An effective strategy to tune supramolecular interaction via a spiro-bridged spacer in oligothiophene-S,S-dioxides and their anomalous photoluminescent behavior

Article abstract of DOI:10.1021/ol070006u

We demonstrate that incorporating nonplanar spiro-bridged structures is an effective strategy for tuning supramolecular interactions of optoelectronic functional moieties. In the model compounds spiro-bridged oligothiophene-S,S- dioxides (BSiSDTFO), unusual dimers constructed by spiro-bridged spacers do not form excimers, which is confirmed by crystallographic data and fluorescent emission spectra.

Full text of DOI:10.1021/ol070006u

ORGANIC
LETTERS
2007
Vol. 9, No. 9
1619-1622
An Effective Strategy to Tune
Supramolecular Interaction via a
Spiro-Bridged Spacer in
Oligothiophene-S,S-dioxides and Their
Anomalous Photoluminescent Behavior
Ling-Hai Xie,† Xiao-Ya Hou,‡ Yu-Ran Hua,‡ Yan-Qin Huang,† Bao-Min Zhao,‡
Feng Liu,‡ Bo Peng,‡ Wei Wei,‡ and Wei Huang*,†
Institute of AdVanced Materials (IAM), Nanjing UniVersity of Posts and
Telecommunications (NUPT), 66 XinMoFan Road, Nanjing 210003, China, and
Institute of AdVanced Materials, Fudan UniVersity, 220 Handan Road,
Shanghai 200433, China
wei-huang@njupt.edu.cn
Received January 2, 2007
ABSTRACT
We demonstrate that incorporating nonplanar spiro-bridged structures is an effective strategy for tuning supramolecular interactions of
optoelectronic functional moieties. In the model compounds spiro-bridged oligothiophene-S,S-dioxides (BSiSDTFO), unusual dimers constructed
by spiro-bridged spacers do not form excimers, which is confirmed by crystallographic data and fluorescent emission spectra.
Optoelectronic materials based on π-conjugated organic mole-
cules have attracted much attention due to their applications
in organic light-emitting diodes, organic field-effect transis-
tors, nonlinear optics, solar cells, and biosensors, etc.1 Explor-
ing good candidates for these applications relies on not only
the design of molecules at the molecular level but also the
elaboration of supramolecular structures because the latter
strongly impacts the electronic structures and photophysical
processes in the solid state.2 For example, the photolumi-
nescence (PL) efficiency of oligothiophene-S,S-dioxides in
the solid state obviously depends on molecular conformations
and supramolecular organizations.3 In addition, weak supra-
molecular interactions based on π-π stacking is common
to many aromatic molecules, under which various supramo-
lecular objects have been formed, such as dimers, excimers,
J-aggregates, H-aggregates, and cross dipole stackings.4 Thus,
one way to further improve the optoelectronic properties of
π-conjugated organic molecules is to control supramolecular
interactions in the solid state.
† Nanjing University of Posts and Telecommunications (NUPT).
‡ Fudan University.
10.1021/ol070006u CCC: $37.00
© 2007 American Chemical Society
Published on Web 03/30/2007

Our objective is to utilize rigid geometric factors, e.g.,
macromolecular disks,5 helical macromolecules,6 and cross-
shaped spiro structures,7 to tune supramolecular interactions
among optoelectronic functional moieties. Different kinds
of geometric conformations could be favorable or unfavor-
able for the alignment of functional groups by participating
in the short intermolecular interactions or acting as spacers.
Therefore, incorporating unique rigid geometric conforma-
tions into the optoelectronic groups is a very good approach
to explore novel optoelectronic materials with high perfor-
mance. In this communication, we demonstrate that incor-
porating nonplanar spiro-bridged frameworks into thiophene-
S,S-dioxides is an effective way to tune supramolecular
interactions of oligothiophene-S,S-dioxide chromophores.
Some unusual dimers constructed by spiro-bridged spacers
do not form excimers, which were confirmed by crystal-
lographic data and fluorescent emission spectra. The weak
intermolecular supramolecular interactions among thiophene-
S,S-dioxides were greatly depressed due to the existence of
biphenyl moieties as an “insulated spacer”, which is ben-
eficial to the fluorescent quantum efficiency in the solid
state.
The synthetic route of model compound BSiSDTFO is
outlined in Scheme 1. The spirodithiophenefluorene (SDTF)
was prepared following the synthetic route of spirobifluorene.
Addition of the 3,3-bithienyl Grignard reagent to fluorenone
gave the tertiary alcohol, followed by Friedel-Crafts dehy-
dration cyclization in a mixture of hydrochloride and acetic
acid to obtain SDTF in an almost quantitative yield.
BSiSDTF was easily obtained by 2 equiv of n-BuLi with
SDTF followed by quenching the Me2-t-BuSiCl reagent.
BSiSDTF was oxidized at the thienyl sulfurs by m-CPBA
to afford stable mono-S,S-dioxides BSiSDTFO in 60% yield.8
Scheme 1. Synthetic Route to BSiSDTFO
However, spiro-bridged bis-S,S-dioxides were still not ob-
tained, although largely excessive m-CPBA was attempted.
The structure of BSiSDTFO was confirmed by 1H and 13
C
NMR, MALDI-TOF MS, elemental analysis, and single-
crystal X-ray diffraction. Single crystals of BSiSDTF and
BSiSDTFO were obtained from a CH2Cl2/alcohol solution.9
There is a slight lengthening of the C-S bond next to the
oxygen atom in BSiSDTFO (from mean values of 1.740 and
1.703 in BSiSDTF to 1.772 and 1.744 in BSiSDTFO), which
implies loss of the aromatic conjugation due to oxidation of
the sulfur atom. Various short intermolecular contacts
participate in the molecular packing of BSiSDTFO, including
C-H‚‚‚O, C-H‚‚‚S, and C-H‚‚‚π hydrogen bonds and π-π
stacking (Figure 1). The typical C-H‚‚‚O distance of 2.591
Å is consistent with 2.57 Å, which was observed in the rigid-
core oligothiophene dioxides.10 However, no extremely short
S‚‚‚O and S‚‚‚S separations in BSiSDTFO were observed,
which are the main driving forces to promote self-assembled
three-dimensional networks in planar oligothiophene-S,S-
dioxides.11 Therefore, it is natural to conclude that spiro
frameworks play an important role in interrupting the
supramolecular interactions between S and O atoms or S and
S atoms that lead to the formation of excimer emission and
the increase of the self-quenching probability in the solid
state.
(1) (a) Skotheim, T. A.; Elsenbaumer, R. L.; Reynolds, J. R. Handbook
of Conducting Polymers; Marcel Dekker Inc.: New York, 1998. (b)
Facchetti, A.; Yoon, M.-H.; Stern, C. L.; Katz, H. E.; Marks, T. J. Angew.
Chem., Int. Ed. 2003, 42, 3900-3903. (c) Mazzeo, M.; Vitale, V.; Sala, F.
D.; Pisignano, D.; Anni, M.; Barbarella, G.; Favaretto, L.; Zaneli, A.;
Cingolani, R.; Gigli, G. AdV. Mater. 2003, 15, 2060-2063. (d) Pisignano,
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(9) Data for X-ray structure analysis were collected at room temperature
on a Bruker SMART 1K CCD area detector with graphite-monochromated
Mo KR radiation (λ ) 0.71073 Å). The absorption correction was applied
by integration based on the crystal shape. Structures were solved by direct
methods and refined against F2 with the full-matrix, least-squares methods
using SHELXS-97 and SHELXL-97, respectively. Hydrogen atoms were
found by difference Fourier syntheses and were refined. Crystal data for
BSiSDTF: C33H40S2Si2, M ) 556.95, colorless cuboid 0.35 × 0.20 × 0.20
mm, monoclinic, P21/c, Z ) 8, a ) 25.681(11) Å, b ) 12.073(5) Å, c )
23.725(10) Å, R ) 90°, â ) 117.111°(5), γ ) 90°, V ) 6547(5) Å3, F(000)
) 2384, Fcalcd ) 1.130 Mg m-3, µ (Mo KR) ) 0.255 mm-1, 2θmax ) 50.02°,
reflections collected/unique 29 035/11 504 (Rint ) 0.0331), parameters 667,
final R1 ) 0.0636 (I > 2σ(I)), wR2 ) 0.1241 (all data), S ) 1.018 (all
data). Crystal data for BSiSDTFO: C33H40O2S2Si2, M ) 588.95, green
needle 0.15 × 0.10 × 0.02 mm, triclinic, P1, Z ) 4, a ) 12.45(4), b )
12.94(4), c ) 22.63(7) (3) Å, R ) 80.74° (5), â ) 86.74° (5), γ ) 79.99°
(6), V ) 3543 (18) Å3, F(000) ) 1256, Fcalcd ) 1.104 Mg m-3, µ (Mo KR)
) 0.243 mm-1, 2θmax ) 50.02°, reflections collected/unique 16 005/12 289
(Rint ) 0.1938), parameters 703, final R1 ) 0.2879 (I > 2σ(I)), wR2 )
0.2273 (all data), S ) 0.556 (all data). CCDC 617286 and 617287 contain
the supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
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H.; Niazimbetova, Z. I.; Bhandari, Y. J.; Galvin, M. E.; Siebbeles, L. D.
A. Chem. Mater. 2006, 18 (8), 2118-2129. (b) Drummy, L. F.; Miska, P.
K.; Alberts, D.; Lee, N.; Martin, D. C. J. Phys. Chem. B 2006, 110 (12),
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G.; Blanchard-Desce, M. Chem.-Eur. J. 2006, 12, 3089-3102.
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Am. Chem. Soc. 2000, 122 (48), 11971-11978.
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14152-14153.
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122, 974-975. (b) Iwasaki, T.; Kohinata, Y.; Nishide, H. Org. Lett. 2005,
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Org. Lett., Vol. 9, No. 9, 2007

Figure 1. Two types of dimeric arrangements within BSiSDTFO
molecule crystals through π-π stacking of the antiparallel interac-
tions of two fluorene planes and C-H‚‚‚O/C-H‚‚‚S hydrogen
bonding of intermolecular interactions.
Despite the fact that thienyl sulfur and the sulfonic oxygen
of BSiSDTFO are involved in many short intermolecular
contacts up to 12 types, most of the short contacts occur
between the carbon of fluorene and the thienyl oxygen or
sulfur instead of among the chromophores of thiophene-S,S-
dioxides. In addition, crystallographic data show that BSiS-
DTFO has two different types of recognizable “dimers” with
antiparallel head-to-tail stacking of fluorene ring interaction
planes, in which three-dimensional organization is achieved
through one face-to-face π-π stacking intermolecular in-
teraction with distances of 3.499 and 3.439 Å, respectively,
and two pairs of C-H‚‚‚O or C-H‚‚‚S intermolecular
interactions (Figure 1). The above X-ray results indicate that
the spiro-type spacers render the molecular structure ex-
tremely bulky compared to the planar structures, which not
only increases the molecular rigidity but also hinders close
packing and intermolecular interaction of chromophores.
Consequently, the introduction of a spiro-type linkage into
thienyl-S,S-dioxides probably is favorable for the improve-
ment of the luminescent quantum yields in the crystallization
state.
Figure 2. UV and PL spectra of BSiSDTFO in cyclohexane,
CHCl3, and a solid film.
ascribed to HOMO-LUMO charge-transfer excitation, has
a positive solvatochromic shift (red shifts) with increasing
solvent polarity, indicating higher polarity exists in the
excited state than in the ground state. On selective excitation
of 310 nm, the PL emission spectrum of BSiSDTFO in
cyclohexane shows a green emission band with a maximum
at 488 nm. However, the PL maximum emission of BSiS-
DTFO in CHCl3 is red shifted by 27-515 nm, indicating a
strong polar character of the excited-state species in the
presence of the thiophene-S,S-dioxide. Compared with that
in solution, the PL maximum emission in the solid film has
a red shift of less than 15 nm, which indicates that no excimer
was formed although two recognizable dimers were found
in crystal analyses. It is worth mentioning that the emission
is originally from energy transfer because the band at ca.
370 nm can always be found in nonpolar cyclohexane,
strongly polar CHCl3, and the solid film. In addition,
BSiSDTFO exhibits much higher quantum efficiency in the
solid state (ca. 20%) than that in the CHCl3 solution (3%).
Quantum chemical calculations based on ab initio B3LYP/
6-31G* gave the electronic densities and energies of the
HOMO and LUMO frontier orbitals of BSiSDTFO. The
lowest singlet excited state of BSiSDTFO is mainly localized
The absorption and PL spectra of BSiSDTFO in 10-5
M
solutions of nonpolar cyclohexane and highly polar CHCl3
and in the solid film are shown in Figure 2. The absorption
spectrum of BSiSDTFO is comprised of two peaks at ca.
258 and ca. 310 nm and a weak and broad band at ca. 400
nm in both solvents. The band at ca. 400 nm, which can be
Org. Lett., Vol. 9, No. 9, 2007
1621

on the oxygenated moieties, whereas the LUMO of the
corresponding BSiSDTF is more dispersed along the entire
aromatic backbone of fluorene.
Table 1. Electrochemical Properties of BSiSDTF and
BSiSDTFOa,b
For comparison, the cyclic voltammograms (CVs) of
BSiSDTFO together with BSiSDTF in the oxidation and
reduction domains are provided in Figure 3. The HOMO
ïx
red
Eonset
(V)
Eonset
HOMO
(eV)
LUMO
(eV)
Eg
(eV)
compound
(V)
BSiSDTF
BSiSDTFO
0.83
1.25
-2.76
-1.51
-5.68
-6.00
-1.99
-3.24
3.59
2.76
a Determined from cyclic voltammetry in CH2Cl2 for oxidation potentials
and in THF for reduction potentials (0.1 M n-Bu4N+PF6 as a supporting
-
electrolyte) using Ag/Ag+ (0.01 M) as a reference electrode at a scan rate
of 100 mV/s. bHOMO and LUMO levels were determined using the
following equations: HOMO/LUMO ) -e(Eonset - 0.0468 V) - 4.8 eV,
where the value 0.0468 V is for FOC vs Ag/Ag+.
i.e., on the electron affinity, and a much smaller effect on
the oxidation process, i.e., on the ionization potential.
In conclusion, a strategy to control the supramolecular
interaction among chromophores has initially been developed
by incorporating spiro-bridged frameworks into oligoth-
iophene-S,S-dioxides, which has been confirmed by a single-
crystal X-ray diffraction study. Recognizable dimers instead
of excimers can be deduced from the combination of
crystallographic data analysis and optical spectra, which is
probably favorable for photoluminescence in the solid state.
The improved electron affinity indicates that spiro-bridged
oligothiophene-S,S-dioxides are potential electron-transport-
ing and hole-blocking materials with high performance.
Figure 3. Cyclic voltammograms of BSiSDTFO and precursor
BSiSDTF measured at a scan rate of 100 mV/s.
and LUMO levels were estimated on the basis of an oxidation
potential of -4.8 eV (below the vacuum level) for the
ferrocene/ferrocenium (Fc/Fc+). The onset potentials of
oxidation and reduction for BSiSDTFO were determined to
be 1.25 and -1.51 V, respectively (vs Ag/Ag+). Thus,
HOMO and LUMO energy levels were estimated to be
-6.00 and -3.24 eV, respectively (Table 1). The oxidation
potential of BSiSDTFO increases only by 0.42 V with respect
to that of parent BSiSDTF, and the reduction potential is
shifted by 1.25 V toward less negative values, resulting in a
substantial increase of the electron affinity and also a
decrease of the energy gap of BSiSDTFO with respect to
DiSiSDTF. The results clearly reveal that the presence of
S,S-dioxides has a drastic effect on the reduction process,
Acknowledgment. This work was financially supported
by the NNSFC under Grants 60235412, 90406021, 50428303,
and 20574012 and by Nanjing University of Posts and
Telecommunications under Grant NY206072. L.-H.X. ap-
preciates the technical support of Ms Ran Sun with Nanjing
University.
Supporting Information Available: The experimental
1
procedures for the new compounds, H NMR, 13C NMR,
and MS spectra, and a molecular packing graph. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL070006U
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Org. Lett., Vol. 9, No. 9, 2007