Aryl- and Heteroaryl-Annulated[a]carbazoles
A R T I C L E S
introduced successfully onto the nitrogen atom of AHA[a]Cs
after the annulation. Thus, annulation product 3a reacted with
di-tert-butyl dicarbonate [(Boc)2O] with the aid of 4-(N,N-
dimethylamino)pyridine (DMAP) to give 13 in a high yield.48
The 3a–Na complex prepared in situ from 3a and NaH reacted
with 2-chloropyrimidine through SNAr process to give 14.49 The
Buchwald N-arylation also worked well for the transformation
of 3a or 3p to 15 or 16, respectively.31 Furthermore, the
treatment of 3d with MeI and KOH in DMSO gave N-
methylated compound 17 in a high yield.50 Such flexible
behavior of AHA[a]Cs that accepts a variety of organic
transformations is promising, for instance, for structural design
in the case of their application to electroactive materials as
shown in the next section.
we were intensely interested in the potential of the AHA[a]Cs
as electroactive materials. We therefore investigated the pho-
tophysical properties of the AHA[a]Cs; the results are collected
in Table 4.
First, we focused on evaluating the effect of substituents on
AA[a]Cs and HA[a]Cs. For example, in the UV-vis spectrum
of 6-methyl-11H-benzo[a]carbazole (3a), the absorption bands
ascribed to the π-π* transitions with the relatively large
extinction coefficients were detected, ranging from 260 to 300
nm.51 It was then found that 3a (ΦF ) 0.165) exhibits purple
emission derived from the emission λmax around 360-400 nm.52
Although the absorption and emission patterns of 3b, 3f, 3j and
3k that have another alkyl group at the different position of 3a
resemble those of 3a, the introduction of the alkyl group was
always accompanied by a red-shift of the emission spectra
regardless of the position of the alkyl group (3a vs 3b, 3f, 3j or
3k). A similar correlation was observed also between indolo[2,3-
a]carbazoles 3q and 3r. Moreover, FL efficiency was found to
be highly dependent on the methoxy group (3a vs 3c or 3d).
Thus, though the ΦF value of 3c slightly decreased compared
with that of 3a, the installation of the methoxy group onto the
C1 position resulted in higher ΦF value. Comparisons between
3a and the N-arylated derivatives (3a vs 3g, 3h, 15 or 3i) also
showed significant differences in photophysical properties.
Phenyl and electron-rich aryl groups on 3a (3g: –Ph, 3h:
–C6H4–p-OMe, 15: –C6H4–p-NPh2) made their absorption bands
around 250-260 nm indefinite, whereas the corresponding band
of 3i having an aryl group with electron-deficient character
(–C6H4–p-CN) was intensified. Furthermore, the structural
change from 3a to 3g, 3h or 15 resulted in a red-shift of the FL
spectra and an increase in the FL quantum yield. Interestingly,
the electron-deficient aryl group caused a drastic color change
from purple to green light (3a vs 3i, ∆λmax ) 92 nm). The effect
of –C6H4–p-CN holds true also for benzofuro[2,3-a]carbazole
3p, where blue-emitting 16 was derived from purple-fluorescent
3p (3p vs 16, ∆λmax ) 51 nm). Compound 14 with a
2.8. Photophysical Properties of AHA[a]Cs. Because of the
broad substrate diversity on the indium-catalyzed annulation,
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