢁ1 (1 eV=1.6ꢆ 10ꢁ19J) and c=3.0ꢆ108 msꢁ1, this equation
may be simplified as: DEopt [eV]=1237.5/l [nm]. Photoluminescence
spectra were recorded with a PTI spectrofluorimeter (PTI-814 PDS, MD
5020, LPS 220B) using a Xenon lamp either in solution (THF). Quantum
=
;
;
2
2
{S[w
(Fo -Fc2)2]/S [w
N
F
values of reflections with Fo2 >2s
(Fo ) (2539 observed reflections); S=
1.224 for 181 parameters. Residual electron density extremes were 0.259
2
and ꢁ0.235 eꢅꢁ3
.
AHCTUNRTGENNG(UN 2,1-a)-DSF-IF: C44H26·CHCl3; M=673.90; colorless prism; crystal size
0.3ꢆ0.2ꢆ0.04 mm; monoclinic; P21/n; a=13.3572(4), b=18.3088(5), c=
14.0511(4) ꢅ; b=109.1210(10); V=3246.67(16) ꢅ3; Z=4; 1calcd
1.379Mgmꢁ3 MoKa radiation (l=0.71073 ꢅ); m=0.316 mmꢁ1
T=
150(2) K; 38036 data (7426 unique, Rint =0.0428, 2.70<q<27.418); wR=
yields in solution (fsol) were calculated relative to quinine sulfate (fsol
=
=
0.546 in H2SO4 1N) using standard procedures.[26] fsol was determined ac-
;
;
cording to the following Equation (1),
2
2
{S[w
(Fo -Fc2)2]/S [w
F
ꢀ
ꢁ
values of reflections with Fo2 >2s
E
2
2
ðTs ꢂ ArÞ ns
ð1Þ
ꢀsol ¼ ꢀref ꢂ 100 ꢂ
nr
ðTr ꢂ AsÞ
and ꢁ1.068 eꢅꢁ3
.
where subscripts s and r refer to the sample and reference, respectively.
The integrated area of the emission peak in arbitrary units is given as T,
n is the refracting index of the solvent (ns =1.407 for THF) and A is the
absorbance. IR spectra were recorded on a VARIAN 640-IR using a
PIKE Technologies MIRacle(TM) ATR (single Attenuated Total Reflec-
tance) with a diamond crystal. Melting points were determined using an
electrothermal melting point apparatus.
Acknowledgements
D.T. thanks the Rꢃgion Bretagne for a studentship. We wish to thank
Thierry Roisnel from CDIFX (Centre de Diffractomꢃtrie X, UMR
CNRS 6226-Rennes) for data collections, C.R.M.P.O (Centre Rꢃgional de
Mesure Physique de L’Ouest) for high resolution mass measurements,
Service de Microanalyse-CNRS (Gif sur Yvette) for CHN analyses,
CINES (Montpellier) for computing time, Maxime Romain for his contri-
bution to the synthesis of (2,1-a)-IF, Dr Rꢃmi Mꢃtivier (ENS Cachan) for
helpful discussions and Stꢃphanie Fryars for invaluable technical assis-
tance.
Electrochemical studies: All electrochemical experiments were per-
formed under argon, using a Pt disk working electrode (diameter 1 mm),
the counter electrode was a vitreous carbon rod and the reference elec-
trode was a silver wire in a 0.1m AgNO3 solution in CH3CN. Ferrocene
was added to the electrolyte solution at the end of a series of experi-
ments. The ferrocene/ferrocenium (Fc/Fc+) couple served as internal
standard. The three electrode cell was connected to a PAR Model 273
potentiostat/galvanostat (PAR, EG&G, USA) monitored with the
ECHEM Software. Dichloromethane with less than 100 ppm of water
(ref. SDS 02910E21) was used without purification. Activated Al2O3 was
added in the electrolytic solution to remove excess moisture. For a fur-
ther comparison of the electrochemical and optical properties, all poten-
tials are referred to the SCE electrode that was calibrated at ꢁ0.405 V
vs. Fc/Fc+ system. Following the work of Jenekhe,[69] we estimated the
electron affinity (EA) or lowest unoccupied molecular orbital (LUMO)
and the ionisation potential (IP) or highest occupied molecular orbital
(HOMO) from the redox data. The LUMO level was calculated from:
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ox
HOMO (eV)=ꢁ[Eonset
ACHTUNGTRENUN(NG vs SCE) + 4.4], based on an SCE energy level
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termination of the onset potential values are ꢃ20 mV for Eonset and
red
ꢃ50 mV for Eonset
.
13656
www.chemeurj.org
ꢄ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 13646 – 13658

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