Dendritic incorporation of quinacridone: Solubility, aggregation, electrochemistry, and solid-state luminescence

Article abstract of DOI:10.1039/b413684e

The first incorporation of quinacridone, a technologically important organic electroluminescent emitter, into dendrimers increases solubility, decreases aggregation, retards heterogeneous electron transfer, and enhances luminescence in condensed phases (powders and thin films).

Full text of DOI:10.1039/b413684e

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Dendritic incorporation of quinacridone: solubility, aggregation,
electrochemistry, and solid-state luminescence{
Adrian Ortiz, Ware H. Flora, Gemma D. D’Ambruoso, Neal R. Armstrong* and Dominic V. McGrath*
Received (in Corvallis, OR, USA) 3rd September 2004, Accepted 20th October 2004
First published as an Advance Article on the web 8th December 2004
DOI: 10.1039/b413684e
heterogeneous conditions (aqueous NaOH/toluene) in the presence
of a phase transfer catalyst (TBAI) that provided dendrimers 2–7
in moderate to good yields (Method B) (ESI{).
All compounds were structurally characterized by a combina-
tion of 1H NMR, mass spectrometry (MALDI) and gel
permeation chromotography (GPC). GPC analysis of 1–7 verified
the expected increasing hydrodynamic volume with increasing
generation for each dendrimer series. No evidence of solution
aggregation was observed.
The first incorporation of quinacridone, a technologically
important organic electroluminescent emitter, into dendrimers
increases solubility, decreases aggregation, retards heteroge-
neous electron transfer, and enhances luminescence in con-
densed phases (powders and thin films).
The linear trans-quinacridone (QA) chromophore1 demonstrates
promising photovoltaic and photoconductive properties and has
become an important dopant dye emitter in electroluminescent
display devices.2,3 Dopant dyes are widely used in small molecule
and polymeric organic light-emitting diodes (OLEDs) to enhance
the electroluminescence (EL) efficiency through both Fo¨rster
energy transfer (ET) and charge transfer/recombination (CT)
pathways.4–6 However, self-quenching and aggregation are pro-
blems for many dopant dyes, including quinacridone, resulting in
significant decreases in luminescence efficiency. Modification to
isolate guest emitters, while maintaining high rates of energy and
charge transfer with the surrounding medium, is critical.7,8
Alkylation at the N,N9 positions of quinacridone inhibits
aggregation of the dopant dye in aluminium quinolate-based
(Alq3) OLEDs.4 However, our own recent work on
N,N9-diisoamylquinacridone (DIQA), which is less aggregating
than N,N9-dimethylquinacridone, still indicates aggregation at
higher concentrations and in neat thin films.5,6 Herein we report
Dendritic encapsulation of quinacridone was successful in
increasing solubility in organic solvents, a critical parameter for
solution processing techniques. Quinacridone dendrimers 1 and 2
exhibited enhanced solubilities in chlorinated solvents (CH2Cl2 and
CHCl3) and 3 was also readily soluble in solvents such as THF,
DMF, DMSO and NMP. The peripheral 3,5-di-tert-butylphenyl
groups on quinacridone dendrimers 4–7 generally rendered these
compounds soluble in an even wider range of organic solvents.
While zeroth-generation 4 was still only soluble in chlorinated
solvents, first-generation 5 was also soluble in THF, NMP and
ethyl acetate, and the range of good solvents for second- and third-
generation dendrimers 6 and 7 included DMF, diethyl ether and
acetone.
Absorbance and photoluminescence (PL) spectra (lex 5 490 nm)
of a dilute solution of 6 are representative of the spectra of all
materials presented herein (Fig. 1). Essentially identical spectra
(lmax 5 513 nm with Stokes shifts of ca. 14 nm) were obtained for
dilute solutions of all other dendrimers, with luminescent quantum
the
dendritic
encapsulation9–11
of
quinacridone
by
N,N9-disubstitution of the chromophore with two types of benzyl
aryl ether dendrons. With increasing generation, these materials
exhibit enhanced solubility in organic solvents, sharply declining
rates of heterogeneous electron transfer in solution, and lower
tendency toward chromophore self-aggregation in thin films that
results in dramatically enhanced thin film luminescence.
Incorporation of quinacridone into the core of benzyl aryl ether
dendrimers was accomplished by alkylation of the vinylogous
amide nitrogens with zeroth through third-generation dendritic
bromides12,13 (Scheme 1). Dipolar aprotic solvent conditions
(NaH/DMF) proved successful for only the lowest-generation
quinacridone dendrimers (Method A) (ESI{). First-generation 1
and zeroth-generation 4 were obtained in 41 and 94% yields,
respectively. However, use of these conditions to prepare higher-
generation dendrimers yielded predominately mono-N-alkylated
quinacridones. Much more successful were previously reported14
{ Electronic supplementary information (ESI) available: Experimental
procedures and characterization data for quinacridone dendrimers:
Fig. S1: PL efficiency of neat thin films of dendrimers 4–7 relative to
DIQA. Fig. S2: Cyclic voltammograms of 1–7. See http://www.rsc.org/
suppdata/cc/b4/b413684e/
*nra@u.arizona.edu (Neal R. Armstrong)
mcgrath@u.arizona.edu (Dominic V. McGrath)
Scheme 1 Synthesis of dendritic quinacridones.
444 | Chem. Commun., 2005, 444–446
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generation is increased, although residual excimer (red-shifted)
emission exists for 7 in these highly-concentrated molecular films.
The impact of this decreasing tendency to aggregate in the solid
state as generation number increases is a logarithmic increase in
relative PL efficiency of the neat films, up to a value 6.8 times that
of DIQA for 7.{ These changes are consistent with qualitative
observations of bulk powders of 4–7 under UV illumination
(Fig. 2). Luminescence intensity becomes greatly enhanced as
generation (MW) is increased and the photo-induced emission
shifts from red to green across the series due to decreased self-
absorption and less excimer (red-shifted) emission.
Dendrimer series 1–3 and 4–7 exhibit attenuation of electron
transfer rates with increasing generation. The formal potentials for
first and second one-electron oxidation and reduction processes
(Table 1) are not significantly changed from DIQA to 1–6.
However, diffusion coefficients, D0, and standard rates for
heterogeneous electron transfer, ku,{ for the one-electron oxidation
of these materials, decrease approximately an order of magnitude
from DIQA to third-generation 3 and second-generation 6,
consistent with previous studies of other dendrimers with redox-
active cores, where no electrochemical interaction exists between
the core and the dendrons.9,10 Note ku is up to 2.5 times slower at
each dendrimer generation for 4–6 vs. 1–3, suggesting that the tert-
butyl groups play a significant role in site isolation and retardation
of solution heterogeneous electron transfer.
Fig. 1 (Left) Solution (CH2Cl2) absorption and photoluminescence
spectra of second-generation 6 and DIQA. (Right) Normalized absor-
bance spectra of thin films of DIQA and dendrimers 1–3.
yields, wf 5 0.85 for DIQA and 1–3 and wf 5 ca. 1.0 for 4–7. Beer’s
law plots were linear for all materials up to concentrations of ca.
1023 M, indicating no solution aggregation in this concentration
regime.
Thin films (ca. 40 nm thickness) of DIQA and 1–7 were
prepared on glass to investigate the nature of the interaction
between quinacridone chromophores in the condensed state.
Comparison of normalized absorbance spectra of films of DIQA
and 1–3 suggest that DIQA, 1 and 2 are aggregated (blue-shifted
absorbance) while the spectrum of 3 approaches that of the
unaggregated dilute solutions (Fig. 1). A similar trend is seen with
increasing generation for dendrimers 4–7 (Fig. 2), with 4 and 5
slightly aggregated and 6 and 7 predominantly unaggregated. The
accompanying PL spectra of the neat thin films of 4–7 (Fig. 2) also
approach the shape and position of the solution spectrum as
We also note that attenuated solution heterogeneous electron
transfer rates with increasing dendrimer size surrounding a redox-
active core9–11 suggests that dendritic encapsulation may be
detrimental to energy harvesting of quinacridone dopants by CT
pathways in a device architecture. However, in preliminary studies
of ITO/PVK–Alq3/Al single-layer OLEDs doped with 4–7
comparisons between PL and EL spectra indicate only a small
attenuation of these CT pathways, suggesting that the PVK
poly(vinylcarbazole) host can achieve close proximity to the
dendrimer core, and high rates of hole-trapping as a consequence,
leading to Alq3 emissive state production.15
In summary, dendrimerization of quinacridone successfully
inhibits aggregation and self-absorption between core molecules in
the solid state, with corresponding increases in luminescence
efficiency. Further studies using these dendrimers as OLED
dopants, especially in wet-processed films, are underway.
This work was supported by the NSF STC-MDITR (DMR-
0120967). W. H. F. and A. O. gratefully acknowledge support
from Pfizer. D. V. M. is a Dreyfus Teacher–Scholar.
Table 1 Electrochemical dataa for quinacridone dendrimers
MW/
g mol21
Eu9Ox
/
Eu9Red
V
/
106D0/
103ku/
V
cm2 s21
cm s21
DIQA
452.60
917.07
1766.06
3464.05
717.01
1365.93
2663.79
5259.49
0.73
0.78
0.76
0.75
0.76
0.79
0.83
—
21.72
21.75
—
14
2.7
—
1.5
5.7
11
3.4
—
1.5
4.8
1
2
3
4
5
6
7
a
b
b
b
—
21.78
21.75
21.70
—
2.6
1.7
2.9
1.2
Fig. 2 (Top) Normalized absorbance and luminescence spectra of DIQA
and dendrimers 4–7. (i) Solution spectra of 6; (ii) absorbance and
photoluminescence spectra of neat thin films of DIQA and 4–7. Thin film
absorbance spectra have been baseline subtracted to emphasize the visible
band. (Bottom) Bulk powders of 4–7 under UV excitation (365 nm).
y0.84c
y0.6c
CH2Cl2 solutions at 293u K with 0.1 M TBAHFP supporting
b
electrolyte.
Extrapolated from values for 4–6.
Anomalous low solubility precluded measurement.
c
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Chem. Commun., 2005, 444–446 | 445

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Adrian Ortiz, Ware H. Flora, Gemma D. D’Ambruoso,
Neal R. Armstrong* and Dominic V. McGrath*
Department of Chemistry, University of Arizona, Tucson,
AZ 85721-0041, USA. E-mail: nra@u.arizona.edu;
mcgrath@u.arizona.edu; Fax: 520-621-8407; Tel: 520-626-4690
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Notes and references
{ D0 was measured from the peak current (ip) of the anodic potential sweep
through the first oxidation. ku was measured using the Nichelson and
Shane treatment of measuring the peak separation (DEp) as a function of
scan rate (v). Mass transport by semi-infinite linear diffusion was confirmed
from the linearity of plots of ip vs. v1/2
.
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446 | Chem. Commun., 2005, 444–446
This journal is ß The Royal Society of Chemistry 2005