A robust pure hydrocarbon derivative based on the (2,1-b)-indenofluorenyl core with high triplet energy level

Article abstract of DOI:10.1039/c1cc14534g

A unique (2,1-b)-indenofluorenyl core flanked with two spirofluorene units, possessing a high triplet energy value and excellent thermal/morphological stability, is reported.

Full text of DOI:10.1039/c1cc14534g

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2011, 47, 11703–11705
www.rsc.org/chemcomm
COMMUNICATION
A robust pure hydrocarbon derivative based on the
(2,1-b)-indenofluorenyl core with high triplet energy levelw
a
b
a
a
a
´ ´ ¨ ´ ´
Cyril Poriel,* Remi Metivier, Joelle Rault-Berthelot, Damien Thirion, Frederic Barriere
and Olivier Jeannina
Received 25th July 2011, Accepted 8th September 2011
DOI: 10.1039/c1cc14534g
A unique (2,1-b)-indenofluorenyl core flanked with two spirofluorene
units, possessing a high triplet energy value and excellent thermal/
morphological stability, is reported.
Phenylene derivatives constitute an important class of molecules,
which have been widely studied in the last decade due to their
potential applications in organic electronics.1 Of particular interest
Fig. 1 (2,1-b)-DSF(t-Bu)4-IF 1 and its positional isomer (1,2-b)-
DSF(t-Bu)4-IF 2.
in oligophenylene chemistry and physics is the (1,2-b)-indeno-
fluorenyl core, which is nowadays an important building block
for blue fluorescent organic light emitting diodes (OLEDs),1–7
organic field effect transistors,8,9 organic solar cells,10 and for
green phosphorescent OLEDs (PhOLEDs).11 More recently the
(2,1-a)-indenofluorenyl core, a positional isomer of the (1,2-b)-
indenofluorenyl core, has also been investigated for potential
organic electronics applications12–15 and has emerged as an
appealing scaffold for the modulation of optical properties of
dispirofluorene-indenofluorene derivatives.13–15 Due to the
importance of the (1,2-b)- and (2,1-a)-indenofluorenyl cores
in organic electronics, it hence appears of great interest to design
other indenofluorene isomers, which might be used in opto-
electronics. For example, the design of organic host materials
for blue phosphorescent emitters possessing a very high triplet
energy value (ET 4 2.7 eV)16,17 represents a highly challenging
task for the future of organic electronics. Of particular interest
for this technology are pure hydrocarbon derivatives, which
have been very rarely used as host materials for blue PhOLED
applications due to the difficulty in achieving high ET, keeping
intact the other physical properties (good carrier transporting,
thermal and morphological properties).18–20 There is indeed a
trade-off between increasing the HOMO–LUMO gap of a
pure hydrocarbon molecule to increase the singlet and triplet
level energy values and decreasing the conjugation length of the
p-aromatic backbone, which may adversely affect the thermal
and morphological stability. In our quest for robust pure
hydrocarbon derivatives with very high ET, we report herein a
unique molecular design based on a meta-substituted terphenylene
core, namely (2,1-b)-indenofluorene, flanked with two spiro-
fluorene units ((2,1-b)-DSF(t-Bu)4-IF 1, Fig. 1). This molecular
design should in principle lead to high ET due to the (2,1-b)-
indenofluorenyl core, keeping however excellent morphological
and thermal properties thanks to the two spirofluorene units.21
This work is to the best of our knowledge the first example of a
(2,1-b)-indenofluorenyl core and one of the highest ET ever
reported for pure hydrocarbon derivatives of interest for organic
electronics.18–20,22
Geometry optimization of 1 and 2 in the singlet and triplet
states was performed using density functional theory at the
Gaussian03 B3LYP/6-31G* level, followed by a single point
calculation on the optimized structures using B3LYP/
6-311+G**. The calculated energies of the frontier molecular
orbitals (and the corresponding HOMO–LUMO gap) are,
respectively, ꢀ5.61 and ꢀ1.32 eV (DE = 4.29 eV) for 1 and
ꢀ5.59 and ꢀ1.52 eV (DE = 4.07 eV) for 2. The computed triplet
adiabatic S0 to T1 excitation energy,23,24 defined as the energy
difference between the highest occupied molecular orbitals in the
relaxed geometry of the molecules in their singlet and triplet
states, is 2.69 eV for 1 and 2.53 eV for 2. Thus, 1 is expected to
have higher ET than that of its related positional isomer 2.
The first step of our synthetic investigations (Scheme 1)
towards the synthesis of 1 was to protect the amino group of
the starting material, 1-bromoaniline, with a tert-butylcarbamoyl
(Boc) group. The use of the non-nucleophilic strong base sodium
hexamethyldisilazane (NaHMDS) prior to the addition of the
di-tert-butyldicarbonate (Boc2O) gave the corresponding
tert-butyl-carbamoylated derivative 3. Reaction of 3 with 1,3-
phenylenebisboronic acid under Suzuky–Miyaura cross-coupling
conditions using [PdIICl2(dppf)]ꢁCH2Cl2 as the catalyst led to the
terphenyl 4 in high yield (87%). Deprotection of the amino
a
´
Universite de Rennes 1, UMR CNRS 6226, Campus de Beaulieu,
35042 Rennes cedex, France. E-mail: cyril.poriel@univ-rennes1.fr
b PPSM, ENS Cachan, UMR CNRS 8531, 61 Avenue du Pre´sident
Wilson, 94235 Cachan, France
w Electronic supplementary information (ESI) available: Synthetic
procedures, complete compounds characterizations, thermal, optical
and electrochemical data and copy of NMR spectra. CCDC 836979.
For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c1cc14534g
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 11703–11705 11703

View Article Online
Scheme 1 Synthesis of 1: (a) 1. NaHMDS, THF, rt; 2. Boc2O; (76%) (b) 1,3-C6H4(B(OH)2)2, [PdIICl2(dppf)]ꢁCH2Cl2, Na2CO3, DMF/H2O, 90 1C
(87%); (c) TFA, CH2Cl2, 0 1C - rt (99%); (d) 1. NaNO2, H2O/HCl, 0 1C; 2. KI/H2O, 0 1C - 60 1C (53%). (e) 1. n-BuLi/THF, ꢀ78 1C 2. 2,7-tert-
butyl-fluoren-9-one, ꢀ78 1C - rt (20%); (f) BF3ꢁOEt2/CH2Cl2, rt (85%).
groups was then performed under acidic conditions and the
diamino derivative 5 was quantitatively obtained. Using a 10-fold
excess of KI as described by Holmes and co-workers,25 key
compound 6 was finally obtained through a Sandmeyer reaction
in 53% yield. Lithium–iodine exchange of 6 with n-butyllithium
followed by the addition of 2,7-tert-butyl-fluoren-9-one26 afforded
the corresponding difluorenol 7 (20%), which was further cyclized
through an intramolecular electrophilic bicyclization reaction to
afford the target (2,1-b)-DSF(t-Bu)4-IF 1 with the (2,1-b)-indeno-
Fig. 3 Absorption (dotted line) and fluorescence emission (solid line,
fluorenyl core. We note that compound 6 reported herein widens
lexc = 330 nm) spectra of 1 (a) and 2 (b) in methylcyclohexane/
2-methylpentane 1 : 1 at rt. Insets: phosphorescence spectra at 77 K.
for the synthesis of spiro derivatives for organics electronics.21,27
the scope of diiodinated terphenyl isomers as key building blocks
The structure of 1 has been confirmed by X-ray diffraction on
single crystals obtained by slow diffusion of pentane in a THF
solution (Fig. 2).z The (2,1-b)-indenofluorenyl core has a max-
imum length of 10.7 A, shorter than their positional isomers (1,2-b)-
or (2,1-a)-indenofluorenyl cores (respectively 11.1 A and 10.8 A).26
This contraction of the (2,1-b)-indenofluorenyl core is
caused by its ‘‘crescent moon’’ shape due to the meta-linkages
of the constituting phenylene rings. The (2,1-b)-indenofluorenyl
core is almost flat with dihedral angles between the plane of the
central phenyl ring and those of the side rings of ca. 3.91 and 0.71.
In addition, the distance between the two spiro carbons is
ca. 5.2 A and the two fluorenyl units display an eclipsed
conformation. These two features clearly indicate that the
two face-to-face fluorene units are too far from one another
to allow intramolecular p-stacking interactions.y
The electrochemical oxidation of 1, investigated by cyclic
voltammetry and differential pulse voltammetry, consists of
two successive isoelectronic reversible oxidations with maxima
at 1.44 and 1.62 V, respectively, followed by a third multi-
electronic oxidation at 1.94 V (see ESIw). Multiple scans includ-
ing the three waves lead to the deposition of an insoluble
electroactive film on the electrode surface due to anodic poly-
merization processes.6 From the onset oxidation potential,28 we
determined the HOMO level of 1 lying at ꢀ5.73 eV, slightly
below that of its isomer 2 (HOMO: ꢀ5.61 eV).26 As the cyclic
voltammetries recorded in the cathodic range do not show any
well-defined reduction wave allowing an accurate electrochemical
band gap measurement (see ESIw), the LUMO of 1 (ꢀ2.19 eV)
was determined from the optical band gap and the HOMO level.
The UV-Vis absorption spectrum of 1 in methylcyclohexane/
2-methylpentane 1 : 1 exhibits five characteristic absorption
bands (302, 314, 327, 334, 342 nm), very similar to those
previously observed for its positional isomer 2 with however, a
slight blue shift (3 nm) between the two maxima (342 nm for 1 vs.
345 nm for 2), Fig. 3. The optical band gaps of 1 and 2 are,
respectively, 3.55 eV and 3.52 eV. The slight band gap contraction
observed in 2 coupled to its higher HOMO level (vide supra)
suggests a better delocalization of p-electrons in 2 compared to
1. Similarly, the fluorescence spectrum of 1 (Fig. 3a), with a
maximum recorded at 343 nm, is also blue-shifted compared
to 2 (lmax = 346 nm, Fig. 3b). The fluorescence spectral shape
of 1 is the mirror image of its absorption spectrum and the
Stokes shift is almost negligible, indicating very little rearrange-
ments in the excited state and hence a highly rigid structure.
The fluorescence quantum yield of 1 was measured to be 0.51
and a fluorescence lifetime of 3.85 ns (in THF) was determined,
showing that 1 is a very efficient blue-violet fluorescent emitter.
At 77 K, in a frozen methylcyclohexane/2-methylpentane 1 : 1
glassy matrix, the phosphorescence spectra of both isomers were
recorded: lmax = 448 and 482 nm for 1 and lmax = 490 and 532
nm for 2. Deactivation of the triplet state of compounds 1 and 2
is very slow under these experimental conditions: the phosphor-
escence decay was measured and the lifetime of the T1 state of 1
(resp. 2) was found to be 4.8 s (resp. 2.9 s), in accordance with
data reported for closely related compounds.29 The corres-
ponding ET values of 1 and 2 obtained by the highest-energy
Fig. 2 ORTEP drawing of 1 (ellipsoid probability at 50% level,
hydrogen atoms have been omitted for clarity).
c
11704 Chem. Commun., 2011, 47, 11703–11705
This journal is The Royal Society of Chemistry 2011

View Article Online
Mo-Ka (l = 0.71073 A) Dc = 1.133 g cmꢀ3, m(Mo) = 0.063 mmꢀ1
15 549 reflections measured, of which 8656 independent (Rint =
,
0.0738), Rf = 0.1044 [4180 data, I 4 2s(I)], wR(F2) = 0.3389,
GOF = 1.056.
y For structural features related to other dispirofluorene-indeno-
fluorenes with interacting face-to-face fluorene units, see ref. 13–15.
1 A. C. Grimsdale and K. Mullen, Macromol. Rapid Commun., 2007,
¨
28, 1676–1702.
2 Z. Ma, P. Sonar and Z.-K. Chen, Curr. Org. Chem., 2010, 18,
2039–2069.
Fig. 4 AC-AFM images of a vacuum-deposited thin-film of 1 (5 ꢂ 5 mm):
non-heated film (a). Film heated (1 h) in air at 130 1C (b) and 160 1C (c).
3 A. C. Grimsdale, K. L. Chan, R. E. Martin, P. G. Jokisz and
A. B. Holmes, Chem. Rev., 2009, 109, 897–1091.
4 J. Jacob, J. Zhang, A. C. Grimsdale, K. Mullen, M. Gaal and E. J. W.
¨
List, Macromolecules, 2003, 36, 8240–8245.
phosphorescent peak were thus, respectively, estimated at ca.
2.76 eV and 2.52 eV. The meta-substituted terphenyl backbone
found in 1 leads hence to a remarkable increase of the ET value
compared to the para-substituted terphenyl backbone found in 2.
Thus, the ET value of 2 appears to be higher than the ET value of
Ir(ppy)3 (ET = 2.42 eV),16 an efficient green phosphorescent
emitter, and the ET value of 1 appears to be higher than the ET
value of FIrpic (ET = 2.64 eV),16 an efficient blue phosphore-
scent emitter.
5 D. Marsitzky, J. C. Scott, J.-P. Chen, V. Y. Lee, R. D. Miller,
S. Setayesh and K. Mullen, Adv. Mater., 2001, 13, 1096–1099.
¨
6 (a) N. Cocherel, C. Poriel, L. Vignau, J.-F. Bergamini and J. Rault-
Berthelot, Org. Lett., 2010, 12, 452–455; (b) D. Thirion, J. Rault-
Berthelot, L. Vignau and C. Poriel, Org. Lett., 2011, 13,
4418–4421.
7 Y. Park, J.-H. Lee, D. H. Jung, S.-H. Liu, Y.-H. Lin, L.-Y. Chen,
C.-C. Wu and J. Park, J. Mater. Chem., 2010, 20, 5930–5936.
8 W. Zhang, J. Smith, R. Hamilton, M. Heeney, J. Kirkpatrick,
K. Song, S. E. Watkins, T. Anthopoulos and I. McCulloch, J. Am.
Chem. Soc., 2009, 131, 10814–10815.
9 H. Usta, C. Risko, Z. Wang, H. Huang, M. K. Deliomeroglu,
A. Zhukhovitskiy, A. Facchetti and T. J. Marks, J. Am. Chem.
Soc., 2009, 131, 5586–5608.
10 J. Kim, S. H. Kim, I. H. Jung, E. Jeong, Y. Xia, S. Cho,
I.-W. Hwang, K. Lee, H. Suh, H.-K. Shim and H. Y. Woo,
J. Mater. Chem., 2010, 20, 1577–1586.
Finally, the morphology of vacuum deposited thin-films of 1
on quartz slides has been studied by atomic force microscopy
in acoustic mode (AC-AFM) in order to evaluate the effect of
temperature and ambient atmosphere on the thin-film structure.
The surface has been exposed to air under thermal stress
conditions from room temperature to 200 1C (Fig. 4 and
figures in ESIw). The non-heated film surface presents a regular
and smooth morphology (Fig. 4a). The surface roughness (Ra)
of the film appears to be very low, around 0.7 nm, and is kept
almost unchanged until 130 1C (Fig. 4b), highlighting the very
good quality of the film surface and the high stability of 1
upon heating. At 160 1C, the surface morphology became
however rough with Ra estimated around 18 nm, likely caused
by degradation and/or crystallization of the material
(Fig. 4c).30–32 1 presents hence an excellent morphological
stability under very harsh conditions and this stability up to
130 1C under an ambient atmosphere is a key point before any
possible OLED applications. Thermogravimetric analysis
(see ESIw) confirms the excellent thermal stability of 1 with a
decomposition temperature21 Td of 387 1C, even higher than
that of its isomer 2 (Td = 350 1C).26
11 L.-C. Chi, W.-Y. Hung, H.-C. Chiu and K.-T. Wong, Chem.
Commun., 2009, 3892–3894.
12 Covion Organic Semiconductors: Eur. Pat. Appl., EP 1491568,
2004.
13 D. Thirion, C. Poriel, J. Rault-Berthelot, F. Barrie
O. Jeannin, Chem.–Eur. J., 2010, 16, 13646–13658.
14 D. Thirion, C. Poriel, F. Barriere, O. Jeannin, R. Me
J. Rault-Berthelot, Org. Lett., 2009, 11, 4794–4797.
15 D. Thirion, C. Poriel, R. Metivier, J. Rault-Berthelot, F. Barrie
and O. Jeannin, Chem.–Eur. J., 2011, 17, 10272–10287.
´
re and
´
´
tivier and
´
´
re
16 H. Yersin, Highly Efficient OLEDs with Phosphorescent Materials,
Wiley-VCH: Verlag GMbH & Co. KGaA, Weinheim, 2007.
17 L. Xiao, Z. Chen, B. Qu, J. Luo, S. Kong, Q. Gong and J. Kido,
Adv. Mater., 2011, 23, 926–952.
18 Y. Tao, C. Yang and J. Qin, Chem. Soc. Rev., 2011, 40, 2943–2970.
19 C. Fan, Y. Chen, P. Gan, C. Yang, C. Zhong, J. Qin and D. Ma,
Org. Lett., 2010, 12, 5648–5651.
20 S. Ye, Y. Liu, C.-a. Di, H. Xi, W. Wu, Y. Wen, K. Lu, C. Du,
Y. Liu and G. Yu, Chem. Mater., 2009, 21, 1333–1342.
21 T. P. I. Saragi, T. Spehr, A. Siebert, T. Fuhrmann-Lieker and
J. Salbeck, Chem. Rev., 2007, 107, 1011–1065.
To summarize, we have designed and synthesized through
an efficient synthetic approach (2,1-b)-DSF(t-Bu)4-IF
1
containing the novel (2,1-b)-indenofluorenyl spirolinked to
two fluorenyl units. This simple molecular concept allows us
to combine within a single pure hydrocarbon molecule a high
ET with a very long phosphorescence lifetime and excellent
thermal and morphological stability. This work is the first
example of a (2,1-b)-indenofluorenyl core and one of the
highest ET ever reported for a morphologically stable pure
hydrocarbon derivative of interest for organic electronics.
We wish to thank the CDIFX, the C.R.M.P.O (Rennes) and
the CINES (Montpellier). We also thank A. Brosseau
(Cachan), Dr J. F. Bergamini (Rennes) for AFM images,
22 Z. Jiang, H. Yao, Z. Zhang, C. Yang, Z. Liu, Y. Tao, J. Qin and
D. Ma, Org. Lett., 2009, 11, 2607–2610.
23 P. Marsal, I. Avilov, D. A. da Silva Filho, J. L. Bredas and
D. Beljonne, Chem. Phys. Lett., 2004, 392, 521–528.
24 H. Sasabe, Y.-J. Pu, K.-I. Nakayama and J. Kido, Chem. Com-
mun., 2009, 6655–6657.
25 K. L. Chan, M. J. McKiernan, C. R. Towns and A. B. Holmes,
J. Am. Chem. Soc., 2005, 127, 7662–7663.
26 C. Poriel, J. Rault-Berthelot, F. Barriere and A. M. Z. Slawin, Org.
´
Lett., 2008, 10, 373–376.
27 D. Horhant, J.-J. Liang, M. Virboul, C. Poriel, G. Alcaraz and
J. Rault-Berthelot, Org. Lett., 2006, 8, 257–260.
28 A. P. Kulkarni, C. J. Tonzola, A. Babel and S. A. Jenekhe, Chem.
Mater., 2004, 16, 4556–4573.
F. Moreau (Rennes) for the TGA and the Region Bretagne
´
29 D. Hertel, S. Setayesh, H. G. Nothofer, U. Scherf, K. Mullen and
H. Bassler, Adv. Mater., 2001, 13, 65–70.
30 S. Liu, F. He, H. Wang, H. Xu, C. Wang, F. Li and Y. Ma,
J. Mater. Chem., 2008, 18, 4802–4807.
31 S.-K. Kim, Y.-I. Park, I.-N. Kang and J.-W. Park, J. Mater.
Chem., 2007, 17, 4670–4678.
32 C.-H. Wu, C.-H. Chien, F.-M. Hsu, P.-I. Shih and C.-F. Shu,
J. Mater. Chem., 2009, 19, 1464–1470.
¨
¨
for a studentship (DT).
Notes and references
z Selected data for 1: C65 H70, M = 851.21, triclinic, space group P-1,
a = 11.383(3) A, b = 12.212(3) A, c = 19.173(5) A, a = 84.385(6)1,
b = 88.389(11)1, g = 70.418(6)1, V = 2494.3(13) A3, Z = 2, T = 150 K,
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 11703–11705 11705