Synthesis, structure, and evaluation of the effect of multiple stacking on the electron-donor properties of π-stacked polyfluorenes

Article abstract of DOI:10.1021/ja035518s

A versatile synthesis of π-stacked polyfluorenes is described. These polyfluorenes retain their cofacial conformations both in solution and in the solid state as was judged by NMR spectroscopy and X-ray crystallography. The experimental electron-detachmen

Full text of DOI:10.1021/ja035518s

Published on Web 06/28/2003
Synthesis, Structure, and Evaluation of the Effect of Multiple Stacking on the
Electron-Donor Properties of π-Stacked Polyfluorenes
Rajendra Rathore,* Sameh H. Abdelwahed, and Ilia A. Guzei†
Department of Chemistry, Marquette UniVersity, P.O. Box 1881, Milwaukee, Wisconsin 53201-1881
Received April 7, 2003; E-mail: rajendra.rathore@marquette.edu
Interactions between aromatic rings via π-stacking are at the
origin of many phenomena of organic material science1 and
biological chemistry including the electron transport in DNA
through stacked π-bases.2 The π-stacking phenomenon has been
studied before, mainly in the form of cyclophanes in which the
two π-systems are forced into a sandwichlike geometry that
generally imparts extensive deformation of the cofacial aromatic
moieties.3 Moreover, because of synthetic difficulties, examples of
such π-stacked molecules with multiple layers are scarce.4 Clearly,
a new synthetic approach is desired for the preparation of
multilayered π-stacked systems which will not only allow the study
of electron-transport phenomenon through stacked aromatic moieties
but will lead to the development of (next-generation) conducting
wirelike materials for practical applications in the emerging field
of nanotechnology.5
follows. For example, Figure 1 shows an upfield shift of the
aromatic protons in F2 when compared to the chemical shifts of
the aromatic protons in F1 due to the anisotropic shielding of the
protons in each fluorene ring. Moreover, such a shielding of the
protons is considerably more pronounced for the central fluorene
moiety in F3 (indicated by red dots), which is sandwiched between
two outer fluorene moieties (signals indicated by blue arrows). The
proton resonances due to the two outer fluorene rings and one
central fluorene ring in the sandwiched structure of F3 were readily
assigned on the basis of the expected 2:1 integration ratio. The
similar cofacial stacking of the fluorene moieties is retained in F4
as was judged by an even further upfield shift of the proton
resonances from the central two fluorene rings as compared to the
proton resonances of the central fluorene ring in F3 (see Figure 1).
We have now designed a versatile class of π-stacked polyfluo-
renes in which the van der Waals contact between the cofacially
oriented fluorene moieties allows effective electronic coupling
among them (see structures F1-F4 in Figure 1). Accordingly, we
report herein the syntheses, the structures in the solid state by X-ray
crystallography and those in solution by NMR spectroscopy, and
the quantitative evaluation of electronic coupling among the
cofacially oriented fluorene moieties in these multiply stacked
polyfluorene systems using photoelectron spectroscopy in the gas
phase and using electrochemistry in solution.
The synthetic strategies utilized for the preparation of polyfluo-
renes F1-F4 are summarized in Scheme 1. The synthesis of F2
was readily accomplished by methylation of easily prepared
difluorenemethane 2 by a reaction of fluorene with paraformalde-
hyde in DMF in the presence of potassium tert-butoxide as a base
in excellent yield. The key steps in the synthesis of F3 involved a
facile palladium-catalyzed coupling of the 2-biphenylmagnesium
bromide to 9,9-bis(2-bromopropenyl)fluorene 3 followed by an acid-
catalyzed (intramolecular) Friedel-Crafts alkylation (see Scheme
1). Similarly, an alkylation of 2 using 2,3-dibromopropene, followed
by coupling with biphenylmagnesium bromide and acid-catalyzed
cyclization, afforded F4 in excellent yield.6
1
Figure 1. Partial H NMR spectra of F1-F4 in CDCl3 at 22 °C.
Scheme 1
To further confirm the cofacial juxtaposition of the fluorene
moieties in F2-F4, we obtained single crystals of F2 and F3 from
a dichloromethane/methanol mixture, and their structures were
determined by X-ray crystallography.7 As expected, on the basis
of the NMR studies (see Figure 1), both of these molecules showed
cofacial juxtaposition of the fluorene moieties with the distances
between the quaternary carbons of the fluorene rings (i.e., C2-
C16 and C16-C30) in F3 being 2.724 and 2.728 Å, respectively.
Note that a similar separation of 2.71 Å between the quaternary
carbons of the fluorene moieties (i.e., C2-C4) was found in F2
(see Supporting Information). Although the fluorene moieties in
these structures are slightly tilted with respect to each other, the
The cofacial juxtaposition of the fluorene moieties in F2-F4,
in solution, was strongly supported by H NMR spectroscopy as
1
† Present address: Molecular Structure Laboratory, Department of Chemistry,
University of Wisconsin, Madison, Wisconsin.
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C O M M U N I C A T I O N S
close van der Waals contact between the fluorene rings is readily
seen in a space-filling representation of F3 (Figure 2).
showed linear correlations with quantity 1/n, where n is the number
of fluorene moieties, as shown in Figure 3B. Extrapolation of the
plots in Figure 3B to 1/n ) 0 gave the vertical ionization potential
of 7.10 eV and the oxidation potential of 0.97 V versus SCE for
the multiply stacked polyfluorene donor with an infinite number
of fluorene moieties. Interestingly, the observed linear relationship
of electron-detachment energies and 1/n in F1-F4 is highly
reminiscent of the ionization energies predicted for varying lengths
of fully conjugated polyacenes by recent theoretical studies by Houk
et al.12
In summary, we have successfully synthesized multiply π-stacked
polyfluorenes which retain their cofacial conformations both in
solution and in the solid state as was judged by NMR spectroscopy
and X-ray crystallography. The availability of the experimental
electron-detachment energies of F1-F4 should spur theoretical
interest which will provide a fundamental understanding that may
prove to be highly relevant to the electron-transport phenomenon
observed in DNA through π-stacked bases.2
We are presently exploring these π-stacked polyfluorene spacers
for the construction of a variety of wirelike materials to examine
the electron-transport phenomenon through these systems using a
variety of spectral methods13 including time-resolved spectroscopy
and X-ray crystallography.
Figure 2. An X-ray structure of F3 (left, ORTEP, and right, space-filling
representation) showing cofacial juxtaposition of the three fluorene moieties.
The consequence of electronic coupling among the stacked
fluorene moieties of F2-F4 in comparison to the model electron-
donor F1 was evaluated by cyclic voltammetry in solution and
photoelectron spectroscopy in the gas phase as follows. Polyfluorene
donors F2-F4 in dichloromethane solution (in the presence of tetra-
n-butylammonium hexafluorophosphate as a supporting electrolyte)
at a scan rate of 200 mV s-1 showed reversible cyclic voltammo-
grams with oxidation potentials (Eox) that progressively decreased
with an increasing number of fluorene moieties.8 Moreover, the
Eox values for F2 (1.42 V), F3 (1.25 V), and F4 (1.14 V) are
significantly lower than that of the parent 9,9-dimethylfluorene9
F1 (1.74 V vs SCE), which only showed an irreversible oxidation
wave.
The measurement of (gas-phase) helium(I) photoelectron spectra
(PES) of F1-F4 (see Figure S1, Supporting Information)10 provided
the vertical ionization energies for various polyfluorene electron
donors, and the values of experimental ionization potentials (IP)
for F1 (7.85 eV), F2 (7.52 eV), F3 (7.33 eV), and F4 (7.28 eV)
decreased in a manner similar to that of the electrochemical
potentials with an increasing number of fluorene moieties. More-
over, the linear relationship between the vertical ionization potentials
(IP, in gas phase) and the oxidation potentials (Eox, in solution) for
F1-F4, illustrated in Figure 3A, suggests that the polyfluorene
donors F1-F4 do not undergo significant structural changes during
(or soon after) electron detachment.11
Acknowledgment. We thank the Petroleum Research Fund
(AC12345) for financial support and Dr. Nadine E. Gruhn
(University of Arizona) for photoelectron spectroscopy.
Supporting Information Available: Various spectral data for F1-
F4 and the X-ray crystallographic data for F2 and F3 (PDF and CIF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
References
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(6) Full details of the syntheses of F1-F4 will be published separately.
(7) X-ray crystallography data for F2 and F3 have been deposited with the
Cambridge Crystallographic Data Center as supplementary publication
nos. CCDC207410 and CCDC207411, respectively. Also see Supporting
Information.
(8) Note that the lowering of redox potentials as a result of cofacial
juxtaposition in arene moieties has ample precedents, for example, see:
Rathore, R.; Kochi, J. K. Can. J. Chem. 1999, 121, 913-921 and
references therein.
(9) Greenhow, E. J.; McNeil, D. J. Chem. Soc. 1956, 3204-3209.
(10) Rathore, R.; Kochi, J. K. AdV. Phys. Org. Chem. 2000, 35, 193-318 and
references therein.
(11) Full analysis of the photoelectron spectra of F1-F4, including molecular
orbital calculations, will be presented separately: Rathore, R.; Gruhn, N.
E.; Abdelwahed, S. H.
(12) Houk, K. N.; Lee, P. S.; Nendel, M. J. Org. Chem. 2001, 66, 5517-
5521. Also see: (a) Klaerner, G.; Miller, R. D. Macromolecules 1998,
31, 2007-2009. (b) Ane´mian, R.; Mulatier, J.-C.; Andraud, C.; Ste´phan,
O.; Vial, J.-C. Chem. Commun. 2002, 1608-1609.
(13) Preliminary spectral studies show strong through-space interactions in F2-
F4 cation radicals as evidenced by the appearance of new broad bands in
the near-IR region. Full details will be disclosed in a separate manuscript.
Figure 3. (A) Correlation of the IP values of F1-F4 in the gas phase and
E
ox values in solution. (B) A plot of the IP (blue axis) values and Eox (red
axis) values versus 1/nF.
Interestingly, a closer inspection of the values of electron-
detachment energies from F1-F4, both in solution and in the gas
phase, indicated that the changes in IP and Eox values are not linear
with an increasing number of fluorene moieties in various stacked
polyfluorene donors. However, both IP and Eox values of F1-F4
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