Multitiered 2D π-stacked conjugated polymers based on pseudo-geminal disubstituted [2.2]paracyclophane

Article abstract of DOI:10.1021/ja104696e

Interchain interactions between π-systems have a strong effect on the electronic structure of conjugated organic materials. This influence has previously been explored by the spectroscopic and electrochemical characterization of molecules in which pairs of conjugated oligomers are held in a stacked fashion by attachment to a rigid scaffold. We have prepared a new polymer which uses a pseudo-geminal disubstituted [2.2]paracyclophane scaffold to hold 1,4-bis(phenylethynyl)-2,5-dialkoxybenzene (PE₃) chomophores in a π-stacked fashion over their entire length and in an extended multitier arrangement. Solutions of this new polymer display a Stokes shift of 171 nm, compared to just ca. 30 nm for previous models in which only the terminal phenyl rings of the PE₃ chromophore are held in a stacked arrangement. This suggests that interchain interactions of π-systems over their entire length in a multitier assembly provides for relaxation of the excited state to a stable phane electronic state which is responsible for emission. This stabilization is not available in the stacked dimer or other regioisomers of the polymer which possess lesser degrees of overlap. Thus, the architecture of the soluble polymer mimics that of segments of conjugated polymers in semiconducting thin films and will provide a platform for the exploration of the nature of charge carriers and excitons in these important materials.

Full text of DOI:10.1021/ja104696e

Published on Web 08/16/2010
Multitiered 2D π-Stacked Conjugated Polymers Based on Pseudo-Geminal
Disubstituted [2.2]Paracyclophane
Subodh P. Jagtap and David M. Collard*
School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400
Received June 8, 2010; E-mail: david.collard@chemistry.gatech.edu
a multitier system, Nakano has demonstrated significant Stokes
Abstract: Interchain interactions between π-systems have a strong
effect on the electronic structure of conjugated organic materials.
shifts for polymers in which individual fluorene chromophores are
organized in a helical stack.16
This influence has previously been explored by the spectroscopic
and electrochemical characterization of molecules in which pairs of
conjugated oligomers are held in a stacked fashion by attachment
to a rigid scaffold. We have prepared a new polymer which uses a
pseudo-geminal disubstituted [2.2]paracyclophane scaffold to hold
1,4-bis(phenylethynyl)-2,5-dialkoxybenzene (PE3) chomophores in
a π-stacked fashion over their entire length and in an extended
multitier arrangement. Solutions of this new polymer display a Stokes
shift of 171 nm, compared to just ca. 30 nm for previous models in
which only the terminal phenyl rings of the PE3 chromophore are
held in a stacked arrangement. This suggests that interchain
interactions of π-systems over their entire length in a multitier
assembly provides for relaxation of the excited state to a stable
“phane” electronic state which is responsible for emission. This
stabilization is not available in the stacked dimer or other regioiso-
Figure 1. 1,4-Bis(phenylethynyl)-2,5-dialkoxybenzene (PE3) containing
mers of the polymer which possess lesser degrees of overlap. Thus,
the architecture of the soluble polymer mimics that of segments of
conjugated polymers in semiconducting thin films and will provide a
platform for the exploration of the nature of charge carriers and
excitons in these important materials.
oligomers: (A) pseudo-para-[2.2]paracyclophane stacked dimer, pp-(PE3)2
(R ) n-C12H25, ref 4); (B) polymeric analog, pp-poly(PE3) (R ) n-C12H25,
ref 19); (C) pseudo-geminal polymer consisting of an extended stack, pg-
poly(PE3), and unstacked Me4PE3, R ) n-C9H19.
Semiconducting conjugated oligomers and polymers have prom-
ise in a variety of applications by virtue of their unique combination
of mechanical, electronic, and optical properties.1 Interchain
interactions between closely packed conjugated chains in the solid
state exert a strong influence on the electronic structures of the
materials: These have been investigated by both computational2
and experimental methods.3 Significant effort has been made to
explore interactions between π-systems by the spectroscopic and
electrochemical characterization of molecules in which pairs of
conjugated chains are held in a two-tier (i.e., double decker)
geometry by covalent attachment to a rigid scaffold. These scaffolds
include [2,2]paracyclophane,4-7 calix[4]arene,8 thieno-annelated
bicyclo[4.4.1]undecane,9 4,5-disubstituted xanthene,10 a m-terphenyl
oxacyclophane,11 and macrocyclic oligothiophenes.12 For example,
pseudo-para (pp) disubstituted [2.2]paracyclophanes have been used
as a scaffold to explore the interactions between pairs of stacked
stilbenes and phenyl ethynylene chromophores, e.g., Figure 1A.
Stacked chromophores with relatively short conjugation lengths
display a large Stokes shift as a result of absorption by a single
tier followed by relaxation of the excited state to a lower energy
“phane” electronic state that emits at a significantly higher
wavelength.4 The pp [2.2]paracyclophane core has also been
incorporated into a number of polymeric structures in which
conjugated oligomeric tiers are arranged such that their terminal
Figure 2. Preparation of pg-poly(PE3) (R ) n-C9H19).
To increase the extent of interaction between conjugated oligo-
mers, we have prepared a new polymeric architecture using pseudo-
geminal (pg) disubstituted [2.2]paracyclophane as a scaffold. The
combination of the U-turn imparted by this core17 with a linear
1,4-phenylene bis(ethynylene) unit provides a three-ring phenylene
ethynylene oligomer (“PE3”) which is held in a stacked arrangement
over its entire length in a multitiered assembly, i.e., pg-poly(PE3),
Figure 1C. The PE3 tier resembles a segment of a poly(1,4-
phenylene ethynylene), PPE, a highly fluorescent class of materials
with potential for use in LEDs and sensors,18 and the stacked
arrangement of the PE3 tiers is similar to the assembly of segments
of conjugated polymers in the solid state. Here we compare the
optical properties of the pseudo-geminal polymer with its unstacked
analogue (Me4PE3), the analogous pp19 and po15 polymers, and
the model π-stacked dimer (pp-(PE3)2),4 Figure 1. This comparison
establishes the influence of extending the length over which the
conjugated units are held in a stacked arrangement (i.e., along the
entire length of the oligomer, as opposed to close contact between
just the terminal units in the pp analog) and the effect of assembling
multiple conjugated chains into extended multitier stacks.
phenyl rings are held in a stacked manner, e.g., Figure 1B.13,14
A
single example of a polymeric pseudo-ortho (po) analog has also
4,15-Diethynyl-[2.2]paracyclophane, 2, was prepared from meth-
been reported.15 In an elegant extension of this approach to prepare
yl [2.2]paracyclophanecarboxylate, 1.20 Formylation of 1 takes place
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10.1021/ja104696e  2010 American Chemical Society

C O M M U N I C A T I O N S
in a regiospecific manner at the pseudo-geminal position, and
conversion to the diethynyl analog was achieved according to
established procedures, Figure 2.21 Monomers 2 and 1,4-diiodo-
2,5-bis(nonyloxy)benzene, 3, were subjected to Sonogashira cross-
coupling condensation polymerization in the presence of a catalytic
amount of Pd(PPh3)4 and CuI in diisopropylamine-THF. The crude
polymer was precipitated by addition of the reaction mixture to
methanol and reprecipitated in acetone to give pg-poly(PE3) as an
orange solid (60% isolated yield). Me4PE3 was prepared as a model
for the isolated (i.e., unstacked) tier of the polymer.
of 380 ( 5 nm; emission maxima at 408 ( 6 nm, and Stokes shifts
of 31 ( 3 nm). In each of these cases, the excited state of a single
conjugated chromophoric tier is sufficiently stable that there is no
driving force for energy transfer to a phane state. In these previous
studies there is limited overlap between the conjugated tiers, and large
Stokes shifts are observed only for shorter chromophores. However,
this does not reflect the extended nature of the interactions between
segments of conjugated polymers in the solid state. The large Stokes
shift of pg-poly(PE3) suggests that interchain interactions of the
chromophoric tiers over the entire length of the conjugated chains,
and stacking in an extended multitier arrangement, provide for
stabilization of a phane state which is not available to the other
regioisomers of the polymer (e.g., pp, po), the pp stacked dimer, or
unstacked analogs.
The pseudo-geminal disubstituted [2.2]paracyclophane core con-
stitutes a versatile scaffold that provides opportunities to vary the
stacking of conjugated tiers in two dimensions, akin to the organization
of segments of conjugated polymers in the solid state (i.e., through
modification of the conjugation length of the tiers and by variation of
the number of tiers held in a stack). The exploration of the effect of
these structural parameters on the optical properties of such ladder
polymers will provide additional insights into the evolution of electronic
structure upon bringing conjugated systems into close proximity.
The 1H NMR spectrum of pg-poly(PE3) consists of a series of broad
peaks with chemical shifts consistent with the expected structure. The
breadth of these peaks arises from the rigidity of the polymer in which
rotation within each oligomeric tier is severely restricted. The molecular
weight of the polymer determined by SEC was 5 kD, with a
polydispersity index (PDI) of 1.7 (THF eluent, calibrated against
polystyrene standards22). This corresponds to an average of eight repeat
units in the polymer chain (i.e., eight tiers). The polymer is soluble in
common organic solvents (CHCl3, CH2Cl2, and toluene) and is easily
processed into thin films by drop casting or spin coating.
The UV-visible and fluorescence spectra of pg-poly(PE3) were
compared to those of the corresponding unstacked model, Me4PE3,
Figure 3. The polymer and unstacked analog have similar absorption
maxima at ca. 320 and 365 nm, but the absorption edge of the
polymeric analog is red-shifted by ca. 50 nm from that of Me4PE3
and lacks the vibronic structure of the latter. The emission spectrum
of the pg polymer shows a broad peak which is significantly red-
shifted (λmax ) 530) and stronger compared to the spectrum of the
linear unstacked model (λmax ) 402).
Acknowledgment. This research was initiated with the support
of the National Science Foundation (ECS 0437925).
Supporting Information Available: Synthetic schemes and char-
acterization data are provided. This material is available free of charge
via the Internet at http://pubs.acs.org.
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Table 1. Absorption and Emission Spectra
Absorption λmax/nma
Emission λmax/nma
Stokes shift nma
pg-poly(PE3)
320, 359
321, 374
b
319, 386
319, 377
530
171
28
b
28
34
Me4PE3
pp-(PE3)2
402 (413)d
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b
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414 (438)d
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While the large Stokes shift of the pg polymer (171 nm) relative to
that of the linear unstacked oligomer (28 nm) can be explained by
energy transfer to a phane electronic state, this behavior is in sharp
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unstacked model Me4PE3, Table 1 (i.e., absorption λmax in the range
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