Switching high two-photon efficiency: From 3,8,13-substituted triindole derivatives to their 2,7,12-isomers

Article abstract of DOI:10.1021/ol102057t

Bridging the triindole core and triarylboryl acceptor by an ethenylene linker at the 3,8,13- or 2,7,12-position, the resultant 3-BET and 2-BET show two-photon absorption (TPA) cross sections of δ = 2100 and 2500 GM (at 810 nm by femtosecond pulses in THF), respectively. The TPA enhancement of the 2,7,12-isomers is also found when comparing 3-BYT and 2-BYT (δ = 870 and 1900 GM) and 3-NET and 2-NET (36 and 400 GM).

Full text of DOI:10.1021/ol102057t

ORGANIC
LETTERS
2010
Vol. 12, No. 22
5192-5195
Switching High Two-Photon Efficiency:
From 3,8,13-Substituted Triindole
Derivatives to Their 2,7,12-Isomers
Lei Ji,‡ Qi Fang,*,† Mao-sen Yuan,† Zhi-qiang Liu,† Yu-xiang Shen,† and
Hong-feng Chen†
State Key Laboratory of Crystal Materials, Shandong UniVersity,
250100, Jinan, P. R. China, and School of Chemistry and Chemical Engineering,
Shandong UniVersity, 250100, Jinan, P. R. China
fangqi@sdu.edu.cn
Received September 15, 2010
ABSTRACT
Bridging the triindole core and triarylboryl acceptor by an ethenylene linker at the 3,8,13- or 2,7,12-position, the resultant 3-BET and 2-BET show
two-photon absorption (TPA) cross sections of δ ) 2100 and 2500 GM (at 810 nm by femtosecond pulses in THF), respectively. The TPA enhancement
of the 2,7,12-isomers is also found when comparing 3-BYT and 2-BYT (δ ) 870 and 1900 GM) and 3-NET and 2-NET (36 and 400 GM).
Two-photon absorption (TPA), in which two photons are
absorbed simultaneously by a molecule, was first predicted
by Go¨ppert-Mayer in the early 1930s.1 Nowadays, TPA-
based achievement covers wide fields in photophysics,
photochemistry, and materials and life sciences, as has been
summarized in several recent reviews.2 In exploring new and
better TPA chromophores, the C3-symmetric octupolar motif
proves to be a successful architecture2-7 in addition to the
well-established dipolar and quadrupolar models.
extended and acceptors (or donors) are end-capped for
promoting intramolecular charge transfer (ICT). The large
TPA cross section of these octupolar molecules may be
interpreted in terms of the excitation model,8 by which an
octupole is treated as three individual dipoles with weak
coupling between them.
Triindole is a highly π-conjugated planar molecule,9 which
resembles truxene. The replacement of three 5,10,15-C atoms
of truxene by three N atoms endows the resultant triindole
Triphenylamine,3 s-triazine,4 1,3,5-tricyanobenzene,5 triph-
enylbenzene,6 and truxene7 have been typically taken as the
octupolar core, from which three π-conjugated arms are
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Sci. 2008, 213, 1–95. (c) He, G. S.; Tan, L.-S.; Zheng, Q.; Prasad, P. N.
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M.; Collins, H. A.; Denning, R. G.; Anderson, H. L. Angew. Chem., Int.
Ed. 2009, 48, 3244–3266.
† State Key Laboratory of Crystal Materials.
‡ School of Chemistry and Chemical Engineering.
(1) Go¨ppert-Mayer, M. Ann. Phys. 1931, 9, 273–294.
10.1021/ol102057t  2010 American Chemical Society
Published on Web 10/25/2010

Scheme 1
with aromaticity and electron-donating potential for ICT.
We noticed that triindole has the 3,8,13- or 2,7,12-
substitution variety, which allows different ways of π-exten-
sion and ICT. In this letter, we report four new D(π-A)3 kinds
of triindole derivatives with a large TPA cross section. By
bridging the triindole core and triarylboryl via the ethenylene
linker at the 3,8,13- or 2,7,12-position, we obtained 3-BET
Triindole derivatives have also acquired several applications
in discotic liquid crystals,10 hole transporting materials,11 and
in OLEDs,12 etc. However, to our knowledge, there are no
reports about TPA properties of triindole derivatives.
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J.; Prasad, P. N. J. Phys. Chem. B 1999, 103, 10741–10745. (b) Yoo, J.;
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Org. Lett., Vol. 12, No. 22, 2010
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Table 1. Linear and Nonlinear Optical Properties
a
a
λabs
εa
λem
∆ν˜b
δ/GM
nm
M-1 cm-1
nm
103cm-1
Φc
(λ/nm)d
Φδ/GM
Tr
8
316
323
335
344
383
402
396
399
412
427
8.0 × 104
3.0 × 104
6.8 × 104
1.4 × 105
1.2 × 105
1.8 × 105
1.2 × 105
1.0 × 105
1.1 × 105
1.1 × 105
397
396
410
428
429
456
506
516
516
543
6.5
5.7
5.5
5.7
2.8
2.9
5.5
5.7
4.9
5.0
0.17
0.15
0.14
0.19
0.55
0.42
0.43
0.60
0.33
0.40
nondetectable
nondetectable
nondetectable
2.1 × 101 (720)
3.6 × 101 (810)
4.0 × 102 (810)
8.7 × 102 (810)
1.9 × 103 (810)
2.1 × 103 (810)
2.5 × 103 (820)
-
-
-
3-SiT
2-SiT
3-NET
2-NET
3-BYT
2-BYT
3-BET
2-BET
0.40
2.0 × 101
1.7 × 102
3.7 × 102
1.1 × 103
6.9 × 102
1.0 × 103
a Linear optical properties in 2 µM THF. b Stokes shift. c Quantum yield. d TPA cross section (100 µM in THF), coumarin 307 (100 µM in MeOH) as
standard,15 1 GM ) 10-50 cm4 s molecule-1 photon-1
.
and 2-BET. The 3-BYT and 2-BYT are the results of
replacing ethenylene by an ethynylene group. The terminal
dimesitylboryl-B(Mes)2 group acts as the electron acceptor
and fluorophore.13 The donor end-capped D(π-D)3 type of
compounds, namely, 3-NET and 2-NET, are also synthesized
for comparison study.
Sonogashira coupling of 2-BrT (or 3-BrT) and trimethylsi-
lylacetylene goes smoothly with high yield when catalyzed by
PdCl2(dppf), whereas Pd(PPh3)2Cl2 resulted in much lower yield
in a similar reaction.14 The resultant 2-SiT and 3-SiT were
deprotected yielding 2-YT and 3-YT, which were then coupled
with 9 to produce 2-BYT and 3-BYT, respectively.
The 3,8,13- or 2,7,12-substitution has been confirmed by
a satisfactory X-ray structure determination of 6, 3-SiT, and
2-SiT (see SI) and can also be conveniently identified by
1H NMR of the proton on C4. As shown in Figure 1, at the
As shown in Scheme 1, 2-BrT and 3-BrT precursors were
synthesized by trimerization of 3 and 6 in POCl3, respectively.14
Direct bromination of 5 will provide 6, rather than 3, as
confirmed by the X-ray structure of 6 (see Supporting Informa-
tion (SI)). Compound 3 was prepared by reduction of 2.
2-BET (or 3-BET) was synthesized through Heck cou-
pling of 2-BrT (or 3-BrT) with 8 in the presence of Et3N,
Pd(OAc)2, and P(o-tol)3 at 110 °C under N2. This concurred
with the reports that the -B(Mes)2 group is stable in
palladium-catalyzed conditions.13b,e
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Figure 1
.
Low-field region of H NMR spectra of some 3,8,13-
and 2,7,12-substituted compounds in CDCl3.
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1
lowest field side of the spectra, the H NMR signal of the
1H-C4 of the 3,8,13-substituted triindole derivatives is
invariably singlet, while that of the 2,7,12-substituted
counterpart is always doublet at relatively higher field.
All the target compounds show high photostability and exhibit
strong one-photon absorption (see Table 1), which can be
thought of as a prerequisite for large TPA. The Tr and 8
building blocks show almost the same peak wavelength, in both
the absorption and emission spectra. When joining them up,
the spectra of the resultant compounds are greatly red-shifted.
The peak wavelength of the absorption and fluorescence
spectra and the Stokes shifts of all the 2,7,12-substituted
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5194
Org. Lett., Vol. 12, No. 22, 2010

Figure 2. Normalized absorption and emission spectra of 2-NET,
2-BET, and 3-BET in THF.
Figure 3. TPA spectra of the title triindole derivatives in THF.
triindole derivatives are larger than those of their 3,8,13-
isomers. The bathochromic of the 2,7,12-compounds indi-
cates that the 2-substitution, in comparison to the 3-substi-
tution, will cause a more effective ICT and a better
conjugation. Indeed, the average C-C bond length between
the central and outer benzene, measured by X-ray diffraction,
is 1.441(3) for 2-SiT and 1.457(3) Å for 3-SiT (see SI),
demonstrating the better conjugation of the 2,7,12-isomers.
The -B(Mes)2-capped compounds show great bathochro-
mic spectral shift to their -N(Ph)2-capped counterparts. The
solvatochromic property of 2-BET and 3-BET of the
fluorescence spectra is also much larger than that of 2-NET
and 3-NET (see SI). It is not unexpected that the photon-
induced ICT of the D(π-A)3 type octupoles is more pro-
nounced than that of the D(π-D)3 type. Both the absorption
and emission of 2-BET (or 3-BET) with the ethenylene
linker are red-shifted relative to those of 2-BYT (or 3-BYT)
with the ethynylene linker.
Excited by the femtosecond pulses of a Ti:sapphire laser,
the molecular TPA cross section, δ, was determined by using
the two-photon excited fluorescence (TPEF) method (see SI
for details).15 As shown in Figure 3 and Table 1, the low-
lying TPA peak wavelength of our target compounds are
basically twice that of the low-lying one-photon absorption,
implying that the one-photon excited state may be identical
with the two-photon excied state.
3-SiT has no detectable TPEF signals, while 2-SiT shows
a sizable TPEF intensity. The δ values of the 2,7,12-
substituted triindole derivatives are overwhelmingly larger
than that of their 3,8,13-isomers in almost all the experi-
mental spectral ranges. The Φδ values (known as TPE action
cross section to characterize the TPEF intensity) of all the
2,7,12-isomers are also invariably larger than that of their
3,8,13-counterparts. The excellent TPA property of the
2,7,12-isomers can be attributed to their better conjugation
of the triindole core and the strong electronic coupling
between the three branches. The 3,8,13-substituted octupoles
may be regarded as three weakly coupled NfB dipoles.
However, the 2,7,12-octupoles can not be treated by this kind
of exciton approach,8 due to their more extended and
complicated way of ICT.
The -B(Mes)2-capped compounds have much larger δ and
Φδ values than their -N(Ph)2-capped counterparts in the
wide spectral region. It seems certain that the structural
factors which bathochromically shift the one-photon spectra
will simultaneously enhance the two-photon response of these
molecules. The δ value of 2-BET (3-BET) is larger than
that of 2-BYT (3-BYT), though 2-BYT shows the highest
Φδ value because of its highest quantum yield Φ. Other
compounds with the ethenylene double-bond π-linker were
also reported to be more TPA active than their ethynylene
triple-bond analogues, and this was attributed to the more
efficient ICT of the formers.3f,16
In summary, this work revealed the 3,8,13- or 2,7,12-
substitution varieties within the octupolar framework of the
triindole derivatives. This “position” variable provides extra
space, in addition to the π-extension and the donor-or-
acceptor functionalization strategies, to fine-tune and opti-
mize molecular structures for TPA. The TPA cross sections
of the 2,7,12-substituted compounds (2-BET, 2-BYT, and
2-NET) are significantly larger than their 3,8,13-isomers (3-
BET, 3-BYT, and 3-NET). We also demonstrated that (1)
the ethenylene π-linker is better than the ethynylene linker,
and (2) the D(π-A)3 structure is superior to the D(π-D)3, for
large TPA. The title compounds or their derivatives may find
some applications as optical limiting or fluorescence imaging
materials based on their large TPA cross section (δ) and TPE
action cross section (Φδ).
Acknowledgment. This work has been supported by the
National Natural Science Foundation of China (Nos. 20972089
and 50673054).
Supporting Information Available: Synthetic procedures
and characterization data, TEPF graphics, absorption and
emission spectra, and X-ray structure data in CIF format.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL102057T
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