Modular synthesis of asymmetric rylene derivatives

Article abstract of DOI:10.1039/c6tc05139a

The modular synthesis of asymmetric rylenes from naphthalic anhydride derivatives is presented. Imidization, Suzuki-Miyaura coupling and cyclodehydrogenation reactions are utilized for the generation of novel functional rylenes with these three core transformations providing significant flexibility over the final structure. The combination of simple purification and high yields enables access to asymmetric rylenes with functional handles at the imide-position and site-specific incorporation of bay position substituents. The resulting library of perylenes and bisnapthalimide-anthracene derivatives showcase the presented methodology and the ability to tune optoelectronic and electrochemical properties.

Full text of DOI:10.1039/c6tc05139a

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Modular synthesis of asymmetric rylene
derivatives†
Cite this:DOI: 10.1039/c6tc05139a
Caitlin S. Sample,a Eisuke Goto,‡a Nisha V. Handa,b Zachariah A. Page,b
Yingdong Luob and Craig J. Hawker*a
The modular synthesis of asymmetric rylenes from naphthalic anhydride derivatives is presented. Imidization,
Suzuki–Miyaura coupling and cyclodehydrogenation reactions are utilized for the generation of novel func-
tional rylenes with these three core transformations providing significant flexibility over the final structure.
The combination of simple purification and high yields enables access to asymmetric rylenes with functional
handles at the imide-position and site-specific incorporation of bay position substituents. The resulting
library of perylenes and bisnapthalimide-anthracene derivatives showcase the presented methodology and
the ability to tune optoelectronic and electrochemical properties.
Received 27th November 2016,
Accepted 2nd January 2017
DOI: 10.1039/c6tc05139a
www.rsc.org/MaterialsC
Instead, the typical approach involves an inefficient (o30%
overall yield) stepwise synthesis; imidization of PTCDA with
Introduction
Rylenes encompass a large family of molecules that are one amine, partial hydrolysis to the monoanhydride monoimide,
comprised of naphthalene units joined at the peri-position,1 and imidization with a second amine.23–25 A one-step synthesis
including well-known and sought-after perylene diimides of asymmetric PDIs in B40% yield was recently developed in our
(PDIs). PDIs were originally used as dyes due to their high group by taking advantage of the difference in reactivity between
molar absorptivity and excellent chemical, thermal, light, and alkyl- and aryl-amines, however the scope of products is limited
air stability.2 More recently, other attractive properties of PDIs to alkyl/aryl combinations.26
have emerged, such as high fluorescence quantum yield3 and
In seeking to further expand the availability and structural
strong electron accepting (n-type) character,4 lending to applica- diversity of asymmetric perylenes, we report a modular approach
tions in fluorescent light collectors,5,6 dye lasers,7–9 electro- to a wider range of functional asymmetric rylene derivatives.
photography,10 organic field-effect transistors,11–13 and organic This strategy is enabled by the disconnection of the perylene/
photovoltaics.12,14 In particular, asymmetric substitution at the rylene core to give the readily available naphthalic anhydride
imide position has resulted in improved solubility and function- starting material (Fig. 1). By having a common initial building
ality, granting access to multichromophoric systems,15,16 supra- block (substituted naphthalimide – one half of PDI), the func-
molecular architectures,17 and incorporation into polymers for tionalization and purification is greatly simplified. As a result,
organic electronic applications.18–20
high yielding imidization, followed by Suzuki–Miyaura coupling
While asymmetric PDIs are attractive, their synthesis has and cyclodehydrogenation reactions could be utilized to
proven challenging. In contrast to their symmetric counter- achieve the asymmetric PDI targets in greater than 40% yield.
parts, which are produced readily through the condensation
of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) with
aliphatic primary amines or anilines,21 asymmetric PDIs are
not typically accessible through direct imidization given the
increased reactivity of the singly substituted monoimide species.22
a Materials Research Laboratory, University of California, Santa Barbara,
California 93106, USA. E-mail: hawker@mrl.ucsb.edu
b Department of Chemistry and Biochemistry, University of California,
Santa Barbara, California 93106, USA
† Electronic supplementary information (ESI) available: Supporting figures,
synthetic details, and compound characterization (PDF). See DOI: 10.1039/
Fig. 1 Retro-synthetic comparison of asymmetric rylenes through the
traditional approach starting from PTCDA versus the presented methodol-
ogy from naphthalic anhydride derivatives.
c6tc05139a
‡ Present address: Department of Polymer Science and Engineering, Yamagata
University, Yamagata, Yamagata Prefecture 990-8560, Japan.
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Moreover, the synthesis could be extended to bay-substituted The conversion from 5 to 6 can be monitored using proton NMR
PDIs as well as larger-core rylenes, highlighting the versatility of (Fig. 2a), particularly in the aromatic region as the five peaks of the
this modular approach.
Suzuki-coupled diad (5) collapse to two doublets (6) with formation
of the perylene core. Additionally, the success of the ring-closing can
also be observed by eye due to the substantial bathochromic shift in
absorbance (B150 nm) from 5 (yellow solution) to 6 (bright red
solution), shown as an inset in Fig. 2b.
Results and discussion
To highlight the versatility of the aforementioned methodology
a library of functional asymmetric PDIs were synthesized, as
shown in Table 1. Given the modular nature of this strategy side
chains can be easily mixed and matched, for example, partnering
naphthalimide 4 containing a solubilizing 2-ethylhexyl chain with
functional naphthalimides bearing a carboxylic acid or alkene
chain to give asymmetric PDIs 7 and 8 respectively. Moreover, two
orthogonal functionalities, such as a carboxylic acid and alkene
can be easily paired with this approach to give 9. Overall, the
yields range from 41% to 51%, as compared to o30% for
traditional methods.
While varying imide substituents on PDIs is a facile route
to increase solubility and incorporate functional handles,
modifications at the 1, 6, 7, and 12 (bay) positions on the
perylene core directly influence the optoelectronic properties of
the molecule.30 As with direct imide substitutions towards
asymmetric PDIs, reactions at the perylene core typically
produce mixtures of mono-, di-, tri-, and tetra-substitution that
are challenging to separate, reducing reaction yields and lead-
ing to impure samples.30 The power of this modular strategy is
that by constructing the central perylene core in a step-wise
fashion starting from discrete naphthalic units, precise control
over the location of the bay substitution in the final PDI is possible.
As shown in Fig. 3a, functional naphthalic anhydrides are com-
mercially available, which provides easy access to alkoxy-functional
naphthalimides, such as methoxy-substituted naphthalimide (10),
through Williamson-ether synthesis (Fig. 3a). Suzuki–Miyaura
coupling of 10 with a boronic ester naphthalimide (11), bearing
no bay-substituents and a functional (alkene) imide chain,
The general synthetic protocol to asymmetric PDIs is represented
in Scheme 1. Bromination of the inexpensive (B$50 kgÀ1
)
commercial reagent ‘‘naphthalic’’, to produce 4-bromo-1,8-
naphthalic anhydride (1), followed by imidization, is easily
achieved by reacting with the desired primary amine in ethanol
(EtOH) at reflux. The reaction mixture changes from an opaque
to translucent solution after 2 hours due to improved solubility
as 1 is converted to the imides, for example 4-bromo-1,8-
naphthalimide 2 (n-octyl) and 3 (2-ethylhexyl) in near quanti-
tative yields. The robust nature of this process allows for facile
reaction monitoring and product purification. Following imide
diversification, Miyaura borylation of 3 then leads to the boronic
ester naphthalimide 4, which could be cross coupled by Suzuki–
Miyaura cross-coupling with a different bromo-naphthalimide 2
derived from the same bromo-substituted starting material, 1.
This results in clean conversion to the desired asymmetric
product (5), which is easily purified through standard techniques.
A
base catalyzed cyclodehydrogenation ring-closure of 5
was then investigated for formation of the asymmetric PDI 6.
Initial cyclodehydrogenation attempts employed potassium
tert-butoxide (t-BuOK) and 1,5-diazabicyclo[4.3.0]non-5-ene (DBN)
in diglyme,27 however the reaction provided inconsistent yields. As
an alternative, potassium carbonate (K2CO3) in ethanolamine at
elevated temperature (130 1C) was investigated for cyclization, but
this procedure leads to transimidation of less sterically hindered
side-chains (e.g., n-alkyl) by ethanolamine (Fig. S1, ESI†).28
Triethanolamine was selected as an alternative to ethanolamine,29
and the change from primary amine to tertiary amine was found to
mitigate transimidation, while providing excellent yields (B94%).
Scheme 1 Representative synthesis of asymmetric PDI 1. (i) EtOH, reflux, 4 h, 87% (2) and 85% (3); (ii) Pd(dppf)Cl2, bis(pinacolato)diboron, KOAc, p-dioxane,
100 1C, 24 h, Bquantitative; (iii) K2CO3 (aq.), Pd(PPh3)4, toluene, 100 1C, 12 h, 67%; (iv) K2CO3, triethanolamine, 130 1C, 24 h, 94%. Comparison between
reaction yields for two ring-closure conditions is provided as an inset.
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Table 1 Asymmetric PDIs (6–9) synthesized with varying imide substituents.
Overall yields account for all synthetic steps
Entry
R1
R2
Overall yield (%)
51
6
7
8
9
44
42
41
Fig. 2 Monitoring of ring-closure reaction to convert 5 to 6. (a) Structures with
corresponding 1H-NMR, showing diagnostic aromatic protons. (b) Absorption
spectra (100 mM in dichloromethane) and corresponding images of 5 and 6
(5 mM in dichloromethane).
produces a PDI that is asymmetric at both the imide and bay
positions (12). It was immediately apparent that methoxy sub-
stitution altered the electronic properties of the perylene core,
given the visual difference in color for solutions of 12 (pink)
relative to non-bay substituted PDIs (6–9, red) (Fig. 3b). UV-Vis
absorption spectroscopy quantified this observation, revealing
a 30 nm bathochromic shift for methoxy-substituted PDI (12)
relative to unsubstituted PDI (6) (Fig. 3b). Additionally, cyclic
voltammetry (CV) measurements show that bay substitution
with an electron-donating methoxy group leads to an increase
in reduction potentials for 12 (À1.19 and À1.50 V) relative to 6
(À1.11 and À1.41 V) (Fig. S2, ESI†), providing further evidence
that bay substitution can be used to directly tune the electronic
properties of PDIs.
Fig. 3 (a) Representative synthesis of bay-substituted PDI 12. (i) K2CO3(aq)
,
Pd(PPh3)4, toluene, 100 1C, 12 h, 63%; (ii) K2CO3, triethanolamine, 130 1C,
20 h, 91%. (b) Absorption spectra (100 mM in dichloromethane) and
corresponding images of 6 and 12 (5 mM in dichloromethane).
As a final demonstration of modularity of this approach a equivalents of borylated naphthalimide (13) with 1,5-dibromo-
novel core-extended rylene derivative was synthesized based on a anthracene (14) produces a dinaphthalimide-anthracene triad
structure inspired by pentarylene30 and an aminoanthraquinone- (15). In contrast to the synthesis of electron deficient cores,
based rylene.31 As shown in Fig. 4a, the reaction of two such as perylene, a two-step ring-closure is required for the
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Fig. 4 Core-extended rylenes, synthesis and absorption. (a) Representative synthesis of 17; (i) K2CO3(aq), Pd(PPh3)4, toluene, 100 1C, 12 h, 68%; (ii) FeCl3,
nitromethane, DCM, 55 1C, 24 h; (iii) K2CO3, ethanolamine, 130 1C, 16 h, 56%. (b) Absorption spectra of 15 and 17 (100 mM in dichloromethane) and
corresponding images of solutions in vials (5 mM in dichloromethane). (c) Summary of core-extended rylene derivatives (18–20) synthesized with varying
imide and bay substituents.
anthracene-based derivative due to the greater electron density which is subsequently coupled with the borylated derivative of
and corresponding lower acidity of the aromatic protons.32 The naphthalimide 2 to yield an asymmetric triad (S2) (chemical
first ring closure was accomplished using traditional Lewis-acid structures given in Chart S1, ESI†). Cyclodehydrogenative ring-
chemistry with iron(III) chloride to give the partially closed closing of the triad provides an asymmetric extended rylene
derivative, 16, which was used directly for the second ring derivative (18), highlighting the ability to incorporate different
closing step. Employing different conditions (K2CO3 in ethanol- side chains at the imide positions for classes of larger rylenes
amine or triethanolamine), then gives the final rylene product beyond PDIs using this methodology. Moreover, the electronic
(17). The dual ring-closures results in a dramatic bathochromic properties of the higher order rylene can be tuned with core
shift in absorption (370 nm) on going from the precursor 15 to substitutions, which was demonstrated for methoxy-substituted
rylene 17 (Fig. 4b) with the distinct change in color, from yellow rylene 19, synthesized in an analogous fashion to 17. As observed
(15) to green-blue (17), being indicative of successful ring- for PDIs, methoxy substitution of extended rylenes in the bay
closing. Additionally, UV-Vis absorption spectroscopy reveals position led to a bathochromic shift (B20 nm) in absorption,
an absorption maximum in the near infrared region (lmax
=
which corresponds with a noticeable color change in solution,
723 nm) for 17, highlighting its narrow energy-gap, which falls going from 17 (blue-green) to 19 (green) (Fig. S3, ESI†). Addi-
between terrylene diimide and quatterylene dimide derivatives, tionally, CV showed a similar increase in reduction potentials
demonstrating the ability to significantly tune the electronic from À0.67 and À0.86 V to À0.79 and À0.96 V for 17 and 19
structure through the introduction of a p-extended core.33 The respectively (Fig. S4, ESI†). The asymmetric and bay-substituted
synthesis of asymmetric extended-core rylenes was achieved in core-extended rylenes showcase the modularity of this approach.
a similar fashion to that described in Fig. 4a, however using a
In summary, we have developed a strategy for the facile
stepwise Suzuki–Miyaura coupling. In short, coupling com- synthesis of asymmetric PDIs that takes advantage of modular
pounds 13 with 14 in a 1 : 1 molar ratio produces a diad (S1), naphthalimide units to provide increased versatility, greater
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Competing financial interest
The authors declare no competing financial interest.
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Acknowledgements
We thank the MRSEC program of the National Science Founda-
tion (Award No. DMR 1121053) and the AFOSR MURI program
(FA9550-12-1-0002) for partial financial support. This work was
also supported by the National Science Foundation Graduate
Research Fellowship under Grant No. 1650114. Any opinion,
findings, and conclusions or recommendations expressed in
this material are those of the authors and do not necessarily
reflect the views of the National Science Foundation.
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