4-Diphenylamino-phenyl substituted pyrazine: Nonlinear optical switching by protonation

Article abstract of DOI:10.1039/c5tc01657f

Through the integration of a 4-diphenylamino-phenyl unit with a pyrazine segment, a series of novel organic conjugated molecules with different spacers and shapes have been synthesized and fully characterized. These as-prepared compounds exhibit reversibl

Full text of DOI:10.1039/c5tc01657f

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4-Diphenylamino-phenyl substituted pyrazine:
nonlinear optical switching by protonation†
Cite this:DOI: 10.1039/c5tc01657f
Liang Xu,‡a Hai Zhu,‡b Guankui Long,a Jun Zhao,c Dongsheng Li,c Rakesh Ganguly,d
Yongxin Li,d Qing-Hua Xu*b and Qichun Zhang*ad
Through the integration of a 4-diphenylamino-phenyl unit with a pyrazine segment, a series of novel
organic conjugated molecules with different spacers and shapes have been synthesized and fully
characterized. These as-prepared compounds exhibit reversible acidochromism in response to
protonation and deprotonation. The investigation of the nonlinear optical (NLO) properties reveals that
the neutral forms of these structures display a reverse saturated absorption (RSA), while the protonated
forms show a saturated absorption (SA).
Received 5th June 2015,
Accepted 10th August 2015
DOI: 10.1039/c5tc01657f
www.rsc.org/MaterialsC
the structure and NLO properties has been strongly investigated by
various classes of conjugated organic molecules with different
Introduction
Two-photon absorption (TPA) is a third-order nonlinear optical conjugation lengths, as well as the strength and substitution
(NLO) process, where two photons are absorbed simultaneously patterns of donors and acceptors.13 Usually, through increasing
to promote a molecule from the ground state to the excited the length of the p-spacer in D–p–A systems, the ground-state D–A
state.1 Normally, organic molecules with large TPA cross sections conjugation is reduced and the HOMO–LUMO gap decreases.14
are important for their potential applications in two-photon Therefore, efforts to further increase the nonlinearity are usually
fluorescence imaging,2 optical power limiting,3 two-photon focused on increasing the conjugation length or increasing the
photodynamic therapy (PDT),4 optical data storage,5 and photo strength of donors or acceptors or both.15
switching.6 TPA cross-section strongly depends on the extent of
Besides the conjugation length, the structures of the
intramolecular charge transfer characteristics of molecules.7 p-conjugated bridges should also be considered in designing
For example, the TPA cross section of fluorescein (water, nonlinear optical materials because this factor would also affect
pH B 13 800 nm),8 Rhodamine B (methanol, 800 nm),9 and the intramolecular charge transfer.16 Recently, some studies
Rhodamine 6G (methanol, 802 nm)10 is reported to be 36, 150 on the novel structures and conjugation patterns of the NLO
and 134 GM, respectively. From the viewpoint of the design chromophores have been reported.17 The strategy to form a rigid
of TPA molecules, their structural factors should include the structure through fusing the donor and the acceptor together
effective conjugation and the polarizability.11 Currently, D–A into one system has been proven to be a smart way to tune the
molecules, which consist of electron donors and electron electro-optical properties. On one hand, the direct attachment of
acceptors connected by a p-conjugated spacer, are the most the donor onto the acceptor unit facilitates a strong donor–
important class of TPA chromophores.12 The relationship between acceptor interaction; on the other hand, such an arrangement is
helpful to achieve a lower band gap and balance dual charge
transport properties.18 Because the 4-diphenylamino-phenyl
a School of Materials Science and Engineering, Nanyang Technological University,
group is a strong electron donor and pyrazine is an electron
acceptor,19 the 4-diphenylamino-phenyl substituted pyrazine is
predicted to have great performance in NLO applications.20
Moreover, such a rigid planar structure has a great contribution
to the extended p-electron delocalization, where p-orbitals in
nitrogen centers can effectively mix with p-electrons. Further-
more, the electro-optical properties could be modulated using
an external trigger.21 In addition, the behavior of the acceptor
pyrazine could be altered by protonation and such alteration
could lead to NLO response at the molecular level. Note that
such NLO variations in response to protonation–deprotonation
would make this material a potential candidate for the application
Singapore 639798, Singapore. E-mail: qczhang@ntu.edu.sg
b Department of Chemistry, National University of Singapore, 3 Science Drive 3,
Singapore 117543, Singapore. E-mail: chmxqh@nus.edu.sg
c College of Materials and Chemical Engineering, Hubei Provincial Collaborative
Innovation Center for New Energy Microgrid, China Three Gorges University,
Yichang 443002, P. R. China
d Division of Chemistry and Biological Chemistry, School of Physical and
Mathematic Sciences, Nanyang Technological University, Singapore 637371,
Singapore
† Electronic supplementary information (ESI) available: Synthesis, characteriza-
tion, photophysical properties, and the theoretical study of compounds 1–3.
CCDC 1403860–1403862. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c5tc01657f
‡ The two authors have equal contribution to this paper.
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in photo switching. Nonlinear optical switching effects are
reported in various materials, such as graphene oxide films,22
gold nanoparticles,23 Rhodamine B24 etc. These materials display
a competition between saturable absorption (SA) and reverse
saturable absorption (RSA) as the excitation fluence increased.
Besides this excitation fluence based switching, a variety of
methods25 are employed to switch quadratic and cubic nonlinear
optical properties, such as oxidation/reduction, photoisomeriza-
tion sequences, and protonation/deprotonation. Protonation
induced switching effects are previously demonstrated to display
a reversible luminescent and second-order nonlinear property.25a
To the best of our knowledge, there is no report on the nonlinear
optical switching between SA and RSA by reversible protonation
and with similar molecular structures. In this research, several
4-diphenylamino-phenyl substituted pyrazines with different
moieties and shapes have been prepared and their NLO proper-
ties with switching behaviors are also investigated.
Fig. 1 The crystal structures of compounds 1–3 (a, c and e) and the crystal
packing diagrams of compounds 1–3 (b, d and f).
Those phenomena indicate that the lone pair electrons on
the nitrogen center of the 4-diphenylamino-phenyl group are
delocalized into the pyrazine segment due to the electron-
accepting tendency of the pyrazine.
Furthermore, the pyrazine ring and the phenyl ring directly
connecting to it are twisted from each other with interplanar
angles of 27.031 and 73.431, which suggests a decoupling effect
between the electron donor 4-diphenylamino-phenyl group
and the electron acceptor pyrazine. The packing diagrams for
compounds 1–3 are also shown in Fig. 1. Note that compound 1
forms a N–H–C weak hydrogen bond.
As shown in Fig. 2, compound 3 exhibits a broad and
moderately intense absorption band (4500 nm), while com-
pounds 1 and 2 show a relatively narrow absorption band
(B400 nm). The peak positions of this band suggest a trend
(2 o 1 o 3), reflecting the substituent effect on the electronic
properties. Considering the phenylene moiety in compound 1
is a neutral group and the dimethylphenylene moiety in com-
pound 2 is a weak electron donor, we believe that the electron
push–pull system between the 4-diphenylamino-phenyl group
Results and discussion
As shown in Scheme 1, the starting material 4,40-bis(N,N-diphenyl-
amino)benzyl was prepared from triphenylamine and oxalyl chloride
through Friedel–Crafts acylation.26 Compounds 1–3 are synthesized
by condensing 4,40-bis(N,N-diphenylamino)benzyl and different
kinds of diamines including 1,2-diaminobenzene, 4,5-dimethyl-
1,2-phenylenediamine or 1,2-diaminoanthraquinone in acetic
acid with a catalytic amount of IBX.27
Suitable crystals of compounds 1–3 (CCDC number: 1403860–
1403862) for single crystal X-ray diffractometer analysis have
been obtained by slow evaporation of dichloromethane solu-
tions over two weeks. As shown in Fig. 1, for compounds 1–3,
the length of the C–C bond connecting the pyrazine ring to the
4-diphenylamino-phenyl group (1.48–1.49 Å) is between the
average bond length of the C–C single bond and the aromatic
C–C bond (C–C single bond 1.54 Å; double bond 1.33 Å;
aromatic 1.40 Å).28 The length of the C–N bond within the
4-diphenylamino-phenyl group (1.41–1.43 Å) falls close to the
average aromatic C–N bond length (C–N single bond 1.47 Å;
double bond 1.34 Å; aromatic 1.43 Å). Especially for compound
3, the shortest C–N bond within the 4-diphenylamino-phenyl
group is 1.38 Å, exhibiting a characteristic C–N double bond.
Scheme 1 Synthetic route for compounds 1–3. Conditions: anhydrous
acetic acid, Ar2, 2-iodoxybenzoic acid, 120 1C, 24 h.
Fig. 2 The normalized UV-Vis and fluorescence spectra of compounds 1–3.
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Table 1 The UV-Vis and fluorescence data of compounds 1–3 before and
after adding TFA
and pyrazine is not significantly altered by the substituents.
However, for compound 3, the 9,10-dioxo-9,10-dihydro-anthracenyl
moiety is a strong electron acceptor, which can effectively enhance
the interaction between the electron donor and the acceptor,
resulting in a bathochromic absorption shift. Note that, the
absorption bands show a remarkable red shift when trifluoro-
acetic acid (TFA) was added to the solution containing com-
pound 1, 2 or 3 (Fig. S2, ESI†). The reason is that the N–H
hydrogen bond is formed between the nitrogen of pyrazine and
the hydrogen of TFA, generating a strongly electron-withdrawing
pyrazinium ion, which triggered a strong intramolecular charge
transfer between the amine donor and the pyrazinium acceptor.
Through an aromatic conjugation, the amine loses one electron
and turns into a strong electron acceptor, while the pyrazinium
ion gets one electron and turns into electron donor amine. This
aromatic conjugation stabilizes the LUMO and causes a dramatic
red-shift of the absorption band. Meanwhile, it induces the dipole
moments with reversed directions. For the 4-diphenylamino-
phenyl segment, the nitrogen cannot be protonated due to the
steric effect. The pyrazine is protonated to pyrazinium for
compounds 1 and 2 while the anthraquinone is protonated to
semiquinone for compound 3. The protonation is completed
upon the addition of 1000 equivalents of TFA. The isosbestic
points indicate the presence of the neutral and protonated
forms in equilibrium and the latter being predominant at
higher concentrations of TFA (Fig. 3). In dichloromethane
solutions, upon the addition of TFA, compounds 1 and 2
display a color change from yellow to violet, while the color
of compound 3 changes from red to green. The original color
could be regained by the addition of trimethylamine (TEA).
UV-Vis spectra of the titration of compound 1 through adding
TFA and TEA are shown in Fig. 3. Upon the addition of TFA to the
solution of compound 1, there is a continuous decrease of the
absorption peaks at 302 and 399 nm, followed by a continuous
increase of the absorption peaks at 338 and 550 nm. Meanwhile,
when TFA was added to the solution of compound 1, the
emission intensity decreased sharply (Fig. S2, ESI†). The specific
data are shown in Table 1. It seems that the protonated species
Abs. lmax Abs. lmax (nm)
Fluor.
(Phi)n
(nm)
(+1000 eq. TFA) lmax (nm) (Phi)n (+1000 eq. TFA)
1
2
3
399
398
523
550
540
702
533
522
—
0.41
0.97
o0.01 o0.01
0.20
0.19
The oxidation and reduction potentials were measured using a
cyclic voltammetric method. As shown in Fig. 4, all compounds
show irreversible oxidation waves due to the removal of an electron
from the 4-diphenylamino-phenyl group. The oxidation and
reduction potentials are affected by the nature of the electronic
interaction between the donor 4-diphenylamino-phenyl group
and the acceptor. For compounds 1 and 2, no reduction wave is
monitored while two reversible reduction waves are found in
compound 3 (Table 2).
The optimization for geometric structures of compounds
1–3 was carried out using the Gaussian 09 program at the
B3LYP/6-31G* level (Table 3). And the frequency analysis was
employed at the same level of theory to check whether the
Fig. 4 The CV spectra of compounds 1–3. The working electrode is
glassy carbon; the counter electrode and the reference electrode are
platinum wires. Potentials are recorded versus Fc+/Fc in a solution of
anhydrous dichloromethane (DCM) with 0.1
M tetrabutylammonium
hexafluorophosphate (TBAPF6) as a supporting electrolyte at a scan rate
has a more pronounced dipolar relaxation from the excited state of 50 mV sꢀ1
.
due to the presence of strong donor–acceptor interactions.
Table 2 The CV data of compounds 1–3
E1ox/V
E2ox/V
E3ox/V
E1red/V
E2red/V
1
2
3
0.42
0.53
0.54
0.70
0.86
0.84
0.94
1.36
—
—
—
ꢀ1.09
—
—
ꢀ1.54
Table 3 The calculated data of neutral and protonated forms of com-
pounds 1–3
HOMO (eV)
LUMO (eV)
Gap (eV)
1
ꢀ4.90
ꢀ7.22
ꢀ4.84
ꢀ7.10
ꢀ4.98
ꢀ7.06
ꢀ1.79
ꢀ5.60
ꢀ1.65
ꢀ5.44
ꢀ2.85
ꢀ6.14
3.11
1.62
3.19
1.66
2.12
0.92
1(H+)
2
2(H+)
3
Fig. 3 The UV-Vis spectra of compound 1 upon adding TFA (the solid line)
and TEA (the dotted line).
3(H+)
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optimized geometrical structure is at a stable point and evaluate segment, while the LUMO is extended to the benzene ring of the
the zero-point vibration energy (ZPE). The excess charges upon other 4-diphenylamino-phenyl group. All these changes indicate
protonation were analyzed through mulliken population analysis that the protonation of pyrazine enhances the electron-
(Fig. S3-1 and Table S3-1, ESI†). Upon protonation, the positive withdrawing ability and drifts the electron-density toward the
charge mainly delocalized on the quinoxaline (compounds 1 pyrazinium ion.
and 2) and anthracene-9,10-dione (compound 3) units. This is
The NLO properties of compounds 1–3 were characterized
also consistent with the calculated results, the excess positive by an open aperture z-scan measurement, shown in Fig. S4-1
charges (q 4 0.3 e) mainly locate at the certain position of (ESI†). The open aperture z-scan measurements of compounds
compounds 1(H+), 2(H+) and 3(H+), respectively. The calculated 1–3 are shown in Fig. 6. All the compounds exhibit reverse
reaction free energy (DG) was summarized in Table S3-2 (ESI†). saturate absorption (RSA) under an excitation power density of
The electronic distributions in HOMO and LUMO for these 80 GW cmꢀ2. Interestingly, they switch from RSA to saturate
compounds are shown in Fig. 5. The HOMOs of these com- absorption (SA) after protonation by adding 1000 eq. TFA into
pounds are delocalized over the 4-diphenylamino-phenyl groups, the solutions. The influence from both the solvents (CH2Cl2
and are extended to the bridge, while the LUMOs are delocalized and TFA) was ruled out based on z-scan curves of the solvents.
on the pyrazine and anthraquinone. The distribution of the
Under the assumption of a spatially and temporally Gaussian
HOMO and the LUMO is not totally separated within the mole- laser beam, the normalized beam transmittance is expressed
cule and a part of overlap is present, which suggests that the by:22,29
HOMO–LUMO transitions cannot be considered as pure charge
ð1
h
i
transfer transitions and can be treated as p–p* transitions with
prominent charge transfer contribution. Because the relationship
between structure–electronic properties could be revealed by DFT
calculation, the protonated forms of these compounds are also
investigated. Except for slight structural variations, no obvious
alternations in the geometry were observed after protonation.
However, obvious changes in the electron distributions were
observed for these compounds due to protonation. After protona-
tion, the HOMO is concentrated on one 4-diphenylamino-phenyl
1
2
pffiffiffi
TðzÞ ¼
ln 1 þ q0ðzÞeꢀt dt
pq0ðzÞ
ꢀ1
q0ðzÞ ¼ bIðzÞLeff
IðzÞ ¼ I0= 1 þ z2=z0
À
Á
2
Fig. 5 The HOMO and the LUMO of neutral and protonated forms of
compounds 1–3.
Fig. 6 The open aperture z-scan measurement of compounds 1–3 (a–c).
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Table 4 The calculated nonlinear coefficient b and corresponding 2PA
cross section s(2) for compounds 1–3 and their protonated forms
(Nanomaterials for Energy and Water Management) from NRF,
and New Initiative Fund from NTU, Singapore.
b (cmꢀ1 GWꢀ1
)
s(2) (GM)
1
0.833
ꢀ15.5
0.784
34.3
—
32.3
—
211
—
1(H+)
2
Notes and references
2(H+)
3
ꢀ0.913
1 H. S. Nalwa, Adv. Mater., 1993, 5, 341–358.
5.14
3(H+)
ꢀ0.674
2 (a) W. Denk, J. Strickler and W. Webb, Science, 1990, 248,
73–76; (b) M. Velusamy, J.-Y. Shen, J. T. Lin, Y.-C. Lin,
C.-C. Hsieh, C.-H. Lai, C.-W. Lai, M.-L. Ho, Y.-C. Chen,
P.-T. Chou and J.-K. Hsiao, Adv. Funct. Mater., 2009, 19,
2388–2397; (c) L. Xu, Y. Zhao, G. Long, Y. Wang, J. Zhao,
D. Li, J. Li, R. Ganguly, Y. Li, H. Sun, X.-W. Sun and
Q. Zhang, RSC Adv., 2015, 5, 63080.
3 (a) H. Zhang, D. E. Zelmon, L. Deng, H.-K. Liu and B. K. Teo,
J. Am. Chem. Soc., 2001, 123, 11300–11301; (b) C. W.
Spangler, J. Mater. Chem., 1999, 9, 2013–2020.
where z is the distance between the sample and the focus; z0 is
the Rayleigh diffraction length; I0 is the peak power density; Leff
is the effective interaction length and b is the nonlinear absorp-
tion coefficient. A positive b describes the two photon absorption
(2PA) of the molecules, while, b is negative in the case of SA. The
saturation intensity term is omitted in this study due to a rather
low linear absorption coefficient at 800 nm.
Using this model, the calculated 2PA cross sections of
compounds 1–3 are 34, 32 and 211. And the calculated non-
linear coefficient b and corresponding 2PA cross section s(2) are
listed in Table 4. In the case of 2PA, as the nonlinear absorption
coefficient is inversely proportional to the energy difference
between the LUMO and the virtual state induced by excitation,
compound 3 possesses a larger 2PA cross section than 1 and 2
(ESI† S4).
After protonation, compounds 1–3 show switching from RSA
to SA. This switching is due to the decrease of the gap between
the HOMO and the LUMO of the compounds as discussed
above. As the gap decreases after protonation, one photon
absorption transition becomes possible. Electrons on the HOMO
are excited to the LUMO transiently thus the HOMO is depleted
and it shows a SA z-scan curve. The TFA concentration dependence
z-scan curves are shown in Fig. S4-2 (ESI†). The RSA gradually
switches to SA as protonation by TFA.
´
4 J. Arnbjerg, A. Jimenez-Banzo, M. J. Paterson, S. Nonell,
J. I. Borrell, O. Christiansen and P. R. Ogilby, J. Am. Chem.
Soc., 2007, 129, 5188–5199.
5 (a) B. H. Cumpston, S. P. Ananthavel, S. Barlow, D. L. Dyer,
J. E. Ehrlich, L. L. Erskine, A. A. Heikal, S. M. Kuebler,
I. Y. S. Lee, D. McCord-Maughon, J. Qin, H. Rockel,
M. Rumi, X.-L. Wu, S. R. Marder and J. W. Perry, Nature,
1999, 398, 51–54; (b) S. Kawata and Y. Kawata, Chem. Rev.,
2000, 100, 1777–1788.
6 (a) M. Irie, T. Fukaminato, T. Sasaki, N. Tamai and T. Kawai,
Nature, 2002, 420, 759–760; (b) M. Bockmann, N. L.
¨
Doltsinis and D. Marx, Angew. Chem., Int. Ed., 2010, 49,
3382–3384; (c) B. Champagne, A. Plaquet, J.-L. Pozzo,
V. Rodriguez and F. Castet, J. Am. Chem. Soc., 2012, 134,
8101–8103; (d) F. Castet, V. Rodriguez, J.-L. Pozzo,
L. Ducasse, A. Plaquet and B. Champagne, Acc. Chem. Res.,
2013, 46, 2656–2665; (e) Y. Wu, Z. Yin, J. Xiao, Y. Liu, F. Wei,
K. J. Tan, C. Kloc, L. Huang, Q. Yan, F. Hu, H. Zhang and
Q. Zhang, ACS Appl. Mater. Interfaces, 2012, 4, 1883–1886;
( f ) P. Serra-Crespo, M. A. van der Veen, E. Gobechiya,
K. Houthoofd, Y. Filinchuk, C. E. A. Kirschhock, J. A.
Martens, B. F. Sels, D. E. De Vos, F. Kapteijn and J. Gascon,
J. Am. Chem. Soc., 2012, 134, 8314–8317.
7 M. A. Albota, C. Xu and W. W. Webb, Appl. Opt., 1998, 37,
7352–7356.
8 C. Xu and W. W. Webb, J. Opt. Soc. Am. B, 1996, 13, 481–491.
9 D. Oulianov, I. Tomov, A. Dvornikov and P. Rentzepis, Opt.
Commun., 2001, 191, 235–243.
10 J. Yoo, S. K. Yang, M.-Y. Jeong, H. C. Ahn, S.-J. Jeon and
B. R. Cho, Org. Lett., 2003, 5, 645–648.
11 J. W. Baur, M. D. Alexander, M. Banach, L. R. Denny, B. A.
Reinhardt, R. A. Vaia, P. A. Fleitz and S. M. Kirkpatrick,
Chem. Mater., 1999, 11, 2899–2906.
12 A. Abbotto, L. Beverina, R. Bozio, A. Facchetti, C. Ferrante,
G. A. Pagani, D. Pedron and R. Signorini, Chem. Commun.,
2003, 2144–2145.
Conclusions
A series of novel 4-diphenylamino-phenyl substituted pyrazines
have been obtained by one-step reaction between 1,2-diamine
and 1,2-diketones. All compounds have been demonstrated to
show unique properties for NLO applications. Upon the addi-
tion of trifluoroacetic acid and triethylamine, these compounds
were subject to protonation and deprotonation on the pyrazine
unit. In the neutral forms, compounds 1–3 exhibited reverse
saturated absorption (RSA) with a 2TPA cross section of 34,
32 and 211 GM, respectively, while in the protonated forms,
they display saturable absorption (SA). These results suggest
that 4-diphenylamino-phenyl substituted pyrazines could have
potential applications in reversible and highly efficient non-
linear optical switches.
Acknowledgements
´
13 (a) M. Albota, D. Beljonne, J.-L. Bredas, J. E. Ehrlich, J.-Y. Fu,
Q.Z. acknowledges the financial support AcRF Tier 1 (RG 133/14)
and Tier 2 (ARC 20/12 and ARC 2/13) from MOE, CREATE program
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 ©The Royal Society of Chemistry 2015
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View Article Online
Paper
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G. Subramaniam, W. W. Webb, X.-L. Wu and C. Xu, Science, 20 (a) K. R. Justin Thomas, J. T. Lin, Y.-T. Tao and C. H. Chuen,
´
1998, 281, 1653–1656; (b) A. Adronov, J. M. J. Frechet,
J. Mater. Chem., 2002, 12, 3516–3522; (b) P. Singh, A. Baheti
and K. R. J. Thomas, J. Org. Chem., 2011, 76, 6134–6145.
G. S. He, K.-S. Kim, S.-J. Chung, J. Swiatkiewicz and P. N.
Prasad, Chem. Mater., 2000, 12, 2838–2841; (c) M. M. Oliva, 21 E. Cariati, C. Dragonetti, E. Lucenti, F. Nisic, S. Righetto,
´
R. Juarez, M. Ramos, J. L. Segura, S. v. Cleuvenbergen,
K. Clays, T. Goodson, J. T. L. Navarrete and J. Casado, 22 X.-F. Jiang, L. Polavarapu, S. T. Neo, T. Venkatesan and
J. Phys. Chem. C, 2013, 117, 626–632. Q.-H. Xu, J. Phys. Chem. Lett., 2012, 3, 785–790.
14 B. A. Reinhardt, L. L. Brott, S. J. Clarson, A. G. Dillard, 23 Y. H. Lee, Y. Yan, L. Polavarapu and Q.-H. Xu, Appl. Phys.
D. Roberto and E. Tordin, Chem. Commun., 2014, 50, 1608–1610.
J. C. Bhatt, R. Kannan, L. Yuan, G. S. He and P. N. Prasad,
Chem. Mater., 1998, 10, 1863–1874.
15 (a) T. Kogej, D. Beljonne, F. Meyers, J. W. Perry, S. R. Marder
Lett., 2009, 95, 023105.
24 N. Srinivas, S. V. Rao and D. N. Rao, J. Opt. Soc. Am. B, 2003,
20, 2470–2479.
´
and J. L. Bredas, Chem. Phys. Lett., 1998, 298, 1–6; 25 (a) B. J. Coe, Chem. – Eur. J., 1999, 5, 2464–2471; (b) J. A.
(b) M. Pawlicki, H. A. Collins, R. G. Denning and H. L.
Anderson, Angew. Chem., Int. Ed., 2009, 48, 3244–3266.
16 H. Reyes, B. M. Munoz, N. Farfan, R. Santillan, S. Rojas-
Lima, P. G. Lacroix and K. Nakatani, J. Mater. Chem., 2002,
12, 2898–2903.
17 (a) X.-H. Zhou, J. Davies, S. Huang, J. Luo, Z. Shi,
B. Polishak, Y.-J. Cheng, T.-D. Kim, L. Johnson and A. Jen,
J. Mater. Chem., 2011, 21, 4437–4444; (b) M. Shimada,
Y. Yamanoi, T. Matsushita, T. Kondo, E. Nishibori,
A. Hatakeyama, K. Sugimoto and H. Nishihara, J. Am. Chem.
Soc., 2015, 137, 1024–1027; (c) S. Thomas, Y. A. Pati and
S. Ramasesha, J. Phys. Chem. A, 2013, 117, 7804–7809.
18 M. Barzoukas and M. Blanchard-Desce, J. Chem. Phys., 2000,
113, 3951–3959.
Delaire and K. Nakatani, Chem. Rev., 2000, 100, 1817–1846;
(c) E. Cariati, C. Dragonetti, E. Lucenti, F. Nisic, S. Righetto,
D. Roberto and E. Tordin, Chem. Commun., 2014, 50,
1608–1610; (d) M. P. Cifuentes, M. G. Humphrey, J. P.
Morrall, M. Samoc, F. Paul, C. Lapinte and T. Roisnel,
Organometallics, 2005, 24, 4280–4288; (e) M. P. Cifuentes,
C. E. Powell, M. G. Humphrey, G. A. Heath, M. Samoc and
B. Luther-Davies, J. Phys. Chem. A, 2001, 105, 9625–9627;
( f ) G. T. Dalton, M. P. Cifuentes, S. Petrie, R. Stranger, M. G.
Humphrey and M. Samoc, J. Am. Chem. Soc., 2007, 129,
11882–11883; (g) C. E. Powell, M. P. Cifuentes, J. P. Morrall,
R. Stranger, M. G. Humphrey, M. Samoc, B. Luther-Davies and
G. A. Heath, J. Am. Chem. Soc., 2003, 125, 602–610; (h) M. Samoc,
N. Gauthier, M. P. Cifuentes, F. Paul, C. Lapinte and M. G.
Humphrey, Angew. Chem., Int. Ed., 2006, 45, 7376–7379.
19 (a) K. Cao, J. Lu, J. Cui, Y. Shen, W. Chen, G. Alemu,
Z. Wang, H. Yuan, J. Xu, M. Wang and Y. Cheng, J. Mater. 26 (a) C. Wang, B. Hu, J. Wang, J. Gao, G. Li, W.-W. Xiong,
Chem. A, 2014, 2, 4945–4953; (b) H. Zhuang, Q. Zhang,
Y. Zhu, X. Xu, H. Liu, N. Li, Q. Xu, H. Li, J. Lu and
L. Wang, J. Mater. Chem. C, 2013, 1, 3816–3824; (c) T. W.
Holcombe, J.-H. Yum, J. Yoon, P. Gao, M. Marszalek, D. D.
Censo, K. Rakstys, M. K. Nazeeruddin and M. Graetzel,
Chem. Commun., 2012, 48, 10724–10726; (d) H.-W. Lin,
L.-Y. Lin, Y.-H. Chen, C.-W. Chen, Y.-T. Lin, S.-W. Chiu
B. Zou, M. Suzuki, N. Aratani, H. Yamada, F. Huo, P. S. Lee
and Q. Zhang, Chem. – Asian J., 2015, 10, 116–119;
(b) C. Wang, M. Yamashita, B. Hu, Y. Zhou, J. Wang, J. Wu,
F. Huo, P. S. Lee, N. Aratani, H. Yamada and Q. Zhang, Asian
J. Org. Chem., 2015, 4, 646–651; (c) C. Wang, J. Wang, P. Li,
J. Gao, S. Y. Tan, W. Xiong, B. Hu, P. S. Lee, Y. Zhao and
Q. Zhang, Chem. – Asian J., 2014, 9, 779–783.
and K.-T. Wong, Chem. Commun., 2011, 47, 7872–7874; 27 B. D. Lindner, J. U. Engelhart, O. Tverskoy, A. L. Appleton,
(e) E. Ripaud, T. Rousseau, P. Leriche and J. Roncali, Adv.
Energy Mater., 2011, 1, 540–545; ( f ) S. Roquet, A. Cravino,
F. Rominger, A. Peters, H.-J. Himmel and U. H. F. Bunz,
Angew. Chem., Int. Ed., 2011, 50, 8588–8591.
´ ˆ
`
P. Leriche, O. Aleveque, P. Frere and J. Roncali, J. Am. Chem. 28 F. Allen, O. Kennard, D. Watson, L. Brammer, A. Orpen and
Soc., 2006, 128, 3459–3466; (g) A. Ajayaghosh, Chem. Soc. R. Taylor, J. Chem. Soc., Perkin Trans. 2, 1987, S1–S19.
Rev., 2003, 32, 181–191; (h) Y. Zhu, R. D. Champion and 29 (a) X.-F. Jiang, L. Polavarapu, S. T. Neo, T. Venkatesan and
S. A. Jenekhe, Macromolecules, 2006, 39, 8712–8719;
(i) E. Zhou, J. Cong, S. Yamakawa, Q. Wei, M. Nakamura,
K. Tajima, C. Yang and K. Hashimoto, Macromolecules,
2010, 43, 2873–2879; ( j) J. Li and Q. Zhang, ACS Appl. Mater.
Interfaces, 2015, DOI: 10.1021/acsami.5b00113.
Q.-H. Xu, J. Phys. Chem. Lett., 2012, 3, 785–790; (b) Y. H. Lee,
Y. Yan, L. Polavarapu and Q.-H. Xu, Appl. Phys. Lett., 2009,
95, 023105; (c) M. Sheik-Bahae, A. A. Said, T. H. Wei,
D. J. Hagan and E. W. Van Stryland, IEEE J. Quantum
Electron., 1990, 26, 760–769.
J. Mater. Chem. C
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