Self-assembled electrooptic thin films with remarkably blue-shifted optical absorption based on an X-shaped chromophore

Article abstract of DOI:10.1021/ja045043k

An "X-shaped" two-dimensional electrooptic (EO) chromophore with extended orthogonal conjugation was designed and synthesized. Self-assembled thin films of this chromophore were fabricated via layer-by-layer chemisorptive siloxane-based self-assembly. The films exhibit a dramatically blue-shifted optical maximum (325 nm) while maintaining a large EO response (χ₃₃⁽²⁾ ≈ 232 pm/V at 1064 nm; r₃₃ ≈ 43 pm/V at 1550 nm). Copyright

Full text of DOI:10.1021/ja045043k

Published on Web 11/18/2004
Self-Assembled Electrooptic Thin Films with Remarkably Blue-Shifted Optical
Absorption Based on an X-Shaped Chromophore
Hu Kang, Peiwang Zhu, Yu Yang, Antonio Facchetti, and Tobin J. Marks*
Department of Chemistry and the Materials Research Center, Northwestern UniVersity,
EVanston, Illinois 60208-3113
Received August 17, 2004; E-mail: t-marks@northwestern.edu
Scheme 1. Synthetic Route to X Chromophore
Molecule-based electrooptic (EO) materials are of great current
interest for optoelectronic and photonic technologies such as high-
speed optical communications, integrated optics, and optical data
processing and storage.1 Crucial synthetic challenges for large bulk
EO response are that individual chromophore components have
large molecular hyperpolarizabilities (â) and that they be arranged
in a noncentrosymmetric architecture. Large-â EO chromophores
are typically devised according to similar design principles: one-
dimensional (1D) planar conjugated π systems end-capped with
donor and acceptor (D, A) moieties. Chromophores of this type
usually exhibit a single, intense, low-lying longitudinal charge-
transfer (CT) excitation, the energy and transition dipole of which
determine the principal â tensor within a two-level model.2
Considerable, elegant efforts have striven to increase â by optimiz-
ing D/A strengths and/or the conjugation pathways of such one-
dimensional (1D) chromophores.3 However, intrinsic to this ap-
proach is the challenge of optimizing the nonlinearity-transparency
tradeoff in which increases in â for such chromophores are almost
invariably accompanied by bathochromic shifts of the optical
maximum. Furthermore, molecules with extended π systems and
low-lying excited states are frequently subject to chemical and
thermal instability.4 Alternative design strategies at the molecular
level are therefore desirable. To this end, nontraditional NLO
chromophores with multiple donor-acceptor substitution have
attracted recent attention, ranging from dipolar “X-shaped”5 and
“Λ-shaped”6 to octopolar molecules.7 Multiple D/A substitution
affords two-dimensional (2D) â tensor character in which off-
diagonal components become significant, offering among other
attractions, possible relaxation of nonlinearity-transparency tradeoffs
and improved phase-matching via the larger off-diagonal compo-
nents.8 However, few experimental studies of dipolar 2D EO
chromophores have been reported to date, with most molecules
exhibiting modest hyperpolarizabilities due to short conjugation
lengths, and only a few incorporated in LB or poled polymer thin
films.5b,c,8
We report here the synthesis and incorporation in intrinsically
acentric EO-active thin films of a novel type of “X-shaped” 2D
chromophore containing a central aromatic core fused to extended
conjugated D/A tetrasubstitution. The optical transition dipoles
couple in such a manner as to drastically blue-shift λmax while
maintaining a very large â response.9 This chromophore was
specifically designed for a covalent layer-by-layer siloxane self-
assembly (SA) approach10 to afford polar, robust, and structurally
regular thin films. To our knowledge, this is the first example of
blue-shifted chromophore incorporation in such a SA approach.
Chromophore precursor 1,2-bis-((E)-2-pyridin-4-yl-vinyl)-4,5-bis-
{(E)-2-[p-N,N-bis(2-tert-butyldimethylsiloxyethyl)amino-phenyl]-
ethenyl}benzene (BPBAB) was synthesized via a sequence of
Wittig-Horner and Heck coupling reactions (Scheme 1). The
dicationic methylpyridinium X chromophore (X-CHR) was next
obtained via MeOTf alkylation of BPBAB. All new compounds
were characterized by conventional analytical/spectroscopic tech-
niques.11 Thermal analysis data show that both BPBAB and X-CHR
have high thermal stability (Td ≈ 340 °C for BPBAB and Td ≈
280 °C for X-CHR).
The X-CHR optical spectrum11 in CH2Cl2 solution consists of
an intense band centered at 357 nm (ꢀ ) 12000 L mol-1 cm-1),
involving πfπ* CT and a less intense, broad band at 440-700
nm, responsible for the red color, and ascribed to aggregation as
supported by concentration-dependent optical absorption spectra
(5 × 10-7 to 3 × 10-8 M), which reveal an increase of the 357 nm
CT band and diminution of the long-λ band upon dilution.12 This
strong tendency for aggregation is also evident in concentration-
dependent NMR experiments.11 The blue-shifted X chromophore
HOMO-LUMO CT excitation can be understood in terms of the
excitonic coupling of transition dipoles,9,13 leading to a first excited
state composed of a pair of excitonic states. The upper excitonic
state is optically allowed, and optical transitions from the ground-
state populate the upper excitonic state, resulting in the spectral
blue shift. In the X chromophore, intramolecular coupling of
transition dipoles in two dimensions is sufficiently strong as to
afford a dramatically blue-shifted optical maximum.
Layer-by-layer self-assembled X chromophore-derived thin films
were fabricated on a variety of substrates (glass, quartz, silicon)
via an iterative three-layer process (Scheme 2): (i) self-limited
chemisorption of 4-Cl2ISiC6H4CH2I onto a hydrophilic substrate
surface, (ii) SA of chromophore precursor BPBAB onto the benzyl
halide-functionalized substrate via quaternization, and (iii) Cl3SiOSi-
(Cl2)OSiCl3 capping to planarize/cross-link the polar structure and
regenerate an active surface for subsequent layer deposition.11
Specular X-ray reflectivity data indicate a narrow distribution of
layer thicknesses and a linear dependence of film thickness on the
number of layers, from which an average interlayer spacing of 40.6
Å can be deduced.11 The resulting films exhibit λmax at the
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J. AM. CHEM. SOC. 2004, 126, 15974-15975
10.1021/ja045043k CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
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Figure 1. (A) Optical absorption spectra of X-chromophore SA films grown
on fused quartz. (Inset) Optical absorbance of films at λmax ) 325 nm as a
function of the number of layers. (B) Square-root of film 532 nm SHG
response (I2ω, arbitrary units) as a function of the number of layers. (Inset)
SHG response as a function of fundamental beam incident angle from a
float glass slide having a film on either side.
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Scheme 2. Self-Assembly of the X Chromophore
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remarkably short wavelength of 325 nm (Figure 1A). The further
blue-shifting of the film λmax vs solution is consistent with
intermolecular dipole-dipole coupling in the closely packed
chromophore layers, where strong π-π interactions are possible.13b,14
The linear dependence of the 325 nm absorbance on the number
of layers indicates that essentially equal quantities of equivalently
oriented chromophore molecules are deposited in forming each
layer. Polarized transmission second harmonic generation (SHG)
measurements at λo ) 1064 nm yield angle-dependent interference
patterns for glass substrates coated on both sides (Figure 1B inset)
that demonstrate essentially identical film quality and uniformity
are achieved on both sides of the substrates. The quadratic
dependence of the 532 nm output intensity (I2ω) on film thickness
(Figure 1B) further demonstrates polar microstructure preservation
as layer-by-layer assembly progresses. The films exhibit large
(9) AM1/ZINDO computations predict âω)0 ) 407 × 10-30 esu (âω)0/Mw
)
0.56 × 10-30 esu) with a HOMO-LUMO CT excitation λmax ) 369 nm
vs âω)0 ) 312 × 10-30 esu (âω)0/Mw ) 0.77 × 10-30 esu) with λmax
)
551 nm for the analogous 1D chromophore:
(10) (a) Facchetti, A.; Abbotto, A.; Beverina, L.; van der Boom, M. E.; Dutta,
P.; Evmenenko, G.; Pagani, G. A.; Marks, T. J. Chem. Mater. 2003, 15,
1064. (b) Zhu, P.; van der Boom, M. E.; Kang, H.; Evmenenko, G.; Dutta,
P.; Marks, T. J. Chem. Mater. 2002, 14, 4982. (c) van der Boom, M. E.;
Zhu, P.; Evmenenko, G.; Malinsky, J. E.; Lin, W.; Dutta, P.; Marks, T.
J. Langmuir 2002, 18, 3704. (d) Lin, W.; Lin, W.; Wong, G. K.; Marks,
T. J. J. Am. Chem. Soc. 1996, 118, 8034. (e) Yitzchaik, S.; Marks, T. J.
Acc. Chem. Res. 1996, 29, 197.
(2)
second-order nonresonant macroscopic NLO responses ø33 ≈ 232
pm/V at 1064 nm (obtained by calibration vs quartz), translating
to an estimated EO coefficient r33 ≈ 43 pm/V at 1550 nm.15
In summary, an “X-shaped” 2D EO chromophore with extended
orthogonal conjugation was designed and synthesized. Self-as-
sembled thin films of this chromophore were fabricated via a layer-
by-layer chemisorptive siloxane-based approach. The chromophoric
film exhibits a dramatically blue-shifted optical maximum (325 nm)
(11) See Supporting Information for details.
(12) Dipolar molecule aggregation in solution is most frequently studied by
optical spectrospy: (a) Wu¨rthner, F.; Yao, S.; Debaerdemaeker, T.;
Wortmann, R. J. Am. Chem. Soc. 2002, 124, 9431. (b) Iverson, I. K.;
Casey, S. M.; Seo, W.; Tam-Chang, S. Langmuir 2002, 18, 3510.
(13) For discussions of dipole-dipole excitionic interactions, see: (a) Guna-
tatne, T.; Kennedy, V. O.; Kenney, M. E.; Rodgers, M. A. J. J. Phys.
Chem. A 2004, 108, 2576. (b) Engelking, J.; Wittemann, M.; Rehahn,
M.; Menzel, H. Langmuir 2000, 16, 3407.
(14) Such spectral shifts are well-known for azobenzene and other dyes in
closely packed self-assembled monolayers and LB films: (a) Nieuwkerk,
A. C.; Marcelis, A. T. M.; Sudho¨ter, E. J. R. Langmuir 1997, 13, 3325.
(b) Everaars, M. D.; Marcelis, A. T. M.; Sudho¨ter, E. J. R. Langmuir
1996, 12, 3964. (c) Shimomura, M.; Aiba, S.; Tagima, N.; Inoue, N.;
Okuyama, K. Langmuir 1995, 11, 969.
(2)
while maintaining a large EO response (ø33 ≈ 232 pm/V at 1064
nm; r33 ≈ 43 pm/V at 1550 nm).
Acknowledgment. We thank DARPA/ONR (SP01P7001R-A1/
N00014-00-C) and the NSF MRSEC program (DMR 0076077) for
support of this research. We thank Dr. S. Keinan and Prof. M.
Ratner for computational collaboration, and Dr. G. Evmenenko and
Prof. P. Dutta for X-ray reflectivity measurements.
(2)
(15) Assuming a two-level dispersion model, the relation between r33 and ø33
(neglecting dispersion of the local field factors) is given by (Sigelle, M.;
Hierle, R. J. Appl. Phys. 1981, 52, 4199):
(3ω02 - ω2)(ω02 - ω′2)(ω02 - 4ω′2)
Supporting Information Available: Experimental details regarding
chromophore synthesis, SA film fabrication, and characterizations. This
material is available free of charge via the Internet at http://pubs.acs.org.
2
n4
(2)
r33
)
ø
33
3ω02(ω02 - ω2)2
where ω, ω′ are EO and SHG fundamental frequencies, respectively (i.e.,
1550 and 1064 nm, respectively), ω0 is the first resonance frequency
(corresponding to λmax), and n is the refractive index.
References
(1) For recent reviews of organic EO materials, see: (a) Dalton, L. R.;
Robinson, B. H.; Nielson, R.; Jen, A. K.; Casmier, D.; Rabiei, P.; Steier,
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