Ortho-Phenylenes: Unusual conjugated oligomers with a surprisingly long effective conjugation length

Article abstract of DOI:10.1021/ja106050s

ortho-Phenylenes represent a fundamental but relatively unexplored class of conjugated molecular architecture. We have developed a robust synthetic approach to monodisperse o-phenylene oligomers which we have demonstrated by synthesizing a homologous series up to the dodecamer. The o-phenylenes exhibit complex conformational behavior but are biased toward a specific 2-fold-symmetric conformation which we believe corresponds to a stacked helix. Surprisingly, the series exhibits long-range delocalization, as measured by bathochromic shifts in UV/vis spectra. Although the overall magnitude of the shifts is modest (but comparable to some other classes of conjugated materials), the effective conjugation length of the series is approximately eight repeat units. The oligomers also exhibit an unusual hypsochromic shift in their fluorescence spectra with increasing length. The origin of these trends is discussed in the context of conformational analysis and DFT calculations of the frontier molecular orbitals for the series.

Full text of DOI:10.1021/ja106050s

Published on Web 09/10/2010
ortho-Phenylenes: Unusual Conjugated Oligomers with a
Surprisingly Long Effective Conjugation Length
Jian He,† Jason L. Crase,† Shriya H. Wadumethrige,‡ Khushabu Thakur,‡ Lin Dai,†
Shouzhong Zou,† Rajendra Rathore,‡ and C. Scott Hartley*,†
Department of Chemistry and Biochemistry, Miami UniVersity, Oxford, Ohio 45056, and
Department of Chemistry, Marquette UniVersity, P.O. Box 1881, Milwaukee, Wisconsin 53201
Received July 8, 2010; E-mail: scott.hartley@muohio.edu
Abstract: ortho-Phenylenes represent a fundamental but relatively unexplored class of conjugated molecular
architecture. We have developed a robust synthetic approach to monodisperse o-phenylene oligomers
which we have demonstrated by synthesizing a homologous series up to the dodecamer. The o-phenylenes
exhibit complex conformational behavior but are biased toward a specific 2-fold-symmetric conformation
which we believe corresponds to a stacked helix. Surprisingly, the series exhibits long-range delocalization,
as measured by bathochromic shifts in UV/vis spectra. Although the overall magnitude of the shifts is modest
(but comparable to some other classes of conjugated materials), the effective conjugation length of the
series is approximately eight repeat units. The oligomers also exhibit an unusual hypsochromic shift in
their fluorescence spectra with increasing length. The origin of these trends is discussed in the context of
conformational analysis and DFT calculations of the frontier molecular orbitals for the series.
to m-phenylenes, which are cross-conjugated6). However, the
inherent twisting of the o-phenylene backbone limits orbital
Introduction
Simple ortho-, meta-, and para-phenylene polymers constitute
fundamental classes of conjugated structures and are the basic
units of poly(phenylene)-based architectures now well estab-
lished in organic electronics and nanotechnology.1 In particular,
the p-phenylenes have been extensively studied as active
materials for organic electronics (e.g., as doped conducting
polymers2 and blue emitting materials for OLEDs3), as single
molecule wires,4 as scaffolds for the organization of other
functionalities,5 and in other applications. In contrast, the
isomeric o-phenylenes have received very little attention. Like
p-phenylenes, o-phenylenes are formally conjugated (in contrast
overlap in the π-system, and they are not expected to exhibit
extensive through-bond conjugation. Accordingly, it has been
reported that o-phenylenes do not exhibit significant π-electron
delocalization, as measured by UV/vis spectroscopy.1a,7
A small number of o-phenylenes and related structures have
been reported in the literature. As early as the 1960s, Kovacic
reported that oxidative polymerization of toluene and chlo-
robenzene results in o-phenylene polymers.8 However, the
resulting materials are not structurally well-defined (e.g.,
believed to contain planar triphenylene-like structural defects),
and polymers obtained by this method have not been extensively
characterized. More recently, the electrochemical polymerization
of catechol and its derivatives was reported to give poly(o-
phenylene)s,9 but these studies have subsequently been ques-
tioned and it is likely that only triphenylene-like molecules result
from these reactions.10
† Miami University.
‡ Marquette University.
(1) (a) Berresheim, A. J.; Mu¨ller, M.; Mu¨llen, K. Chem. ReV. 1999, 99,
1747–1785. (b) Grimsdale, A. C.; Mu¨llen, K. Macromol. Rapid
Commun. 2007, 28, 1676–1702. (c) Schmaltz, B.; Weil, T.; Mu¨llen,
K. AdV. Mater. 2009, 21, 1067–1078.
The most direct parallels to the work reported in this paper
are Ito’s poly(2,3-quinoxaline)s11 and Simpkins’ synthesis of a
genuine series of o-phenylene oligomers (up to the nonamer).12
Both of these systems exhibit well-defined, but very different,
(2) Ivory, D. M.; Miller, G. G.; Sowa, J. M.; Shacklette, L. W.; Chance,
R. R.; Baughman, R. H. J. Chem. Phys. 1979, 71, 1506–1507.
(3) (a) Grem, G.; Leditzky, G.; Ullrich, B.; Leising, G. AdV. Mater. 1992,
4, 36–37. (b) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew.
Chem., Int. Ed. 1998, 37, 402–428. (c) Grimsdale, A. C.; Chan, K. L.;
Martin, R. E.; Jokisz, P. G.; Holmes, A. B. Chem. ReV. 2009, 109,
897–1091.
(6) Hong, S. Y.; Kim, D. Y.; Kim, C. Y.; Hoffmann, R. Macromolecules
2001, 34, 6474–6481.
(4) (a) Schlicke, B.; Belser, P.; De Cola, L.; Sabbioni, E.; Balzani, V.
J. Am. Chem. Soc. 1999, 121, 4207–4214. (b) Weiss, E. A.; Ahrens,
M. J.; Sinks, L. E.; Gusev, A. V.; Ratner, M. A.; Wasielewski, M. R.
J. Am. Chem. Soc. 2004, 126, 5577–5584. (c) Holman, M. W.; Liu,
R.; Zang, L.; Yan, P.; DiBenedetto, S. A.; Bowers, R. D.; Adams,
D. M. J. Am. Chem. Soc. 2004, 126, 16126–16133. (d) Banerjee, M.;
Shukla, R.; Rathore, R. J. Am. Chem. Soc. 2009, 131, 1780–1786.
(5) (a) Sakai, N.; Mareda, J.; Matile, S. Acc. Chem. Res. 2005, 38, 79–
87. (b) Bhosale, S.; Sisson, A. L.; Talukdar, P.; Fu¨rstenberg, A.;
Banerji, N.; Vauthey, E.; Bollot, G.; Mareda, J.; Ro¨ger, C.; Wu¨rthner,
F.; Sakai, N.; Matile, S. Science 2006, 313, 84–86. (c) Sisson, A. L.;
Sakai, N.; Banerji, N.; Fu¨rstenberg, A.; Vauthey, E.; Matile, S. Angew.
Chem., Int. Ed. 2008, 47, 3727–3729.
(7) (a) Noren, G. K.; Stille, J. K. Macromol. ReV. 1971, 5, 385–430. (b)
Tour, J. M. AdV. Mater. 1994, 6, 190–198.
(8) (a) Kovacic, P.; Uchic, J. T.; Hsu, L.-C. J. Polym. Sci., Part A: Polym.
Chem. 1967, 5, 945–964. (b) Kovacic, P.; Ramsey, J. S. J. Polym.
Sci., Part A: Polym. Chem. 1969, 7, 111–125. (c) Hsing, C.-F.; Jones,
M. B.; Kovacic, P. J. Polym. Sci., Part A: Polym. Chem. 1981, 19,
973–984.
(9) (a) Xu, J.; Liu, H.; Pu, S.; Li, F.; Luo, M. Macromolecules 2006, 39,
5611–5616. (b) Dong, B.; Zheng, L.; Xu, J.; Liu, H.; Pu, S. Polymer
2007, 48, 5548–5555. (c) Ma, M.; Liu, H.; Xu, J.; Li, Y.; Wan, Y. J.
Phys. Chem. C 2007, 111, 6889–6896.
(10) Voisin, E.; Williams, V. E. Macromolecules 2008, 41, 2994–2997.
9
13848 J. AM. CHEM. SOC. 2010, 132, 13848–13857
10.1021/ja106050s  2010 American Chemical Society

ortho-Phenylenes
A R T I C L E S
secondary structures: unlike the rod-like p-phenylenes, o-
phenylenes have the potential for a rich conformational behavior,
as the backbone can assume a variety of distinct conformational
states due to hindered rotation and torsional biases about the
biaryl bonds. In principle, this could lead to intractable mixtures
of random, slowly interconverting conformers. However, ex-
tensive studies of the poly(2,3-quinoxaline)s have established
that they adopt an extended helical conformation both in solution
and in the solid state.11 The o-phenylenes themselves have
received much less attention, but Simpkins’ study does include
a crystal structure of an o-phenylene hexamer that establishes
a compact helical conformation.12
compared to typical conjugated architectures, including a
surprisingly long effective conjugation length as measured by
UV/vis spectroscopy and a hypsochromic shift in fluorescence
spectra with increasing length. These results are discussed in
the context of the secondary structure of the oligomers and ab
initio calculations of their frontier molecular orbitals.
Even if o-phenylenes do not exhibit extensive through-bond
delocalization, we reasoned that they still have considerable
potential as functional organic nanostructures (e.g., in terms of
charge or energy transport). For example, Mu¨llen-type polyphe-
nylene dendrimers have been used to spatially organize chro-
mophores for efficient energy transport,13 and interarene elec-
tronic interactions within the dendritic arms have been
demonstrated despite high bond torsions.14 Porous polyphe-
nylene frameworks have also recently been shown to undergo
efficient energy transfer.15 Further, DNA has attracted consider-
able attention as a charge-transport medium,16 even though the
UV/vis spectrum of an extended DNA polymer is essentially a
superposition of the spectra of the constituent bases,17 implying
limited ground-state conjugation. It is the spatial organization
of the chromophores that makes these materials interesting.
Analogously, the o-phenylene architecture may provide a
convenient means to organize chromophores into dense arrays.
If the secondary structure of o-phenylenes is well-defined and
can be established and controlled, then they may exhibit
interesting through-space interactions between aromatic mono-
mers. Ultimately, we believe these systems may be relevant as
novel approaches to molecular wires and also as models for
transport phenomena in DNA, organic thin films, and π-stacked
columnar aggregates.
Nevertheless, very few well-characterized examples of o-
phenylenes have been synthesized, and very little is known of
their conformational behavior and electronic structure. Here we
present a robust synthetic approach to monodisperse o-phenylene
oligomers, which we have demonstrated by synthesizing the
homologous series oPn up to the dodecamer. This is significantly
longer than previously reported o-phenylenes, which has allowed
us to observe systematic changes in electronic properties as a
function of the number of arene repeat units (n). We have found
that the oPn series exhibits several very unusual properties
Results
Synthesis. In general, the synthesis of monodisperse oligomers
requires bifunctional monomers in which the reactivity of at
least one reactive group can be controlled.18 In Simpkins’
original synthesis of o-phenylenes, this was accomplished by
taking advantage of the greater reactivity of aryl iodides,
compared to aryl bromides, toward Suzuki-Miyaura coupling;
however, only low yields were obtained for the higher oligo-
mers.12 For our series, we wished to apply recent developments
in iterative oligo(phenylene) synthesis via Suzuki-Miyaura
coupling.19 In our hands, methods19a,b based on the deactivation
of the boronic acid were unsuccessful in the synthesis of oPn,
due to undesired side reactions or difficulties in purifying and
characterizing the resulting sterically hindered products. How-
ever, Manabe’s approach based on the use of phenols as masked
triflates, which they had demonstrated with the synthesis of an
o-phenylene pentamer,19c was readily applied to the synthesis
of the oPn series.
We based our synthesis on 9,10-boroxarophenanthrene de-
rivative 1, as it allowed two arene units to be added in a single
step, and because the desired substitution pattern was readily
installed. Similar boroxarenes have previously been shown to
undergo Suzuki-Miyaura coupling.20 Traditionally, these com-
pounds have been prepared by treatment of the appropriate
2-hydroxybiphenyl with BCl3;21 however, we found it conve-
nient to prepare monomer 1 from readily available 2,2′-dibromo-
5,5′-dimethoxybiphenyl22 via a series of metal-halogen ex-
changes, as shown in Scheme 1.
(11) (a) Ito, Y.; Ihara, E.; Murakami, M. J. Am. Chem. Soc. 1990, 112,
6446–6447. (b) Ito, Y.; Ihara, E.; Murakami, M. Angew. Chem., Int.
Ed. Engl. 1992, 31, 1509–1510. (c) Ito, Y.; Miyake, T.; Hatano, S.;
Shima, R.; Ohara, T.; Suginome, M. J. Am. Chem. Soc. 1998, 120,
11880–11893. (d) Ito, Y.; Ohara, T.; Shima, R.; Suginome, M. J. Am.
Chem. Soc. 1996, 118, 9188–9189. (e) Suginome, M.; Collet, S.; Ito,
Y. Org. Lett. 2002, 4, 351–354.
With monomer 1 in hand, the synthesis of the oPn series was
carried out according to Scheme 2. Suzuki-Miyaura coupling
of 1 with 4-iodoanisole using Buchwald’s SPhos catalyst23 gave
trimer oP3-OH, which was then activated toward further
(12) Blake, A. J.; Cooke, P. A.; Doyle, K. J.; Gair, S.; Simpkins, N. S.
Tetrahedron Lett. 1998, 39, 9093–9096.
(18) Franz, N.; Kreutzer, G.; Klok, H.-A. Synlett 2006, 1793–1815.
(19) (a) Noguchi, H.; Hojo, K.; Suginome, M. J. Am. Chem. Soc. 2007,
129, 758–759. (b) Gillis, E. P.; Burke, M. D. J. Am. Chem. Soc. 2007,
129, 6716–6717. (c) Ishikawa, S.; Manabe, K. Chem. Lett. 2006, 35,
164–165.
(13) (a) Weil, T.; Reuther, E.; Mu¨llen, K. Angew. Chem., Int. Ed. 2002,
41, 1900–1904. (b) Bauer, R. E.; Grimsdale, A. C.; Mu¨llen, K. Top.
Curr. Chem. 2005, 245, 253–286.
(14) Liu, D. J.; De Feyter, S.; Cotlet, M.; Stefan, A.; Wiesler, U.-M.;
Herrmann, A.; Grebel-Koehler, D.; Qu, J.; Mu¨llen, K.; De Schryver,
F. C. Macromolecules 2003, 36, 5918–5925.
(20) Zhou, Q. J.; Worm, K.; Dolle, R. E. J. Org. Chem. 2004, 69, 5147–
5149.
(21) (a) Dewar, M.; Dietz, R. J. Chem. Soc. 1960, 1344–1347. (b) Greig,
L. M.; Kariuki, B. M.; Habershon, S.; Spencer, N.; Johnston, R. L.;
Harris, K. D. M.; Philp, D. New J. Chem. 2002, 26, 701–710.
(22) Campbell, N.; Scott, A. H. J. Chem. Soc. C 1966, 1050–1052.
(23) Walker, S. D.; Barder, T. E.; Martinelli, J. R.; Buchwald, S. L. Angew.
Chem., Int. Ed. 2004, 43, 1871–1876.
(15) Chen, L.; Honsho, Y.; Seki, S.; Jiang, D. J. Am. Chem. Soc. 2010,
132, 6742–6748.
(16) Porath, D.; Cuniberti, G.; Di Felice, R. Top. Curr. Chem. 2004, 237,
183–227.
(17) Eisinger, J.; Shulman, R. G. Science 1968, 161, 1311–1319.
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He et al.
Scheme 1. Synthesis of Monomer 1a
of oP6-oP12 also indicated high purity, with single monomodal
peaks for each oligomer (Figure S6; oP4 elutes too close to the
permeation limit of our column). The oligomers were also
characterized by elemental analysis.
Crystals of oP4 and oP6 were grown by crystallization from
CH2Cl2/MeOH and CH2Cl2/MeCN, respectively, and their
structures were determined by X-ray crystallography, as shown
in Figure 2. The overall backbone conformation of an oPn
oligomer is defined by the dihedral angles of its biaryl bonds
(φi, where i indexes the bonds starting at one end; see Figure 2
for examples). In the solid state, oP4 adopts a quasi-C2-
symmetric helical conformation with backbone bond torsions
of φ1 ) 42.4°, φ2 ) 55.6°, and φ3 ) 44.9°. The outermost rings
are offset stacked, with 4.16 Å between their centers and a very
close contact of 3.15 Å between C14 and C44. This structure
is similar to that of another recently reported o-quaterphenyl.25
Conversely, the solid-state structure of oP6 is not purely helical.
The backbone again assumes a quasi-C2-symmetric conforma-
tion (φ1 ) 127.4°, φ2 ) 135.2°, φ3 ) -74.4°, φ4 ) 138.0°, φ5
) 131.4°), although the orientations of the terminal methoxy
groups break the overall symmetry of the molecule. The four
central arenes in oP6 are arranged as a stacked helix analogous
to the conformation of oP4; however, the outermost rings are
flipped away from the helical path. Interestingly, this contrasts
with the π-stacked, entirely helical conformation observed by
Simpkins and co-workers for their o-phenylene hexamer.12
UV/vis Spectroscopy. UV/vis spectra of oP2-oP12 in dichlo-
romethane are given in Figure 3 (oP2 ) 4,4′-dimethoxybiphe-
nyl). In general, the spectra are broad and featureless, with little
qualitative change in shape for n g 4. The oligomers exhibit
no significant UV/vis solvatochromism, as indicated by com-
parison with spectra of methanol and hexanes solutions (Figure
S7). There is also very little difference between the solution
and solid-state spectra of oP12 (compare Figures 3 and S8).
Normalization of the spectra with respect to n (Figure S9)
reveals a substantial hypochromic effect; i.e., the intensities of
the bands centered around 270 nm decrease substantially with
oligomer length, with a corresponding increase in the intensities
at both longer (>300 nm) and shorter (<250 nm) wavelengths.
This effect is largest for oP12, which has a hypochromicity of
approximately 60% at 270 nm vs oP2. This behavior is highly
reminiscent of DNA, where it arises from the parallel stacking
of the transition dipole moments of the bases.26 Hypochromicity
has also been observed in other π-stacked systems.27 In the
present case, analysis of this effect is complicated as the
individual monomers likely cannot be treated as independent
chromophores. However, it is noteworthy that for arenes it is
generally assigned to parallel stacked geometries, which is
consistent with the conformational analysis discussed below.28
a Reagents and conditions: (a) (i) nBuLi, THF, -78 °C, 1 h; (ii) B(OiPr)3,
12 h; (iii) NaOH, H2O2, H2O, 5 h; (b) dihydropyran, PPTS, CH2Cl2,
n
overnight; (c) (i) BuLi, THF, -78 °C; (ii) B(OiPr)3, 12 h; (iii) HCl(aq),
1.5 h.
coupling by triflation (to give oP3-OTf). The oligomer could
be extended to oP5-OH at this point by coupling with another
equivalent of 1 or capped by coupling with 4-methoxyphenyl-
boronic acid to give oP4 (alternatively, oP4 can be prepared
directly by coupling of 2,2′-dibromo-5,5′-dimethoxybiphenyl
with 4-methoxyphenylboronic acid). Repetition of this sequence
allowed the higher oligomers oP6-oP12 to be prepared. This
method affords consistently good yields at each step and should
be readily extended beyond the dodecamer. Using this approach,
we were able to prepare >300 mg of each member of the series
in a single synthetic effort.
In contrast to p-phenylenes, all compounds in Scheme 2
exhibit excellent solubility in organic solvents despite lacking
explicit solubilizing groups, as is expected for o-phenylenes.1a,7b
NMR spectra of the longer oligomers, beginning with oP5-OH,
are complicated by slow conformational exchange on the NMR
time scale, which is unsurprising given the steric hindrance of
the o-phenylene backbone. Thus, while oP4 exhibits a readily
interpreted first-order 1H NMR spectrum (Figure S1), the
spectrum of oP6 consists of broad signals at room temperature
(Figure S2), although sharp, well-resolved signals are obtained
by cooling to -5 °C. The major signals in this spectrum can be
fully assigned to the proposed structure, including its 2-fold
symmetry, although small unassigned signals remain. The higher
1
oligomers oP8-oP12 give relatively sharp H NMR spectra at
room temperature (Figures S3-S5); again, the major signals in
the spectra can be fully assigned to the expected structures, but
there are additional minor signals. We believe that the small
signals in the NMR spectra result not from impurities but from
less-significant conformational states in slow exchange on the
NMR time scale. This is supported by the observation that all
1H NMR signals for each oligomer broaden and coalesce at
elevated temperature (spectra at 100 °C in DMSO-d6 are
included in the Supporting Information). In another report, a
related o-phenylene hexamer required elevated temperatures for
NMR characterization.24 Unfortunately, we were unable to reach
a fast exchange regime for our oligomers within the limitations
of our instrumentation (150 °C).
(25) Nehls, B. S.; Galbrecht, F.; Bilge, A.; Brauer, D. J.; Lehmann, C. W.;
Scherf, U.; Farrel, T. Org. Biomol. Chem. 2005, 3, 3213–3219.
(26) (a) Bloomfield, V. A.; Crothers, D. M.; Tinoco, I., Jr. Physical
Chemistry of Nucleic Acids; Harper & Row: New York, 1974. (b)
Tinoco, I., Jr. J. Am. Chem. Soc. 1960, 82, 4785–4790.
All isolated oPn exhibit exceptionally clean MALDI mass
spectra (Figure 1), which are qualitatively consistent with high
purity given that all other synthetic intermediates also give
strong MALDI signals under the same conditions. The elemental
composition of the oligomers is confirmed by the excellent
match between the experimental and theoretical isotopic dis-
tributions of the MALDI peaks. Gel permeation chromatography
(27) (a) Lokey, R. S.; Iverson, B. L. Nature 1995, 375, 303–305. (b) Nelson,
J. C.; Saven, J. G.; Moore, J. S.; Wolynes, P. G. Science 1997, 277,
1793–1796. (c) Nakano, T.; Yade, T. J. Am. Chem. Soc. 2003, 125,
15474–15484. (d) Goto, H.; Katagiri, H.; Furusho, Y.; Yashima, E.
J. Am. Chem. Soc. 2006, 128, 7176–7178. (e) Sinkeldam, R. W.;
Hoeben, F. J. M.; Pouderoijen, M. J.; De Cat, I.; Zhang, J.; Furukawa,
S.; De Feyter, S.; Vekemans, J. A. J. M.; Meijer, E. W. J. Am. Chem.
Soc. 2006, 128, 16113–16121.
(24) Ormsby, J. L.; Black, T. D.; Hilton, C. L.; Bharat; King, B. T.
(28) Cantor, C. R.; Schimmel, P. R. Biophysical Chemistry; W. H. Freeman
and Company: San Francisco, 1980.
Tetrahedron 2008, 64, 11370–11378.
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ortho-Phenylenes
A R T I C L E S
Scheme 2. Iterative Synthesis of oPn Oligomersa
a Reagents and conditions: (a) 4-iodoanisole, Pd(OAc)2, SPhos, K3PO4, THF, 90 °C, overnight; (b) Tf2O, pyridine, CH2Cl2, overnight; (c)
4-methoxyphenylboronic acid, Pd(OAc)2, SPhos, K3PO4, THF/H2O (4:1), 100 °C, overnight; (d) 1, Pd(OAc)2, SPhos, K3PO4, THF/H2O (4:1), 90 °C, overnight.
The evolution of electronic spectra as a function of oligomer
length has long been established as a means to probe the extent
of conjugative effects.29 The degree of conjugation in conjugated
oligomers is often evaluated by plotting observed UV/vis
transition energies vs 1/n, which generally gives a good linear
fit for relatively short oligomers (i.e., away from the asymp-
totic polymer limit as n f ∞). To account for the observed
deviations from linearity as n increases, Meier has developed
an empirical method for the quantification of the extent of
conjugation in conjugated oligomeric/polymeric systems
based on a fit to eq 1:
∆λ ) λ∞ - λ1 and b quantify the overall effect of conjugation
and the extent of conjugation, respectively, allowing com-
parisons to be made between different conjugated systems.
From these parameters, it is possible to calculate the effective
conjugation length of the system (necl), which represents the
number of monomer units over which there is significant
delocalization. By definition, the effective conjugation length
is the value of n for which the difference between λ∞ and λn
is less than 1 nm, which gives
ln ∆λ
b
necl
)
+ 1
(2)
λn ) λ∞ - ∆λ · e-b(n-1)
(1)
The UV/vis spectra of the oPn series undergo a clear red shift
from oP2 to oP6; however, the absence of distinct maxima
beyond n ) 4 makes quantitative evaluation of the change in
where λn is the wavelength of the relevant transition for an
oligomer with n monomer units. The parameter λ∞ is the
extrapolation of λn to the polymer limit, and the parameters
λmax values, as is common for conjugated oligomers, impossible.
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A R T I C L E S
He et al.
Figure 3. UV/vis spectra of oP2-oP12 in dichloromethane.
Figure 1. MALDI mass spectra of oPn (dithranol matrix). Bottom: full
experimental spectra. Top: experimental (bottom) and theoretical (top)
isotopic distributions for each M+ peak. oP4: calcd 426.18, found 425.96;
oP6: calcd 638.27, found 638.22; oP8: calcd 850.35, found 850.42; oP10:
calcd 1062.43, found 1062.50; oP12: calcd 1274.52, found 1274.66.
Instead, we have evaluated onset absorption values (λext,UV
)
obtained by extrapolation of the first turning point in the spectra
to ε ) 0 (Figure S10).30 Similar results are obtained if the
absorption onset is estimated as the point at which the ε values
pass beyond a low threshold. In his original paper, Meier showed
that similar data provide an equivalent measure of both the
magnitude (∆λ) and degree (b) of conjugation for other
systems.31 Further, analysis of the data for the structurally related
p-phenylenes4d indicates that the parameters (∆λUV, bUV) are
insensitive to the use of extrapolated onset data vs the more
conventional peak maxima, confirming that λext,UV should
provide useful parameters for the oPn series that are appropriate
for semiquantitative comparisons with other systems.
As shown in Figure 4, the obtained λext,UV values show very
good fits to eq 1 and also show a clear linear dependence on
1/n, implying that significant changes in the spectra are observed
even up to n ) 12. Fitting to eq 1, we extract values of λ∞,UV
) 325.5 ( 0.5 nm, ∆λUV ) 39 ( 2 nm, and bUV ) 0.52 (
Figure 4. (a) Plot of λext,UV vs n; the solid line represents a fit to eq 1 (ꢀ2
) 1.50). (b) Plot of λext,UV vs 1/n; the solid line is a linear least-squares fit
(R2 ) 0.994).
0.05. For the oPn system, we estimate therefore that necl ≈ 8,
based on eq 2.
Fluorescence Spectroscopy. Fluorescence spectra for the oPn
series are shown in Figure 5. Overall, the oligomers are
moderately fluorescent. Quantum yields were determined relative
to DPA (Φ ) 0.90) following the established procedure.32 The
quantum yield for oP4 is comparatively low (Φ ) 0.040), but
for the remaining oligomers it is moderate and relatively
consistent (oP6: Φ ) 0.12; oP8: Φ ) 0.19; oP10: Φ ) 0.19;
Figure 2. ORTEP representations (50% ellipsoid probability) of the crystal structures of oP4 and oP6. A molecule of acetonitrile has been removed from
the structure of oP6 for clarity.
9
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ortho-Phenylenes
A R T I C L E S
Figure 5. Normalized fluorescence spectra of oPn in dichloromethane
(excitation at 260 nm).
Figure 7. Excitation spectra of oP4 as a function of monitored emission
wavelength (in dichloromethane). The extremes, 375 and 550 nm, are shown
in blue and red, respectively. Intermediate emission wavelengths, in 25 nm
increments, are shown in black. The spectra are normalized to the same
total area. Note that in a 1.0 cm cuvette dichloromethane begins to absorb
significantly for λ < 245 nm (A ) 0.05).
Figure 6. (a) Plot of the observed λmax,FL vs n; the solid line represents a
fit to eq 1 (ꢀ2 ) 5.13). (b) Plot of λmax,FL vs 1/n; the solid line represents a
linear least-squares fit (R2 ) 0.96).
oP12: Φ ) 0.15). As was the case for the UV/vis spectra, the
oligomers exhibit essentially no solvatochromism in their
fluorescence spectra (Figure S11), and the solid state spectrum
of oP12 is essentially identical to that in solution (compare
Figures 5 and S8).
Figure 8. Cyclic voltammetry of oPn vs Ag/AgNO3 in anhydrous
acetonitrile at a scan rate of 100 mV/s with tetra-n-butylammonium
hexafluorophosphate as the supporting electrolyte. The dotted line is at E
) 0.99 V. Each successive oligomer is offset by 1 × 10-4 A.
Electrochemistry. In order to probe differences in the abilities
of the oligomers to delocalize charge, the oPn series was studied
by cyclic voltammetry as solutions in anhydrous acetonitrile
with tetra-n-butylammonium hexafluorophosphate as the sup-
porting electrolyte. As shown in Figure 8, the compounds
oP4-oP12 show clear oxidation peaks without corresponding
reduction peaks on the return scan, suggesting that oxidation is
chemically irreversible.33 For all of the oligomers there is
essentially no change in the position of the first oxidation peak
as a function of n (Ep ≈ +0.99 V vs Ag/AgNO3), which occurs
at the same potential as oP2. Therefore, based on the cyclic
voltammogram of oP2, the entire oPn series has oxidation
potentials of Eox ) 1.28 V vs SCE.34 Similarly, we estimate
their HOMO levels at 5.6 eV.35
Surprisingly, while there is a substantial bathochromic shift
in the fluorescence when comparing oP2 to oP4, the overall trend
is a gradual hypsochromic shift in the fluorescence maximum
as n increases, with simultaneous reduction of a shoulder at the
long wavelength end of the spectrum. Although we do not
believe that this is an electronic effect (see below), for the
purpose of quantifying the magnitude of this shift it is interesting
that the λmax,FL values also show a reasonable fit to eq 1 and a
linear dependence on 1/n (Figure 6). The fit to eq 1 yields λ∞,FL
) 384 ( 6 nm, ∆λFL ) -44 ( 6 nm, and bFL ) 0.2 ( 0.1.
We were interested in further probing the nature of the two
bands observed in the fluorescence spectra, hypothesizing, based
on the NMR data, that they may arise from different emitting
conformations. To this end, we acquired excitation spectra of
oP4 as a function of emission wavelength, shown in Figure 7.
Clearly, the excitation spectra are highly dependent on the
monitored emission wavelength. Emission from the blue-shifted
band in the fluorescence spectrum is preferentially excited by
the extremes of short (ca. 240 nm) and long (300 nm)
wavelengths, whereas emission from the red-shifted band is
preferentially excited by intermediate (270 nm) wavelengths.
These differences are also reflected in the emission spectra as
a function of excitation wavelength (Figure S12). Similar, but
less pronounced, behavior is observed for oP6. The excitation
spectra of oP2, however, are independent of the monitored
emission wavelength, as expected.
Discussion
Arguably, the defining feature of o-phenylenes compared to
most other conjugated architectures is their inherently complex
(29) (a) Electronic Materials: The Oligomer Approach; Mu¨llen, K., Wegner,
G., Eds.; Wiley-VCH: Weinheim, 1998. (b) Martin, R. E.; Diederich,
F. Angew. Chem., Int. Ed. 1999, 38, 1350–1377.
(30) Zhao, Y.; Campbell, K.; Tykwinski, R. R. J. Org. Chem. 2002, 67,
336–344.
(31) Meier, H.; Stalmach, U.; Kolshorn, H. Acta Polym. 1997, 48, 379–
384.
(32) Eaton, D. F. Pure Appl. Chem. 1988, 60, 1107–1114.
(33) Repeat scans on the same solutions and experiments which stopped
after the first oxidation (i.e., at 1.1 V) further indicated decomposition.
9
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A R T I C L E S
He et al.
connected arenes are closer to coplanarity, whereas the closed
helix should exhibit closer through-space contacts between
arenes. In the case of Ito’s poly(2,3-quinoxaline)s, most
compounds studied were shown to favor the open helical
conformation. Conversely, the solid-state structure of Simpkins’
o-phenylene hexamer corresponds to the closed helix.
As discussed, the NMR characterization of the oPn series
suggests that interconversion between these various conforma-
tions is slow on the NMR time scale and that the overall pool
is dominated by a 2-fold-symmetric (presumably C2-symmetric)
conformer. With this in mind, we turned our attention to the
fluorescence spectra of oP4 (Figure 5). The distinct excitation
spectra for the two bands suggest that they arise from different
emitting species. Since oP4 itself has only one internal biaryl
bond (φ2, see Figure 5), we believe that these two bands likely
represent emission from the two distinct conformational states
(i.e., φ2 ≈ (70° vs (130°). It follows, since the red-shifted
emission band decreases in relative intensity as n increases, that
one particular torsional state becomes more prevalent for the
longer oligomers. This assertion is reasonable if one assumes
that the two biaryl bonds near the ends of the oligomers (e.g.,
φ2 and φ6 in Figure 9) behave differently from those closer to
the center (e.g., φ3, φ4, and φ5). Obviously, as the oligomer is
lengthened, it is only the number of central biaryl bonds that
increases.
Our hypothesis is that, in solution, the internal biaryl torsion
angles (φ3-φ5 for oP8) are biased toward the (70° states and
that the external dihedrals (φ2/φ6) are more conformationally
mobile, in that they are also able to sample the (130° states.
In other words, we believe that the closed helix is a better
representation of the overall conformation of these structures,
with some conformational disorder at the ends. This hypothesis
is supported by several pieces of indirect evidence.
Figure 9. Limiting conformations of oP8 optimized at the B3LYP/6-31G(d)
level. (a) “Closed” helix, with φ2 ) φ6 ) 67°, φ3 ) φ5 ) 70°, and φ4 )
67° (φ1 ) φ7 ) 46°). (b) “Open” helix, with φ2 ) φ6 ) -130°, φ3 ) φ5 )
-128°, and φ4 ) -131° (φ1 ) φ7 ) -133°).
conformational behavior, due to various combinations of
torsional angles along the internal biaryl bonds φ2-φn-2 (rotation
about the terminal bonds φ1 and φn-1 is degenerate). Confor-
mational analysis of the structurally related poly(2,3-quinoxa-
line)s has shown that, in an o-arylene polymer, these torsions
can assume two stable limiting values (approximately 45° and
130° for the specific case of 2,3-quinoxalines).36 Analysis of
the oPn series suggests that the behavior of these materials
should be similar, as exemplified by the representative confor-
mations of oP8 shown in Figure 9. In general, we have found
that the internal biaryl bonds in the oPn series can assume
dihedrals of approximately (70° or (130°, based on minimiza-
tions at the B3LYP/6-31G(d) level (calculated using Gaussian
0337).
Although the various combinations of φi describe many
possible conformations available to these structures, we begin
by considering the two limiting cases. If all φi ≈ 70°, the
conformation is best described as a tightly packed, stacked
helical conformation that we refer to as the “closed” helix
(Figure 9a). Alternatively, if all φi ≈ 130°, the oligomer is in
an extended helical conformation we call the “open” helix
(Figure 9b). The specific conformational state should have a
substantial impact on the properties of an o-phenylene: the open
helix should exhibit more effective orbital overlap since directly
First, analogues of the closed helical conformation are favored
in all similar structures in the literature of which we are aware.
The most significant of these is the solid-state structure of the
o-phenylene hexamer reported by Simpkins and solution-phase
analysis (ROESY NMR) of a related o-phenylene tetramer.25
Similar conformations have also been reported for the structur-
ally similar oligo(ꢁ-pyrrole)s38 and oligo(2,3-thiophene)s.39 The
key exceptions to this behavior are Ito’s poly(2,3-quinoxaline)s,
but these architectures are typically functionalized with bulky
substituents that would favor the open helical conformation due
to sterics. Moreover, the 2,3-quinoxaline backbone, which lacks
hydrogen atoms ortho to the biaryl bonds, should be predisposed
toward the more planar open helical conformation (i.e., just as
2-phenylpyrimidine is planar in the ground state, whereas
biphenyl is twisted40). It is noteworthy that the Ito group has
reported the crystal structure of an oligo(2,3-naphthalene)
tetramer that lacks bulky substituents and does possess the ortho
hydrogen atoms; in contrast to their other reported structures,
it does indeed appear to adopt a closed helical conformation in
the solid state.41
Second, the solid-state structures of oP4 and oP6 (Figure 2)
suggest that the (70° state is favored for the internal bonds.
(38) Magnus, P.; Danikiewicz, W.; Katoh, T.; Huffman, J. C.; Folting, K.
J. Am. Chem. Soc. 1990, 112, 2465–2468.
(34) Determined as the average of the anodic and cathodic peak potentials,
referenced to ferrocene (Eox ) 0.450 vs SCE).
(39) (a) Marsella, M. J.; Yoon, K.; Almutairi, A.; Butt, S. K.; Tham, F. S.
J. Am. Chem. Soc. 2003, 125, 13928–13929. (b) Almutairi, A.; Tham,
F. S.; Marsella, M. J. Tetrahedron 2004, 60, 7187–7190.
(40) Barone, V.; Commisso, L.; Lelj, F.; Russo, N. Tetrahedron 1985, 41,
1915–1918.
(35) Estimated from the onsets of the oxidation peak relative to ferrocene
n
as a standard, EHOMO ) -(E′oP - E′Fc) + 4.8 eV.
(36) Ito, Y.; Ihara, E.; Murakami, M.; Sisido, M. Macromolecules 1992,
25, 6810–6813.
(37) Frisch, M. J.; et al. Gaussian 03, revision D.02; Gaussian, Inc.:
Wallingford, CT, 2004.
(41) Motomura, T.; Nakamura, H.; Suginome, M.; Murakami, M.; Ito, Y.
Bull. Chem. Soc. Jpn. 2005, 78, 142–146.
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ortho-Phenylenes
A R T I C L E S
oP4 adopts the closed helical conformation. oP6 does not;
however, it is the ends of the oligomer (φ2 ≈ φ4 ≈ 137°) that
deviate from the closed helical state, and not the internal bond
(φ3 ≈ 74°). For convenience, we define this type of conforma-
tion, a closed helix except that the monomers at the ends of the
oligomer are flipped away from the helical path, as a “quasi-
closed” helix. We were, unfortunately, unable to obtain crystal
structures of the longer oligomers; however, conformational
energy searches at the MMFF level (which give high-quality
geometries for the related oligo(2,3-thiophene)s39a) invariably
identify the closed helical conformation as the most stable
2-fold-symmetric conformer for each of oP4-oP12.42 Unfortu-
nately, gas-phase B3LYP/6-31G(d) calculations (described
below) do not provide clear guidance as to the relative stability
of the conformers, as all optimized geometries were found to
be very close energetically (e.g., ∆E < 1.1 kcal/mol for all
considered conformations of oP6).
Third, the emission and excitation spectra support the
prevalence of the closed helical conformers. The open helix
would be expected to exhibit stronger through-bond conjugation
simply because the arene subunits are closer to coplanarity
(twists of ca. 50° vs 75° for the open and closed helices,
respectively). For oP4, the longer wavelength emission does
indeed correspond to the longer wavelength excitation maxi-
mum, suggesting that it arises from the open conformer. This
assignment of the excitation spectra is qualitatively supported
by TD-DFT calculations of the two oP4 conformers at the PCM-
TD-PBE0/6-311+G(2d,p)//B3LYP/6-31G(d) level (Figure
S13).43 The calculations confirm that the most intense transitions
for the open helical conformer should be red-shifted compared
to the closed helix. Further, the predicted UV/vis transitions
for the closed helix do include a weak but significant long-
wavelength transition, which is consistent with the shoulder in
the excitation spectra. These theoretical results do not predict
that this weak low-energy transition for the closed helix will
be red-shifted relative to the open helix, but we note that the
difference between these transitions is well within the expected
error for these calculations (ca. 0.1 eV, which is 7 nm at 300
nm)43 and also consistent with the known tendency of TD-DFT
calculations to overestimate the extent of conjugation in
oligomeric π-systems.44
Strikingly, the calculated HOMOs of the oPn oligomers are
localized at their ends, presumably because of improved orbital
overlap between the terminal monomers (e.g., for oP12, φ1 )
φ11 ) 46°, whereas the mean interior φi is 69°). This conse-
quence of the geometry of the o-phenylene backbone is very
unusual, as the frontier orbitals of unfunctionalized conjugated
oligomers are typically located near their centers.4d,45 The
LUMOs, conversely, are localized in the interiors of the oligo-
mers. In the case of oP12, the LUMO spans essentially the
second through fifth monomers (from either end) with signifi-
cantly less density at the direct center. Interestingly, close
examination of the calculated LUMOs does suggest some
through-space overlap between the stacked arenes.
This picture of the electronic structure of the oPn series can
be used to rationalize some of their unusual properties. For
example, by cyclic voltammetry, we had observed that there
was no substantial change in the first oxidation potential over
the entire series, implying that there is little change in HOMO
energies (5.6 eV) as the oligomers are lengthened. From the
DFT calculations, it is now clear why this is the case: for all
oligomers, the HOMOs are delocalized over just two units at
the termini, and these subunits are directly analogous to oP2.
Interestingly, the actual oxidation potentials (Eox ) 1.28 V vs
SCE) are quite similar to that of an infinite unsubstituted
p-phenylene polymer (ca. 1.3 V vs SCE),4d which can be
rationalized by assuming that the electron-donating methoxy
groups compensate for the reduced conjugation. The calculated
HOMO levels are similar to those of polyphenylenes in general
(5.8-6.0 eV)46 and are comparable to other classes of structur-
ally related conjugated polymers, such as dialkoxy-substituted
poly(p-phenylene ethynylene)s (5.8 eV)47 and polyfluorenes
(5.65 eV).48
One of the most surprising findings of our study is the
observation that changes in the UV/vis spectra of the oPn series
are observed even at large n. Comparison of the overall effect
of conjugation determined for the oPn series, ∆λUV ) 39 ( 2
nm, with those determined for other conjugated oligomers
suggests that it is modest and roughly comparable to some
p-phenylene ethynylenes (∆λUV ≈ 40 nm),49 although in general
it is a factor of 2 or 3 lower than most related conjugated systems
(e.g., ∆λUV ) 100-140 nm for p-phenylenes, p-phenylene
vinylenes, or other p-phenylene ethynylenes).31,50 Conversely,
the extent of conjugation, b ) 0.52 ( 0.05, implies that the
delocalization spans a remarkably long range (b ) 0.5-0.7 for
p-phenylenes, ca. 0.5 for phenylene vinylenes, 1.0 for phenylene
ethynylenes). This gives rise to the surprisingly long effective
conjugation length of necl ≈ 8. This value is quite similar to
that for alkoxy-substituted p-phenylenes with relatively large
torsional angles between the repeat units (53°),51 for which necl
On the basis of the hypothesis that the oPn oligomers can be
broadly described by the closed helix model, we carried out
geometry minimizations (B3LYP/6-31G(d)) of this conformer
for each member of the series, shown in Figure 10. For
comparison, we also carried out similar calculations for the open
helix and the quasi-closed helix conformers, with their structures
and frontier orbitals shown in Figures S14 and S15. It must be
noted that, while there are very obvious and significant
differences between the calculated electronic structures of the
fully open and fully closed helices, the quasi-closed and fully
closed helices are functionally equivalent in the sense that they
provide identical qualitative interpretations of the experimental
data.
(45) (a) Bruschi, M.; Giuffreda, M. G.; Lu¨thi, H. P. ChemPhysChem 2005,
6, 511–519. (b) Yang, L.; Ren, A.-M.; Feng, J.-K.; Wang, J.-F. J.
Org. Chem. 2005, 70, 3009–3020. (c) Ran, X.-Q.; Feng, J.-K.; Liu,
Y.-L.; Ren, A.-M.; Zou, L.-Y.; Sun, C.-C. J. Phys. Chem. A 2008,
112, 10904–10911. (d) Lukesˇ, V.; Aquino, A. J. A.; Lischka, H.;
Kauffmann, H.-F. J. Phys. Chem. B 2007, 111, 7954–7962.
(46) Grimsdale, A. C.; Mu¨llen, K. AdV. Polym. Sci. 2008, 212, 1–48.
(47) Montali, A.; Smith, P.; Weder, C. Synth. Met. 1998, 97, 123–126.
(48) Beaupre´, S.; Leclerc, M. Macromolecules 2003, 36, 8986–8991.
(49) Schumm, J. S.; Pearson, D. L.; Tour, J. M. Angew. Chem., Int. Ed.
Engl. 1994, 33, 1360–1363.
(42) These calculations consider all possible 2-fold permutations of the
biaryl torsions. For oP4-oP10, the closed helix is identified as the most
stable conformer. For oP12 it is the most stable 2-fold-symmetric
conformer (i.e., consistent with the major conformer identified by
NMR).
(43) Jacquemin, D.; Perpe`te, E. A.; Ciofini, I.; Adamo, C. Acc. Chem. Res.
2009, 42, 326–334.
(50) Ickenroth, D.; Weissmawnn, S.; Rumpf, N.; Meier, H. Eur. J. Org.
Chem. 2002, 2808–2814.
(44) Gierschner, J.; Cornil, J.; Egelhaaf, H.-J. AdV. Mater. 2007, 19, 173–
191.
(51) Based on optimization at the B3LYP/6-31G(d) level.
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A R T I C L E S
He et al.
Figure 10. Calculated B3LYP/6-31G(d) geometries (top), HOMOs (middle), and LUMOs (bottom) for the closed helical conformations. The orbitals are
displayed with isosurface values of 0.02.
≈ 9,52 although it is significantly lower than that for ladder
p-phenylenes with enforced planarity between the rings, for
which necl g 12.46,53,54 The experimental effective conjugation
length is in good agreement with the extent of the calculated
LUMO levels, which appear to be delocalized over at most five
consecutive arenes, and thus would be expected to be unaffected
beyond n ≈ 10.
The unusual conformational behavior of these oligomers can
be used to rationalize their UV/vis behavior compared to most
other conjugated systems. Significant conjugative effects are
observed over a relatively large number of arene units, which
we ascribe to the rigidity of the o-phenylene architecture.
Conversely, because of the considerable twisting of the o-
phenylene backbone, the overall effect of conjugation (∆λ) is
small, although it is surprisingly large considering that previous
discussion in the literature had suggested that delocalization in
an o-phenylene should extend over only biphenyl-like subunits.7a
As a consequence of this modest ∆λ, the UV/vis spectra of the
oPn series are in general blue-shifted by ca. 50 nm compared
to p-phenylenes of similar lengths (both unsubstituted and
(52) (a) Remmers, M.; Mu¨ller, B.; Martin, K.; Ra¨der, H.-J.; Ko¨hler, W.
Macromolecules 1999, 32, 1073–1079. (b) Grimsdale, A. C.; Mu¨llen,
K. AdV. Polym. Sci. 2006, 199, 1–82.
(53) Grimme, J.; Kreyenschmidt, M.; Uckert, F.; Mu¨llen, K.; Scherf, U.
AdV. Mater. 1995, 7, 292–295.
(54) The necl values quoted here differ slightly from the literature reports,
as we have reanalyzed the data with eqs 1 and 2 for consistency.
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ortho-Phenylenes
A R T I C L E S
alkoxy-substituted).4d,52a We are currently investigating how this
weak but long-range conjugation will affect the wire-like
behavior of these systems.
studies of excited state geometries will be required to fully
understand this phenomenon.
Conclusions
The fluorescence spectra for oP4-oP12 undergo a blue shift
with increasing length; consequently, while the emission of oP4
is actually red-shifted relative to p-phenylene tetramers, ex-
trapolation suggests that an oPn polymer should exhibit blue-
shifted emission compared to poly(p-phenylene)s.4d,52a This
unusual hypsochromic shift with increasing n is almost certainly
not due to an electronic effect, since it is not mirrored in the
UV/vis spectra (and would appear to violate the basic principles
of a simple particle-in-a-box model). It has been shown that
certain donor-acceptor-substituted conjugated oligomers exhibit
hypsochromic shifts with increasing length due to attenuation
of intramolecular charge-transfer interactions.55 There is, how-
ever, no suggestion of significant charge transfer in the excited
states of the oPn series, given the lack of solvatochromism in
the fluorescence spectra. Instead, we believe that the observed
hypsochromic shift is likely due to varying abilities of the
oligomers to undergo geometric changes in the excited state. It
is well-known that p-phenylenes undergo planarization in the
excited state, leading to large Stokes shifts.56 Planarization
should obviously not be possible for the much more sterically
encumbered o-phenylenes, where, for closed helices, it es-
sentially corresponds to compression of the spring-like conform-
ers. We suspect, however, that the longer oPn oligomers will
be less able to accommodate these geometric changes since they
would have to occur within their sterically less-forgiving
interiors. In essence, the longer oligomers appear to undergo
lessened geometric rearrangements in their excited states, leading
to reduced Stokes shifts. Alternatively, it may be that the blue
shift of the fluorescence maxima simply reflects the changing
conformational pool as n increases (i.e., decrease of φi subunits
in the 70° state). Additional experiments to more accurately
assess the conformational behavior and careful theoretical
We have developed an iterative synthetic approach to
o-phenylene oligomers, which has allowed the synthesis of
a homologous series of these compounds up to the dodecam-
er. Evaluation of their absorption, emission, and electro-
chemical properties as a function of oligomer length reveals
very unusual behavior, including the following: (1) significant
bathochromic shifts to the UV/vis spectra, suggesting that
conjugation extends much farther along the o-phenylene
backbone than previously thought; (2) significant hypsochro-
mic shifts to the fluorescence spectra; and (3) oxidation
potentials (and HOMO energies) that are largely independent
of length. These unusual properties appear to result from the
conformational behavior of the oligomers, which is more
complex than that of typical conjugated materials. We are
currently exploring methods to probe this conformational
behavior directly and the use of the o-phenylene motif as a
means to organize other chromophores.
Acknowledgment. C.S.H., J.H., and J.L.C. acknowledge support
by the National Science Foundation (CHE-0910477), Miami
University, and the donors of the American Chemical Society
Petroleum Research Fund (47926-G7). R.R., S.H.W., and K.T.
acknowledge support from the National Science Foundation
(Chemistry). L.D. and S.Z. acknowledge support from the National
Science Foundation (CHM-0616436).
Supporting Information Available: NMR spectra of oPn, GPC
traces, UV/vis spectra in methanol and cyclohexane, solid-state
UV/vis and fluorescence spectra of oP12, normalized UV/vis
spectra and hypochromicity, extrapolations used for UV/vis λext
values, fluorescence spectra in methanol and cyclohexane,
emission spectra of oP4 as a function of excitation wavelength,
calculated oscillator strengths of oP4, calculated geometries and
FMOs for open helical conformers, calculated geometries and
FMOs for quasi-closed helical conformers, full experimental
procedures, and complete ref 37. This material is available free
of charge via the Internet at http://pubs.acs.org.
(55) (a) Meier, H. Angew. Chem., Int. Ed. 2005, 44, 2482–2506. (b) Meier,
H.; Gerold, J.; Kolshorn, H.; Mu¨hling, B. Chem.sEur. J. 2004, 10,
360–370.
(56) (a) Momicchioli, F.; Bruni, M. C.; Baraldi, I. J. Phys. Chem. 1972,
76, 3983–3990. (b) Heimel, G.; Daghofer, M.; Gierschner, J.; List,
E. J. W.; Grimsdale, A. C.; Mu¨llen, K.; Beljonne, D.; Bre´das, J.-L.;
Zojer, E. J. Chem. Phys. 2005, 122, 054501.
JA106050S
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