Tuning two-photon absorption cross-sections for triphenylamine derivatives

Article abstract of DOI:10.1039/c3ra42789g

A methylene bridge link connecting the ortho-positions within the triphenylamine scaffold increases the molecular planarity significantly. The one-photon spectroscopies of bridged triphenylamine molecules show considerable extended conjugation relative to their triphenylamine counterparts, leading to enhanced two-photon absorption (TPA) cross-sections. Various substituents at the scaffold have been prepared and studied. Using a femtosecond laser source Z-scan method, the TPA cross-sections were characterized. The maximum magnitude of the TPA cross-section is ～4800 GM, which is four times that of its triphenylamine counterpart. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3ra42789g

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Tuning two-photon absorption cross-sections for
triphenylamine derivatives3
Cite this: RSC Advances, 2013, 3,
17914
Zhen Fang,*ab Richard D. Webster,c Marek Samocde and Yee-Hing Lai*a
A methylene bridge link connecting the ortho-positions within the triphenylamine scaffold increases the
molecular planarity significantly. The one-photon spectroscopies of bridged triphenylamine molecules
show considerable extended conjugation relative to their triphenylamine counterparts, leading to
enhanced two-photon absorption (TPA) cross-sections. Various substituents at the scaffold have been
prepared and studied. Using a femtosecond laser source Z-scan method, the TPA cross-sections were
characterized. The maximum magnitude of the TPA cross-section is y4800 GM, which is four times that of
its triphenylamine counterpart.
Received 5th June 2013,
Accepted 29th July 2013
DOI: 10.1039/c3ra42789g
www.rsc.org/advances
a strong donor framework for electroluminescence and TPA
molecules,18,19 which produce significantly enhanced TPA
Introduction
cross-sections.20,21 Based on the single crystal diffraction data
of B1, we found that although the dihedral angle between the
phenyl ring and the central N-bonded C plane is dramatically
reduced, the central nitrogen atom has a deviation from the
plane, which leads to less conjugation than the T1 molecule.18
Surprisingly, when the para-positions of the three phenyl rings
are attached with bulky units, e.g. –Br, the central nitrogen
atom is suppressed within the 3C plane and the conjugation
length is effectively increased.18,20 Taking advantage of
modifying the bridged triphenylamine with bulky units at
para-positions, a substantial opportunity exists in molecular
structure optimization for large TPA cross-sections. Chart 1
shows the structures of triphenylamine based chromophores.
Two-photon absorption (TPA) has been widely investigated
because of its potential applications in optical limiting, 3-D
data storage, photodynamic therapy and two-photon fluores-
cence microscopic bioimaging (TPFM);1–4 while TPFM is
highly dependent on the material’s TPA action cross-sections
(gs2), where s2 is the TPA cross-section and g is the
fluorescence quantum yield.5 Within the last two decades,
many strategies have been developed for constructing mole-
cules with large TPA cross-sections, which involves increasing
p-conjugation length, introducing donor–p-acceptor systems6
and multipolar and dendritic structures,7 and improving
molecular planarity.8
Triphenylamine (T1 in Chart 1) has been applied as an
electron donating function group in the design of TPA
chromophores.9–16 However, due to the large dihedral angle
between the phenyl ring plane and the plane of the nitrogen-
bonded carbon atoms, the overlap of the p orbital carrying the
lone pair of electrons on the nitrogen with the p orbitals of the
phenyl rings is relatively poor.17 To solve this problem, a
bridged triphenylamine (B1 in Chart 1) has been developed as
Results and discussion
Triphenylamine was iodinated via a typical method under
KIO3 and KI in acetic acid to yield T2, followed by a
aNational University of Singapore, Department of Chemistry, 3 Science Drive 3,
117543, Singapore. E-mail: fangzh78@unc.edu; chmlaiyh@nus.edu.sg
bDepartment of Chemistry, The University of North Carolina at Chapel Hill, Chapel
Hill, NC, 27514, USA
cDivision of Chemistry and Biological Chemistry, School of Physical and
Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link,
637371, Singapore
dLaser Physics Centre, Research School of Physics and Engineering, Australian
National University, Canberra, ACT 0200, Australia
eInstitute of Physical and Theoretical Chemistry, Wroclaw University of Technology,
50-370 Wroclaw, Poland
Electronic supplementary information (ESI) available: Materials,
3
instrumentation methods, synthesis details and characterization. See DOI:
10.1039/c3ra42789g
Chart 1 Structures of the T and B series.
17914 | RSC Adv., 2013, 3, 17914–17917
This journal is ß The Royal Society of Chemistry 2013

View Article Online
RSC Advances
Paper
Scheme 1 Synthesis of T3, B3 and T4.
Sonogashira coupling with 2-ethynyl-9,9-dihexyl-9H-fluorene (4)
to afford T3, or coupling with 4-ethynyl-N,N-diphenylaniline (6)
to afford T4 (see Scheme 1). Note that a typical bromination of
fluorene has been conducted with bromine or NBS in hot
propylene carbonate,22,23 which will lead to a large amount of
2,7-dibromofluorene or 9-bromofluorene. The presence of 1,3-
dibromo-5,5-dimethylhydantoin at ambient condition affords
clean bromination without the risk of a hazardous bromine
reagent.24 Using
a similar procedure, B2 and B3 were
synthesized with moderate yields (see details in the ESI ). The
3
core units of bridged triphenylamine (B1), and B4 have been
reported elsewhere using mild conditions with a Grignard
reagent.21 The introduction of tert-butyl groups at the para
positions of phenylene rings is to improve the solubility of B4,
diminishing the probability of aggregation.
Fig. 1 Normalized absorption spectra of T1 and B1 (a), absorption and emission
spectra of T3 and B3 (b), T4 and B4 (c), concentration: 5 mM in toluene, emission
spectra were recorded when excited with the maximum absorption wave-
length.
The one-photon absorption and emission spectra of T3, B3,
T4 and B4 were measured in toluene solutions. The spectra are
shown in Fig. 1, and a tabulation of absorption and emission
maxima and fluorescence quantum yields (g) is provided in
Table 1. Absorption spectra of T1 and B1 are shown to
illustrate the impact of –C(CH3)2 linkage on the parent
molecules (Fig. 1a). As we found previously, there is deviation
of the N atom from the central N-bonded carbon plane in B1,
resulting in a nitrogen floating above the structure compared
to T1,21 while the dihedral angles between the phenyl rings
and the central carbon plane decrease significantly. However,
we note reverse phenomena when bulky groups are attached at
the para positions, i.e. B3 and B4. As shown in Fig. 1b, T3
features peak absorption at 385 nm, whereas B3 exhibits a
maximum at 394 nm, which red-shifts by 9 nm, indicating an
increase of conjugation length from T3 to B3. Although the
parent molecule B1 has a smaller conjugation length than T1,
substituting bulky groups at the para positions of the three
phenylenes increases the conjugation effectively, suppressing
the deviation between nitrogen and the central plane of
N-bonded C atoms. The emission spectra of T3 and B3 a show
similar maximum red shift, i.e. from 427 nm for T3 to 437 nm
for B3. As shown in Fig. 1c, once the periphery groups are
replaced by triphenylamine (T4) or bridged triphenylamine
(B4), a magnified variation in both absorption and emission
between T4 and B4 is observed. There is a large red shift of
y21 nm in maximum wavelength for the absorption spectra
from T4 (390 nm) to B4 (411 nm), and 13 nm in emission
maximum from T4 (425 nm) to B4 (438 nm). Interestingly, all
four molecules show similar quantum yields (g) of y0.52–
0.60.
This journal is ß The Royal Society of Chemistry 2013
RSC Adv., 2013, 3, 17914–17917 | 17915

View Article Online
Paper
RSC Advances
Table 1 One-photon photophysical of T3, T4, B3 and B4 in toluene
Compound
labs/nm
lem/nm
ga
T3
B3
T4
B4
385
394
390
411
427
437
425
438
0.55
0.60
0.52
0.53
a
Determined using quinine sulfate in 0.1 M H2SO4 as the standard,
quantum yield (g) = 0.55.
The studies of the molecular conformation effect on TPA
have revealed that the magnitude of the TPA cross-section
depends on favorable molecular structures that optimize
intramolecular charge transfer.25–28 An extended molecular
planarity resulting from the enhanced electronic interaction
suggests a strong correlation to larger TPA cross-sections,
Fig. 2 Cyclic voltammograms spectra. 1 mM solution in CH2Cl2 with 0.1 mM
n-Bu4NPF6 at a scan rate of 100 mV s21; 1 mm diameter planar Pt electrode at
293 ¡ 5 K.
which has been highlighted in our previous studies.21
A
bridged triphenylamine moieties, which leads the magnitude
of the cross-section to be superior to that of T4. Interestingly,
the introduction of iodine atoms at the phenylene para
positions of B1 makes the cross-section increase from 30 GM
to 110 GM. This result gives us confidence that the planar
bridged triphenylamine, combined with polar or multipolar
units, would be an efficient strategy for building up molecules
with large TPA cross-sections.
The electrochemical behaviour of T3, B3, T4 and B4 (1 mM
in CH2Cl2) was examined by cyclic voltammetry (CV) and
square wave voltammetry (SWV) and the spectra are presented
in Fig. 2. T1 and B1 are presented for comparison. T1 displays
a one-electron oxidation process at 0.535 V vs. Fc/Fc+, while B1
displays a one-electron oxidation process at 0.340 V vs. Fc/Fc+.
We have revealed that B1’s oxidized form, i.e. the radical
cation, is more stable than that of T1 due to the stabilization
from the –C(CH3)2 linkages.18 We note that both T3 and B3
show only a one-electron oxidation process, which is identical
to T1 and B1, respectively. The reason is that the fluorene
moiety has a higher oxidative potential (.1 V),32 which is not
observed in the scan range. T4 displays a one-electron
oxidation, followed by a two-electron oxidation and then
another one-electron oxidation at intervals of 160 mV. As
presented in Fig. 3, the presence of ethynyl conjugated
linkages allows T4 to stabilize its monocation through more
effective and extended conjugation, which is identical to that
proposed by us previously.18 In contrast, B4 displays a three-
tabulation of TPA cross-sections is provided in Table 2. All
results were obtained by a femtosecond Z-scan method, with a
light intensity of about 70 GW cm22 at a repetition rate of 250
Hz. A wavelength of 650 nm, also the only wavelength we used,
was chosen as we found the maximum TPA absorption for
triphenylamine derivatives at this wavelength.16,20,21 This
wavelength is significantly lower than the doubled wavelength
(y800 nm). Similar findings have been observed in our and
other groups’ earlier work,20,21,29,30 attributed to the fact that a
higher excited energy state with larger probability is popu-
lated, as compared with one-photon excitation.31
Compared with the triphenylamine series, the bridged
triphenylamine series exhibits significantly larger TPA cross-
sections for similar numbers of pi electrons (Np). We observed
strong photo darkening in T3, which may result in an
overestimated TPA cross-section value. However, B3 (1350
GM, 1 GM = 1 6 10250 cm4 s per photon) displays a larger
cross-section relative to T3 (1100 GM). This result indicates the
improved molecular planarity in the triphenylamine core unit
to be the only contribution to the increase of the cross-section,
which is in agreement with the red shift in the absorption and
emission spectra (Fig. 1). Another impressive increase is that
the cross-section of B4 is over 4 times that of T4. In fact, the
comparison of their relative efficiencies in terms of s2/Np
clearly suggests the enhancement. Note that B4 has four
Table 2 TPA properties of T3, T4, B3 and B4 in toluene
Mw
245.32
365.51
623.0
Np
Cross-section s2 (GM)a
s2/Np
gs2
T1
B1
T2
B2
T3
B3
T4
B4
20
20
20
20
68
68
92
92
5(2)
30(5)
22(3)
110(40)
1100 (100)
1350 (120)
1200(200)
4800(500)
0.25
1.5
1.1
—
—
—
—
605
810
624
2544
743.2
5.5
1314.95
1435.14
1047.29
1864.69
16.2
19.9
13.0
52.2
a
Numbers in parentheses represent the uncertainty.
Fig. 3 Oxidation process of T4 (upper) and B4 (lower) up to tetracation.
This journal is ß The Royal Society of Chemistry 2013
17916 | RSC Adv., 2013, 3, 17914–17917

View Article Online
RSC Advances
Paper
C. Subrananiam, W. W. Webb, X. L. Wu and C. Xu, Science,
1998, 281, 1653.
Table 3 Approximate formal potentials (E0) and number of electrons trans-
ferred during the oxidations
7 J. J. Zyss, Chem. Phys., 1993, 98, 6583.
E0/Va,b
8 B. A. Reinhardt, L. L. Brott, S. J. Clarson, A. G. Dillard, J.
C. Bhatt, R. Kannan, L. X. Yuan, G. S. He and P. N. Prasad,
Chem. Mater., 1998, 10, 1863.
9 H. J. Lee, J. Sohn, J. Hwang, S. Y. Park, H. Choi and M. Cha,
Chem. Mater., 2004, 16, 456.
10 Y. X. Yan, X. T. Tao, Y. H. Sun, G. B. Xu, C. K. Wang, T.
X. Yang, Y. Z. Wu, Y. Ren and M. H. Jiang, Mater. Chem.
Phys., 2005, 90, 139.
Process 1
Process 2
Process 3
+0.695 (1)
Process 4
T1
B1
T3
B3
T4
B4
0.535 (1)
0.340 (1)
+0.510 (1)
+0.375 (1)
+0.375 (1)
+0.230 (3)
+0.535 (2)
+0.530 (1)
11 P. Wei, X. D. Bi, Z. Wu and Z. Xu, Org. Lett., 2005, 7, 3199.
12 Z. Fang, M. Samoc, R. D. Webster, A. Samoc and Y.-H. Lai,
Tetrahedron Lett., 2012, 53, 4885.
a
Peak potential vs. Fc/Fc+ to the nearest 5 mV measured by square
wave voltammetry and assuming electrochemical reversibility.
b
Number of electrons associated with each process are given in
parentheses.
13 S.-J. Chung, K.-S. Kim, T.-C. Lin, G. S. He, J. Swiatkiewicz
and P. N. Prasad, J. Phys. Chem. B, 1999, 103, 10741.
14 G. S. He, J. Swiatkiewicz, Y. Jiang, P. N. Prasad, B.
A. Reinhardt, L.-S. Tan and R. Kannan, J. Phys. Chem. A,
2000, 104, 4805.
electron oxidation followed by a one-electron oxidation,
represented in Fig. 3. A summary of the formal oxidation
potentials measured by SWV is given in Table 3. Compared
with the T series, the B series shows 140–200 mV less for the
first oxidation attributed to the stabilization of radical cations,
as shown in B1 and T1.
`
15 L. Porres, O. Mongin, C. Katan, M. Charlot, T. Pons,
J. Mertz and M. Blanchard-Desce, Org. Lett., 2004, 6, 47.
16 O. Varnavski, X. Yan, O. Mongin, M. Blanchard-Desce and
T. Goodson III, J. Phys. Chem. C, 2007, 111, 149.
17 A. N. Sobolev, V. K. Belsky, I. P. Romm, N. Y. Chernikova
and E. N. Guryanova, Acta Crystallogr., Sect. C: Cryst. Struct.
Commun., 1985, 41, 967.
18 Z. Fang, V. Chellappan, R. D. Webster, L. Ke, T. Zhang,
B. Liu and Y.-H. Lai, J. Mater. Chem., 2012, 22, 15397.
19 Z. Jiang, T. Ye, C. Yang, D. Yang, M. Zhu, C. Zhong, J. Qin
and D. Ma, Chem. Mater., 2011, 23, 771.
20 Z. Fang, X. Zhang, Y.-H. Lai and B. Liu, Chem. Commun.,
2009, 920.
21 Z. Fang, T.-T. Teo, L. Cai, Y.-H. Lai, A. Samoc and
M. Samoc, Org. Lett., 2009, 11, 1–4.
22 H. Firouzabadi, N. Iranpoor, K. Amani and S. D. Ross, J.
Mol. Catal. A: Chem., 2003, 195, 289.
23 M. Finkelstein and R. C. Petersen, J. Am. Chem. Soc., 1958,
80, 4327.
24 H. Eguchi, H. Kawaguchi, S. Yoshinaga, A. Nishida,
T. Nishiguchi and S. Fujisaki, Bull. Chem. Soc. Jpn., 1994,
67, 1918.
Conclusions
In summary, we synthesize both triphenylamine and bridged
triphenylamine based triads. Their one-photon and two-
photon photophysical properties are discussed. Due to the
enhanced planarity of the triphenylamine moiety, extended
conjugation has been found in both B3 and B4, which provide
larger two-photon absorption cross-sections than T3 and T4.
Together with the studies of oxidation under cyclic voltam-
metry conditions, an over 150 mV lower oxidation potential in
B series affirms the stabilization of radical cations by the
planar structures. This shows potential in electroluminescent
applications as well as the considerable TPA performance for
bridged triphenylamine.
25 T. K. Ahn, K. S. Kim, D. Y. Kim, S. B. Noh, N. Aratani,
C. Ikeda, A. Osuka and D. Kim, J. Am. Chem. Soc., 2006, 128,
1700.
26 Z. S. Yoon, J. H. Kwon, M.-C. Yoon, M. K. Koh, S. B. Noh, J.
L. Sessler, J. T. Lee, D. Seidel, A. Aguilar, S. Shimizu,
M. Suzuki, A. Osuka and D. Kim, J. Am. Chem. Soc., 2006,
128, 14128.
Notes and references
1 J. E. Ehrlich, X. L. Wu, L. Y. S. Lee, Z. Y. Hu, H. Rockel, S.
R. Marder and J. W. Perry, Opt. Lett., 1997, 22, 1843.
2 D. R. Larson, W. R. Zipfel, R. M. Williams, S. W. Clark, M.
P. Bruchez, F. W. Wise and W. W. Webb, Science, 2003, 300,
1434.
27 X. Zhou, A.-M. Ren, J.-K. Feng, X.-J. Liu, G.-B. Yang and W.-
J. J. Tian, Opt. Mater., 2004, 27, 315.
28 G. Xu, X. Zhao, D. Hu, Z. Shao, H. Liu and Y. Tian, Opt.
Commun., 2006, 260, 292.
29 C. Xu, R. M. Williams, W. Zipfel and W. W. Webb,
Bioimaging, 1996, 4, 198.
3 C. Xu, W. Zipfel, J. B. Shear, R. M. Williams and W.
W. Webb, Proc. Natl. Acad. Sci. U. S. A., 1996, 93, 10763.
4 S. Kim, H. E. Pudavar, A. Bonoiu and P. N. Prasad, Adv.
Mater., 2007, 19, 3791.
30 Y. P. Meshalkin and S. S. Chunosova, Quantum Electron.,
2005, 35, 527.
31 J. P. Hermann and J. Ducuing, Opt. Commun., 1972, 6, 101.
32 J. Rault-Berthelot, M. Cariou and J. J. Tahri-Hassani, J.
Electroanal. Chem., 1996, 402, 203.
5 H. Y. Woo, B. Liu, B. Kohler, D. Korystov, A. Mikhailovsky
and G. C. Bazan, J. Am. Chem. Soc., 2005, 127, 14721.
6 M. Albota, D. Beljonne, J. L. Bredas, J. E. Ehrlich, J. Y. Fu, A.
A. Heikal, S. E. Hess, T. Kogej, M. D. Levin, S. R. Marder,
D. McCord-Maughon, J. W. Perry, H. Rockel, M. Rumi,
This journal is ß The Royal Society of Chemistry 2013
RSC Adv., 2013, 3, 17914–17917 | 17917