Tuning the optical properties of aryl-substituted dispirofluorenelndenofluorene isomers through intramolecular excimer formation

Article abstract of DOI:10.1021/ol901750x

Two familles of positional Isomers of displrofluorene-lndenofluorene substituted by phenyl groups at the 2,7-posltlons of the fluorene moieties present drastically different optical properties. The emission wavelengths may be gradually and conveniently mo

Full text of DOI:10.1021/ol901750x

ORGANIC
LETTERS
2009
Vol. 11, No. 21
4794-4797
Tuning the Optical Properties of
Aryl-Substituted Dispirofluorene-
Indenofluorene Isomers through
Intramolecular Excimer Formation
Damien Thirion,† Cyril Poriel,*,† Fre´de´ric Barrie`re,† Re´mi Me´tivier,§
Olivier Jeannin,† and Joe¨lle Rault-Berthelot*,†
UniVersite´ de Rennes 1, UMR CNRS 6226 “Sciences Chimiques de Rennes”,
MaCSE group, Bat 10C, Campus de Beaulieu, 35042 Rennes cedex France, and PPSM,
Institut d’Alembert, ENS Cachan, UMR CNRS 8531, 61 AVenue du Pre´sident Wilson,
94235 Cachan, France
cyril.poriel@uniV-rennes1.fr; joelle.rault-berthelot@uniV-rennes1.fr
Received July 30, 2009
ABSTRACT
Two families of positional isomers of dispirofluorene-indenofluorene substituted by phenyl groups at the 2,7-positions of the fluorene
moieties present drastically different optical properties. The emission wavelengths may be gradually and conveniently modulated for
one of the two isomers by the phenyl group’s substituent whose bulkiness controls the extent of the excimeric interaction evidenced
in this paper.
The synthesis of new materials with well-defined geometry
and specific properties is strongly sought worldwide for
applications in organic electronics.1-5 As the molecular
structure has a significant impact on the properties of organic
materials, it seems of great interest to prepare, through a
common intermediate, different molecules that possess
different and tunable properties. Such an approach is
illustrated by the synthesis, Via a common intermediate, of
aryl-substituted (1,2-b)-DSF(R)4-IF 1 and (2,1-a)-DSF(R)4-
IF 2 (DSF, dispirofluorene; IF, indenofluorene), presenting
distinct geometry profiles that translate into different phys-
icochemical properties (Scheme 1). After consideration of
structural features present within 2b, one representative of
DSF-IFs 2, we report the marked difference observed in the
electrochemical and optical properties of the DSF-IF 1 and
the DSF-IF 2 families. We then address the tuning of the
emission band within the DSF-IF 2 family, through the
† Universite´ de Rennes 1, UMR CNRS 6226.
§ Institut d’Alembert, ENS Cachan, UMR CNRS 8531.
(1) Chem. ReV. 2007, 107, 4 (Thematic issue: Organic Electronics and
Optoelectronics. Guest editors: Forrest, S. R., Thompson, M. E.)
.
(2) Grimsdale, A. C.; Mu¨llen, K. Macromol. Rapid Commun. 2007, 28,
1676–1702
.
(3) Pudzich, R.; Fuhrmann-Lieker, T.; Salbeck, J. AdV. Polym. Sci. 2006,
199, 83–142
.
(5) 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.
(4) Saragi, T. P. I.; Spehr, T.; Siebert, A.; Fuhrmann-Lieker, T.; Salbeck,
J. Chem. ReV. 2007, 107, 1011–1065
.
10.1021/ol901750x CCC: $40.75
Published on Web 10/01/2009
 2009 American Chemical Society

slippage angles are very low, i.e. approximately 14° and 18°
(see SI). As discussed by Janiak, such values of ring-centroid/
ring-centroid distance, vertical displacements, and ring
slippage angles may indicate strong π/π interactions in the
solid state.9,10
Scheme 1. DSF-IFs 1a-d and 2a-d
nature and size of the para-substituent of the phenyl groups
borne by the fluorene moieties.
The intramolecular cyclization6,7 of a difluorenol derivative
in the presence of a Lewis acid (Scheme 1) leads to the
formation of two positional isomers possessing distinct
geometry: (1,2-b)-DSF-IF 1 and (2,1-a)-DSF-IF 2, which
were separated by column chromatography. We note that,
with identical substituents, the DSF-IFs 2 are always more
soluble than their corresponding DSF-IF 1 congeners.
Figure 2.
Cyclic voltammetry13 (and DPVs as insets) recorded in
CH2Cl2-[NBu4][PF6] 0.2 M, using Pt anode; sweep rate: 100
mV·s-1. Calculated nature of the highest occupied orbitals of 1b
and 2b and their cation radicals.
The general electrochemical behaviors of 1b-d and 2b-d
(see SI) are exemplified with 1b and 2b (Figure 2). Each
compound is oxidized along three successive isoelectronic
oxidation steps as more clearly depicted with the differential
pulse voltamogram (DPV) (inset Figure 2). The first mono-
electronic oxidation (E1; peak potential obtained from CV)
occurs at 1.45 V for 1b and at a significantly lower potential
(1.20 V) for 2b. It is well-known that the oxidation of
π-stacked systems is more facile than their nonstacked
analogues.11,12 The “phenyl-fluorene-phenyl” cofacial ar-
rangement present in 2b is thus likely at the origin of its
observed low oxidation potential. Analogous observations,
highlighting interactions between two stacked aryl moieties,
were reported by Rathore and co-workers for bichromophoric
dibenzobicyclo[4.4.1]undecane or oligofluorene deriva-
tives.11,12 Theoretical calculations indicate that the HOMOs
of 1b and 2b have a mixed character with nevertheless major
coefficients found on the indenofluorene core for 1b and on
the “phenyl-fluorene-phenyl” moieties for 2b (Figure 2).
Accordingly, the SOMOs of 1b•+ and 2b•+, involved in the
second monoelectronic oxidation (E2 ) 1.55 V for 1b and
1.36 V for 2b), are centered on the “phenyl-fluorene-phenyl”
moieties for 1b•+ and on the indenofluorene core for 2b•+.
The shape of the 1b-d UV-vis spectra consists of a large
absorption band comprising four maxima at ca. 314, 330,
Figure 1. Two different views of 2b from X-ray data.
The X-ray diffraction data of single crystals of 2b reveal
an asymmetric unit containing two independent molecules
(Figure 1, only one of the two independent molecules is
represented).8 Due to the face-to-face arrangement of the
fluorene units in 2b, one of the spiro carbon points slightly
above the indeno[2,1-a]fluorene plane (defined through the
central phenyl ring) while the other spiro carbon points below
this plane.6 The dihedral angles between the fluorene moiety
and its 4-fluorophenyl substituent span a wide range, i.e. from
26° to 41° (see Supporting Information (SI)). The two lowest
dihedral angles (ca. 26.3° and 27.4°) correspond to two face-
to-face phenyl rings (Figure 1, left) with a ring-centroid/
ring-centroid distance estimated at ca. 3.76 Å. The corre-
sponding vertical displacements between these two phenyl
rings are 0.94 and 1.14 Å, and their corresponding ring
(9) Janiak, C. J. Chem. Soc., Dalton Trans. 2000, 3885–3896
.
(6) Poriel, C.; Rault-Berthelot, J.; Barrie`re, F.; Slawin, A. M. Z. Org.
(10) Dorn, T.; Janiak, C.; Abu-Shandi, K. CrystEngComm 2005, 7, 633–
Lett. 2008, 10, 373–376
.
641
.
(7) Poriel, C.; Barrie`re, F.; Thirion, D.; Rault-Berthelot, J. Chem.sEur.
(11) Rathore, R.; Abdelwahed, S. H.; Guzei, I. A. J. Am. Chem. Soc.
J., in press
.
2003, 125, 8712–8713
(12) Chebny, V. J.; Shukla, R.; Linderman, S. V.; Rathore, R. Org. Lett.
2009, 11, 1939–1942
(13) Comparison of the cyclic voltammograms shows the different
solubility of 1b and 2b polycations. Indeed, an adsorption/desorption process
is evident on the CV of 1b, while the three redox waves of 2b are all
diffusion controlled. This observation is consistent with the fact that DSF-
IFs 2 are always more soluble than their corresponding DSF-IF 1 congeners.
.
(8) Selected data for 2b: C141H88F8, M ) 1934.11, triclinic, space group
j
P1, a ) 14.2342(8) Å, b ) 16.3982(11) Å, c ) 23.0578(15) Å, R )
106.176(2)°, ꢀ ) 91.373(2)°, γ ) 106.571(2)°, V ) 4922.9(5) Å3, Z ) 2,
T ) 100 K, Mo KR (λ ) 0.710 73 Å) Dc ) 1.305 g·cm-3, m(Mo) ) 0.085
.
mm-1, 57 009 reflections measured, of which 22 263 independent (Rint
)
0.0959), Rf ) 0.0758 [15 093 data, I > 2σ(I)], wR(F2) ) 0.2013, GOF )
1.082.
Org. Lett., Vol. 11, No. 21, 2009
4795

The fluorescence spectra of 1b-d in THF (Figure 4a)
display an intense contribution with well-resolved vibrational
features at 360/368 and 377/385 nm and a relatively small
Stokes shift21 of 15/23 nm. The fact that the fluorescence
spectra are better resolved than the absorption spectra
suggests that, in the excited state, the bonds joining the
fluorene and the phenyl rings acquire some double bond
character, giving rise to a more rigid/planar structure.16
Moreover, the emission maxima of 1b-d are consistent with
those reported in the literature for chromophores containing
the “phenyl-fluorene-phenyl” moieties.14,15 The emission
band for 1b-d is red-shifted with respect to 1a due to the
important contributions of the phenyl rings leading to a more
conjugated excited state. This effect has been previously
observed with indenofluorene derivatives.22 The fluorescence
spectra of DSF-IFs 2 were also studied in solution (Figure
4b). 2a presents two well-resolved emission bands at 346
and 365 nm.6 The very small Stokes shift (6 nm) leads us to
conclude that the DSF-IF 2 core is highly rigid, restricting
structural deformation upon photoexcitation. Interestingly,
DSF-IFs 2b-d exhibit a significantly different behavior than
2a. Indeed, for 2b-d, a large, structureless, and red-shifted
Figure 3.
Absorption spectra of 1a-d and 2a-d (10-5 M in THF).
Inset: focus on the 335/365 nm portion of the spectra.
336, and 345 nm (Figure 3, solid lines). Despite the broadness
of these absorption bands, the four maxima fit well with both
those of 1a6 and those of known chromophores containing
the “phenyl-fluorene-phenyl” moieties.14,15 The broader
spectra of 1b-d, compared with that of 1a, are rationalized
by the combined contributions of two chromophores (namely,
the “phenyl-fluorene-phenyl” moieties and the indeno[1,2-
b]fluorene core) and by the rotational freedom of the phenyl
rings around the C-C bonds joining the fluorene and phenyl
units.16 Thus, despite the extension of conjugation of the
fluorene units with the phenyl rings, the lowest energy
transition is assigned to the indeno[1,2-b]fluorene core and
is detected at 345 nm for all molecules, as previously
observed for 1a. For the phenyl-substituted DSF-IFs 2b-d,
the UV-vis absorption spectra (Figure 3, dashed lines) also
exhibit broad bands, with two maxima at ca. 323 and 340
nm. The lowest energy transition at 340 nm is found for all
DSF-IFs 2b-d and fits well with that of 2a. This transition
is ascribed to the indeno[2,1-a]fluorene core, similarly to the
above discussion on the DSF-IF 1 molecules. In addition,
the band in 2b-d observed at ca. 320 nm is assigned to the
“phenyl-fluorene-phenyl” moieties and is blue-shifted com-
pared to the corresponding large split band (330/336 nm) of
1b-d. Another significant feature is the absorption onset of
2b-d, which is always found at higher intensity and
wavelength than those of 1b-d, respectively (see inset of
Figure 3 focusing on the low energy part of the spectra).
Similar features in the absorption spectra of other molecular
systems containing two chromophores in a face-to-face
arrangement have been previously reported in the literature
and assigned to intramolecular excitonic interactions in the
ground state.17-20
Figure 4. )
Emission spectra (10-6 M in THF) of (a) 1a-d (λexc
345 nm); (b) 2a-d (λexc ) 340 nm). (c) Fluorescence decay curves
of 1a-b/2a-b (THF). λexc ) 330 nm, λem ) 375 nm (1a-b, 2a)
or 450 nm (2b).
band (with respect to 1b-d) is observed with maxima,
respectively, at 450, 413, and 454 nm (Figure 4b). At this
stage, this large band can be tentatively assigned to an
intramolecular excimer formation, as previously observed for
example by Scherf and co-workers for oligophenyl based
cruciforms.23 Moreover, and in contrast to 1b-d, the Stokes
shifts become extremely large, i.e. 110 nm (2b), 73 nm (2c),
and 114 nm (2d), highlighting the dramatic effect of the face-
(14) Liao, Y.-L.; Lin, C.-Y.; Liu, Y.-H.; Wong, K.-T.; Hung, W.-Y.;
Chen, W.-J. Chem. Commun. 2007, 1831–1833
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Johannes, H.-H.; Kowalsky, W.; Weimann, T.; Wang, J.; Hinze, P.; Gerhard,
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A.; Sto¨ssel, P.; Vestweber, H. AdV. Mater. 2005, 17, 31–34
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(16) Belleteˆte, M.; Ranger, M.; Beaupre´, S.; Leclerc, M.; Durocher, G.
Chem. Phys. Lett. 2000, 316, 101–107.
(17) Person, R. V.; Peterson, B. R.; Lightner, D. A. J. Am. Chem. Soc.
1994, 116, 42–59
(18) Lewis, F. D.; Kurth, T. L.; Liu, W. Photochem. Photobiol. Sci.
2002, 1, 30–37
.
.
(19) Lewis, F. D.; Kurth, T. L. Can. J. Chem. 2003, 81, 770–776
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(21) The Stokes shift is defined in this work as λem - λabs (in nm) .
(22) Merlet, S.; Birau, M.; Wang, Z. Y. Org. Lett. 2002, 4, 2157–2159.
(23) Nehls, B. S.; Galbrecht, F.; Bilge, A.; Brauer, D. J.; Lehmann,
C. W.; Scherf, U.; Farrell, T. Org. Biomol. Chem. 2003, 3, 3213–3219.
(20) Benten, H.; Ohkita, H.; Ito, S.; Yamamoto, M.; Sakumoto, N.; Hori,
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2005, 109, 19681–19687
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4796
Org. Lett., Vol. 11, No. 21, 2009

a broad and a red-shifted emission band23 associated with
long and multiexponential decay curves20 and a lower
quantum yield. Thus, it is reasonable to contend that the
emission in 2b-d arises from intramolecular exci-
mers.11,20,24,25 Finally, in 2b-d, the emission wavelength
can be easily tuned by the careful choice of the steric bulk
of the substituents borne by the phenyl rings. Indeed, in the
2b-d series, 2c possesses the smallest Stokes shift (73 nm),
the highest quantum yield (48%), and a dissymmetric blue-
shifted emission band (λ ) 413 nm) (Figure 4b). These
differences found for 2c have been ascribed to a larger steric
hindrance between the face-to-face phenyl rings, induced by
their tert-butyl substituents, leading to weaker π-π interac-
tions in the excited state. It can thus be concluded that the
structural environment of the phenyl rings plays a crucial
role in the tuning of the emission color of this unique new
family of fluorophores.
In summary, we have discussed and rationalized the
properties of two new families of aryl-substituted DSF-IFs
1 and 2, possessing different geometric profiles. The original
geometry of the DSF-IF 2b-d family, with face-to-face
“phenyl-fluorene-phenyl” moieties, leads to excimer emission
through intramolecular interactions in the excited state.
Moreover, these molecules possess not only large Stokes
shifts but also remarkable tunable optical properties. Indeed,
the emission colors can be easily modulated through the steric
hindrance between the adjacent substituted phenyl rings.
Table 1. Optical Properties of DSF-IFs 1 and 2
λabs
(nm)a
λem
(nm)a
λem - λabs Φsol
(nm)
(%)b
1b 313, 329, 336, 345 360, 377, 397c
2b 308,c 323, 340
450
1c 314, 331, 337, 345 365, 384, 403c
2c 310,c 323, 340
413
1d 314, 331, 337, 345 368, 385, 406,c 432c
2d 310,c 324, 340
454
15
110
20
73
23
77
30
77
48
77
30
114
a Measured in THF (C ) 10-6 M), b Quantum yields in solution (φsol
)
were calculated relative to quinine sulfate (see Supporting Information).22
c Shoulder.
to-face “phenyl-fluorene-phenyl” arrangement in these mol-
ecules (Table 1).
The fluorescence decay curves also show a marked
difference between DSF-IFs 1 and 2. Indeed, the decay
curves of 1a-d and 2a were successfully fitted by single
exponentials (Figure 4c), and their lifetimes were measured
in the range 1.1 to 2.0 ns. On the other hand, 2b-d showed
a much more complex behavior. The emission decays were
found to be multiexponential, and the mean time-constants
were calculated to be much longer (〈τ〉 ≈ 5-15 ns). As
typical examples, fluorescence decay curves of 1a-b and
2a-b are shown in Figure 4c. Despite their different
geometries, 1a and 2a show almost identical lifetimes,
respectively τF ) 1.93 ( 0.07 ns and τF ) 1.83 ( 0.02 ns.
The lifetime of 1b is slightly shorter (τF ) 1.34 ( 0.02 ns).
A totally different behavior is observed for 2b. Hence, its
fluorescence decay curve needed at least three time constants
to reach a satisfactory fit (ꢁR2 < 1.2) with a major contribution
of a quite slow component (13.0 ( 0.1 ns). This marked
difference in the fluorescence decay behaviors provides
another eloquent illustration of the unique effect of the
“phenyl-fluorene-phenyl” cofacial arrangement.20
Acknowledgment. D.T. thanks the Re´gion Bretagne for
a studentship. We thank the CRMPO (Rennes) for mass
measuments and the CINES (Montpellier) for computing
time.
Supporting Information Available: Full characterization
1
of DSF-IF 1b-d and 2b-d, H/13C NMR spectra, electro-
chemical and crystallographic data, and computational
details. This material is available free of charge via the
Internet at http://pubs.acs.org.
DSF-IFs 1a and 2a both possess a high quantum yield
(ca. 70%).6 The substitution of the fluorene rings with phenyl
arms in 1b-d does not affect the quantum yield that remains
at ∼75% (Table 1). However in 2b-d, the introduction of
the phenyl rings leads to a strong decrease in the quantum
yield, i.e. 30/48% (Table 1). Hence, compared to their
congeners in the DSF-IF 1 family, DSF-IFs 2b-d possess
OL901750X
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C.; Freeman, S. Chem. Commun. 2008, 5161–5168
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