New dispiro compounds: Synthesis and properties

Article abstract of DOI:10.1021/ol7026202

We report the synthesis and structural characterization of two dispiro compounds. These two positional isomers have been designed and synthesized through an efficient method. Because of the rigidity and orthogonality of the spiro bridge, both molecules ex

Full text of DOI:10.1021/ol7026202

ORGANIC
LETTERS
2008
Vol. 10, No. 3
373-376
New Dispiro Compounds: Synthesis
and Properties
Cyril Poriel,*,†,‡ Joelle Rault-Berthelot,*,†,‡ Fre´de´ric Barrie`re,†,‡ and
1
Alexandra M. Z. Slawin§
UniVersite´ de Rennes 1 and CNRS UMR 6226 “Sciences Chimiques de Rennes”
Equipe ‘Matie`re Condense´e et Syste`mes Electroactifs’ (MaCSE) Bat 10C, Campus de
Beaulieu, 35042 Rennes cedex, France, and School of Chemistry, UniVersity of St.
Andrews, Fife, Scotland KY16 9ST, U.K.
cyril.poriel@uniV-rennes1.fr; joelle.rault-berthelot@uniV-rennes1.fr
Received October 29, 2007
ABSTRACT
We report the synthesis and structural characterization of two dispiro compounds. These two positional isomers have been designed and
synthesized through an efficient method. Because of the rigidity and orthogonality of the spiro bridge, both molecules exhibit a well-defined
architecture which consists of two fluorene rings connecting to an indenofluorenyl unit via two sp3 carbon atoms. The structural, electrochemical,
optical, and thermal properties of these dispiro isomers are discussed.
Organic molecules/polymers with a π aromatic backbone are
capable of transporting charge, and thus, these systems may
behave as semiconductors in opto-electronic devices.1 In
addition, organic chemistry offers a versatile means of
tailoring the properties of molecules/polymers, which is not
possible for traditional inorganic materials. Compared to
extended π conjugated polymers, small molecular units with
well-defined conjugation lengths are characterized by the
absence of chain defects, are easier to purify, and are prone
to full characterization.2,3 This makes these molecules more
attractive than extended π-conjugated polymers for inves-
tigations of the structure-property relationships.4 In this
context, several families of molecular architectures have been
developed and exploited, such as twin molecules, orthogo-
nally fused spiro-type molecules, paracyclophane, dendri-
mers, or C3 symmetry starburst molecules.4 In our quest for
efficient materials for blue organic light emitting diodes
(OLED), we recently designed a new family of luminescent
dispiro derivatives, dispiro[fluorene-9,6′-indeno[1,2-b]fluo-
rene-12′,9′′-fluorene called (1,2-b)-DSF-IFs.5,6 As recently
reported by different groups, such dispiro structures appear
to be highly promising for blue OLED application.7-11 On
the other hand, (1,2-b)-indenofluorene (1,2-b-IF) has been
(5) Horhant, D.; Liang, J.-J.; Virboul, M.; Poriel, C.; Alcaraz, G.; Rault-
Berthelot, J. Org. Lett. 2006, 8, 257-260.
(6) Poriel, C.; Liang, J.-J.; Rault-Berthelot, J.; Barrie`re, F.; Cocherel,
N.; Slawin, A. M. Z.; Horhant, D.; Virboul, M.; Alcaraz, G.; Audebrand,
N.; Vignau, L.; Huby, N.; Hirsch, L.; Wantz, G. Chem. Eur. J. 2007, 13,
10055-10069.
(7) Kimura, M.; Kuwano, S.; Sawaki, Y.; Fujikawa, H.; Noda, K.; Taga,
Y.; Takagi, K. J. Mater. Chem. 2005, 15, 2393-2398.
(8) Xie, L. H.; Hou, X.-Y.; Tang, c.; Hua, Y.-R.; Wang, R.-J.; Chen,
R.-F.; Fan, Q.-L.; Wang, L.-H.; Wei, w.; Peng, B.; Huang, W. Org. Lett.
2006, 8, 1363-1366.
(9) Vak, D.; Lim, B.; Lee, S.-H.; Kim, D.-Y. Org. Lett. 2005, 7, 4229-
4232.
(10) Harrington, L. E.; Britten, J. F.; Mc Glinchey, M. J. Org. Lett. 2004,
6, 787-790.
(11) Wu, Y.; Zhang, J.; Bo, Z. Org. Lett. 2007, 9, 4435-4438.
† Universite´ de Rennes 1.
‡ CNRS UMR 6226.
§ University of St. Andrews.
(1) Mu¨llen, K.; Scherf, U. Organic Light-Emitting DeVices: Synthesis,
Properties and Applications; Wiley-VCH Verlag GMbH & Co. KGaA:
Weinheim, 2006.
(2) Tour, J. M.; Wu, R.; Schumm, J. S. J. Am. Chem. Soc. 1990, 112,
5662-5663.
(3) Wu, R.; Schumm, J. S.; Pearson, D. L.; Tour, J. M. J. Org. Chem.
1996, 61, 6906-6921.
(4) Cao, X.-Y.; Zhang, W.; Pei, J. Org. Lett. 2004, 6, 4845-4848 and
references therein.
10.1021/ol7026202 CCC: $40.75
© 2008 American Chemical Society
Published on Web 01/10/2008

widely used for the last 10 years as a central building block
within the backbone of polymers or oligomers leading to a
strong enhancement of the OLED properties.9,12-14 However,
to the best of our knowledge, the (2,1-a)-IF positional isomer
has only been reported in two instances15,16 and is unknown
with spiro linkages. In this context, we thus designed and
prepared, through an expedient synthesis, two new dispiro
chromophores, (1,2-b)-DSF(t-Bu)4-IF 5 and (2,1-a)-DSF-
(t-Bu)4-IF 6, with shape persistent molecular architectures
and different geometric profiles, i.e., linear antarafacial and
suprafacial geometries.8 The synthetic route is presented in
Scheme 1. It is based on a coupling reaction between
the difluorenol 4 in moderate yield (50%). This reaction
appeared to be highly sensitive to the dilithiate intermediary
formation time. Indeed, a byproduct, identified as the
monoalcohol 4′ by X-ray crystallography is always formed
(cf. the Supporting Information). Very short reaction time
for metal-halogen exchange followed by immediate quench-
ing with 3 is essential to minimize the formation of 4′. The
intramolecular cyclization reaction performed in acidic
medium leads to the expected formation of 5 and 6. These
molecules possess two spiro-linked fluorene rings and a
central IF backbone (1,2-b- or 2,1-a-substituted). Compound
6 appears to be the first example of a spiro-linked (2,1-a)-
IF core. As shown in Table 1, the conversion of the diol 4
Scheme 1. Synthesis of 5 and 6
Table 1. 6/5 Ratio for Different Reaction Conditions
solvent
T (°C)
6/5 ratioa (%) reaction time (min)
acid
CH2Cl2
CH2Cl2 rt
AcOH
AcOH
AcOH
0
19/81
24/76
40/60
50/50
60/40
30
30
overnight
180
120
H2SO4
H2SO4
HCl
HCl
HCl
5 to rtb
rt to 70
reflux
a The 6/5 ratio was determined from the 1H NMR spectrum of the crude
reaction before any treatment. b 3 h at 5 °C and overnight at rt.
to 5 and 6 is highly sensitive to the reaction conditions,
including the acid, the solvent, and the reaction temperature.
Preliminary studies indicate that the ratio of these two
isomers may be strongly modulated going to a 6/5 ratio of
19/81 to 60/40 depending on these reaction conditions. The
isolated yields of both isomers are in each case greater than
80%.
diiodinated terphenyl (2,2′′-DITP)6 and 2,7-tert-butylfluo-
renone 3, followed by a key intramolecular cyclization
reaction. From a statistical point of view, this cyclization
step may occur on either side of the terphenyl backbone and
should lead to the (1,2-b)-DSF(t-Bu)4-IF 5 and (2,1-a)-DSF-
(t-Bu)4-IF 6. Although the bulkiness of the tert-butyl
substituents should in principle favor the less sterically
hindered positional isomer we expected the formation of both
isomers. Indeed, the formation of conceptually related
positional isomers from a terphenyl backbone has been
previously observed with indolocarbazole derivatives by
Leclerc and co-workers.17,18 Theoretical calculations (full
optimization on simplified models of the two isomers without
tert-butyl groups at the B3LYP/6-31G* level of theory vide
infra) indicate that the 5-like isomer is only less than 8 kcal
mol-1 more stable than the 6-like isomer.
The two isomers 5 and 6 were easily separated by column
chromatography. Heteronuclear multiple bond correlation
(HMBC) spectra were performed for both isomers in order
to ascertain their molecular structures through long-range
shift correlations. In this context, the spiro carbons are
powerful probes to detect neighboring hydrogens. For 5, three
cross-peaks are observed for the spiro carbon atoms corre-
sponding to three-bond correlations (3J) with Ha (7.21 ppm),
Hb (6.74 ppm), and Hc (6.65 ppm), Figure 1a. The spectrum
3
of 6 exhibits only two J correlations with the spiro carbon
As shown in Scheme 1, the lithium-iodine exchange of
2,2′′-DITP with n-butyllithium at low temperature followed
by quenching with 2,7-tert-butylfluorenone 319,20 afforded
(12) Grimsdale, A. C.; Mu¨llen, K. Macromol. Rapid. Commun. 2007,
28, 1676-1702.
(13) Jacob, J.; Zhang, J.; Grimsdale, A. C.; Mu¨llen, K.; Gaal, M.; List,
E. J. W. Macromolecules 2003, 36, 8240-8245.
(14) Merlet, S.; Birau, M.; Wang, Z. Y. Org. Lett. 2002, 4, 2157-2159.
(15) Deuschel, W. HelV. Chim. Acta 1951, 34, 2403-2416.
(16) Covion Organic Semiconductors: Eur. Pat. Appl., EP 1491568,
2004.
(17) Blouin, N.; Michaud, A.; Wakim, S.; Boudreault, P.-L. T.; Leclerc,
M.; Vercelli, B.; Zecchin, S.; Zotti, G. Macromol. Chem. Phys. 2006, 207,
166-174.
(18) Wakim, S.; Leclerc, M. Synlett 2005, 8, 1223-1234.
(19) Weber, E.; Do¨rpinghaus, N.; Cso¨regh, I. J. Chem. Soc., Perkin Trans.
2 1990, 2167-2177.
(20) Stigers, K. D.; Koutroulis, M. R.; Chung, D. M.; Nowick, J. S. J.
Org. Chem. 2000, 65, 3858-3860.
Figure 1. Portion of the HMBC (CDCl3) spectra of (a) 5 and (b)
6.
374
Org. Lett., Vol. 10, No. 3, 2008

atoms assigned to Ha (6.20 ppm) and Hb (5.87 ppm), Figure
1b. The absence of a third correlation is consistent with the
long distance separating the hydrogen atoms of the (2,1-a)-
IF central phenyl ring and the spiro carbon atoms.
Single crystals of 5 and 6 obtained by slow diffusion of
hexane into a CDCl3 solution were analyzed by X-ray
diffraction in order to study their detailed molecular structure
(Figure 2). In these two structures, the fluorene rings adopt
Figure 3. Anodic oxidation of a 1.35 10-3 M solution of 5 (a-c)
and 1.30 ×10-3 M of 6 (d-f) in CH2Cl2 + Bu4NPF6 0.2 M. Sweep
rate: 100 mV/s. S ) 4 µA. Pt disk working electrode (diameter
1 mm).
oxidation wave at 1.44 V (cf. the Supporting Information).
In the cathodic range (data not shown), the reversible
monoelectronic reduction occur at -2.36 and -2.52 V for
Figure 2. Molecular structure of 5 and 6. Top (a) and side views
(b and c), from single-crystal X-ray data.
a “saddle shape” twist (cf. the Supporting Information for
definitions and calculations of the folding angles of the
fluorene rings). In 5, the fluorene rings are folded, with two
minor distortions on each side, caused by the two tert-butyl
substituents, that stretch inside of the rigid H shape (Figure
2, view 5b). In 6, the folding of the fluorene rings is more
pronounced. Contrary to the isomer 5 and in order to avoid
any strong steric interactions, the fluorene rings are stretched
outside of the H shape (Figure 2, view 6b). The distance
between the two opposite carbon atoms bearing tert-butyl
substituents is ca. 4.4 Å. The shortest distance between
opposite carbon atoms is 3.3 Å (Figure 2, view 6a), which
is smaller than the sum of the van der Waals radii. It should
also be noted that, for both structures, the spiro carbons are
not perfectly in the plane of the indenofluorenyl moieties
(cf. the Supporting Information). Indeed, in the solid-state
structure of 6, a slight tilt is observed for the two spiro
carbons, one pointing above the indenofluorenyl plane
(through the central phenyl ring) and the other pointing below
this plane (Figure 2, view 6c). This up/down structural feature
is assigned to the spiro linkage of the (2,1-a)-IF core. In the
case of 5, the spiro carbons are both pointing on the same
side of the indenofluorenyl plane.
Table 2. Electrochemical Properties and HOMO/LUMO
Levels of 5 and 6 Compared with (1,2-b)-DSF-IF, (1,2-b)-IF,
and 2
1
EOx
ERed HOMO LUMO ∆Eel ∆Eop ∆Ecal
(V)f
(V)g
(eV)a
(eV)b (eV)c (eV)d (eV)e
5
6
1.33, 1.61 -2.36 -5.61 -2.20 3.41 3.49 4.19
1.79, 2.03
1.24, 1.55 -2.52 -5.48 -2.05 3.43 3.53 4.16
1.84, 1.96
(1,2-b)-IF
(1,2-b)-
DSF-IF
2
1.31, 2.02 -2.56 -5.62 -1.99 3.63 3.61 4.30
1.43, 1.87 -2.25 -5.76 -2.17 3.59 3.51 4.18
1.44, 1.91 -2.73 -5.45 -1.03 4.42 3.93
-
a From the onset oxidation potential. b From the onset reduction potential.
c ∆Eel ) |HOMO-LUMO| from redox data. d From the edge of the
absorption spectrum (in CH2Cl2). e ∆Ecal ) |HOMO-LUMO| using Gauss-
ian 03 B3LYP/6-31G* calculation on simplified models of 5 and 6. f In
CH2Cl2. g In DMF.
5 and 6, respectively. These values are closer to the reduction
potential of (1,2-b)-IF (-2.56 V) than to that of 2 (-2.73
V). Therefore, it is rational to assign the first oxidation and
reduction process of 5 and 6 to indenofluorene centered
electron transfers. Theoretical calculations on simplified
models of both isomers corroborate the electrochemical
assignments pointing to the indenofluorenyl character of the
HOMO and LUMO for 5 and 6 (cf. the Supporting
Information). The lower first oxidation potential of 6 with
respect to 5 is due to the different structure of the two
indenofluorenyl moieties. With related 1,2-benzofluorene and
2,3-benzofluorene positional isomers, a similar negative shift
The electrochemical properties of 5 and 6 were investi-
gated by cyclic voltammetry (CV) and compared with the
nonsubstituted (1,2-b)-DSF-IF and with the two molecular
building blocks of 5, i.e., (1,2-b)-IF and 2,7-tert-butyl
fluorene 2 (Figure 3, Table 2). The first oxidation of (1,2-
b)-IF is monoelectronic and reversible and occurs at 1.31
V. A similar oxidation process occurs at 1.33 V for 5. On
the other hand, the CV of 2 exhibits a first reversible
Org. Lett., Vol. 10, No. 3, 2008
375

of the first oxidation potential has been reported.21 Contrary
to the nonsubstituted (1,2-b)-DSF-IF,6 anodic oxidation of
5 and 6 does not lead to any electrodeposition process as
anticipated by the introduction of the blocking tert-butyl
substituents on the fluorenyl units.
Following the work of Jenekhe,22 we estimated the
HOMO/LUMO levels (Table 2). Compared to (1,2-b)-DSF-
IF the HOMO level of 5 appears to be slightly higher and
the LUMO level slightly lower. It therefore appears that,
although the fluorene moieties are not involved in the primary
oxidation and reduction, the donor effect of their tert-butyl
substituents is felt by the spiro-linked indenofluorenyl
backbone. The HOMO and LUMO levels of 6 lie at -5.48
and -2.05 eV, respectively. The similar electrochemical band
gap values for 5 (3.41 eV) and 6 (3.43 eV) are consistent
with the measured optical band gaps and also qualitatively
consistent with the theoretical values obtained with DFT
calculations. The UV-vis absorption spectrum of 5 exhibits
the five characteristic absorption bands of its central (1,2-
b)-IF building block.6,14 The spectrum of 6 exhibits a main
absorption band slightly hypsochromically shifted by 4 nm,
with respect to 5, in accordance with previous observations
on the (1,2-b)-IF and (2,1-a)-IF core.16 The photolumines-
cence spectra of 5 and 6 revealed a very small Stokes shift
(3/4 nm) that confirms that both structures are very rigid.
Compounds 5 and 6 are highly fluorescent with quantum
yields of ca. 70% (Table 3, Figure 4).
Figure 4. (a) UV-vis spectra of 5, 6, and (1,2-b)-IF in CH2Cl2.
(b) Emission spectra of 5 and 6 in cyclohexane (10-6 M).
350 and 320 °C, respectively) as expected by the presence
of the rigid, spiro-fused orthogonal linkages.23-25 Such
behavior is highly important to improve the lifetime of blue
OLED.
In conclusion, we have designed and synthesized two new
dispiro isomer chromophores. These molecules are large band
gap compounds with a high photoluminescence quantum
yield in solution and are thermally stable. We demonstrated
an efficient method for preparing these two positional isomers
in different ratios depending on the reaction conditions. This
method appears to be promising for preparing numerous
compounds with the unique spirolinked (2,1-a)-IF backbone.
Work is in progress on the elucidation of the mechanism of
the cyclization reaction. An intimate understanding of the
reaction is of great interest in preparing luminophores with
well-defined geometry for future optoelectronic applications.
These studies are underway in our laboratory.
Table 3. Optical and Thermal Properties of 5 and 6
λExc
λEm
φ
Td
Acknowledgment. We thank the C.R.M.P.O (Rennes),
S. Sinbandhit for the 2D NMR study, the CINES (Montpel-
lier) for computing time, Dr N. Audebrand for TGA, and S.
Fryars for invaluable technical assistance.
λ
Αbs (nm)a
(nm)a
(nm)a
(%)b (°C)c
5
6
308, 314, 330, 337, 339, 345 345 348, 369
323, 338, 341
70 350
341 345, 363, 381 75 320
a In cyclohexane. b The relative quantum yield φ was measured using
Supporting Information Available: Experimental pro-
cedures and spectroscopic characterizations of new com-
pounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
standard procedures with reference to quinine sulfate in 1 N H2SO4 (φ )
0.546).14 c Decomposition temperature Td is defined as the temperature at
which 5% loss occurs during heating.23
OL7026202
Thermogravimetric analysis (cf. the Supporting Informa-
tion) confirmed the good thermal stability of 5 and 6 (Td of
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J. Chem. ReV. 2007, 107, 1011-1065.
(21) Bordwell, F. G.; Cheng, J.-P.; Bausch, M. J. J. Am. Chem. Soc.
1988, 110, 2867-2872.
(22) Kulkarni, A. P.; Tonzola, C. J.; Babel, A.; Jenekhe, S. A. Chem.
Mater. 2004, 16, 4559-4573.
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Lett. 2005, 7, 5131-5134.
(25) Kim, Y.-H.; Shin, D.-C.; Kim, S.-H.; Ko, C.-H.; Yu, H.-S.; Chae,
Y.-S.; Kwon, S.-K. AdV. Mater. 2001, 13, 1690-1693.
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