Luminescent dendrons with oligo(phenylenevinylene) core branches and oligo(ethylene oxide) terminal chains

Article abstract of DOI:10.1021/ma0506233

Farication of novel amphiphilic light-emitting dendrons with oligo(phenylenevinylene) core branches and oligo(ethylene oxide) terminal chains (OPV-d-OEO) was presented. It was observed that dendrons possess CAC values of 1.60 and ).40 mg/mL, respectively. It was found that the amphiphilic characteristics and resultant aggregation behaviors led to concentration and solvent polarity-dependent fluorescence in solutions. The results show that the solution photoluminescence (PL) spectra for these dendrons display red shifts with increasing concentration and/or solvent polarity.

Full text of DOI:10.1021/ma0506233

Macromolecules 2005, 38, 9389-9392
9389
Scheme 1. Synthetic Routes to G1 and G2
Luminescent Dendrons with
Oligo(phenylenevinylene) Core Branches and
Oligo(ethylene oxide) Terminal Chains
Liming Ding, Dongwook Chang, and Liming Dai*
Department of Chemical and Materials Engineering, School
of Engineering, University of Dayton, Dayton, Ohio 45469
Tao Ji and Sinan Li
Department of Polymer Engineering, University of Akron,
Akron, Ohio 44325
Jianping Lu and Ye Tao
Institute for Microstructural Sciences, National Research
Council, Ottawa, Canada
Donavon Delozier and John Connell
NASA Langley Research Center, Hampton, Virginia 23681
Received March 24, 2005
Revised Manuscript Received August 15, 2005
Introduction
The use of polymeric light-emitting diodes (LEDs) for
flat panel displays offers many advantages, including
large-area fabrication, light weight, flexibility, low
operating voltage, and ease of color tuning.1,2 Since the
discovery of electroluminescence from poly(p-phenyle-
nevinylene) (PPV) in 1990,3 many light-emitting poly-
mers have been developed,4 and polymer light-emitting
devices of novel features (e.g., patterned multicolor
emissions) attractive for practical applications have
been made through intriguing molecule/device designs
and constructions.1,4,5 Chemical modification can often
adjust the band gap of light-emitting polymers, leading
to effective color tuning of the devices.6 Continued
interest in the exploration of new chromophore struc-
tures, and hence new emission properties, has spurred
considerable attention on the design and synthesis of
novel light-emitting materials. Dendritic macromol-
ecules provide unique molecular architectures for vari-
ous use in LEDs.7 They can be designed to possess
conjugated core branches for an efficient emission and/
or charge transportation with appropriate terminal
groups/chains for good processability. For instance,
Jenekhe et al.8 have developed n-type light-emitting
conjugated dendrimers based on a benzene core and
diphenylquinoline peripheral groups. LEDs based on
these dendrimers as electron-transport layers showed
good performance.
We have previously demonstrated that PPV deriva-
tives with oligo(ethylene oxide) side chains show proper-
ties characteristic of two components with synergetic
effects.5 The covalent linkages between PPV and oligo-
(ethylene oxide) constituents can effectively minimize
the phase separation problem often associated with
conventional light-emitting electrochemical cells (LECs).9
The oligo(ethylene oxide) side chains have also been
demonstrated to facilitate the construction of light-
emitting micropatterns by region-selective deposition of
the chromophores through the oligo(ethylene oxide)
polar groups.5
In this work, novel amphiphilic light-emitting den-
drons with oligo(phenylenevinylene) core branches and
oligo(ethylene oxide) terminal chains (OPV-d-OEO)
were prepared. These dendrons combine the stiff and
insoluble conjugated core branches with the highly
flexible and soluble oligo(ethylene oxide) terminal chains
* Corresponding author. E-mail: ldai@udayton.edu.
10.1021/ma0506233 CCC: $30.25 © 2005 American Chemical Society
Published on Web 09/30/2005

9390 Notes
Macromolecules, Vol. 38, No. 22, 2005
Figure 1. 1H NMR spectrum of G2 in CDCl3.
into one macromolecule. Because of the amphiphilic
properties arising from the large difference in solubility
between the two constituent components, unique con-
centration-dependent photoluminescence (PL) was ob-
served for aqueous solutions of these dendrons. The PL
emission depends also strongly on solvent polarity. This
paper presents the synthesis and characterization of
OPV-d-OEO with a generation number of 1 (G1) and 2
(G2), along with their fluorescence properties in various
solvents of different polarities.
Results and Discussion
As shown in Scheme 1, the amphiphilic dendrons are
composed of an aldehyde focal point, conjugated stilbene
core branch, and multiple triethylene glycol monomethyl
ether groups at the periphery. To synthesize dendrons
G1 and G2, 3,5-dibromobenzaldehyde (2) was prepared
by reacting 1,3,5-tribromobenzene (1) with DMF in the
presence of n-BuLi (n-butyllithium), followed by hy-
drolysis in 2 N HCl and purification via recrystalliza-
tion. In the meantime, 3,5-triethyleneglycobenzaldehyde
(5) was prepared via the Mitsunobu reaction10 of phenol
with alcohol, which was then converted to compound 6
by the Wittig reaction.11 Dendron G1 was then prepared
in a high yield (80%) by Heck reaction12 of 6 and 3,5-
dibromobenzaldehyde. Dendron G2 was synthesized by
first converting G1 into compound 7 via the Wittig
reaction and then reacting 7 with 3,5-dibromobenzal-
dehyde through the Heck reaction. The yield of G2 by
the Heck reaction is low (30%), partially due to the
difficulties in the purification. Since crystallization is
not applicable to the viscous liquid of G2, the most
effective purification method for G2 is column chroma-
tography. However, the much higher polarity of G2
relative to G1 makes the column chromatography of G2
very difficult. Both compound 7 and G2 have very strong
interactions with silica gel matrix in the column, so high
polar solvents are required for the elution. However,
high polar solvents could also elute the side products
together with G2 and 7. Therefore, the suitable eluents
for an effective separation of the desired product from
Figure 2. Concentration dependence of surface tension for
G1 and G2 water solution at 20 °C.
the side products have to be those solvents of a relatively
low polarity, though some loss of the product is inevi-
table due to the failure of completely eluting the product
out from the silica gel column.
As expected, both dendrons G1 and G2 are soluble in
most common organic solvents, such as chloroform,
methylene dichloride, THF, methanol, and acetone. The
good solubility facilitates their characterization by vari-
ous solution methods. Figure 1 shows the 1H NMR
spectrum of G2 in CDCl3, which has been fully assigned.
Since G1 and G2 contain hydrophobic and hydrophilic
parts in one molecule, typical amphiphilic behavior can
be expected in their solutions. Indeed, Figure 2 shows
a decrease in the surface tension with increasing den-
dron concentration to a certain value (ca. 48.4 and 45.7
dyn/cm for G1 and G2, respectively) before leveling off
with further increase in concentration due to the den-
dron aggregation. The critical aggregation concentration
(CACsa concentration above which aggregation occurs)
for G1 and G2 was estimated to be 1.60 and 0.40 mg/
mL, respectively. The much slower decrease of the
surface tension with increasing concentration and the
significantly higher CAC value observed for G1 with

Macromolecules, Vol. 38, No. 22, 2005
Notes 9391
Figure 3. Absorption spectra for G1 and G2 in chloroform.
respect to G2 indicate a weaker tendency toward ag-
gregation for G1 due to its relatively smaller molecular
size. The longer stilbene branches and more oligo-
(ethylene oxide) terminal chains in G2 lead to relatively
stronger intermolecular interactions and easier chain
entanglements in solution and hence aggregation at a
relatively lower concentration.
The absorption spectra of G1 and G2 in chloroform
are given in Figure 3 with absorption peaks at 309 and
318 nm, respectively. Compared with G1, the 9 nm red
shift in absorption for G2 is attributable to the longer
conjugation length of its stilbene core. The fluorescence
of G1 and G2 in solution was also investigated. Figure
4a shows a PL peak at 411 nm for an aqueous solution
of G1 at concentration below CAC at room temperature.
Above CAC, the PL peak red shifts to 418 nm due to
the aggregation-induced π-π stacking which enhances
π-electron delocalization over the aggregated stilbene
cores. Similarly, G2 also shows a red shift in solution
PL from 434 nm below CAC to 457 nm above CAC
(Figure 4b). The larger red shift in PL for G2 solution
(23 nm) compared with G1 solution proves again the
stronger aggregation tendency for G2 (vide supra).
As G1 and G2 molecules consist of nonpolar stilbene
core branches and highly polar ethylene oxide units,
they can be regarded as polar molecules. By analogy to
other bipolar molecules, therefore, G1 and G2 could
show certain solvent-polarity-dependent molecular in-
teractions and aggregate structures in solution. As
expected, we observed a general trend, with some
fluctuation, in the red shift of G2 PL with increasing
solvent polarity above the CAC (see Table S1, Figure
5). In particular, the PL peak at 457 nm for G2 in water
blue-shifted to 407 nm when a less polar solvent such
as p-xylene was used. Since G2 is more soluble in a more
polar solvent and the concentration used for all G2
solutions (0.67 mg/mL) is higher than the CAC in water
(0.40 mg/mL), we believe that G2 molecules exist in
aggregation form in all studied solutions. In a nonpolar
solvent, the stilbene chromophore units are more ran-
domly distributed in the solution due to the relatively
strong interaction between the nonpolar solvent and
stilbene units. In a polar solvent (e.g., water), however,
strong interactions between the solvent molecules and
the polar ethylene oxide chains could “stretch” the
ethylene oxide chains relatively away from the stilbene
chromophore core, facilitating the intermolecular inter-
action (aggregation) between the “naked” stilbene cores.
This could lead to the formation of a 3-D supramolecular
Figure 4. PL spectra for G1 (a) and G2 (b) in water.
Figure 5. PL spectra of G2 in different solvents (0.67 mg/
mL). The inset shows the effect of solvent polarity14 on PL peak
wavelength for the G2 solutions (0.67 mg/mL).
assembly (possibly cylindrical or spherical dendrimer)13
with longer effective conjugation lengths and hence the
remarkable red shift in PL, though the assembled
supramolecular structures are unlikely to be monodis-
persed. The observed PL spectral broadening for water
solution (Figure 5) can be attributable to possible
excimer formation. The film PL spectra for G1 and G2
are given in Figure 6, which shows the same peak
position at 437 nm with the FWHM values of 55 and
69 nm, respectively. The broader PL spectrum for G2
film suggests, once again, the significance of the excimer
formation.
In summary, we have synthesized two novel am-
phiphilic light-emitting dendrons consisting of hydro-
phobic oligo(phenylenevinylene) core branches and hy-
drophilic oligo(ethylene oxide) terminal chains with a

9392 Notes
Macromolecules, Vol. 38, No. 22, 2005
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polarity-dependent fluorescence in solutions. Solution
PL spectra for these amphiphilic fluorescent dendrons
display red shifts with increasing concentration and/or
solvent polarity. The newly discovered solvatochromism
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Acknowledgment. We thank NASA, AFRL/ML,
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of Dayton for financial support.
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