Novel periphery-functionalized solvatochromic nitrostilbenes as precursors for class II hybrid materials

Article abstract of DOI:10.1021/cm903048u

Novel donor-acceptor-substituted stilbenes were synthesized by means of nucleophilic aromatic substitution starting from 1-fluoro-2-nitro-4-[(E)-2-(4- nitrophenyl)vinyl]benzene and n-propylamine, L-proline, and 2-aminopropane-1,3- diol as model compounds

Full text of DOI:10.1021/cm903048u

2720 Chem. Mater. 2010, 22, 2720–2729
DOI:10.1021/cm903048u
Novel Periphery-Functionalized Solvatochromic Nitrostilbenes As
Precursors for Class II Hybrid Materials
Katja Schreiter,† Katja Hofmann,† Andreas Seifert,† Alexander Oehlke,†
†
Katharina Ladewig, Tobias Ruffer, Heinrich Lang, and Stefan Spange*
‡
‡
,†
€
†Department of Polymer Chemistry and ‡Department of Inorganic Chemistry, Institute of Chemistry,
Chemnitz University of Technology, Strasse der Nationen 62, 09111 Chemnitz, Germany
Received October 1, 2009. Revised Manuscript Received March 25, 2010
Novel donor-acceptor-substituted stilbenes were synthesized by means of nucleophilic aromatic
substitution starting from 1-fluoro-2-nitro-4-[(E)-2-(4-nitrophenyl)vinyl]benzene and n-propyl-
amine, L-proline, and 2-aminopropane-1,3-diol as model compounds for the study of molecular
interactions in the solid-state and liquid environments of different polarity. The solvatochromic
behavior of the chromophoric building blocks has been studied and interpreted using the empirical
Kamlet-Taft solvent parameters π* (dipolarity/polarizability), a (hydrogen bond donating
capacity), and β (hydrogen bond accepting ability) in terms of the well-established linear solvation
energy relationship (LSER): νmax = νmax,0 þ sπ* þ aa þ bβ. The most dominant effect on the
solvatochromic behavior of nitrostilbenes is caused by interactions with solvents of different polarity/
polarizability. To evaluate the environmental effects the nitrostilbenes are subjected to, we also
investigated the UV/vis spectroscopic behavior as pure crystal powders and as an organic component
in class II xerogels. Starting from these chromophores we were able to use three different synthetic
routes in order to bind the chromophore covalently to the trialkoxysilane via an amine, amide or
urethane function, respectively. The precursors thus obtained were converted into the corresponding
xerogels in situ in a one-pot procedure. Two routes can be followed-one involving incorporation of
the chromophore in the main chain, and the other involving incorporation in the side chain. All
obtained organic-inorganic hybrid materials were characterized in detail using solid-state NMR,
FT-IR, and UV/vis spectroscopy.
1. Introduction
possible on a molecular level.9-16 Consequently, there is a
significant potential for application in organometallic cata-
lysis,17 sensors,18 and for photo-optic19,20 and nonlinear
optical (NLO) materials.21-24 Most recently chiral sol-gel
materials17,25,26 have been developed, for example, by the
entrapment of chiral molecules in a silica matrix. This
concept has been applied successfully to a large number of
Sol-gel processes have been investigated extensively1 and
are well established in materials science2-4 and catalysis.5-8
Completely new perspectives for the synthesis and structural
design of complex hybrid materials have been opened up and
the combination of specific properties of organic functional
groups with those of a silicate network has been made
(16) Corriu, R.; Leclercq, D. Angew. Chem., Int. Ed. 1996, 35, 1420–
1436.
(17) Defreese, J. L.; Katz, A. Chem. Mater. 2005, 17, 6503–6506.
´
(18) Garcıa-Acosta, B.; Comes, M.; Bricks, J. L.; Kudinova, M. A.;
*Corresponding author. E-mail: stefan.spange@chemie.tu-chemnitz.de.
(1) Hench, L. L.; West, J. K. Chem. Rev. 1990, 90, 33–72.
(2) Philipp, G.; Schmidt, H. J. Non-Cryst. Solids 1984, 63, 283–92.
(3) Boury, B.; Corriu, R. J. P. Adv. Mater. 2000, 12, 989–992.
(4) Novak, B. M. Adv. Mater. 1993, 5, 422–433.
(5) Lu, Z.-I.; Lindner, E.; Mayer, H. A. Chem. Rev. 2002, 102, 3543–
3578.
(6) Kingsbury, J. S.; Garber, S. B.; Giftos, J. M.; Gray, B. L.;
Okamoto, M. M.; Farrer, R. A.; Fourkas, J. T.; Hoveyda, A. H.
Angew. Chem. 2001, 113, 4381–4386.
Kurdyukov, V. V.; Tolmachev, A. I.; Descalzo, A. B.; Marcos,
ꢀ~
ꢀ
M. D.; Martınez-Manez, R.; Moreno, A.; Sancenon, F.; Soto, J.;
´
ꢀ
Villaescusa, L. A.; Rurack, K.; Barat, J. M.; Escriche, I.; Amoros,
P. Chem. Commun. 2006, 2239–2241.
(19) Levy, D.; Avnir, D. J. Phys. Chem. 1988, 92, 4734–4738.
(20) Li, H.; Lin, N.; Wang, Y.; Feng, Y.; Gan, Q.; Zhang, H.; Dong, Q.;
Chen, Y. Eur. J. Inorg. Chem. 2009, 519–523.
(21) Chaumel, F.; Jiang, H.; Kakkar, A. Chem. Mater. 2001, 13, 3389–
3395.
(22) Oviatt, H. W., Jr.; Shea, K. J. Chem. Mater. 1995, 7, 493–498.
(23) Yang, Z.; Xu, C.; Wu, B.; Dalton, L. R. Chem. Mater. 1994, 6,
1899–1901.
(24) Nosaka, Y.; Tohriiwa, N.; Kobayashi, T.; Fujii, N. Chem. Mater.
1993, 5, 930–932.
(7) Elias, X.; Pleixats, R.; Man, M. W. C.; Moreau, J. J. E. Adv. Synth.
Catal. 2007, 349, 1701–1713.
(8) Adima, A.; Moreau, J. J. E.; Man, M. W. C. Chirality 2000, 12,
411–420.
(9) Judeinstein, P.; Sanchez, C. J. Mater. Chem. 1996, 6, 511–525.
ꢀ
(10) Corriu, R. J. P.; Mehdi, A.; Reye, C. J. Mater. Chem. 2005, 15,
4285–4294.
(11) Corriu, R. J. P. Angew. Chem., Int. Ed. 2000, 39, 1376–1398.
(12) Corriu, R. Polyhedron 1998, 17, 925–934.
(13) Moreau, J. J. E.; Pichon, B. P.; Man, M. W. C.; Bied, C.; Pritzkow,
(25) Marx, S.; Avnir, D. Acc. Chem. Res. 2007, 40, 768–776.
(26) Adima, A.; Moreau, J. J. E.; Man, M. W. C. Chirality 2000, 12,
411–420.
ꢀ
H.; Bantignies, J.-L.; Dieudonne; Sauvajol, J.-L. Angew. Chem.,
(27) Arrachart, G.; Carcel, C.; Moreau, J. J. E.; Hartmeyer, G.; Alonso,
B.; Massiot, D.; Creff, G.; Bantignies, J.-L.; Dieudonne, P.; Man,
M. W. C.; Althoff, G.; Babonneau, F.; Bonhomme, C. J. Mater.
Chem. 2008, 18, 392–399.
Int. Ed. 2004, 43, 203–206.
(14) Wen, J.; Wilkes, G. L. Chem. Mater. 1996, 8, 1667–1681.
€
€
(15) Spange, S.; Seifert, A.; Muller, H.; Hesse, S.; Jager, C. Angew.
Chem., Int. Ed. 2002, 41, 1729–1732.
r
2010 American Chemical Society
pubs.acs.org/cm
Published on Web 04/06/2010

Article
Chem. Mater., Vol. 22, No. 9, 2010 2721
organic compounds and biomolecules, e.g., amino acids,
sugars, enzymes, antibodies, surfactants, and chiral poly-
mers.25,27 Organic functional moieties can be incorporated
into the silica network in various ways. On the one hand,
chromophoric molecules can be held in an inorganic network
by weak interactions like van der Waals forces or hydrogen
bonds (class I, host-guest concept).9,28,29 On the other hand,
it is possible to bind the chromophore to the matrix directly
by one or more covalent bonds (class II).9,28,29
Scheme 1. Combination of Nucleophilic Aromatic Substitution
and Sol-Gel Process in a One-Pot (X = F, OCH3) and Two-Step
Synthesis (n = 1, 2) Opens the Possibility of Producing
Chromophoric Class II Hybrid Materials
Functional stilbenes have gained wide interest and
found a vast range of useful applications in the areas of
optical switching, image storage devices, and molecular
function regulation and are thus interesting chromo-
phores as organic component in organic-inorganic
hybrid materials.30,31 The sol-gel technique represents
an attractive means of synthesizing materials which en-
sure the excellent chemical stability of their chromophoric
systems. Through the incorporation of chromophores
into a silica matrix, these materials are predestined for
pigment applications.32 With regard to this and other
potential applications, it is important to know whether
and how the nitrostilbene unit interacts with its surround-
ings (e.g., self-interaction, interaction with environment).
The probing of dye aggregates and supramolecular struc-
tures by means of UV/vis spectroscopy is an experimental
challenge and of importance for both academic research
and practical application in nanoscience technology. For
this objective, the exact knowledge of the solid-state
structures is necessary, and the respective UV/vis spectra
have to be significantly different as a function of structure
variation. This issue requires the choice of a suitable
model systems that show color changes as a function of
the nature of accumulation processes. Nitrostilbenes shows
sufficient solvatochromic and crystallochromic effects de-
pending on structure variation and environmental influ-
ences. In this context, we here report novel amino acid- and
diol-functionalized nitrostilbenes 2 and 3, which can be
synthesized from fluorinated nitrostilbene 1.33,34 A specific
object is the study of solvent influence on the UV/vis
absorption spectra of these aminonitrostilbenes.
effects.35-40 Multiple intermolecular solute/solvent
interactions can be described using linear free energy
relationships (LSER).35,36 To separate the effects of non-
specific chemical interactions including electrostatic effe-
cts (dipolarity/polarizability) from specific interactions
(hydrogen bonding), we used the simplified Kamlet-Taft
equation (eq 1)41-45
ꢀ
~
~
νmax ¼ νmax,0 þ aa þ bβ þ sπ
ð1Þ
Using eq 1, the effect of hydrogen bonding donor capacity
(HBD) a,41 the hydrogen bonding acceptor capacity (HBA)
β,42 and the dipolarity/polarizability π*43,44 of a solvent can
be expressed. ν~max,0 corresponds to a standard process,
referenced to a nonpolar medium. a, b, and s represent
solvent-independent correlation coefficients, which reflect
the effect of each parameter.
A complementary point of investigations is the study of
environmental effects in the solid state. Therefore, functio-
nalization with fluorine (1), amino acid (2), and diol substi-
tuents (3) allows reaction with trialkoxysilanes to give the
chromophoric trialkoxysilanes 1-PR-3-PR (precursor),
which can be converted into organically modified class II
silica gel (1-XG-3-XG) by addition of tetraethoxysilane in a
sol-gel process (Scheme 1). The properties of the organic-
inorganic hybrid materials can be discussed in context of the
results obtained from studies of solvatochromism.
Interactions of solvatochromic dyes with pure solvents
or solvent mixtures arise from a combination of many
In the present paper, the properties of these amino
nitrostilbenes were investigated in solution (solvatochro-
mism in terms of Kamlet and Taft), in the solid state (X-ray
diffraction), and after embedding into a silica matrix
(13C-CP-MAS NMR, 29Si-CP-MAS NMR, thermogravi-
metric measurements, FT-IR, UV/vis spectroscopy).
(28) Chaumel, F.; Jiang, H.; Kakkar, A. Chem. Mater. 2001, 13, 3389–
3395.
(29) Pandey, S.; Baker, G. A.; Kane, M. A.; Bonzagni, N. J.; Bright,
F. V. Chem. Mater. 2000, 12, 3547–3551.
(30) Wolf, M. O.; Fox, M. A. Langmuir 1996, 12, 955–962.
(31) Marten, J.; Erbe, A.; Critchley, K.; Bramble, J. P.; Weber, E.;
Evans, S. D. Langmuir 2008, 24, 2479–2486.
€
(32) Seifert, A.; Ladewig, K.; Schonherr, P.; Hofmann, K.; Lungwitz,
2. Experimental Section
R.; Roth, I; Pohlers, A.; Hoyer, W.; Baumann, G.; Schulze, S.;
Hietschold, M.; Moszner, N.; Burtscher, P.; Spange, S. J. Sol-Gel
Sci. Technol. 2010, 53, 328–341.
General Remarks. Solvents from Merck, Fluka, Lancaster,
and Aldrich were redistilled over appropriate drying agents
prior to use. All commercial reagents were used without further
ꢀ
(33) Jourdant, A.; Gonzalez-Zamora, E.; Zhu, J. J. Org. Chem. 2002,
67, 3163–3164.
(34) Roth, I.; Simon, F.; Bellmann, C.; Seifert, A.; Spange, S. Chem.
Mater. 2006, 18, 4730–4739.
(41) Taft, R. W.; Kamlet, M. J. J. Am. Chem. Soc. 1976, 98, 2886–2894.
(42) Kamlet, M. J.; Taft, R. W J. Am. Chem. Soc. 1976, 98, 377–383.
(43) Kamlet, M. J.; Hall, T. N.; Boykin, J.; Taft, R. W. J. Org. Chem.
1979, 44, 2599–2604.
(44) Kamlet, M. J.; Abboud, J. L. M.; Taft, R. W. J. Am. Chem. Soc.
1977, 99, 6027–6038.
(35) Reichardt, C. In Solvents and Solvent Effects in Organic Chemistry,
2nd ed.; VCH: Weinheim, Germany, 1988, and references therein.
(36) Reichardt, C. Chem. Rev. 1994, 94, 2319–2358.
€
(37) Muller, P. Pure Appl. Chem. 1994, 66, 1077–1079.
(38) Marcus, Y. Chem. Soc. Rev. 1993, 22, 409–416.
(39) Gutmann, V. Coord. Chem. Rev. 1976, 18, 225–240.
(40) Gutmann, V. The Donor-Acceptor Approach to Molecular Inter-
actions; Plenum Press: New York, 1978.
(45) Kamlet, M. J.; Abboud, J.-L. M.; Abraham, M. H.; Taft, R. W. J.
Org. Chem. 1983, 48, 2877–2887.

2722 Chem. Mater., Vol. 22, No. 9, 2010
Schreiter et al.
purification, and were purchased from the following supplier:
Merck: L(-)-proline (g99%); 2-aminopropane-1,3-diol (97%).
Compound 1 was prepared as described in the literature.33,34
All melting points (Mp) were measured on a Boetius melting
point apparatus and were uncorrected. The thermogravimetric
measurements were performed in a helium atmosphere with a
10 K/min heating rate (temperature range: 30-800 ꢀC) using a
Thermogravimetric Analyzer 7 (Perkin-Elmer). The UV/vis
absorption spectra were obtained with an MCS 400 diode array
UV/vis spectrometer from Carl Zeiss, Jena, connected via glass-
fiber optics. Specific surface area Nitrogen adsorption isotherms
were measured at 77.6 K using Areameter II setup from
3J=9.0 Hz, ArH), 8.29 (1 H, d, 4J=2.1 Hz, ArH). 13C NMR (69.9
MHz, DMSO-d6, 25 ꢀC): δ=11.4, 21.9, 44.2, 115.5, 123.9, 124.2,
124.3, 125.8, 127.0, 130.8, 131.9, 134.2, 144.5, 145.4, 146.0. IR
(KBr): ν~/cm-1=3366, 3075, 2932, 1625, 1588, 1512, 1333, 1242,
1199. Anal. Calcd for C17H17N3O4: C, 62.38; H, 5.23; N, 12.84.
Found: C, 62.32; H, 5.18; N, 12.70. UV/vis: λmax(MeOH)/nm 389
(ε/L mol-1 cm-1 31200).
(2S)-1-{2-Nitro-4-[(E)-2-(4-nitrophenyl)vinyl]phenyl}pyrroli-
dine-2-carboxylic acid 2. L-Proline (0.966 g, 8.655 mmol) and
NaHCO3 (0.145 g, 1.731 mmol) were dissolved in water
(10 mL). To this solution was added 1-fluoro-2-nitro-4-[(E)-
2-(4-nitrophenyl)vinyl]benzene (1) (0.499 g, 1.731 mmol) dis-
solved in acetone (40 mL) and water (10 mL). The reaction
mixture was stirred for 72 h at room temperature and then poured
into 2 M HCl (50 mL) in a single portion. The resulting pale red
precipitate was recovered by filtration and washed with water.
This material was purified by flash column chromatography
(silica gel, diethyl ether, ethanol), affording 2 as a pale red solid
(79% yield). Mp 130-132 ꢀC. 1H NMR (250 MHz, DMSO-d6,
25 ꢀC): δ=1.84-2.03 (3 H, m, proH-3a, proH-4), 2.36-2.45 (1 H,
m, proH-3b), 3.02-3.11 (1 H, m, proH-5a), 3.31-3.40 (1 H, m,
proH-5b), 4.43-4.48 (1 H, m, proH-2), 6.94 (1 H, d, 3J=9.0 Hz,
ArH), 7.30 (1 H, d, 3J=16.4 Hz, CH), 7.50 (1 H, d, 3J=16.4 Hz,
CH), 7.78-7.81 (3 H, m, ArH), 7.99 (1 H, d, 4J=2.1 Hz, ArH),
8.21 (2 H, d, 3J=8.9 Hz, ArH), 12.90 (1 H, bs, COOH). 13C NMR
(69.9 MHz, DMSO-d6, 25 ꢀC): δ=24.5, 30.5, 51.4, 61.8, 117.6,
124.3, 124.7, 125.2, 125.3, 127.1, 131.1, 131.6, 137.3, 141.3, 144.5,
146.0, 173.4. IR (KBr): ν~/cm-1=3078, 2981, 1716, 1589, 1520,
1337, 1189. Anal. Calcd for C19H17N3O6: C, 59.53; H, 4.47; N,
10.96. Found: C, 59.21; H, 4.59; N, 10.84. UV/vis: λmax(DCM)/
nm 390 (ε/L mol-1 cm-1 27000).
€
Strohlein Instruments.
1H NMR and 13C NMR spectra were measured at 20 ꢀC on a
Bruker Avance 250 NMR spectrometer at 250 and 69.9 MHz.
The residual solvent signals (DMSO-d6) were used as internal
standards. The solid-state 13C-{1H}-CP-MAS NMR experi-
ments (100.6 MHz) were recorded on a Bruker Digital Avance
400 spectrometer equipped with 7 mm double-tuned probes
capable of MAS at 12 kHz. Data acquisition was performed
with proton decoupling (TPPM). Spectra were referenced ex-
ternally to tetramethyl silane (TMS) as well as to adamantane
(δ=38.48 ppm) as a secondary standard. 29Si-{1H}-CP-MAS
NMR spectroscopy was performed at 79.5 MHz using 7 mm
rotors spinning at 5 kHz. The contact time was 3 ms and the
recycle delay 3 s. Shifts were referenced externally to tetra-
methylsilane (0 ppm) with the secondary standard being
tetrakis(trimethylsilyl)silane (-9.8, -135.2 ppm). All spectra
were collected with 1H decoupling using a TPPM pulse
sequence.
The FT-IR spectra were measured by means of diffuse
reflection diluted with KBr at room temperature in the wave-
number range 400 to 4000 cm-1 on a Perkin-Elmer Fourier
transform 1000 spectrometer. Elemental analysis was deter-
mined with a Vario-EL analyzer.
2-({2-Nitro-4-[(E)-2-(4-nitrophenyl)vinyl]phenyl}amino)pro-
pane-1,3-diol 3. 2-Amino-propane-1,3-diol (1.146 g, 12.578
mmol), NaHCO3 (0.211 g, 2.512 mmol) and 1-fluoro-2-nitro-
4-[(E)-2-(4-nitrophenyl)vinyl]benzene (1) (0.724 g, 2.512 mmol)
were dissolved in acetone (50 mL). The reaction mixture was
stirred for 72 h at 70 ꢀC and then poured into water (50 mL). The
resulting red precipitate was recovered by filtration and washed
with chloroform, affording 3 as a red solid (82% yield). Mp
182 ꢀC. 1H NMR (250 MHz, DMSO-d6, 25 ꢀC): δ=3.58-3.64
(4 H, m, CH2), 3.79-3.86 (1 H, m, CH), 5.02 (2 H, t, 3J=5.7 Hz,
OH), 7.25 (1 H, d, 3J=9.2 Hz, ArH), 7.31 (1 H, d, 3J=16.3 Hz,
CH), 7.54 (1 H, d, 3J=16.4 Hz, CH), 7.82 (2 H, d, 3J=8.8 Hz,
ArH), 7.95 (1 H, dd, 3J=9.2 Hz, 4J=2.1 Hz, ArH), 8.22 (2 H, d,
3J=8.8 Hz, ArH), 8.31 (1 H, d, 4J=2.1 Hz, ArH), 8.55 (1 H, d,
3J=8.4 Hz, NH). 13C NMR (62 MHz, DMSO-d6, 25 ꢀC): δ=
55.6, 60.0, 116.2, 124.0, 124.3, 124.4, 135.9, 127.1, 130.9, 132.0,
134.1, 144.6, 145.5, 146.0. IR (KBr): ν~/cm-1=3324, 3226, 3074,
2943, 1626, 1588, 1506, 1338, 1290, 1213. Anal. Calcd for
C17H17N3O6: C, 56.82; H, 4.77; N, 11.69. Found: C, 56.01; H,
4.64; N, 11.37. UV/vis: λmax(MeOH)/nm 389 (ε/L mol-1 cm-1
31200).
Crystal data were collected on a Oxford Gemini Diffracto-
meter at low temperature (100 K) using Cu-KR-radiation (λ=
1.54184 A). The structure was solved by direct methods using
SHELXS-97.46 The structure was refined by full-matrix least-
squares procedures on F2 using SHELXL-97.47 All non-hydrogen
atoms were refined anisotropically. All hydrogen atoms were added
to the calculated positions, except for OH and NH which were
found by difference Fourier synthesis. For more details, see the
Supporting Information.
Correlation Analysis. Multiple regression analysis (according
Kamlet-Taft eq 1) was performed using the Origin 5.0 statistics
program.
Syntheses. {2-Nitro-4-[(E)-2-(4-nitrophenyl)vinyl]phenyl}pro-
pylamine 1-M. 1-Fluoro-2-nitro-4-[(E)-2-(4-nitrophenyl)vinyl]-
benzene (1) (0.70 g, 2.429 mmol) was dissolved in toluene
(40 mL) and n-propylamine (1.0 mL, 0.718 g, 12.144 mmol) was
addedin a single portion. The reactionmixturewas heatedfor 10 h
at 70 ꢀC. Afterward the excess n-propylamine and solvent were
distilled off. The solid red residue was crystallized from ethyl
acetate to give the title compound as red crystals (75% yield). Mp
Preparation of Hybrid Precursors by the Sol-Gel Process.
Reaction of 1-fluoro-2-nitro-4-[(E)-2-(4-nitrophenyl)vinyl]ben-
zene (1) with aminopropylsilane (APS) and methylaminopro-
pylsilane (MAPS), respectively, in tetraethoxysilane (TEOS): 1
1
157-158 ꢀC. H NMR (250 MHz, DMSO-d6, 25 ꢀC): δ=0.96
(3 H, t, 3J=7.4 Hz, CH3), 1.58-1.73 (2 H, m, CH2), 3.29-3.42 (2
H, m, CH2), 7.13 (1 H, d, 3J=9.2 Hz, ArH), 7.27 (1 H, d, 3J=16.4
Hz, CH), 7.51 (1 H, d, 3J=16.4 Hz, CH), 7.80 (2 H, d, 3J=9.0 Hz,
ArH), 7.92 (1 H, dd, 3J=9.2 Hz, 4J=2.1 Hz, ArH), 8.20 (2 H, d,
and aminosilane were dissolved in TEOS (n1:naminosilane:nTEOS
=
1:1:10) with stirring under an argon atmosphere. The mixture
was heated to reflux for 5 h. An intense yellow color is developed
in the course of the reaction and a nonviscous yellow solution is
formed. After cooling the reaction solution to room tempera-
ture, the dark yellow solution was treated first with ethanol and
then with distilled water. The amount of water added to initiate
the sol-gel process is equimolar to the number of hydrolyzable
(46) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467–473.
(47) Sheldrick, G. M. SHELXL-97, Program for Crystal Structure
€
€
Refinement; University of Gottingen: Gottingen, Germany, 1997.

Article
Chem. Mater., Vol. 22, No. 9, 2010 2723
Scheme 2. Synthesis of Compounds 1-M, 2, and 3 by Nucleo-
philic Aromatic Substitution of Fluorostilbene 1 with n-Propyl-
amine, L-Proline, and 2-Aminopropane-1,3-diol
Si-OR groups (nwater=3naminosilane þ 4nTEOS), with the reaction
mixture being diluted in a 10-fold excess of ethanol compared to
water prior to initiation (nEtOH =10nwater). When the gel had
solidified, it was dried at 25 ꢀC in vacuo and then unreacted 1
was removed by Soxhlet extraction with acetone. The xerogels
1a-XG and 1b-XG were then redried in vacuo.
The alkoxysilanes 2-XG and 3-XG containing the nitrostil-
bene unit were synthesized by a coupling reaction between
3-isocyanatopropyltriethoxysilane (ICPS) and a carboxylic
acid- (2) and a diol-functionalized nitrostilbene (3), respectively.
A multinecked flask was repeatedly evacuated and flushed with
argon. Two precursors were placed in the flask with the solvent
(THF). The mixture was stirred for several minutes and then
refluxed in an oil bath for 48 h. After cooling to room tempera-
ture, TEOS (n2:nICPS:nTEOS = 1:1:10; n3:nICPS:nTEOS = 1:2:10)
was added in a single portion to the resulting solution in THF.
Aqueous acid (HCl, pH 1) was then added to promote the
hydrolysis/condensation process. The amount of water added to
initiate the sol-gel process is equimolar to the number of
hydrolyzable Si-OR groups (nwater = 3naminosilane þ 4nTEOS).
The solution was stirred at room temperature for 5 h and aged
for 10 days to increase viscosity. When the gel had solidified, it
was dried at 25 ꢀC in vacuo and then unreacted chromophore
was removed by Soxhlet extraction with acetone. The xerogels
2-XG and 3-XG were then redried in vacuo.
Figure 1. ORTEP diagrams of the molecular structure of (A) 1-M and
(B) 3, respectively, with displacement ellipsoids shown at the 50%
probability level. H-atoms, except OH and NH, are omitted for clarity.
the diol-functionalized stilbene 3 crystallizes in the mono-
clinic space group P2(1)/n. The molecular structures of
both compounds are shown in Figure 1 together with the
atomic labeling scheme. The crystallographic data, col-
lection parameters, bond lengths (A) and angles (deg) are
provided as Supporting Information.
In the solid state, 1-M and 3 possess a trans-configura-
tion with respect to the central CdC double bond
(C7-C8), as typically observed for stilbene compounds.56
As expected,57 there are intramolecular hydrogen bonds
between the amino bonded hydrogen atoms and the
ortho-nitro group (1-M: N2-H2N O3, d = 2.621(2)
3 3 3
A, — =130(2)ꢀ; 3: N2-H2N O5, d=2.624(4) A, — =
3 3 3
The surface modification of 1b-XG was accomplished by
allowing the hydroxyl groups on the dry gel surface to react
with 20% HMDS, hexamethyldisilazane, in toluene for 5 h at
90 ꢀC. Finally, the sample was washed with toluene to remove
the unreacted surface modification agents and reaction pro-
ducts. The modified xerogel 1b-XG was then redried in vacuo.
120(4)ꢀ). Additional, molecule 3 forms a further intramo-
lecular hydrogen bond between the amino hydrogen and
the hydroxyl oxygen atom (N2-H2N O4, d=2.778(4)
3 3 3
A, — = 118(4)ꢀ). Compound 1-M has no further func-
tional groups except the nitro and the amino units and
hence intermolecular hydrogen bonds are not observed.
However, between two carbon atoms of two molecules
3. Results and Discussion
(C12 C1A) of 1-M, a very short distance is found
3 3 3
Synthesis and Characterization. The novel donor-
acceptor-substituted stilbenes 1-M, 2, and 3 (Scheme 2)
were obtained by nucleophilic aromatic substitution48-55
of the activated fluoroaromatic 133,34 with the corre-
sponding amines.
Single-Crystal X-ray Analyses. Crystals of 1-M and 3
suitable for single-crystal X-ray structure analysis were
obtained by slow diffusion of n-hexane into an acetone
solution of 1-M and 3, respectively. Compound 1-M
crystallizes in the orthorhombic space group Pbca and
(C12 C1A, d=3.347 A), obviously due to π-π inter-
3 3 3
actions. Considering these interactions in the solid state,
molecules of 1-M are able to form a 2D layer along the
crystallographic b- and c-axes (Figure 2).
In the solid state, a 3D network is obtained in the case
of stilbene 3. In contrast to 1-M, it consists of 2D layers
along the crystallographic b- and c-axes formed by inter-
molecular hydrogen bonds between the hydroxyl hydro-
gen atoms, the ortho-nitro group (O3A-H3OA O6,
3 3 3
d=2.792(4) A, —=172(3)ꢀ), and the diol groups (O4B-
H4OB O3A, d=2.765(4) A, — =169(4)ꢀ) (Figure 3),
3 3 3
respectively. Because of π-π stacking interactions along
the crystallographic a-axes between the ortho-nitro
(48) Bunett, J. F.; Zahler, R. E. Chem. Rev. 1951, 49, 273–412.
(49) Sauer, J.; Huisgen, R. Angew. Chem. 1960, 9, 294–315.
(50) Schmid, G. H. Organic Chemistry, 1st ed.; Mosby-Verlag: St. Louis,
MO, 1996.
(51) Suhr, H. Chem. Ber. 1964, 97, 3277–3283.
(56) Moran, A. M.; Bartholomew, G. P.; Bazan, G. C.; Meyers Kelley,
A. J. Phys. Chem. A 2002, 106, 4928–4937.
(52) Suhr, H. Liebigs Ann. Chem. 1965, 687, 175–182.
(53) Suhr, H. Liebigs Ann. Chem. 1965, 689, 109–117.
(54) Suhr, H. Chem.Ber. 1964, 97, 3268–3276.
ꢀ
(57) Panunto, T. W.; Urbanczyk-Lipkoswska, Z.; Johnson, R.; Etter,
(55) Grube, H.; Suhr, H. Chem. Ber. 1969, 102, 1570–1579.
M. C. J. Am. Chem. Soc. 1987, 109, 7786–7797.

2724 Chem. Mater., Vol. 22, No. 9, 2010
Schreiter et al.
Figure 2. Part of the crystal packing of 1-M. π-π interactions are indicated with orange dotted lines.
Figure 3. Selected part of the 3D network of 3 caused by intermolecular hydrogen bonds, indicated with dotted lines, displaying the alignment of one 2D
layer. The gray molecules in the background represent a further 2D layer. The π-π interactions between the nitro groups and the aromatic ring
superimposed 2D layer are not illustrated here for clarity.
group and the aromatic unit (C1-C6), the 2D layers are
connected to give a 3D network. A further graphical
illustration of the 3D network is shown in Figure S1 in
the Supporting Information.
Solvent Effects on the UV/Vis Absorption Spectra. The
novel dyes 1-M, 2, and 3 (Figure 4) were investigated with
respect to their activity as solvent-sensitive indicators.
The UV/vis absorption maximum of 2 in methanol
Figure 4. Chromophores 1-M, 2, and 3.
shifts hypsochromically from ν~max=407 to 399 nm in a
concentration range from 0.407 ꢁ 10-5 to 4.070 ꢁ
Concentration-dependent UV/vis spectroscopic investi-
gations of 1-M and 3 did not show a shift in the UV/vis
absorption maximum. The solvatochromic properties of
compounds 1-M and 3, which do not undergo self-
10-5 M. It is a fact that carboxylic acids form inter-
molecular hydrogen bonds and that the aggregation
behavior is dependent on parameters such as acid con-
centration, temperature and solvent polarity.58-63
aggregation, can therefore be investigated (Table 1).
Hence the solvatochromism of 2 is not investigated.
Compound 3 shows the shortest wavelength UV/vis
absorption band at λmax=383 nm in tetrachloromethane,
whereas 1-M shows the strongest hypsochromic shift at
(58) Schreiter, K.; Spange, S. J. Phys. Org. Chem. 2008, 21, 242–250.
(59) Bulmer, J. T.; Shurvell, H. F. J. Phys. Chem. 1973, 77, 256–262.
(60) Tanaka, N.; Kitano, H.; Ise, N. J. Phys. Chem. 1990, 94, 6290–
λ
max =382 nm in n-hexane, in which 3 is insoluble. For
both chromophores, the longest wavelength UV/vis ab-
6292.
(61) Laborie, F. J. Polym. Sci. 1977, 15, 1255–1274.
(62) Kuppens, T.; Herrebout, W.; Veken, B.; Bultnick, P. J. Phys.
Chem. A 2006, 110, 10191–10200.
sorption maximum is observed at λmax(3) = 415 nm,
λmax(1-M) = 412 nm in hexamethylphosphoramide
(HMPA). These shifts correspond to a solvatochromic
(63) Wolfs, I.; Desseyn, H. O. J. Mol. Struct. (THEOCHEM) 1996, 360,
81–97.

Article
Table 1. UV/Vis Absorption Maxima of 1-M and 3 Measured in 36
Solvents of Different Polarity and Hydrogen-Bond Ability
Chem. Mater., Vol. 22, No. 9, 2010 2725
well in agreement with a bathochromic shift of the UV/vis
absorption maxima with increased polarity of the solvent.
For the nitrostilbene 3, a negative correlation coeffi-
cient b is determined. On the other hand, hydrogen-bond
acceptor (HBA) solvents do not have any effect on the
position of the UV/vis absorption maximum of 1-M
(b=0). For 1-M, interactions between the NH-function
and HBA solvents are merely conceivable (b < 0). How-
ever, this interaction competes with the formation of an
intramolecular hydrogen bond to the ortho-nitro group64
(b = 0). Compound 3 can also form an intramolecular
hydrogen bond as observed for 1-M. Solvents with HBA
ability can also interact with 3 via specific solvation of the
diol group.65 This result is initially surprising, because the
diol function is separated from the aromatic push-pull
system by the propyl group spacer. However, this is con-
firmed by the results of the linear multiple regression
analysis of 1-M. Furthermore, we found a similar effect
on the β-parameter in solvatochromic investigations on
N-(2-hydroxyethyl)-substituted Michlers ketone and
N-(methyl-N-[1-(2,3-dihydroxypropyl)]-4-nitroaniline de-
rivatives.66-68
The positive sign of parameter a for 1-M and 3 indicates
that there is a hypsochromic shift of λmax with increasing
hydrogen-bond donor capacity of the solvent (negative
solvatochromism). This suggests that the ground state is
more stabilized than the first excited state. This can be
explained in terms of a possible interaction with the free
electron pair of the nitrogen or the amino function.
Preparation of Chromophoric Class II Xerogels. Start-
ing from the nitrostilbenes 1-3, we prepared organo-
functionalized class II silicate-based xerogels by the com-
bination of first nucleophilic aromatic substitution (Ia),
amide formation (Ib), or urethane formation (Ic) and a
subsequent sol-gel process (II) in tetraethoxysilane
(TEOS) in a one-pot synthesis (Scheme 3, Table 3).
The advantage of the one-pot syntheses of 1a-XG and
1b-XG is the use of TEOS as solvent and cross-linking
agent in the sol-gel process, which is a key feature of this
synthesis route. The advantage of the two-step synthesis
route for 2-XG and 3-XG is the possibility to vary the
linker between the chromophore and the silicate network
and thus allow the incorporation of tailor-made organic
components.
ν~max (ꢁ 103 cm-1
1-M
)
solvent
3
a
a
cyclohexane
n-hexane
triethylamine
tetrachloromethane
p-xylene
toluene
benzene
diethyl ether
1,4-dioxane
anisole
tetrahydrofuran
ethyl acetate
chloroform
1,1,2,2-tetrachloroethane
pyridine
dichloromethane
hexamethylphosphoramide
tetramethylurea
1,2-dichloroethane
benzonitrile
25.97
26.18
26.11
25.71
25.45
25.45
25.38
26.04
25.51
25.00
25.32
25.58
25.25
24.88
24.63
25.13
24.27
24.75
25.06
24.69
25.32
24.75
24.88
24.45
25.32
25.13
25.58
25.51
25.77
25.58
25.64
25.71
24.75
24.63
25.71
25.19
1906
25.32
26.11
25.58
25.58
25.58
25.77
25.51
25.06
25.13
25.45
25.71
25.13
24.39
25.45
24.10
24.51
25.38
24.63
25.38
24.63
24.75
24.39
25.44
25.06
25.25
25.38
25.45
25.45
25.58
25.71
25.06
24.69
26.10
25.84
2013
acetone
N,N-dimethylacetamide
N,N-dimethylformamide
dimethyl sulfoxide
acetonitrile
nitromethane
1-decanol
1-butanol
2-propanol
1-propanol
ethanol
methanol
ethane-1,2-diol
formamide
2,2,2-trifluoroethanol
1,1,1,3,3,3-hexafluoro-2-propanol
Δν~ (cm-1
)
a Probe is insoluble in this solvent.
range of Δν~(3)=2013 cm-1 and Δν~(1-M)=1906 cm-1
respectively.
LSE Correlation Analyses. The solvent-dependent UV/
vis absorption of 1-M and 3 can be interpreted with
regard to the dipolarity/polarizability and hydrogen
bonding capacity of the solvent using the Kamlet-Taft
equation (eq 1).
The solvent parameters a, β, and π* used for the
multiple linear regression analysis are given in the Sup-
porting Information. The qualitatively best regressions of
1-M and 3 are shown in Table 2 as a function of the
Kamlet-Taft solvent scale.
The correlation coefficient r is greater than 0.90 for LSE
relationships, which indicates a high validity of the multi-
parameter equations and allows significant conclusions to
be drawn. The most dominant effect on the solvatochromic
behavior of compounds 1-M and 3 is caused by interactions
with solvents of different dipolarity/polarizability and is
reflected in parameters. The negative sign of sindicates that
the electronically excited state of these molecules becomes
stronger solvated and consequently stabilized with increas-
ing the solvents dipolarity/polarizability. On the strength of
the higher dipole moment, the energy of the electronically
excited state decreases more than the ground state. This is
,
A summary of the xerogel syntheses carried out and the
characterization of the xerogels 1-XG-3-XG obtained is
given in Table 3. To obtain 1-XG and 2-XG a 1:1:10 nC:
nTAS:nTEOS molar ratio was used. Because of the two
hydroxyl functionalities in 3, a 1:2:10 n3:nTAS:nTEOS
molar ratio was used for 3-XG, incorporating the chro-
mophoric building block in the main chain of the silica
(64) Cattana, R.; Silber, J. J.; Anunziata, J. Can. J. Chem. 1992, 70,
2677–2682.
€
(65) Spange, S.; Hofmann, K.; Walfort, B.; Ruffer, T.; Lang, H. J. Org.
Chem. 2005, 70, 8564–8567.
€
(66) El-Sayed, M. M.; Muller, H.; Rheinwald, G.; Lang, H.; Spange, S.
J. Phys. Org. Chem. 2001, 14, 247–255.
€
(67) Spange, S.; El-Sayed, M. M.; Muller, H.; Rheinwald, G.; Lang, H.;
Poppitz, W. Eur. J. Org. Chem. 2002, 4159–4168.
€
(68) El-Sayed, M. M.; Muller, H.; Rheinwald, G.; Lang, H.; Spange, S.
Chem. Mater. 2003, 15, 746–754.

2726 Chem. Mater., Vol. 22, No. 9, 2010
Schreiter et al.
Table 2. Solvent-Independent Correlation Coefficients a, b, and s of the Kamlet-Taft Parameters r, β, and π*; Solute Property of the Reference System
ν~max,0, Significance ( f ), Correlation Coefficient (r), Standard Deviation (sd), and Number of Solvents (n) Calculated for the Solvatochromism of 1-M and 3
eq
compd
ν~max,0
a
b
s
n
f
r
sd
1
1
1-M
3
26.262 ((0.091)
26.515 ((0.125)
0.182 ((0.070)
0.348 ((0.069)
0
-1.641 ((0.131)
-1.490 ((0.157)
36
34
<0.0001
<0.0001
0.911
0.925
0.205
0.197
-0.849 ((0.109)
Scheme 3. One-Pot Synthesis of Chromophoric Sol-Gel Hybrid Materials (Class II) 1-XG-3-XG: Nucleophilic Aromatic Substitution
(Ia), Amide Synthesis (Ib), and Urethane-Forming Reaction (Ic), Followed by Sol-Gel Processing (II)
Table 3. Experimental Conditions and Characterization of 1-XG-3-XG
results
Tda (ꢀC)
via DSC
chromophore
(C)
trialkoxy-
silane (TAS)
Tgb (ꢀC)
via DSC
Tdc (ꢀC)
via TGA
nC:nTAS:nTEOS ratio
λmax (nm)
BET (m2/g)
1
APS
1:1:10
1:1:10
1:1:10
1:2:10
1a-XG
1b-XG
2-XG
392
392
383
393
114
52
127
40
269
230
242
275
93
85
89
89
213
247
241
216
MAPS
ICPS
ICPS
2
3
3-XG
a Td = Onset decomposition temperature of chromophore incorporated in silica network. b Tg = Glass transition temperature. c Td
Decomposition temperature where there is 5 wt % loss under nitrogen.
=
network. The covalent bonding of the organic dye mole-
cule in an inorganic matrix can occur in various ways. The
surface areas of the xerogels were determined by the BET
method (Table 3). A low surface area was found for all
xerogels under study.
1688 cm-1 corresponding to the amide group appears. The
FTIR spectrum of the diol-substituted nitrostilbene 3
shows typical -OH and -NH stretching vibrations be-
tween 3400-3200 cm-1. The corresponding xerogel 3-XG
shows stretching vibrations at 3352 and 1702 cm-1 (νN-H
and νCdO), which are evidence of urethane formation. In all
xerogels 1-XG-3-XG, a strong absorption band is seen in
the range of 1200-1000 cm-1 corresponding to the
Si-O-Si stretching vibrations.
Glass-Transition Temperatures. The glass transition
temperature (Tg) is an important parameter for determin-
ing the stability of dipolar orientations of chromophores
in a matrix. Tg can be affected by various factors such as
the degree of cross-linking, the size of the chromophore
and the ratio of reactants used.69 For our synthesis we
FT-IR Spectra. The successful incorporation of the
nitrostilbene unit into a silica network via various linkers
could be established using FTIR spectroscopy. The FTIR
spectra of 1-M, 2, and 3 and their corresponding xerogels
(1-XG-3-XG) each exhibit two sharp absorption bands
in the range of 1506-1530 and 1333-1341 cm-1, respec-
tively, which correspond to the asymmetric and sym-
metric stretching vibrations of the nitro group of the
chromophoric building block. Furthermore, for the
carboxylic-acid-functionalized stilbene 2 a νCdO absorp-
tion at 1716 cm-1 is observed. After conversion into
the xerogel (2-XG), this band is, as expected, no
longer observed. Instead a νCdO stretching vibration at
(69) Silva, S. S.; Ferriera, R. A. S.; Fu, L.; Carlos, L. D.; Mano, J. F. R.;
Reis, L.; Rocha, J. J. Mater. Chem. 2005, 15, 3952–3961.

Article
Chem. Mater., Vol. 22, No. 9, 2010 2727
Figure 6. (A) Solid state 13C-{1H}-CP-MAS NMR spectra of xerogels
1a-XG (12.5 kHz MAS frequency); (B) 13C-{1H} NMR spectrum of 1-M
(DMSO-d6). The rhomb (#) indicates unhydrolyzed ethoxy groups in the
xerogel.
Figure 5. 29Si-{1H}-CP-MAS NMR spectra of xerogels 1-XG-3-XG
(nomenclature, see Scheme 2).
always chose the same chromophoric building block and
similar molar ratios of the reactants, thus obtaining
hybrid materials with Tg in the relatively narrow range
of 85-93 ꢀC (Table 1). These data were determined by
differential scanning calorimetry (DSC).
Figure 7. Photograph of stilbenes 1-M, 2, 3, and their xerogels 1-XG-
3-XG.
1b-XG: -109.4 (Q4), -101.8 (Q3), -93.5 (Q2), -65.4 (T3),
-59.2 (T2); 2-XG: -107.5 (Q4), -100.2 (Q3), -92.5 (Q2),
-65.3 (T3), -58.1 (T2). For 3-XG, T0 (-45.6) and T1
(-52.7) signals can also be observed in the 29Si-{1H}-CP-
MAS NMR spectrum, besides T2 (-59.3) and T3 (-65.7)
signals. The T0 signal can be assigned to the 3-isocyanato-
propyltriethoxysilane (ICPS)70 or the precursor 3-Pr. How-
ever, the completely unreacted educts (ICPS, 3-Pr) should
be removed by Soxhlet extraction. A T0 signal is also
observable in the case that only one of these two groups is
bound to the silica matrix and the other one remains
unhydrolized. However, under the current conditions of
measurement, the intensity ratios of 29Si signals, e.g., T0, do
not correspond to the actual concentrations in the xerogel.
13C-{1H}-CP-MAS NMR Spectroscopy. In Figure 6,
the 13C-{1H}-CP-MAS NMR spectrum of 1a-XG (A) and
the 13C-{1H} NMR spectrum of 1-M (B) measured in
DMSO-d6 is depicted. On comparison of these spectra, it
can be seen that the resonances observed for 1-M are also
visible in the 13C-{1H}-CP-MAS NMR spectrum of 1a-
XG. This is evidence for the integrity of the chromophoric
building block even after incorporation into the class II
xerogel. Furthermore, the spectrum has two further
signals at 16.4 and 64.3 ppm due to the nonhydrolyzed
Si-OEt groups. These results are in agreement with those
from the 29Si-{1H}-CP-MAS NMR spectroscopy (Q3 and
Q2 signals).
Thermal Stability. The thermal stability of the hybrid
materials obtained was determined by thermogravimetric
analysis under an inert gas atmosphere (Table 3). The
results of the TGA show that the onset of decomposition
of xerogels with a secondary amine function as the
structural element begins in a similar temperature range:
1a-XG Td = 213 ꢀC, 3-XG Td = 216 ꢀC. The initial
decomposition temperature for 1b-XG (Td = 247 ꢀC)
and 2-XG (Td = 241 ꢀC), which have a tertiary amine
function as the structure-forming building block, is about
30 ꢀC higher in each case.
29Si-{1H}-CP-MAS NMR Spectroscopy. 29Si-{1H}-
CP-MAS NMR spectra of the xerogels 1-XG-3-XG were
recorded (Figure 5). Using cross-polarization, qualitative
spectra with good signal-to-noise ratios could be obtained
within a short time. However, only the presence of the indi-
vidual signals can be used as reliable evidence; the intensity
ratios of the individual signals do not correspond to the
actual concentrations in the xerogel. The 29Si-{1H}-CP-
MAS NMR spectrum of 1-XG-3-XG confirmed the co-
valent bonding of the organic moiety to the matrix by the
appearance of T2 and T3 signals due to the ligand in
addition to the characteristic Q3 and Q4 signals due to con-
densed TEOS. 29Si-{1H}-CP-MAS NMR spectrum of 1a-
XG shows five signals. The signals at δ=-66.3 ppm (T3)
and δ = -58.9 ppm (T2) arise from the trialkoxysilane
component, which is embedded into the silicate matrix. The
signals at δ=-110.0 ppm (Q4), δ=-101.8 ppm (Q3), and
δ=-93.6 ppm (Q2) can be assigned to the silanol groups
and siloxane bridges in the silicate matrix. In the 29Si-{1H}-
CP-MAS NMR spectra of the networks 1b-XG and 2-XG,
besides the Q4 signal, as in 1a-XG, there are Q3 and Q2
signals arising from tetraethoxysilane, which is hydrolyzed
and condensed partially. The T3 signal and a small shoulder
of very low intensity for the T2 signal can also be seen.
UV/Vis Reflection Spectra. The class II xerogels
1-XG-3-XG obtained have orange to orange-red color
shades (Figure 7). An interesting application for these
types of chromophores, incorporated in an inorganic
matrix, may be in the area of substantial pigments.
(70) Lebeau, B.; Maquet, J.; Sanchez, C.; Toussaere, E.; Hierle, R.;
Zyss, J. J. Mater. Chem. 1994, 4, 1855–1860.

2728 Chem. Mater., Vol. 22, No. 9, 2010
Schreiter et al.
Figure 8. UV/vis absorption spectra of (A) 1-M, (B) 2, (C) 3, and their xerogels 1-XG-3-XG, respectively, as solid powders and dissolved in methanol.
UV/vis absorption spectra of class II xerogels 1-XG-3-
XG have been measured by means of diffuse reflectance
spectroscopy. The polarity in terms of the ET(30) scale of
non chromophoric derivatized amino-xerogel (ET(30)=
55 kcal/mol) is close to that of methanol (ET(30) =
55.4 kcal/mol).71 Referring to ET(30) values in the same
order of magnitude, protonation of the probe at both the
nitro group oxygen and the amino group nitrogen atom
can not be expected. Representative UV/vis spectra of
1-XG-3-XG, and 1-M, 2, and 3, measured as powders
and dissolved in methanol for comparison are shown in
Figure 8A-C. Generally, the half-with of the UV/vis
absorption band of the respective xerogels are broader
than those in well-behaved regular solvents, e.g., methanol.
This result indicates a wider polarity distribution compared
to that of a solvent.
The UV/vis absorption bands are nonsymmetric and
show several UV/vis absorption maxima. The UV/vis
diffuse reflectance spectra of the xerogels 1-XG-3-XG
exhibit UV/vis absorption maxima at 392 nm (1-XG,
3-XG) and 383 nm (2-XG). For the corresponding nitro-
stilbenes 1-M, 2, and 3 a distinct UV/vis absorption
maximum at 352 nm is present.
In the solid state the chromophores 1-M, 2, and 3 form
intra- and intermolecular hydrogen bonds in addition to
strong dipolar interactions. This is verified by the single
crystal X-ray structures of 1-M and 3. The directed
specific intermolecular interaction between the nitro
group oxygen and one HBD moiety (specifically ob-
served for 3) causes a significant bathochromic UV/vis
shift. As the chromophore is encapsulated by the silica
network (1-XG-3-XG), the formation of dipolar inter-
actions between the chromophores is limited. This can
be the explanation for the fact that the width of the
UV/vis reflection bands of the stilbenoid building
blocks is clearly broader than that of the corresponding
xerogels.
The treatment of 1b-XG with hexamethyldisilazane
(HMDS) leads to a replacement of the surface hydroxyl
groups by trimethylsilyloxy groups, resulting in a de-
crease in the acidity of the xerogel surface. Furthermore,
a decrease in the term dipolarity/polarizability occurs.
From the results of the Kamlet-Taft analysis of 1-M and
3, it can be concluded that the influence of changes in
the dipolarity/polarizability on the UV/vis behavior of
1b-XG is more dominant than variations of surface
acidity. The UV/vis absorption maximum of the tri-
methylsilyl modified 1b-XG is shifted to slightly higher
energies (Δν~=330 cm-1) in comparison to the unmodi-
fied material. This observation is in agreement with the
solvatochromic study.
4. Conclusion
We have presented a series of variously functionalized
nitrostilbenes (1-M, 1-3). In the solid state, the chromo-
phores 1-M, 2, and 3 form intra- and intermolecular
hydrogen bonds in addition to strong dipolar interac-
tions. This is verified by the single crystal X-ray structures
and UV/vis diffuse reflectance spectroscopy of 1-M and 3.
A comprehensive study of the solvent effects on the
position of the wavelength absorption band of 1-M
and 3 is presented. The absorption maxima have been
measured in a variety of HBD and non-HBD solvents.
Overall, the two nitrostilbenes show a positive solvato-
chromism and a solvatochromic range of Δν~(3) =
2013 cm-1 and Δν~(1-M)=1906 cm-1, respectively. The
result of such correlations suggests that the influence of
solvent dipolarity/polarizability on the long wavelength
absorption maximum for 1-M and 3 is more predomi-
nant. The functionalization of nitrostilbenes 1-3 allows
reaction with trialkoxysilanes to give the chromophoric
trialkoxysilanes 1-PR-3-PR, which were converted into
the corresponding organically modified class II silica gels
(1-XG-3-XG). Structural characterization by a range of
techniques (FT-IR, 29Si-CP-MAS NMR, 13C-CP-MAS
NMR, thermogravimetric analysis) confirmed that class
II hybrid materials are available. The absence of T0
signals in 1-XG-2-XG shows that unreacted precursor
is not present in the materials obtained. However, a T0
signal observed for 3-XG is originated by one of two
possible triethoxysilyl groups from 3-Pr. All materials
obtained show similar thermal behavior.
In such xerogels, the formation of dipolar interactions
between the chromophores is limited in comparison to the
pure chromophores.
Acknowledgment. Financial support from the Deutsche
Forschungsgemeinschaft and the Fonds der Chemischen
Industrie is gratefully acknowledged. The authors thank R.
€
Lungwitz, P. Schonherr, and R. Jaeschke, Department of
Polymer Chemistry, Chemnitz University of Technology, for
solid-state NMR, TGA, and DSC measurements.
€
€
(71) Seifert, A.; Spange, S.; Muller, H.; Hesse, S.; Jager, C J. Sol-Gel
Sci. Technol. 2003, 26, 77–81.

Article
Supporting Information Available: Crystal data for 1-M and
Chem. Mater., Vol. 22, No. 9, 2010 2729
CB2 1ET, UK. Kamlet-Taft parameter set, all results of
the multiple linear regression analyses, X-ray crystallogra-
phic data, collection parameters of 1-M and 3 (PDF). This
material is available free of charge via the Internet at http://
pubs.acs.org.
3 have been deposited with the Cambridge Crystallographic
Data Center as supplementary publication no. 745588 and no.
745589. Copies of this information can be obtained free of
charge on application to CCDC, 12 Union Road, Cambridge