Frustration of condensed phase aggregation of naphthalocyanine by dendritic site-isolation

Article abstract of DOI:10.1021/ma100902m

The synthesis and aggregation studies of a pair of 3, 4,12,13,21,22,30,31- octasubstituted 2,3-Naphthalocyanine (Ncs) 1 and 2 was reported. The synthetic route leading to these naphthalonitriles started from dimethyl 4,5-dihydroxyphthalate onto which the

Full text of DOI:10.1021/ma100902m

5512 Macromolecules 2010, 43, 5512–5514
DOI: 10.1021/ma100902m
Scheme 1. Synthesis of 0th and 2nd Generation Dendritic Ncs 1 and 2
Frustration of Condensed Phase Aggregation of
Naphthalocyanine by Dendritic Site-Isolation
Xiaochun Chen, Nilmi Fernando, and Dominic V. McGrath*
Department of Chemistry and Biochemistry, The University of
Arizona, Tucson, Arizona 85721
Received April 24, 2010
Revised Manuscript Received May 26, 2010
2,3-Naphthalocyanine (Nc) is a tetrapyrrolic macrocycle de-
rived from the linear benzoannulation of phthalocyanine (Pc).
Owing to the aromaticity of the extended π system, Nc has high
chemical stability and effectively absorbs in near-IR region, which
makes this organic semiconductor highly suitable for use in optical
based technologies such as optical limiting,1,2 OLEDs,3 OFETs,4
OPVs,5 and photodynamic therapy (PDT).6 However, a decisive
disadvantage of Nc and corresponding metal complexes is low
solubility in organic solvents and high tendency to self-associate
into dimers and higher order aggregates, even in dilute solution.7,8
Much attention has been focused on controlling the solubility
and self-association of Nc using peripheral substutituents,1c,9-11
while little work has been carried out on dendrimer-incorporated
Ncs,12-14 despite an active effort in dendritic phthalocyanine
(Pc) materials.15-19 It is known that the sphere-like dendritic
macromolecules can mimic Nature’s principle of site isolation by
encapsulating and shielding the active core (e.g., photoactive,
electroactive, and catalytic moieties) with the bulky dendritic shell.20
Astheactivemoietiesbecomeisolated from one another, detrimental
processes arising from intermolecular interactions such as aggrega-
tion and self-quenching can be prevented.21-26 Here, we report the
synthesis and aggregation studies of a pair of 3,4,12,13,21,22,30,31-
octasubstituted Ncs 1 and 2 (Scheme 1), the first examples of
octasubstituted dendritic Ncs. The eight sterically bulky dendrons
located on the periphery of the macrocycle effectively eliminate
Nc core aggregation in second generation Nc 2 both in solution and
the condensed phase.
The synthesis of dendritic Ncs utilized 6,7-disubstituted naphtha-
lonitriles 12 and 13 as precursors (Scheme 1). The synthetic route
leading to these naphthalonitriles started from dimethyl 4,5-dihy-
droxyphthalate 5,27 onto which the previously synthesized28 den-
dritic alcohols 3 and 4 were introduced via the Mitsunobu protocol
(Scheme 2). The obtained diesters 6 and 7 were reduced to the
corresponding 1,2-dimethanols (8 and 9), and subsequent Swern
oxidation provided the disubstituted dendritic phthalaldehydes 10
and 11.29 Finally, 10 and 11 were converted to naphthalonitriles 12
and 13 via base-promoted condensation with succinonitrile in
DMSO. The metal-free 3,4,12,13,21,22,30,31-octasubstituted den-
dritic Ncs 1 and 2 were prepared by cyclotetramerization of 12 and
13 using a catalytic amount of LiBr and 1 equiv of DBU in refluxing
1-pentanol.
Scheme 2. Synthesis of Dendritic Naphthalonitriles 12 and 13
Both 1 and 2 were characterized by various spectroscopic
methods and elemental analysis. When the 1H NMR spectrum of
1 was obtained in 95:5 CDCl3-DMSO-d6, a solvent system with
which we have success in disrupting molecular aggregation asso-
ciated with π-π interactions, we observed well-resolved signals
for the substituent groups although signals for the macrocyclic aro-
matic protons were broadened. In contrast, 2 was readily soluble
in CDCl3, and its 1H NMR spectrum showed well-resolved
signals for all protons of the substituent dendrons as well as the
macrocycle (see Supporting Information).
Initial molecular aggregation studies were performed in solu-
tion as a function of increasing volume fraction of EtOH in
EtOH-CH2Cl2 solutions of 1 and 2. Upon initial increase in
EtOH volume fraction (i.e., 0 f 10 f 20%) aggregation of the Nc
core of 1 was indicated by a marked decrease in the absorptivity
of the Q-band at 783 nm with concomitant increase in aggregate
absorbance at 717 nm (Figure 1a). The hypsochromic shift of
the Q-band from 783 to 717 nm indicates that aggregation of 1
under these solvent conditions was predominantly cofacial H-ag-
gregation. The B-bands at 332 and 402 nm exhibited only slight
hypochromicity during this change in solvent conditions. Further
addition of EtOH (30 f 40 f 50%) caused further decrease of
the Q-band at 783 nm, but now accompanied by a decrease in
absorbance across the entire wavelength range, suggesting exclu-
sion of both 1 and its aggregates from solution due to their low
solubility in CH2Cl2-EtOH mixtures.
Compounds 1 and 2 were purified by flash column chroma-
tography, although the expected poorer solubility of zeroth
generation 1 versus second generation 2 in common aprotic
solvents such as THF, chloroform, ether, and CH2Cl2 resulted
in strong adsorption of 1 to the column material (SiO2) resulting
in low yield (7%) following isolation. Second generation 2 was
obtained in moderate yield (40%).
When second generation 2 was assayed under the same con-
ditions, a different response was observed (Figure 1b). Dendrimer
2 exhibited no significant change in its absorption spectrum until
*Corresponding author. E-mail: mcgrath@u.arizona.edu. Telephone:
520-626-4690. Fax: 520-621-8407.
pubs.acs.org/Macromolecules
Published on Web 06/09/2010
r 2010 American Chemical Society

Communication
Macromolecules, Vol. 43, No. 13, 2010 5513
the Nc ring in the condensed phase, although the slight bath-
ochromic shift suggests weak edge-to-edge exciton interactions
(J-aggregation) occurring in the solid phase.7,8,30
Thermal analysis by differential scanning calorimetry (DSC)
reveals a glasstransition (Tg) for 2 at 80.1 °C as the only transition
between 25 and 200 °C (see Supporting Information), while the
DSC of 1 indicates no transitions over this same temperature
range. This result is consistent with 2 providing an amorphous
environment for dendritic site isolation of the naphthalocyanine
chromophore, and should allow for melt processing of thin-films
of 2. This glass transition for naphthalocyanine 2 is commensu-
rate with other benzyl aryl ether dendrimers, and represents a
decrease in Tg from the smaller compound naphthalonitrile 13
(Tg = 90.5 °C), which in turn possess a higher Tg than the second
generation dendron 4 (Tg = 63.8 °C). Deviation from the tradi-
tional relationship between Tg and molecular weight for linear
polymers when comparing dendrimers of increasing generation
has been previously observed.31
Figure 1. UV-vis spectra for Ncs (a) 1 and (b) 2 in CH2Cl2 and
CH2Cl2-EtOH mixtures; (c) Q-band absorptivity at 783 nm.
In summary, we have synthesized two dendritic octasubsti-
tuted Ncs that demonstrate the ability of peripheral dendritic
substituents of sufficient size to prevent Nc aggregation in
solution and the condensed phase. These materials allow for
solution processing techniques for nonaggregated thin-films of
these ubiquitous organic dyes in applications that rely on site-
isolation, such as emissive and optical limiting devices.
Acknowledgment. This work was supported by the NSF
(CHE-0719437). We thank Jeff Pyun for helpful discussions
regarding thermal analysis.
Supporting Information Available: Text giving experimental
details and additional characterization data for all new com-
pounds and figures showing NMR spectra, GPC chromato-
grams, and DSC curves. This material is available free of charge
via the Internet at http://pubs.acs.org.
Figure 2. Normalized UV-vis spectra for (a) 1 and (b) 2 in thin film
and CH2Cl2 solution (1 Â 10-5 M).
References and Notes
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Q-band to 722 nm in a hypochromic fashion, indicating predomi-
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change in solvent quality, reflecting the increased solubility im-
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