Decreasing aromaticity in π-conjugated materials: Efficient synthesis and electronic structure identification of cyclopentadiene-containing systems

Article abstract of DOI:10.1021/ol402824m

An efficient route to cyclopentadiene-containing π-conjugated molecules is reported. A comparative analysis between the aryl/dienyl hybrids and their aromatic congeners shows a propensity of the diene moiety to reduce the optical band gap of a π-conjugated system without compromising a planar structural topology. Moreover, a novel poly(fluorene) derivative bearing alternating cyclopentadiene repeat units was synthesized to demonstrate the applicability of this method in polymer synthesis.

Full text of DOI:10.1021/ol402824m

ORGANIC
LETTERS
2013
Vol. 15, No. 23
5970–5973
Decreasing Aromaticity in π‑Conjugated
Materials: Efficient Synthesis and
Electronic Structure Identification of
Cyclopentadiene-Containing Systems
Lei Chen, Sufian M. Mahmoud, Xiaodong Yin, Roger A. Lalancette, and
Agostino Pietrangelo*
Department of Chemistry, Rutgers University;Newark, 73 Warren Street, Newark,
New Jersey 07102, United States
a.pietrangelo@rutgers.edu
Received October 1, 2013
ABSTRACT
An efficient route to cyclopentadiene-containing π-conjugated molecules is reported. A comparative analysis between the aryl/dienyl hybrids and
their aromatic congeners shows a propensity of the diene moiety to reduce the optical band gap of a π-conjugated system without compromising a
planar structural topology. Moreover, a novel poly(fluorene) derivative bearing alternating cyclopentadiene repeat units was synthesized to
demonstrate the applicability of this method in polymer synthesis.
π-Conjugated organic materials are studied extensively
for their (opto)electronic properties that are attractive for
device applications such as organic field-effect transistors,1
light-emitting diodes,2 electrochromic displays,3 sensors,4
and photovoltaics.5 The properties of these materials are a
consequence of successive sp2/sp-hybridized carbon atoms
along the polymer backbone that give rise to extended
π-conjugated networks. (Macro)molecules of this sort
typically comprise aromatic repeat units (e.g., phenylene,
(hetero)arylenes, fluorenylenes, carbazolylenes, etc., and
fused derivatives thereof) that lend chemical tunability,
processability, and atmospheric stability to these systems,
despite inherent resonance stabilization energies that can
suppress the formation of polyene-like quinoidal struc-
tures that are necessary for charge carrier delocalization
(Figure 1).6 Recently, there has beeninterest in introducing
(1) Menard, E.; Meitl, M. A.; Sun, Y.; Park, J.-U.; Shir, D. J.-L.;
Nam, Y.-S.; Jeon, S.; Rogers, J. A. Chem. Rev. 2007, 107, 1117–1160.
(2) Karl, N. In Organic Electronic Materials: Conjugated Polymers
and Low Molecular Weight Organic Solids; Farchioni, R., Grosso, G., Eds.;
Springer-Verlag: Berlin, 2001; pp 215ꢀ240.
(3) Beaujuge, P. M.; Reynolds, J. R. Chem. Rev. 2010, 110, 268–320.
(4) (a) Thomas, S. W., III; Joly, G. D.; Swager, T. M. Chem. Rev.
2007, 107, 1339–1386. (b) McQuade, D. T.; Pullen, A. E.; Swager, T. M.
Chem. Rev. 2000, 100, 2537–2574.
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5868–5923.
Figure 1. π-Conjugated systems and their mesomeric (quinoid)
forms.
r
10.1021/ol402824m
Published on Web 11/14/2013
2013 American Chemical Society

Table 1. Characterization Data for Aryl/Dienyl Hybrids and Their Aromatic Congeners
a UV/vis absorption maximum, λmax (nm), in dichloromethane (CH2Cl2). b Emission maximum, λem (nm), in CH2Cl2. c Oxidation potential, Eox (V vs
SCE), in CH2Cl2 containing 0.1 M tetrabutylammonium hexafluorophosphate. d Data acquired in acetonitrile (CH3CN). e Data obtained from ref 8.
f Data obtained from ref 9.
subunits into (macro)molecules that reduce a material’s
aromatic character as a means to extend effective conjuga-
tion lengths and improve delocalization,7 attributes that
Scheme 1. Synthesis of 3 and 4
areessentialtoadvancing bandgapengineeringandcharge
transport control technologies. Unfortunately, the pool of
nonaromatic π-conjugated precursors10 is severely limited
compared to its aromatic counterpart, an issue that must be
addressed if materials that exhibit the desirable attributes of
both a polyene and a polyaromatic are to be realized.
Supplanting aromatic units with cyclopentadienyl-based
alternatives is an attractive approach to reducing the aro-
matic character of a π-conjugated system. In this context,
topology of the system. Literature reports on the synthesis
the diene moiety is inherently devoid of the reorganization
and reactivity of bifunctional cyclopentadienyl-based pre-
energy that is required to break aromaticity upon doping
cursors are rare,11 and to the best of our knowledge, a
while its cyclic nature is analogous to many common aro-
(convenient) route to the aryl/dienyl hybrids described
matics (e.g., thienyl, furyl, and N-methylpyrrolyl) and, there-
herein has not been reported. In this communication, we
fore, should not significantly affect the overall structural
disclose a convenient route to such systems and examine
their spectroscopic properties and structure. In all cases,
(6) Roncali, J. Macromol. Rapid Commun. 2007, 28, 1761–1775.
(7) (a) Elbaz, G. A.; Repka, L. M.; Tovar, J. D. ACS Appl. Mater.
the hybrids exhibit reduced optical band gaps compared to
their aromatic congeners. Given the reaction scope of the
reagents used in this work and the optical band gap
reducing effect of the diene moiety, the methods described
herein could serve as a general synthetic tool toward the
preparation of novel π-conjugated materials with attrac-
tive (opto)electronic properties.
The syntheses of the diene precursors are outlined in
Scheme 1. Dione 2 is prepared by reacting 1 with base and
methyl iodide (MeI)12 and converted to 3 upon treatment
with potassium bis(trimethylsilyl)amide (KHMDS) and
N-(5-chloro-2-pyridyl)bis(trifluoromethanesulfonimide)
(ClPyN(Tf)2). Subsequently, 4 was prepared via palla-
dium-catalyzed cross-coupling of bis(pinacolato)diboron
(B2(Pin)2) and 3 using methods reported by Miyaura
et al.13 Single-crystal X-ray diffraction analysis confirmed
the targeted structure with boronate ester groups located
on the 1 and 4 positions of the planar cyclic scaffold.
Interfaces 2011, 3, 2551–2556. (b) Peart, P. A.; Tovar, J. D. Org. Lett.
ꢀ
2007, 9, 3041–3044. (c) Frere, P.; Raimundo, J.-M.; Blanchard, P.;
Delaunay, J.; Richomme, P.; Sauvajol, J.-L.; Orduna, J.; Garin, J.;
Roncali, J. J. Org. Chem. 2003, 68, 7254–7265. (d) Speros, J. C.; Paulsen,
B. D.; White, S. P.; Wu, Y.; Jackson, E. A.; Slowinski, B. S.; Frisbie,
C. D.; Hillmyer, M. A. Macromolecules 2012, 45, 2190–2199. (e) Lim, E.;
Lee, S.; Lee, K. K. J. Nanosci. Nanotechnol. 2012, 12, 4194–4198. (f)
Hong, S. Y.; Kwon, S. J.; Kim, S. C. J. Chem. Phys. 1996, 104, 1140–
1146. (g) Wong, B. M.; Cordaro, J. G. J. Phys. Chem. C 2011, 115,
18333–18341.
(8) (a) Seixas de Melo, J.; Elisei, F.; Gartner, C.; Aloisi, G. G.;
Becker, R. S. J. Phys. Chem. A 2000, 104, 6907–6911. (b) Glenis, S.;
Benz, M.; LeGoff, E.; Schindler, J. L.; Kannewurf, C. R.; Kanatzidis,
M. G. J. Am. Chem. Soc. 1993, 115, 12519–12525. (c) Kauffmann, T.;
Lexy, H. Chem. Ber. 1981, 114, 3667–3673.
(9) (a) Rohde, N.; Eh, M.; Geißler, U.; Hallensleben, M. L.; Voigt, B.;
Voigt, M. Adv. Mater. 1995, 7, 401–404. (b) Geißler, U.; Hallensleben,
M. L.; Rohde, N. Macromol. Chem. Phys. 1996, 197, 2565–2576. (c)
Kauffmann, T.; Lexy, H. Chem. Ber. 1981, 114, 3674–3683.
(10) For examples, see: (a) Yamamoto, T.; Saito, H.; Osakada, K.;
Ando, I.; Kikuchi, M. Polym. Bull. 1992, 29, 597–603. (b) He, G.; Kang,
L.; Delgado, W. T.; Shynkaruk, O.; Ferguson, M. J.; McDonald, R.;
Rivard, E. J. Am. Chem. Soc. 2013, 135, 5360–5363.
(11) For examples, see: (a) Yamaguchi, S.; Tamao, K. Tetrahedron
Lett. 1996, 37, 2983–2986. (b) Yamaguchi, S.; Itami, Y.; Tamao, K.
Organometallics 1998, 17, 4910–4916. (c) Willis, M. C.; Claverie, C. K.
Tetrahedron Lett. 2001, 42, 5105–5107. (d) Willis, M. C.; Powell,
L. H. W.; Claverie, C. K.; Watson, S. J. Angew. Chem., Int. Ed. 2004,
43, 1249–1251.
(12) Agosta, W. C.;Smith, A. B., III. J. Org. Chem. 1970, 35, 3856–3860.
(13) Takagi, J.; Takahashi, K.; Ishiyama, T.; Miyaura, N. J. Am.
Chem. Soc. 2002, 124, 8001–8006.
Org. Lett., Vol. 15, No. 23, 2013
5971

Scheme 2. Synthesis of Aryl/Dienyl Hybrids
Figure 2. (a) UV/vis absorption spectra and (b) cyclic voltam-
mograms of PhꢀC5ꢀPh (CH2Cl2), PyrꢀC5ꢀPyr (CH2Cl2),
FurꢀC5ꢀFur (CH3CN), and ThꢀC5ꢀTh (CH2Cl2). (c) Energy
level diagram showing the calculated HOMO/LUMO levels
(B3LYP/6-31þG*).
In 2001, Willis and Claverie reported on the Suzukiꢀ
Miyaura cross-coupling of a 1,4-ditriflate-1,3-cyclopentadiene
with excess 4-methoxyphenylboronic acid that afforded
only monocoupled product.11c In subsequent studies, di-
coupled products were isolated in good yield at the expense
of using two separate catalytic systems to complete the
transformation.11d Here, dicoupled π-conjugated products
(entries 1ꢀ4, Table 1) were prepared in good to excellent
yield (ca. 81ꢀ98%) by reacting 3 with 2.2 equiv of aryl
boronic acid (ester) at ambient temperature using a mod-
ification of a procedure (Scheme 2, Method A) reported by
Fu et al.14 Ambient temperature is a critical condition for
this reaction as experiments conducted at ca. 60 °C afford
mostly starting material after workup, inferring that elevated
temperatures give rise to catalyst poisoning. The scope of this
reaction was examined further by reversing the functional
group identities on the diene and aromatic reagents. Using
experimental conditions outlined in Scheme 2, Method A,
diboronate ester 4 was treated with 3 equiv of either bromo-
benzene or 2-bromothiophene; however, only starting mate-
rial was recovered. Indeed, the yields of PhꢀC5ꢀPh15 and
ThꢀC5ꢀTh were improved to 81 and 40% respectively,
when more insistent conditions (Scheme 2, Method B) were
employed, thus prompting a mechanistic investigation into
reactivity differences between 3 and 4 that is currently
underway.
to their aromatic congeners (entries 5ꢀ8, Table 1), a trend
that is supported by the fluorescence emission profiles of Phꢀ
C5ꢀPh, ThꢀC5ꢀTh, and FurꢀC5ꢀFur (Figures S12ꢀS14).
When compared against their silole-containing congeners,
PhꢀC5ꢀPh and ThꢀC5ꢀTh exhibit both blue-shifted λmax
(by ca. 36 and 29 nm, respectively) and λem (by ca. 43 and
48 nm, respectively).17
Figure 3. Representations of the X-ray crystal structure of
PyrꢀC5ꢀPyr as 50% thermal ellipsoids (except H atoms). (a)
View down the b-axis of the unit cell. (b) View down the c-axis of
the unit cell. Key: C = gray, N = purple.
Characterization of the diene-containing compounds
was accomplished using spectroscopic and XRD methods.
All 1H NMR spectra possess singlets at ca. 6.94 and 1.26 ppm
that are assigned to the olefinic and aliphatic protons of the
diene unit, respectively, suggesting that competing Heck
reactions16 are minimized under the aforementioned condi-
tions. A red shift in the absorption maxima λmax of the aryl/
dienyl hybrids (entries 1ꢀ4, Table 1, Figure 2a) clearly
indicates a reduction of the optical band gap when compared
The electronic structures of the hybrids were examined
using cyclic voltammetry (CV) and DFT methods. High-
lights include oxidation potentials (Eox, Table 1, Figure 2b)
that decrease in the order of PhꢀC5ꢀPh, ThꢀC5ꢀTh,
FurꢀC5ꢀFur, and PyrꢀC5ꢀPyr, a trend that is consistent
among the aromatic congeners under near identical condi-
tions. Moreover, the calculated (DFT(B3LYP)/6-31þG*)
HOMO and LUMO energy levels (Figure 2c) are in
(14) Littke, A. F.; Dai, C.; Fu, G. C. J. Am. Chem. Soc. 2000, 122,
4020–4028.
(15) PhꢀC5ꢀPh has been prepared by an alternative route. See:
Paquette, L. A.; Leichter, L. M. J. Org. Chem. 1974, 39, 461–467.
(17) (a) Katkevics, M.; Yamaguchi, S.; Toshimitsu, A.; Tamao, K.
Organometallics 1998, 17, 5796–5800. (b) Zhan, X.; Barlow, S.; Marder,
S. R. Chem. Commun. 2009, 1948–1955.
ꢀ
ꢀ
(16) Petz, A.; Peczely, G.; Pinter, Z.; Kollar, L. J. Mol. Catal. A:
ꢀ
Chem. 2006, 97–102.
5972
Org. Lett., Vol. 15, No. 23, 2013

reasonable agreement with experimental values (Supporting
Information, Table S1) determined by CV and UV/vis
spectroscopy. In terms of shape, all frontier orbital densities
were found to be comparable with each other (Figure S16)
with some showing conjugation across the entire molecular
scaffold.
Topological affects by the diene moiety were examined
by XRD. In short, PyrꢀC5ꢀPyr packs into a Pmmn space
group (Figure 3) with the diene ring inverted with respect
to the outer N-methylpyrrolyl rings. Perhaps the most
intriguing aspect of the structure is the high degree of
planarity across the system, a packing motif that is markedly
different from tert-(1-methylpyrrole), where the central ring
is inclined ca. 44.9(2) and 46.6(2)° with respect to the outer
rings.18 Indeed, XRD studies also show ThꢀC5ꢀTh,19
FurꢀC5ꢀFur, and PhꢀC5ꢀPh (Supporting Information)
to possess small mean dihedral angles (ca. < 25.4(6)°),
confirming that the planarity of these systems are not
compromised by the diene moiety.
Figure 4. (a) MALDI-TOF mass spectrum of 5. (b) Solution
(black) and solid-state (red) UV/vis absorption spectra of 5.
Solution (blue) and solid-state (green) emission spectra of 5. All
spectra were obtained in dichloromethane at ca. 25 °C.
The solution and solid-state UV/vis absorption and
emission spectra of 5 are shown in Figure 4b. Notable
features include (1) a solid-state absorption maximum
(ca. 432 nm) that is ca. 20 nm blue-shifted with respect to
λmax in solution and (2) a solid-state emission maximum
(ca. 560 nm) thatisca. 50nm red-shifted withrespect toλem
in solution. To assess the effect of the diene moieties on the
optoelectronic properties of a π-conjugated macromolecule,
low molecular weight 5 was prepared (Mn = 6.9 kg/mol,
PDI = 2.09) and characterized against a poly(fluorene)
derivative (PFT) of comparable molecular weight (Mn =
6.0 kg/mol, PDI = 1.30) bearing thienyl repeat units in lieu
of diene units.20 In line with observations described above, the
UV/vis absorption and emission spectra of 5 (Supporting
Information)21 exhibit maxima that are indeed red-shifted
(λmax = 15 and 35 nm, respectively) compared to the aromatic
congener.
In conclusion, we report a convenient route to cyclo-
pentadiene-containing π-conjugated (macro)molecules.
Diene precursors bearing either triflate or boronate ester
groups were used in the syntheses; however, only the
SuzukiꢀMiyaura reactions employing the former afford
products in good to excellent yield. Based on the results
reported here, a facile means to reducing aromatic character
without compromising coplanarity can be achieved in π-
conjugated systems by introducing cyclopentadienes into the
(macro)molecular scaffolds. Incorporating cyclopentadiene-
containing (macro)molecules into (opto)electronic devices is
a topic of future studies.
Scheme 3. Synthesis of 5
As a natural extension of this work, we applied this
methodology toward the synthesis of a polymer composed
of alternating cyclopentadiene and aromatic repeat units.
Toward this end, ditriflate 3 and 9,9-dihexylfluorene-2,7-
diboronic acid were used as comonomers to afford poly-
mer 5 (Scheme 3). Consistent with the small-molecule
systems described above, the 1H NMR spectrum possesses
a single resonance at ca. 6.94 ppm that is assigned to the
olefinic protons of the diene repeat units. The number-
average molecular weight (Mn) was estimated by GPC
analysis to be ca. 21.8 kg/mol with a broad PDI of 2.65,
typical of SuzukiꢀMiyaura polycondensation reactions.
Additional evidence of structural composition was ac-
quired from a matrix-assisted laser desorption/ionization
time-of-flight mass spectrum (Figure 4a) that illustrates
spectralpatternswitha repeatunitofca. m/z424, a value in
line with the assigned repeat unit structure. Differential
scanning calorimetry (DSC) revealed a glass transition
temperature (Tg) of 286 °C and a crystalline melting
temperature (Tm) of 331 °C.
Acknowledgment. The authors thank Prof. Frieder
Jakle (Rutgers University) for useful discussions. This
(18) Lynch, D. E.; Geissler, U.; Byriel, K. A. Synth. Met. 2001, 124,
385–391.
(19) ThꢀC5ꢀTh has been prepared as an oil. See: Berlin, A.; Zotti,
G.; Zecchin, S.; Schiavon, B. Macromol. Chem. Phys. 2002, 203, 1228–
1237.
€
work was supported by Rutgers University.
Supporting Information Available. Synthetic details
and characterization data. This material is available free
of charge via the Internet at http://pubs.acs.org.
(20) (a) Davis, A. R.; Peterson, J. J.; Carter, K. R. ACS Macro Lett.
€
T.; Scherf, U. Synthesis 2002, 9, 1136–1142.
(21) Samples of low molecular weight 5 exhibit a wavelength absorp-
tion band at ca. 550 nm that is not observed in high molecular weight
samples. The origin of this peak is currently under investigation.
2012, 1, 469–472. (b) Asawapirom, U.; Guntner, R.; Forster, M.; Farrell,
The authors declare no competing financial interest.
Org. Lett., Vol. 15, No. 23, 2013
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