Triphenylamine derivatized phenylacetylene macrocycle with large two-photon absorption cross-section

Article abstract of DOI:10.1016/j.tetlet.2012.07.003

A phenylacetylene macrocycle (PAM) derivative containing triphenylamine as the framework was synthesized in one-step Sonogashira coupling. The photophysical and electrochemical properties were investigated in details. This hexamer shows significant enhanc

Full text of DOI:10.1016/j.tetlet.2012.07.003

Tetrahedron Letters 53 (2012) 4885–4888
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Tetrahedron Letters
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Triphenylamine derivatized phenylacetylene macrocycle with large
two-photon absorption cross-section
Zhen Fang a, , Marek Samoc b, Richard D. Webster c, Anna Samoc b, Yee-Hing Lai a,
⇑
⇑
a Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
b Laser Physics Centre, Research School of Physics and Engineering, Australian National University, Canberra ACT 0200, Australia
c Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history:
A phenylacetylene macrocycle (PAM) derivative containing triphenylamine as the framework was syn-
thesized in one-step Sonogashira coupling. The photophysical and electrochemical properties were inves-
tigated in details. This hexamer shows significant enhancement in two-photon absorption cross-section
relative to reported PAM derivatives.
Received 17 May 2012
Revised 30 June 2012
Accepted 2 July 2012
Available online 10 July 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Triphenylamine
Phenylacetylene macrocycle
One-step
Two-photon absorption
Electrochemistry
Phenylacetylene macrocycles (PAMs) have been extensively
studied,1 since the first synthesis of unsubstituted PAM in 1970s.2
The conformationally rigid and shape persistent molecules with
nanoscale dimensions have attracted much interests because of
their potentials in molecular crystals,3 conducting tubular liquid
crystals,4 and molecular turnstiles.5 Due to the large cavity, PAMs
with multipolarstructures have beendeveloped tobind organicsub-
strates and catalyze chemical reactions functioning as natural cyclo-
dextrins.6 Meanwhile, multipolar PAMs with large two-photon
absorption cross-section have been investigated for potential appli-
cation in imaging.7 Their electronic and optical properties can be
fine-tuned by modifying the size and shape of the macrocycles and
the characteristics of side functional groups.
Triphenylamine (TPHA) has attracted considerable attention in
lightemittingdiode,8 andnonlinearopticsbecauseofitsuniquestruc-
ture.9 Crystallography and quantum mechanical calculation have
confirmed that the three N–C bonds are in one common plane and
continuous
p conjugation extends through the lone electron of nitro-
gen atom,10 which makes it a good chromophore for luminescent de-
vices as well as particularly for two-photon absorption application.
This leads us hereby present a macrocycle (hexamer 3, in Chart 1) as
a connection of triphenylamine and PAM moieties, exploring the
structure–property relationship. We report the syntheses, electro-
chemical and optical properties of 3, while dimmer 1 and tetramer
2 are demonstrated for comparison. Through a femtosecond Z-scan
technique, we have observed significant improvement for two-photo
absorption cross-section from dimmer, tetramer to hexamer.
The bifunctional monomer 4-ethynyl-N-(4-ethynylphenyl)-N-
phenylbenzenamine (6) was synthesized by Sonogashira coupling
from 4-iodo-N-(4-iodophenyl)-N-phenylbenzenamine (4) and tri-
methylsilyl acetylene followed by the removal of trimethylsilyl
group in a basic solution with a high yield. Hexamer 3 was ob-
tained in 17% yield by coupling from 4 and 6. The target compound
was prepared by one-step synthesis and further purification was
realized by flash chromatography and precipitation from CH2Cl2
and methanol. To avoid cross-coupling polymerization of 4 and 6,
PAM
⇑
Corresponding author at present address: Department of Chemistry, University
of North Carolina at Chapel Hill, NC 27599, USA. Tel.: +1 919 962 0315; fax: +1 919
962 2388 (Z.F.).
E-mail addresses: fangzh78@email.unc.edu (Z. Fang), chmlaiyh@nus.edu.sg
(Y.-H. Lai).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.07.003

4886
Z. Fang et al. / Tetrahedron Letters 53 (2012) 4885–4888
N
N
N
1
N
N
N
N
N
N
N
N
N
3
2
Chart 1. Structures of dimmer 1, tetramer 2, and hexamer 3.
Pd(PPh3)2Cl2, CuI, Et3N
NIS, CHCl3
N
N
trimethylsilylacetylene
2h, 70ºC, Ar
N
I
I
Si
Si
4
75%
5
93%
Pd(PPh3)2Cl2, CuI, Et3N
2days, 70ºC, Ar
4
MeOH, THF, H2O
KOH, 2h
3
N
17%
6
98%
Scheme 1. Synthesis of 3.
a dilute concentration is required with the addition of catalysts
well controlled. Compared with reported methods via a trimmer
halide and corresponding trimmer acetylene, this method affords
an overall moderate yield with straight forward and convenient
work up.6,7 The synthesis route is summarized in Scheme 1 (details
in Supplementary data).
Figure 1 shows the normalized absorption and emission of 1, 2,
and 3. The absorption spectrum of TPHA is shown for reference.
Compound 1–3 all show maxima at ꢀ380 nm attributable to the
p–
p⁄ transition, while a shoulder at 300 nm in 1 and 2 corresponds
to the triphenylamine unit in agreement of the absorption of TPHA.
The ratio of the absorbance at 300 nm to the maximum one de-
creases from 1 to 2 and finally diminishes in 3 indicating that
the electron delocalization completely through the TPHA and ethy-
nyl segment in 3. Meanwhile, the maximum absorption red-shifts
from 1 to 3 suggest the finding of extended conjugation length,
corresponding to the
As shown in emission spectra, compound 1–3 emit blue light max-
imized at ꢀ430 nm. Their quantum yields ( ) have been measured
with quinoline sulphate in 0.1 M H2SO4 as the reference. Com-
pounds 1, 2, and 3 show comparable quantum yields, with of
p–
p⁄ transition of the molecular backbone.
U
Absorbance
Emission
TPHA
U
45%, 60%, and 58%, respectively. It indicates no significant aggrega-
tion, in agreement with the UV absorption spectra. While there is
no appreciable variation in their fluorescence maxima going from
1 to 3, indicating that there is no significant change in conjugation
effect in extending the conjugated molecular framework from dim-
mer to hexamer, although the slight onset emission redshift may
be attributed to the more extended conjugation.
1.0
0.8
0.6
0.4
0.2
1
2
3
Figure 2 shows the electrochemical behavior of TPHA and 1–3
examined by cyclic voltammetry (CV) and square wave voltamme-
try (SWV). TPHA displays a one-electron oxidation process at
0.535 V versus Fc/Fc+, where the anodic (ipox) to cathodic (ipred) peak
current ratio (ipox/ipred) is >1 at a scan rate of 100 mV sÀ1. This
indicates chemical instability of the oxidized compound, that is
the radical cation of 1 (an EC mechanism, where E signifies an elec-
tron transfer and C represents a chemical step).11 As the scan rate
300
350
400
450
500
550
Wavelength/nm
increases to approximately 1 V sÀ1, the ipox/ip value approaches
red
unity, due to the chemical step being outrun.
Figure 1. Absorption and emission of 1–3 in chloroform (5
lM), kex: 380 nm.

Z. Fang et al. / Tetrahedron Letters 53 (2012) 4885–4888
4887
20
15
10
5
TPHA
1
2
3
0
0.2
0.4
0.6
0.8
1.0
E/V vs Fc+/Fc
Figure 4. Simulated structure of 3, Blue: N, Gray: C, H atoms are omitted.
Figure 2. Cyclic voltammetry of TPHA and 1–3, 1 mM solution in CH2Cl2 with 0.1 M
Bu4NPF6 at a scan rate of 100 mV sÀ1
cation stability, where trication shows a higher symmetry than
the monocation and dication in the fast oxidation process of 3.
The structure of hexamer 3 is calculated with minimized energy
as shown in Figure 4. There is no stereochemical structure found.
For the individual TPHA unit, the three N–C bonds are localized
in a same plane as the single TPHA molecule shows.10 The non-
bonding distance between two opposite carbons is 19.55 Å, which
is comparable with the reported 1,4-diethynylphenyl derivatized
PAM,6 and thus giving us an idea of the size of the interior of the
macrocycle. This large cavity provides opportunities for further
modification of amphiphilic function groups at the interior of the
macrocycle, achieving biocompatible materials in biotechnology.
The two-photon absorption was measured by a femtosecond la-
ser source Z-scan method.13 The cross-section was calculated from
dependence of the nonlinear absorption of solutions on the con-
centration. Surprisingly, 1–3 show peak two-photon absorption
cross-section at 650 nm, which is less than the double wavelength
(ꢀ760 nm) of their linear absorption maxima. Similar phenomena
have been observed in our and other groups’ researches.14 The pos-
sible reason is that two-photon excitation might populate the mol-
.
Dimmer 1 displays two one-electron reversible oxidation pro-
cesses with the second process 210 mV more positive that the first
process (0.390 V vs Fc/Fc+). Compound 2 displays a one-electron
oxidation, followed by a two-electron oxidation and then another
one-electron oxidation. The first oxidation processes in 1 and 2
to give the corresponding monocation at TPHA unit should be sim-
ilar. While the second two-electron process in 2 forms a symmet-
rical trication (Fig. 3). Surprisingly, there is a significant difference
in the CV curve of 3 regarding to the number of electrons trans-
ferred and the chemical reversibility, which leads to two broad oxi-
dation processes observed, that is 0.410 V and 0.640 V. According
to the CV curve, possible oxidation processes are proposed as
shown in Figure 3. Although the CV shows only two oxidation
curves with identical current suggesting two three-electron pro-
cesses, there might be radical cation intermediates from one-elec-
tron oxidation. It has been found that the difference in peak
potentials (
D
E° = |E1°ÀE2°|) is equal to 35.6 mV if there is no inter-
action between the redox centers.12 Weak interaction may yield
non-distinguishable processes as shown in both two processes in
3. Meanwhile, symmetry may play a significant role for radical
ecule to
a higher energy excited singlet state with larger
probability as compared to that by one-photon excitation.15 The
+
N
N
N
N
N
+
N
e
2e
e
+
+
N
N
N
N
N
N
N
N
+
N
N
+
+
+
N
+
+
+
N
N
N
N
N
N
N
N
N
N
3e
3e
+
N
N
N
+
+
N
N
N +
N +
+
e
e
e
e
+
+
+
+
+
N
+
N
N
N
N
N
N
N
N
N
N
N
+
e
e
N
N
N
N
N
N
N
N
N
N
N
N
+
+
+
+
+
Figure 3. Oxidation process of 2 and 3.

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Z. Fang et al. / Tetrahedron Letters 53 (2012) 4885–4888
cross-section of dimmer 1 is 420 GM (1 GM = 1 Â 10À50 cm4s/pho-
ton). With the growth of molecular size, the cross-section dramat-
ically increases to 1200 GM for tetramer 2. However, the increase
from 2 to 3 (1300 GM) is somehow saturated, which is somehow
in agreement with the linear absorption. We reported dendritic
hexamer with 30% enhancement relative to tetramer.16 Compared
with PAM structure, dendritic hexamer features a more branched
structure, which facilitates the growth of two-photon absorption.17
To achieve higher two-photon absorption cross-section, multifunc-
tional groups, for example electron-donating and withdrawing
groups could be introduced in future work.
In conclusion, we synthesize a triphenylamine-containing PAM
by one-step Sonogashira reaction. Two three-electron oxidation
processes have been observed in cyclic voltammetry. The two-
photon absorption cross section is significantly larger than the
phenylacetylene macrocycle with donor and acceptor reported
with ꢀ125 GM.7 Future studies will be focusing on the modification
of exterior and interior function groups for higher cross-section and
biocompatible character.
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Acknowledgments
This work was supported by the National University of Singa-
pore (NUS). The authors thank the staff at the Chemical, Molecular
and Materials Analysis Centre, Department of Chemistry, NUS, for
their technical assistance.
Supplementary data
17. McDonagh, A. M.; Humphrey, M. G.; Samoc, M.; Luther-Davies, B.
Organometallics 1999, 18, 5195–5197.
Supplementary data associated with this article can be found, in
the onlineversion, at http://dx.doi.org/10.1016/j.tetlet.2012.07.003.