Improved nonlinearity-transparency-thermal stability trade-off with spirobifluorene-bridged donor-π-acceptor chromophores

Article abstract of DOI:10.1021/ol302443z

A series of novel 9,9′-spirobifluorene-bridged donor-π-acceptor chromophores containing triarylamine moieties were easily synthesized starting from the readily available reagent 4,4′-dimethylbiphenyl. These chromophores were found to combine excellent transparency in the visible region, high thermal stabilities, and large optical nonlinearity.

Full text of DOI:10.1021/ol302443z

ORGANIC
LETTERS
2012
Vol. 14, No. 20
5282–5285
Improved NonlinearityꢀTransparencyꢀ
Thermal Stability Trade-Off with
Spirobifluorene-Bridged
Donor-π-Acceptor Chromophores
Haibo Xiao,* Hongyao Yin, and Xiaoying Zhang
Department of Chemistry, Shanghai Normal University, Shanghai 200234, P.R. China
xiaohb@shnu.edu.cn
Received September 3, 2012
ABSTRACT
A series of novel 9,90-spirobifluorene-bridged donor-π-acceptor chromophores containing triarylamine moieties were easily synthesized starting
from the readily available reagent 4,40-dimethylbiphenyl. These chromophores were found to combine excellent transparency in the visible region,
high thermal stabilities, and large optical nonlinearity.
In recent years the design and synthesis of organic
compounds that exhibit nonlinear optical (NLO) properties
(first and second hyperpolarizability β and γ, respectively)
have received particular attention because of their possible
applications in various fields, including telecommunica-
tions, optical data storage, optical power limiting, etc. This
interest is motivated not only by the large NLO response but
also by the versatility, ease of processing, and possibility of
tailoring the physicochemical properties by the molecular
engineering approach.1 To date, the vast majority of per-
forming NLO chromophores obey a few general structural
requirements: they are planar conjugated structures end-
capped with strong electron-donor (D) and electron-acceptor
(A) residues leading to effective polarization along the
π-conjugated axis.2 Although, increasing π-conjugation gen-
erally translates into higher molecular hyperpolarizability,
the increase in molecular hyperpolarizability of such
chromophores is usually accompanied by a decrease in the
optical transparency and thermal stability, which limits their
practical applications in photonics.3,4 As a consequence, a
variety of dual (mutiple) charge-transfer chromophores, such
as octupolar, Λ-shaped, and X-shaped molecules have been
designed and investigated in recent years.5 However, achiev-
ing the nonlinearityꢀtransparencyꢀthermal stability trade-
off is still a major challenge.2,5
Recently, three-dimensional materials based on spirobi-
fluorene-bridged conjugated systems became of particular
interest.6 Generally, spiro compounds are known as glass-
forming materials with a high thermal stability.7 The intro-
duction of electron donor and electron acceptor groups in
(3) Mashraqui, S. H.; Ghadigaonkar, S.; Ashraf, M.; Ranjini, A. S.;
Ghoshb, S.; Dasb, P. K. Tetrahedron. 2007, 63, 10011.
(4) Qian, Y.; Lin, B. P.; Xiao, G. M.; Li, H. H.; Yuan, C. W. Opt.
Mater. 2004, 27, 125.
(5) Zhang, C.-Z.; Lu, C. G.; Zhu, J.; Lu, G.-Y.; Wang, X.; Shi, Z.-W.;
Liu, F.; Cui, Y. P. Chem. Mater. 2006, 18, 6091.
(6) Chien, Y.-Y.; Wong, K.-T.; Chou, P.-T.; Cheng, Y.-M. Chem.
Commun. 2002, 2874.
(7) Saragi, T. P. I.; Pudzich, R.; Fuhrmann, T.; Salbeck, J. Appl.
Phys. Lett. 2004, 84, 2334.
(1) He, M.; Zhou, Y. M.; Liu, R.; Dai, J.; Cui, Y. P.; Zhang, T. Dyes
Pigm. 2009, 80, 6.
(2) Beverina, L.; Sanguineti, A.; Battagliarin, G.; Ruffo, R.; Roberto,
D.; Righetto, S.; Soave, R.; Presti, L. L.; Ugob, R.; Pagani, G. A. Chem.
Commun. 2011, 47, 292.
r
10.1021/ol302443z
Published on Web 10/04/2012
2012 American Chemical Society

two different biphenyl branches of the spirobifluorene core
affords a class of spiro compounds with a “top-down”
asymmetric 2, 7-A, 20, 70-D substitution pattern,8 which
may be good candidates for the construction of highly
transparent NLO materials due to the spiroconjugation
effects between the two fluorene units.9,10 Most spiro-linked
compounds are synthesized from the central spirobifluorene
with equal or different substituents in the 2,20 and 7,70
positions,8,11 but it is difficult to obtain “top-down” asym-
metric compounds directly from the spirobifluorene core.
Some groups reported the synthesis of “top-down” asym-
metric spiro compounds by introducing halogeno groups in
the precursor 9, 90-fluorenone before spiro linkage.8
In this communication, we synthesized three new “top-
down” asymmetric spirobifluorene compounds, SPF-1,
SPF-2, and SPF-3 starting from 4,40-dimethylbiphenyl.
The advantages of this synthetic method are that it
avoids the use of expensive reagent 2-bromobiphenyl as
thestarting materialandthemethylgroup isconvenientfor
subsequent transformations to appropriate functional
groups such as bromomethyl, aldehyde, carboxylic acid,
etc. The synthetic pathways of 9,90-spirobifluorene-bridged
D-π-A chromophores SPF-1, SPF-2, and SPF-3 are shown
in Scheme 1. In these D-π-A chromophores, the cyano,
dicyanomethylene, and nitrophenyl vinyl groups serve as
the electron acceptor, respectively, while the triarylamine
moiety acts as the donor counterpart. The role being played
by spirobifluorene is as a π-conjugated bridge because of the
presence of the spiroconjugation effect between the electron
acceptor and the donor. For the purpose of comparison, four
model compounds SPF-1a, SPF-2a, SPF-3a, and SPF-1b
(Scheme 1) have been chosen as the reference compounds in
this work. SPF-1, SPF-2, and SPF-3 are distinguished by
good transparency, high thermal stability, and large optical
nonlinearity for nonlinear optical application. We emphasize
that although a few “top-down” asymmetric spirobifluorene
compounds have been synthesized by several groups6,12ꢀ14
and many asymmetric spirobifluorene compounds have been
employed as NLO materials,15 the potentialities of “top-
down” asymmetric spirobifluorene compounds for NLO
have not been considered yet.15
Scheme 1. Reagents and Conditions
The UVꢀvis absorption spectra of the chromophores
SPF-1, SPF-2, and SPF-3 and model compounds SPF-1a,
SPF-2a, SPF-3a, and SPF-1b in toluene are shown in
Figure 1. It can be seen that the maximum absorption
wavelengths of SPF-1, SPF-2, and SPF-3 are less than
Figure 1. UVꢀvis spectra of chromophores in toluene.
(12) Ku, S.-Y.; Hung, W.-Y.; Chen, C.-W.; Yang, S.-W.; Mondal, E.;
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A. P. J. Phys. Chem. B 2008, 112, 16300.
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S.; Quici, S. Eur. J. Org. Chem. 2010, 4004.
Org. Lett., Vol. 14, No. 20, 2012
5283

400 nm and the cutoff wavelengths were found to be as low
as 460 nm. Accordingly, these molecules maintain almost
full transparency in the visible region. We observed two
distinct absorption bands in the absorption spectra of
SPF-1, SPF-2, and SPF-3: one in the range 280ꢀ320 nm,
corresponding to the πꢀπ* electronic transition of the
spirobifluorene core, and the other in the range 350ꢀ460 nm,
corresponding to the intramolecular charge transfer (ICT)
band, as confirmed by the solvatochromism (Table 1, Sup-
porting Information (SI)).15,16 The ICT bands red-shifted in
the order SPF-1 < SPF-2 ≈ SPF-3. The absorption maxima
for the dicyanovinyl substituted SPF-2 and that of the
4-nitrostyryl analog SPF-3 appear at nearly similar
noted as evidence for the interaction between the upper and
lower parts of the molecule in the ground state.13,19ꢀ23
The thermal stabilities of SPF-1, SPF-2, and SPF-3 are
investigated by thermal gravity analysis (TGA) and differ-
ential scanning calorimetry (DSC). The decomposition
temperature (Td, temperature at which 5% mass loss
occurs during heating), as shown in Figure 2, is 268 °C for
SPF-1, 264 °C for SPF-2, and 235 °C for SPF-3, respec-
tively. Their Td’s are very close to those of commercially
available NLO dyes, such as DR1 (Td = 254 °C), DO3
(Td = 235 °C), and DR19 (Td = 260 °C),24 and consistent
with the appropriate temperature for materials working
and processing.25 The decomposition temperature of chro-
mophore SPF-3 was decreased due to its greater degree of
π-electron conjugation. With extension of the π-conjuga-
tion system comes an increased thermal instability of the
compounds.26 No melting was noticed for chromophores
SPF-1, SPF-2, and SPF-3 in the DSC curves (see SI), and
an obvious exothermic peak due to crystallization of SPF-1
at 240 °C is observed.
λ
max (around 384 nm), thereby suggesting their comparable
electron-withdrawing properties.3 In general, the absorption
spectra of the target compound exhibit a composite char-
acteristic of the acceptor and donor spirobifluorene model
compounds, with negligible interactions among the substi-
tuents in the ground state.6,17 In contrast, the absorption
The TPA cross sections of chromophores SPF-1, SPF-2,
and SPF-3 have been measured by open aperture Z-scan
experiments performed with a femtosecond (fs) laser
Table 1. Linear and Nonlinear Properties of the Chromophores
and Their Thermal Stabilities
ΔVc
βCTμg ꢁ 10ꢀ30
σe
Td
a
b
f
λmax
(nm) (nm) (cmꢀ1
λmax
compds
)
(esu D)d
(GM) (°C)
SPF-1a 338
SPF-1b 370
341
372
376
384
384
503h
243
275
626
626
554
1083
SPF-1
SPT-2
SPF-3
DR1
368
376
378
477g
19138
12429
15134
4340
62949 268
2928
3773
264
235
235i
a In acetonitrile, the absorption data were taken from the first
vibronic peak. b In toluene, the absorption data were taken from
the first vibronic peak. c Solvatochromic shift = 1/λmax(acetonitrile) ꢀ
1/λmax(toluene). d The values were measured by the solvatochromic
method. e The two-photon absorption cross sections. f Determined by
TGA. g Measured in 1,4-dioxane; the λmax value of DR1 is from ref 27.
h Measured in DMF; the λmax value of DR1 is from ref 27. i The Td value
of DR1 is from ref 24.
Figure 2. TGA curves of SPF-1, SPF-2, and SPF-3.
profiles of the SPF compounds (SPF-1, SPF-2, and SPF-3)
are different from those of the model compounds (SPF-1a,
SPF-2a, SPF-3a, and SPF-1b). Careful spectral analyses
(from Figure 1 and Table 1) indicate that the lowest energy
absorption of SPF-1 displays a bathochromic shift and
exhibits a more pronounced solvatochromism as compared
to those of monochromophore model compounds SPF-1a
and SPF-1b. From Figure 1, it can be seen that the lowest
energy absorptions of SPF-2 and SPF-3 in toluene appear at
shorter wavelengths than those of SPF-2a (at 418 nm) and
SPF-3a (at ∼410 nm), respectively. Such a hypsochromic
shift of the lowest energy absorption can be observed in other
spiro-bridged conjugated systems.18,19 These results can be
source. Figure 3 shows the Z-scan data of chromophores
SPF-1, SPF-2, and SPF-3 in THF at the nonoptimized
800 nm wavelength position. The σ-values of SPF-1, SPF-2,
and SPF-3 were 62949 GM, 2928 GM, and 3773 GM
respectively. With respect to the π-attached acceptor
(20) Hohlneicher, G.; Bremm, D.; Wytko, J.; Bley-Escrich, J.;
Gisselbrecht, J.-P.; Gross, M.; Michels, M.; Lex, J.; Vogel, E. Chem.;
Eur. J. 2003, 9, 5636.
(21) King, S. M.; Hintschich, S. I.; Dai, D.; Rothe, C.; Monkman,
A. P. J. Phys. Chem. C 2007, 111, 18759.
(22) Chiang, C.-L.; Tseng, S.-M.; Chen, C.-T.; Hsu, C.-P.; Shu, C.-F.
Adv. Funct. Mater. 2008, 18, 248.
(23) Hung, W.-Y; Tsai, T.-C.; Ku, S.-Y.; Chi, L.-C.; Wong, K.-T.
(16) Heredia, D.; Natera, J.; Gervaldo, M.; Otero, L.; Fungo, F.; Lin,
C.-Y.; Wong, K.-Y. Org. Lett. 2010, 12, 12.
(17) Fungo, F.; Wong, K.-T.; Ku, S.-Y.; Hung, Y.-Y.; Baed, A. J.
Phys. Chem. Chem. Phys. 2008, 10, 5822.
(24) Kuo, W.-J.; Hsiue, G.-H.; Jeng, R.-J. Macromolecules 2001, 34,
2373.
J. Phys. Chem. B 2005, 109, 3984.
(25) Moylan, C. R.; Miller, R. D.; Twieg, R. J.; Betterton, K. M.; Lee,
V. Y.; Matray, T. J.; Nguyen, C. Chem. Mater. 1993, 10, 1499.
(26) Cho, M. J.; Choi, D. H.; Sullivanb, P. A.; Akelaitis, A. J. P.;
Dalton, L. R. Prog. Polym. Sci. 2008, 33, 1013.
€
(18) Durr, H.; Ruge, B.; Schmitz, H. Angew. Chem., lnt. Ed. Engl.
1973, 12, 577.
(19) Kao, J.; Radom, L. J. Am. Chem. Soc. 1978, 100, 760.
5284
Org. Lett., Vol. 14, No. 20, 2012

We report here the measurement and calculation of
the values of βCTμg using the solvatochromic method
(see SI),27ꢀ29 although the values obtained are not as
accurate as the values measured with the EFISHG or
HRS method. The experimental values of SPF-1, SPF-2,
SPF-3, and DR1 are listedin Table 1, where DR1 data have
been enclosed as reference. It can be found that the values
of SPF-1, SPF-2, and SPF-3 were 19138 esu D, 12429 esu
D, and 15134 esu D, respectively, larger than that of DR1.
The evolutionofthe second-order NLO properties of SPF-
1, SPF-2, and SPF-3 with the changes in the acceptor
follows a similar tendency to that of two-photon absorp-
tion properties.
In summary, we have easily synthesized a series of novel
9,90-spirobifluorene-bridged D-π-A chromophores contain-
ing triarylamine moieties starting from the readily available
reagent 4,40-dimethylbiphenyl and demonstrated that the
introduction of electron donor and acceptor groups in two
different fluorene units of the spiro-bridged conjugated
systems provides a promising and innovative route for the
optimization of the nonlinearityꢀtransparencyꢀthermal
stability trade-off.
Figure 3. Z-scan experimental data of chromophores SPF-1
(10ꢀ3 M), SPF-2 (10ꢀ2 M), and SPF-3 (10ꢀ2 M) in THF.
chromophores, the σ-values of SPF-1, SPF-2, and SPF-3
followed the order dicyanovinyl < 4-nitrostyryl < ꢀCN. A
higher σ-value is recorded for the case of SPF-1 although its
ICT band is lower compared to those of SPF-2 and SPF-3
(Figure 1). This observation implies that factor(s) other
than molecular polarizability contribute in part to the non-
linear optic effect.3
It is known that the second-order polarizabilities (βxxx),
the composite magnitude (βCTμg) of molecular hypersus-
ceptibilities, and the second harmonic generation (SHG)
values of microcrystals are usually used to characterize the
second-order NLO performance of organic compounds.
The most common method used to measure organic
compounds is the EFISHG or HRS method, but this
experimentation is not easy to set up in most laboratories.
Acknowledgment. This work was financially supported
by the Innovation Program of Shanghai Municipal Edu-
cation Commission (No. 13ZZ105), the ResearchProgram
of Shanghai Normal University (SK201239) and the Nat-
ural Science Foundation of Shanghai City of China (No.
11ZR1426200).
Supporting Information Available. Experimental pro-
cedures and characterization for 4ꢀ7, SPF-1, SPF-2, and
SPF-3. Copies of 1H and 13C NMR spectra. Determina-
tion of the second-order molecular nonlinearity and thermal
stabilities. This material is available free of charge via the
Internet at http://pubs.acs.org.
(27) Chen, L. J.; Cui, Y. J.; Mei, X. N.; Qian, G. D.; Wang, M. Q.
Dyes Pigm. 2007, 72, 293.
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(29) Song, H.-C.; Chen, Y.-W.; Zheng, X.-L.; Ying, B.-N. Spectro-
chim. Acta, Part A 2001, 57, 1717.
The authors declare no competing financial interest.
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