observed in the previous report, where yields were
20ꢀ30%.9,10 These two intermediates could be obtained
in good yield (54% for 10 and 26% for 11) when the reac-
tion is carried out in boiling water (Scheme 1, bottom).
Because 10 and 11 are almost insoluble in water while the
starting materials are soluble, the precipitate of 10 and 11
can be easily collected and separated. Both 10 and 11 can
be converted to 2a during the sublimation and therefore do
not have to be separated. This aqueous method yields 2a in
an 80% yield after heating at 250ꢀ300 °C and sublimation
(Table 1). While the aqueous synthesis of 10 and 11 adds
another synthetic step, it is still a nontoxic and inexpensive
solvent. Pyromellitic dianhydride 3 also afforded 3a in a
42% yield after sublimation (Table 1).
Table 1. Summary Yields of Condensation Products after Train
Sublimation (Photographs below Show Sublimed Products).
Conditions: Series 1 and 2, Diamine Anhydride 1:1; Series 3, 2
Equiv of Diamine; Series 4, 3 Equiv of Diamine; Series 5, 4
Equiv of Diamine
a At 220 °C, 2 h with catalytic amount of Zn(OAc)2. b At 100 °C, 1 h in
H2O, then heated at 300 °C for 1 h. c At 250 °C, 3 h with catalytic amount
of Zn(OAc)2. d At 100 °C, 1 h in H2O, then heated at 300 °C for 2 h. e At
300 °C, 3 h with catalytic amount of Zn(OAc)2. f 5d appears to be
decomposing at 600 °C required for the sublimation.
semiconductors in organic electronic applications. 1bꢀ5b were
successfully synthesized in good yield, although the reaction,
due to lower amine nucleophilicity, required a higher tempera-
ture (300 °C). At temperatures <300 °C, the reaction was
incomplete, showing a mixture of imido compounds.
Figure 2. Effect of the amount of a diamine on product yield.
Reactions of 1,8-diaminonaphthalene c and naphtha-
lene carboxylic anhydride derivatives1, 4, and 5 gaveyields
similar to the model reactions with 1,2-diaminobenzene a.
The reactions of anhydrides 2 and 3 differed from the
model reaction that yielded 2a. Here, the SSC-DS method
with the zinc acetate catalyst gave yields superior to the
aqueous conditions, which is attributed to less strained
fused six- and five-membered rings in the 12H-isoindolo-
[2,1-a]perimidin-12-one system.8b
From the perspective of potential applications, the NTCBI
4a and PTCBI 5a are the more important products and
particular attention was devoted to their synthesis. As
alluded to above, the diamine anhydride stoichiometry is
a factor important for achieving not only a high product
yield but also a high purity product in SSC-DS. Figure 2
shows 1a, 4a, and 5a yields as a function of the 1,2-
diaminobenzene proportion. In the case of the model reac-
tions, excess diamine did not improve the product yields;
however, in the case of 4a and 5a, improved yields were
observed. The excess of a volatile diamine fraction is recov-
ered during the sublimation. As a result, better yields and
easy separation were achieved for NTCBI 4a and PTCBI 5a.
The yields of benzimidazole and bisbenzimidazole pro-
ducts obtained by SSC-DS and the aqueous condensation
method are summarized in Table 1,11 with photos of
selected products “as sublimed”. 3aꢀd, 4aꢀd, and 5aꢀd
are obtained as mixtures of isomers.
Because the bisbenzimidazole compounds such as
PTCBI and NTCBI are n-type semiconductors (and elec-
tron transporting materials) with high potential impact in
solar cells (OPVs) and OTFTs, we were quite interested in
their octa-fluoro analogues obtained from the tetrafluoro-1,2-
diaminobenzene b.12 So far, fluorinated bisbenzimidazoles
attracted little attention despite their potentially high
electron affinity, which could make them valuable n-type
With the products in hand, we investigated their photo-
physical and electrochemical properties. The results for the
series 1aꢀd and 4aꢀd are summarized in Figure 3 and Table
2. For solubility reasons, some materials were investigated
as thin films. The UVꢀvis spectra of compounds 1aꢀd and
4aꢀd are characterized by broad πꢀπ* transition and ex-
tinction coefficients of 8000 Mꢀ1 cmꢀ1 1b,4b In compounds
.
4, this band is broader, presumably due to the presence of
aggregates in the solutions.4d Compounds 1a and 1b show
the πꢀπ* transition at 350ꢀ420 nm, while 1c and 1d show
broad πꢀπ* transition at 390ꢀ600 and 370ꢀ500 nm respec-
tively; these broad and red-shifted bands attributed to a
charge transfer character and the extended π conjugation
in the naphthalene and phenanthrene moieties.13,14 In the
case of the series 1aꢀd and between 4a and 4b, the emission
(13) Goswami, S.; Sen, D.; Das., N. K.; Hazra, G. Tetrahedron Lett.
2010, 51, 5563.
(14) Erten, S.; Meghdadi, F.; Gunes, S.; Koeppe, R.; Sariciftci, N.;
(11) Known compounds: 1ꢀ5a and 1ꢀ5c. For the comparison of
reaction yields and conditions, see the Supporting Information.
(12) Heaton, A.; Hill, M.; Drakesmith, F. J. Fluorine Chem. 1997, 81,
133.
Icli, S. Eur. Phys. J. Appl. Phys. 2007, 36, 225.
(15) (a) Pohl, R.; Anzenbacher, P., Jr. Org. Lett. 2003, 5, 2769.
(b) Pohl, R.; Montes, V. A.; Shinar, J.; Anzenbacher, P., Jr. J. Org.
Chem. 2004, 69, 1723.
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Org. Lett., Vol. 13, No. 18, 2011