Site-isolated, intermolecularly photocrosslinkable and patternable dendritic quinacridones

Article abstract of DOI:10.1039/b901897b

Quinacridone-cored dendrimers with photocrosslinkable cinnamate moieties on the periphery can be patterned down to 5 micron features while retaining luminescence.

Full text of DOI:10.1039/b901897b

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Site-isolated, intermolecularly photocrosslinkable and patternable
dendritic quinacridonesw
Gemma D. D’Ambruoso, Eric E. Ross, Neal R. Armstrong and Dominic V. McGrath*
Received (in Berkeley, CA, USA) 29th January 2009, Accepted 23rd April 2009
First published as an Advance Article on the web 12th May 2009
DOI: 10.1039/b901897b
Quinacridone-cored dendrimers with photocrosslinkable
cinnamate moieties on the periphery can be patterned down to
5 micron features while retaining luminescence.
generation, and the crosslinking can be monitored by UV
spectroscopy.19–21 The dendrimers were synthesized in a
convergent fashion from the peripheral cinnamate moieties
to a benzyl chloride at the focal point (ESIw). First and second
generation dendrons (with benzyl chlorides at the focal point)
were coupled to quinacridone (QA) under basic conditions
(KOH–DMSO) to yield crosslinkable quinacridone dendrimers
1 and 2, respectively (Chart 1). The temperature of these
reaction mixtures was carefully maintained at the maximum
necessary (55 1C) to deprotonate QA as indicated by a change
from a violet suspension to a homogeneous blue solution.
Temperatures in excess of this resulted in significant amounts
of additional C-alkylation as evidenced by a higher MW peak
in the gel permeation chromatogram (GPC) of the resulting
product. A lower yield of 2 (15%) relative to 1 (65%) was
attributed to poorer solubility of the [G2] dendron relative
to the [G1] dendron in DMSO. Dendrimers 1 and 2 were
both characterized by 1H and 13C NMR, MS, GPC, and
combustion analysis.
We demonstrate here the formation of photopatternable thin
films of dendritic quinacridones (Chart 1) which address the
need for easily processed, patternable, emissive materials for
application in organic light emitting diodes (OLEDs) and
related thin film light sources. The quinacridone core is
related to established (green emitting) dopants (e.g. DMQA)
for aluminium quinolate-based OLEDs,1 which enhance
electroluminescence efficiency and stability of the OLED.
We have previously demonstrated site-isolation of this
quinacridone core using [G0]–[G4] dendrimers, which
increased the photoluminescence efficiency by 7-fold relative
to N,N0-diisoamylquinacridone (DIQA),2
a quinacridone
derivative we developed as a dopant in solution-processed
single layer OLEDs.3,4 Because dendrimers are capable of site-
isolating chromophores at their core, they have been used to
effectively encapsulate photoactive, electroactive, and catalytic
moieties.5–11 Dendrimers have been incorporated into organic
light emitting diodes (OLEDs) because the site-isolation of
small molecule emitting chromophores prevents aggregation
and self-quenching for films with high effective concentrations
of the emissive core molecule.12–18 Additionally, increased
solubility lent by the dendritic structure renders these materials
amenable to solution-processing of large area thin films.
Here, we have incorporated photoactivated cinnamate
crosslinking groups onto the periphery of first and second
generation quinacridone dendrimers, and demonstrate that
standard photocrosslinking technology can be employed to
create patterned thin films down to 6 nm in thickness, with
lateral feature sizes as small as 5 mm. These cinnamate–
quinacridone dendrimers are the first example of inter-
molecularly crosslinked emissive dendrimers for use in
photopatterned luminescent thin film architectures.
Absorbance spectra of dendrimers 1 and 2 (Fig. 1) exhibited
the expected cinnamate (295 and 310 nm) and quinacridone
(512 nm) absorbance maxima. Photoluminescence (PL)
spectra (lex = 490 nm) in dilute chloroform solution (Fig. 1)
show similar spectral profiles to DIQA with quinacridone
absorbance lmax of 512 nm and a Stokes shift of ca. 13 nm
(Table 1). Quantum yields, measured versus a fluorescein
standard, were 0.86 for 1 (compared with 0.85 for DIQA)
and 1.0 for 2. Linear Beer’s law plots indicated that no
solution aggregation existed up to ca. 10ꢀ3 M.
Two thicknesses of thin films (ca. 300 nm and 6 nm by
ellipsometry) of both 1 and 2 were made by spin-casting from
ca. 10ꢀ6 M CHCl3 solutions onto quartz. The 300 nm films
were used to characterize the aggregation state of the
chromophores before (Fig. 2) and after crosslinking (ESIw),
and to investigate the conditions necessary for successful
crosslinking (Fig. 3).
The dendrimers prepared for this study consisted of a
quinacridone core, benzyl aryl ether dendritic subunits, and
cinnamate-based peripheral subunits.
A cinnamic acid
derivative was chosen for the periphery because it can be
photocrosslinked without a chemical co-initiator, is chemically
inert to the reaction conditions necessary for increase of
Carl S. Marvel Laboratories, Department of Chemistry,
University of Arizona, Tucson, AZ 85721, USA.
E-mail: mcgrath@email.arizona.edu
w Electronic supplementary information (ESI) available: Synthesis
scheme, experimental procedures and characterization data for quina-
cridone dendrimers. See DOI: 10.1039/b901897b
Chart 1 Quinacridone derivatives and dendrimers 1 and 2.
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Fig. 1 Absorbance spectra (CHCl3) of 1 (black) and 2 (red). Inset
shows the absorbance and photoluminescence in the quinacridone
region.
Fig. 2 Thin film (300 nm) absorbance and photoluminescence of 1
(red) and 2 (black).
Comparison of normalized absorbance spectra of 300 nm
thin films of dendrimers 1 and 2 to solution spectra confirms
non-aggregating chromophore cores brought about by
dendron-induced site-isolation (Fig. 2). Decreasing PL lmax
(Table 1) and narrower peak widths in thin film PL spectra for
dendrimers 1 and 2 indicate decreased amounts of red-shifted
(excimer) emission with increasing generation. Interestingly,
the PL lmax of both 1 and 2 are blue-shifted compared to the
tBu[G3] quinacridone dendrimer reported previously by us,2
although the PL efficiencies are somewhat lower. Although the
cinnamate dendrons provide physical encapsulation of the
chromophores so that aggregation is prevented, it is likely
that the dendrons do not spatially isolate the chromophores at
sufficient distances to prevent self-quenching which lowers the
PL efficiency.
Fig. 3 Absorbance monitoring of crosslinking for (a) 1 and (b) 2 in
300 nm thin films. The dark lines are the spectra measured following
wet development in CHCl3.
The 300 nm thin films of dendrimers 1 and 2 were subjected
to irradiation (l = 310 nm, 6 nm slits) using a 75 W Xe arc
lamp. The solid state [2 + 2] photodimerization of cinnamic
acid and its derivatives is easily monitored by changes in the
UV absorbance (lmax = 310–315 nm).19–21 Continuous
irradiation clearly showed a decrease in the cinnamate peaks
development (18 hours) for 2. Hence, dendrimer 1 was only
moderately successful in achieving a crosslinked film, while 2
formed an insoluble network. Longer irradiation times
provided similar results with incomplete crosslinking of 1 in
all cases. Several trials determined that at least a 40% loss of
cinnamate absorbance was necessary to form an insoluble
network for dendrimer 2. This 40% loss corresponded to
ca. 60–90 minutes of irradiation depending on film thickness
(thicker films required longer irradiation times).
in the UV for both 1 (30% overall decrease of 292 nm, lmax
)
and 2 (40% overall decrease at 292, lmax) after 90 minutes
(Fig. 3). The decrease in cinnamate absorbance is most likely
due to both E - Z isomerization as well as [2 + 2] photo-
dimerization. During wet development (CHCl3 for 3 minutes),
a major portion of the film of 1 washed away leaving only 28%
of the original absorbing cinnamate peak, while the film of 2
exhibited no change in absorbance subsequent to wet develop-
ment. These results were the same even after extended wet
Photoluminescence measurements of dendrimer 2 in 300 nm
films indicated an integrated loss of emission of ca. 20% after
Table 1 Photophysical characterization of 1 and 2 in CHCl3 solution and thin film (in parentheses)
MW/g molꢀ1
lmax,Abs/nm
lmax,PL/nm
10ꢀ4 elmax/Mꢀ1 cmꢀ1
Film PL efficiencyb
a
Ffl
DIQA
452.6
1485.7
2903.3
5259.4
524 (498)
512 (516)
512 (519)
513 (516)
537 (594)
526 (558)
525 (551)
526 (567)
1.79
1.78
1.68
1.78
0.85
0.86
1.0
1
[G1], 1
[G2], 2
tBu[G3]2
4.1
6.4
6.8
1.0
a
b
Measured versus a fluorescein standard (Ffl = 0.95) 0.1 M aq. NaOH solution. Measured by integrating the corrected PL spectrum
(lex = 470 nm) and dividing by the absorbance at 470 nm. This value was then divided by the analogous value for a DIQA film.
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characterized them by fluorescence microscopy and AFM.
Fluorescence microscopy images of the patterned 6 nm films
(Fig. 4c) are identical to those obtained after patterning
300 nm films (Fig. 4b). However, differences were observed
by AFM. Whereas the 300 nm films were found to contain
channels after patterning and wet development (ESIw), the
ultra thin films showed no evidence of channels within the
patterned rows (Fig. 4d). We believe that this is due to
complete polymerization through the entire thickness of the
6 nm film. The similar thickness of the ultra thin film before
and after crosslinking (ca. 6 nm), as well as the similar
RMS surface roughness, is a further evidence of complete
polymerization. In the 300 nm films, the majority of material
is removed upon wet development due to incomplete
polymerization.
Dendrimers can simultaneously serve multiple roles in a
complex system due to their varied yet discrete architecture.
The dendrimers presented herein contain all the necessary
components that render them easily processed, patternable,
emissive materials for the fabrication of stand-alone emissive
thin film light sources. These materials indicate enhanced
utility for dendrimers in OLEDs and other solution-processed,
multilayer luminescent devices.
Fig. 4 Fluorescence microscopy images (a), (b), and (c) of patterned
dendrimer 2: (a) was made from TEM grids (ca. 85 mm feature sizes),
while (b) and (c) are 5 mm lines; (b) shows an area in which the
dendrimer has become delaminated and then re-adsorbed to the
surface; (c) is 5 mm lines patterned on a ca. 6 nm thin film; (d) is an
AFM image (full images in ESIw) of 5 mm patterned lines of dendrimer
2 on a ca. 6 nm thin film.
The authors acknowledge the National Science Foundation
(DMR-0120967) for support.
Notes and references
crosslinking and wet development. This corresponded to a
similar 20% decrease in quinacridone absorbance during
crosslinking (ESIw), suggesting that PL loss was solely due
to degradation of the film during crosslinking and that loss of
emission due to red-shifting was negligible. Since the bulk of
the site-isolation properties of the material remain intact,
observed loss in emission is not detrimental to the usefulness
of the material in luminescent thin film architectures.
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