Many poly(phenylenevinylene), or PPV,

derivatives have been evaluated for

light-emitting diode (LED) applications,

but few possess the necessary high

photoluminescence efficiencies.

Now, Kung-Hwa Wei and coworkers at

Taiwan’s National Chiao Tung

University have synthesized highly

fluorescent polymers that contain

dendritic phenyl side chains (DENPPV)

[Dinakaran

et al., Macromolecules

(2005) 38, 10429].

DENPPV homopolymers with a

molecular weight of 300 000 Da have

very low solubility in toluene and

dimethylformamide, making it

unfeasible to prepare thin films using

spin coating. This can be overcome by

preparing copolymers of DENPPV and

2-methoxy-5-(2-ethylhexyloxy)-1,4-

phenylenevinylene (MEHPPV). A

copolymer consisting of 75% DENPPV

and 25% MEHPPV displays excellent

photoluminescence with quantum

yields as high as 82% in solution and

the solid state, making the copolymers

potentially useful for use LEDs.

The researchers were able to make

electroluminescent devices by spin-

coating the DENPPV-MEHPPV

copolymer onto a device and adding a

top electrode.

The thermal stability and glass

transition temperature of the dendritic

copolymer is greater than those

reported for other PPV polymers

because the bulky, rigid dendritic

phenyl groups provide restricted

polymer chain mobility and resistance

to thermal oxidation.

Without altering the electronic

properties of PPV, the dendritic side

chains result in better

electroluminescence efficiency by

minimizing self-quenching and

preventing polymer self-aggregation.

John K. Borchardt

Dendritic side
groups for high
fluorescence

POLYMERS

Controlling cell adhesion to polymer surfaces is essential for

the success of biomaterial implants in many medical

applications.

The tripeptide sequence Arg-Gly-Asp, or RGD, binds to protein

receptors called integrins on cell surfaces, and can be used to

promote cell adhesion to substrate surfaces.

While coating polymer surfaces with RGD peptides is well

known, researchers at the Technical University of Munich,

Germany have developed a method of controlling cell

adhesion by changing the distance and orientation of RGD

peptides relative to a polymer surface [Auernheimer

et al.,

J. Am. Chem. Soc. (2005) 127, 16107].

Horst Kessler and coworkers synthesized a set of peptides

containing a photoswitchable spacer between the surface

anchoring group and the RGD peptide. The

photoisomerization of the spacer changes the distance

between the anchor and the RGD protein. The researchers

coated poly(methyl methacrylate), or PMMA, surfaces with

these peptides.

Irradiation of the surfaces with 366 nm wavelength light

results in a shorter distance between the RGD peptide and

the surface. The switching in the conformation of the peptide

coating can be reversed using 450 nm radiation.

The expanded geometry provides stronger cell adhesion to the

PMMA surfaces. The shorter spacer configuration reduces the

cell adhesion to almost the same level as that of uncoated

PMMA.

This control of the spacer geometry could provide a means of

generating patterns of cell-adhesive and cell-repulsive areas

on biomaterial surfaces. Such patterns could be generated

using photolithography techniques.

John K. Borchardt

Switching cell adhesion on and off

BIOMATERIALS

All-organic frameworks are light, rigid, and stable

POROUS MATERIALS

Chemists at the University of Michigan, Ann Arbor and
Arizona State University have synthesized highly
porous structures that consist of common organic
building blocks covalently bonded into extended,
crystalline frameworks [Côté

et al., Science (2005) 310,

1166]. The covalent organic frameworks (COFs) have
rigid structures, exceptional thermal stability, and low
densities as a result of the strong bonds between
B, C, and O atoms. With specific surface areas
surpassing those of well-known zeolites and porous
silicates, the team led by Omar M. Yaghi is hopeful
that the new lightweight materials can be optimized
for gas storage, photonic, and catalytic applications.
Yaghi’s group have previously produced porous
materials called metal-organic frameworks (MOFs), in
which organic molecules are held in a three-
dimensional lattice through metal oxide linkers. By
modifying the components, the porosity and
functionality of MOFs can be tailored. The team take a
similar approach for making COFs, suggesting similar
modifications could be incorporated.
The general strategy for synthesizing COFs is based on
a simple one-step condensation reaction that has a
high yield and uses mild reaction conditions in which
three boronic acid molecules converge to form a cyclic,
planar boroxine anhydride molecule with the loss of
three water molecules. By using 1,4-benzenediboronic
acid (BDBA) instead, an extended hexagonal sheet is
formed. Heating BDBA at 120°C for 72 hours under a
mesitylene dioxane solution allows the reaction to
proceed slowly. A crystalline material, COF-1, is

isolated in 71% yield with a graphite-like structure of
stacked two-dimensional hexagonal sheets. COF-1 has
pores of 6-12 Å in size and a surface area of 640 m

2

g

-1

.

Another compound, COF-5, was constructed using an
analogous condensation reaction between BDBA and
hexahydroxyl triphenylene under similar conditions.
COF-5 has a larger pore width of 27 Å and a surface
area of 1590 m

2

g

-1

.

Jonathan Wood

The covalently bonded, crystalline sheets of COF-1.
(Courtesy of Adrien Côté.)

JAN-FEB 2006 | VOLUME 9 | NUMBER 1-2

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RESEARCH NEWS