Versatile Synthesis of Small NLO-Active Molecules Forming Amorphous Materials with Spontaneous Second-Order NLO Response

Full text of DOI:10.1021/ja038207q

Published on Web 11/26/2003
Versatile Synthesis of Small NLO-Active Molecules Forming Amorphous
Materials with Spontaneous Second-Order NLO Response
Ele´na Ishow,* Cyril Bella¨ıche, Laurent Bouteiller,§ Keitaro Nakatani, and Jacques A. Delaire
ENS Cachan, PPSM (CNRS UMR 8531), 61 aVenue du Pre´sident Wilson, 94235 Cachan Cedex, France
Received August 29, 2003; E-mail: ishow@ppsm.ens-cachan.fr
Scheme 1. General Synthesis of DBAB-Xa
Low-molecular-weight organic glasses have gained an increasing
interest in optoelectronics during the last 5 years.1 Compared to
polymers, they offer well-defined molecular structure, ensuring
reproducible properties as well as reduced time aging. Such
materials have been involved in electroluminescent devices,2
photorefractivity,3 and holographic data storage4 where triarylamine
derivatives play an essential role due to their remarkable electron-
donor capability. On the contrary, very few papers deal with second-
order nonlinear optically (NLO) active materials based on small
organic molecules giving efficient and orientationaly stable non-
centrosymmetric materials for electrooptic modulators and fre-
quency converters applications.5 We report in this contribution a
versatile synthesis of push-pull compounds comprising a bulky
electron-donating triarylamino group linked to varied electron-
withdrawing groups. Moreover, to the best of our knowledge, these
a Conditions: i) triphenylamine, NaDBS, CH2Cl2/H2O, rt, 18 h. ii) NBS,
molecules are the first example of compounds forming amorphous
CH2Cl2 reflux, 4 h. iii) Pd(PPh3)4, PhB(OH)2, 1 mol‚L-1 Na2CO3, toluene,
80 °C. iv) Malononitrile, pyridine.
materials with spontaneous NLO activity without requiring any
external poling process.
Classical synthesis of substituted triarylamines involves phase-
transfer Ullman or Hartwig coupling conditions which require
aromatic halides and electron-rich amino compounds.6,7 This
limitation prohibited us from using aminoazobenzene derivatives
while synthesis of bromo- or iodoazobenzene substituted by an
electron-deficient group encounters low-yield reactions. We adopted
the following three-step synthetic pathway which permitted varying
the nature of the electron-withdrawing group and the introduction
of the bulky substituents at the very final stage; hence, compounds
with various thermal properties can be synthesized (Scheme 1).
The corresponding freshly prepared 4-substituted phenyldiazo-
nium salt8 was reacted with 1 equiv of triphenylamine under phase-
transfer conditions at room temperature over 1 day to afford the
azobenzene derivative 2.9
Bromination of 2 by 2 equiv of N-bromosuccimide in refluxing
CHCl3 for 4 h gave the sole dibromo compound 3. Subsequent
Suzuki cross-coupling between 2 equiv of phenylboronic acid and
3 was carried out in toluene overnight in the presence of a 1 mol‚L-1
Na2CO3 aqueous solution and Pd(Ph3)4 as catalyst, and yielded
DBAB-X 4 in 50-70% yield. Compared to the initial synthesis of
DBAB-H (4-[di(biphenyl-4-yl)amino]azobenzene)4b following an
Ullman phase-transfer coupling at 190 °C, this mild three-step
sequence avoids deleterious high temperatures and can be extended
to less chemically inert electron-accepting groups.
above 200 °C. This thermal behavior as well as the absence of
X-ray diffraction peaks in the melt product signifies amorphous
properties. The ability of compounds 4a-e to form stable glassy
materials compared to their analogous N,N-diphenylamino-
substituted compounds undoubtedly originates from the biphenyl
moieties which prevent close packing of the molecules and
efficiently reduce their tendency to crystallization.
Elaboration of thin films with a high optical quality plainly
benefits from this amorphous property. Vacuum evaporation of
compounds around 200 °C at a pressure of 5 × 10-5 mbar onto a
glass substrate led to transparent films with a rms roughness of
less than 0.7 nm from AFM topographic measurements. Such a
deposition process offers the great advantage to discard any solvent
pollution and subsequent time-consuming drying steps. Moreover,
it suits need of multilayer materials for which the spin-coating
technique requires elaborate insoluble polymers.10
UV-vis absorption spectra of both evaporated films and
compounds in various solvents exhibit two bands (Table 1). The
first one, centered around 335 nm, is insensitive to the nature of
the electron-withdrawing group and the solvent polarity, contrarily
to the second band which appears in the visible range and is strongly
red-shifted when going from the less to the most polar compounds
(4a to 4e). The visible absorption maximum of evaporated films
made from polar DBAB-X compounds undergoes a slight batho-
chromic shift, compared to that of films in DMSO solutions; this
expresses the actual polar influence exerted on each chromophore
by the neighboring molecules.
From comparison with N,N-diphenylamino model compounds,
the visible and UV bands have been reasonably assigned to
azobenzene-centered charge transfer and biphenyl-centered π-π*
transitions, respectively.11 In addition, semiempirical AM1 com-
putations show an extended electron density from the biphenyl ends
to the azo core for the HOMO, and from the azo core to the
Thermal analyses were carried out at a 20 °C‚min-1 scan rate
under a nitrogen flow and involved two heating cycles from 25 to
300 °C (Table 1). For all of the compounds 4a-e, the first
temperature sweep showed a sharp endothermic peak featuring
product melting (Tm) around 200 °C without any material loss. After
cooling to room temperature and heating again, a slope change was
clearly detected around 80 °C, and no melting peak was observed
§ Current address: Universite´ Pierre et Marie Curie, 4 place Jussieu, 75252 Paris
Cedex 05, France.
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J. AM. CHEM. SOC. 2003, 125, 15744-15745
10.1021/ja038207q CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Thermal, UV-Visible Absorption, and Second-Order NLO Properties of DBAB-X Compounds
λ
max (nm)
λ
max (nm)
λ
max (nm)
µâ1907
(10-48 esu)b,c
µâ0
(10-48 esu)c
a
compound
in toluene
in DMSO
in film
T (°C)
g
Tm (°C)
d
33 (pm/V)d
4a
4b
4c
4d
4e
435, 335
475, 328
475, 329
493, 331
524, 341
438, 330
481, 326
482, 329
501, 329
544, 345
335, 435
483, 328
485, 330
509, 333
541, 345
74
85
91
76
96
225
239
197
227
216
170
484
430
828
1073
126
337
302
555
667
<0.1
1.3
2.1
3
17
a Absorbance of 500 nm-thick evaporated films. b CHCl3 solution. c Accuracy (10%. d 1.5 µm-thick evaporated films.
electron-attracting group for the LUMO. Consequently, the resulting
charge-transfer transition presents a bathochromic shift compared
to that of the phenyl-substituted analogues. Finally, some occupied
and vacant MOs close in energy to the HOMO and LUMO are
located on biphenyl groups, and should be responsible for the UV
transition.
The second-order NLO properties of DBAB-X compounds were
investigated in CHCl3 solutions and in solid state (powder and
evaporated thin films) by second harmonic generation (SHG) at
1907 nm where resonance and second-harmonic reabsorption effects
are ruled out. From electric field-induced second harmonic (EFISH)
measurements and using the two-level model12 for extrapolated first-
order hyperpolarizability to zero-frequency â0, the product µâ0 (µ
describing the dipole moment) was obtained. This value increases
with the electron-accepting strength of X, being the highest one
for the strongest electron-withdrawing dicyanovinylene moiety
(Table 1).13 Contrarily to previous studies on polymers, introduction
of the dicyanovinylene group substantially raises the NLO activity
and the glass transition temperature without generating lower
thermal stability.11
withdrawing groups has been developed which allows an easy
tuning of their thermal (Tg above 80 °C) and NLO properties. Upon
vacuum deposition, they form auto-organized amorphous glasses
with high optical quality and spontaneous second-order NLO
activity stable over one year. This unexpected result offers new
developments for orientationaly stable noncentrosymmetric organic
materials based on the clean and reproducible vacuum deposition
process.
Acknowledgment. P. Yu, A. Le´austic, and R. Cle´ment from
LCI (Orsay) and Veeco expertise laboratory (Dourdan-France) are
deeply acknowledged for powder X-ray diffraction (Siemens
diffractometer) and AFM measurements (model Microscope Ex-
plorer), respectively. We also thank J. Chauvin and J. Poly for their
help in running synthesis and optical experiments.
Supporting Information Available: Experimental procedures for
4b-e, characterizations, and SHG equipment (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
References
Whereas the crystallized powder exhibited no spontaneous SHG
activity when probing at 1907 nm, vacuum-deposited films formed
from polar compounds 4b-e unexpectedly gave a significant
spontaneous signal whose intensity remains stable up to one year.
The NLO coefficient d33 ranged from 1.3 to 17 pm/V, scaling with
the strength of the electron-withdrawing group (Table 1). Formation
of acentric crystallites has been ruled out as no X-ray diffraction
peak was detected and no roughness feature was evidenced by
surface AFM imaging. Bulk homogeneity was checked through the
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In summary, a novel mild and modular synthetic approach to
bulky triarylamine compounds substituted by strong electron-
JA038207Q
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