Bis-silylated terephthalate as a building block precursor for highly fluorescent organic-inorganic hybrid materials

Article abstract of DOI:10.1039/c2nj40434f

A bis-silylated diethyl 2,5-bis[N,N-(3-triethoxysilyl)propylurea] terephthalate precursor (5) was synthesized from a terephthalate derivative (3) and successfully used to prepare highly fluorescent organosilicas by hydrolytic self-condensation (DPM1) and co-condensation (DPM2) with tetraethoxysilane. The dyes 3 and 5 present absorption in the UV-Vis region and fluorescence emission in the yellow and blue regions, respectively. The red-shifted bands in the absorption spectra of dye 3 with respect to dye 5 can be related to the formation of intramolecular charge transfer (ICT) state even in the ground-state due to the strong electron-donor amino groups and moderate electron-withdrawing carbonyl groups present in the benzenic ring. The small Stokes' shift, as well as the high fluorescence quantum yield values discards the intramolecular proton transfer mechanism in the excited state. The fluorescence emission dependence on the solvent polarity indicates that an intramolecular charge transfer (ICT) or a locally excited (LE) state is taking place in the excited state to the dyes 3 and 5, respectively. The new fluorescent hybrid materials (DPM1 and DPM2) show absorption maxima in the solid state located at around 390 nm, indicating that the electronic structure of the fluorescent precursor was not significantly perturbed in the ground state after its self-condensation or co-condensation with TEOS. The fluorescent material DPM2 presents a photophysical behavior quite similar to the precursor in solution. On the other hand, the self-condensed material DPM1 presents a fluorescence emission maximum red-shifted by 33 nm, which can be probably attributed to π-π stacking between the terephthalate cores. Any evidence of auto-organization could be observed in these materials. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012.

Full text of DOI:10.1039/c2nj40434f

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PAPER
Bis-silylated terephthalate as a building block precursor for highly
fluorescent organic–inorganic hybrid materialsw
Daniela Pletsch, Fabiano da Silveira Santos, Fabiano Severo Rodembusch,
Valter Stefani* and Leandra Franciscato Campo*
Received (in Montpellier, France) 25th May 2012, Accepted 14th September 2012
DOI: 10.1039/c2nj40434f
A bis-silylated diethyl 2,5-bis[N,N-(3-triethoxysilyl)propylurea]terephthalate precursor (5) was
synthesized from a terephthalate derivative (3) and successfully used to prepare highly fluorescent
organosilicas by hydrolytic self-condensation (DPM1) and co-condensation (DPM2) with
tetraethoxysilane. The dyes 3 and 5 present absorption in the UV-Vis region and fluorescence
emission in the yellow and blue regions, respectively. The red-shifted bands in the absorption
spectra of dye 3 with respect to dye 5 can be related to the formation of intramolecular charge
transfer (ICT) state even in the ground-state due to the strong electron-donor amino groups and
moderate electron-withdrawing carbonyl groups present in the benzenic ring. The small Stokes’
shift, as well as the high fluorescence quantum yield values discards the intramolecular proton
transfer mechanism in the excited state. The fluorescence emission dependence on the solvent
polarity indicates that an intramolecular charge transfer (ICT) or a locally excited (LE) state is
taking place in the excited state to the dyes 3 and 5, respectively. The new fluorescent hybrid
materials (DPM1 and DPM2) show absorption maxima in the solid state located at around
390 nm, indicating that the electronic structure of the fluorescent precursor was not significantly
perturbed in the ground state after its self-condensation or co-condensation with TEOS. The
fluorescent material DPM2 presents a photophysical behavior quite similar to the precursor in
solution. On the other hand, the self-condensed material DPM1 presents a fluorescence emission
maximum red-shifted by 33 nm, which can be probably attributed to p–p stacking between the
terephthalate cores. Any evidence of auto-organization could be observed in these materials.
Introduction
of the organic substructure. Since then several bridged
The incorporation of photoresponsive precursors into the
sol–gel matrix as organic building blocks can be a powerful
tool for developing new fluorescent organosilicas for optical
applications such as photostabilizers, waveguides, laser materials,
and organic light emitting diodes (OLEDs).1–10 In particular, self-
assembly of bridged silsesquioxanes represents a typical and
efficient approach to design and fabricate silica-based hybrid
materials based on a sol–gel process.11 The concept of self-
directed assembly in sol–gel derived bridged silsesquioxane hybrid
materials was first presented by Moreau et al.,12 where new hybrid
materials were reported with helical morphology via H-bond
mediated hydrolysis of a single chiral precursor. A left- or right-
handed helix was auto-generated, according to the configuration
silsesquioxanes with photoluminescent properties have been
published.13–16 All the molecular organic precursors containing
at least two alcoxysilyl groups can be transformed into nanostruc-
tured hybrid materials by hydrolytic sol–gel polycondensation.17
A
wide variety of organic functionalities can be incorporated into
the silica network,18 including alkyl, aromatic, heterocyclic
groups, chelating systems (cyclames, crown ethers, porphyrins,
etc.), chiral molecules,19,20 as well as polymers.2,9,21 The most
common self-organization of organic units can be obtained by
van der Waals interactions, for example p–p interaction between
the aromatic rings and/or by intermolecular H-bonding, such as
ureido groups between organic groups.
In this work, we present the synthesis and photophysical
characterization of a new bridged silsesquioxane with a planar
structure due to intramolecular H-bonding in combination
with core-to-core intermolecular H-bonding associated with
ureido groups.22,23 The new fluorescent silsesquioxane was
used to produce photoresponsive organic–inorganic hybrid
materials via hydrolytic self-condensation and co-condensation
with tetraethoxysilane (TEOS), which can be very interesting
Universidade Federal do Rio Grande do Sul, Instituto de Quı´mica,
´
ˆ
Laboratorio de Novos Materiais Organicos, Av. Bento Gonc¸alves,
9500. CP 15003, CEP 91501-970, Porto Alegre-RS, Brazil.
E-mail: vstefani@iq.ufrgs.br, campo@iq.ufrgs.br;
Fax: +55-51-33087304; Tel: +55-51-33087206
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c2nj40434f
c
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since fluorescent terephthalates have received much attention
due to their applications in electronic materials, light-emitting
diodes (LEDs), and optical sensors.24–31
CuKa as a radiation source, l = 0.15418 nm. The distances
were calculated using the Bragg equation. In order to correlate
the X-ray patterns with the dye structures, calculations
were performed employing the Gaussian 03 software37 and
MOPAC93 packages. A simple conformational analysis was
carried out at the semiempirical level (PM3 parametrization)
for the precursor (Fig. S8, ESIw).
Experimental
Materials and methods
Synthesis of the fluorescent terephthalates
All the solvents and reagents were used as received or purified
using standard procedures.32 Spectroscopic grade solvents
(Merck or Aldrich) were used for fluorescence emission and
UV-Vis absorption measurements. The compound 1,4-cyclo-
hexanedione-2,5-dicarboxylate (1) was synthesized according
to the literature.33 Elemental analyses were performed by
Perkin–Elmer model 2400. Melting points were measured
using a Gehaka PF 1000 apparatus and are uncorrected.
Infrared spectra were recorded using a Shimadzu Prestige 21
in KBr pellets or in ATR mode. 1H and 13C-NMR spectra
were performed using a VARIAN INOVA YH300 using
tetramethylsilane (TMS) as the internal standard and CDCl3
(Aldrich) as the solvent at room temperature. UV-Vis absorp-
tion spectra were performed on a Shimadzu UV-2450 spectro-
photometer. Steady state fluorescence spectra were measured
using a Shimadzu spectrofluorometer model RF-5301PC.
Spectrum correction was performed to enable measuring a
true spectrum by eliminating instrumental response such as
wavelength characteristics of the monochromator or a detector
using Rhodamine B as an external standard (quantum counter).
All experiments were performed at room temperature in a
concentration of 10À5 M. The diffuse reflectance spectra were
measured using an ISR-2200 Integrating Sphere Attachment.
For the measurements in the solid state, the hybrid materials
were treated as powder. The experiments were performed at
room temperature and the baseline in the solid-state was
obtained using BaSO4 (Wako Pure Chemical Industries,
Ltd.). The quantum yields of fluorescence (Ffl) in solution
were determined at 25 1C using the optically dilute method
(absorbance lower than 0.05) with anthracene (Ffl = 0.29)34
and fluorescein (Ffl = 0.79)35 in ethanol as quantum yield
standards for the dyes 3 and 5 respectively. Sodium salicylate
(Ffl = 0.42) was used as a quantum yield standard in the solid-
state.36 Time-resolved fluorescence measurements were made by
time-correlated, single-photon counting using an EasyLife V
spectrophotometer at 22 1C. To the dyes 3 and 5, excitation
wavelengths of 405 and 380 nm, respectively, were applied using
a fast laser diode (Table S2, ESIw). The fluorescence emissions
from 3 and 5 were isolated from scattered laser light using a
rejection filter at 400 and 380 nm, respectively. The chi-squared
(w2) statistics were about one and randomness in the fit residuals
was obtained. Scanning electron microscopy (SEM) was
performed using a JEOL LSM 5800 microscope with 20 kV
and 850 times of magnification (Fig. S11 and S12, ESIw).
Samples were fixed on carbon tape on a stub, followed by gold
sputtering. TGA analyses were conducted using a Universal
Analyzer V2.6D (TA Instruments). Dry samples, 3–4 mg, were
directly weighed into aluminum pans. A heating rate of 20 1C
minÀ1 was maintained from 0 to 700 1C with a nitrogen purge.
X-ray diffraction patterns of the powdered samples were
obtained using a Siemens Diffractometer model D500 using
The terephthalate derivative 3 was prepared as already pre-
sented in the literature.38 The silsesquioxane 5 was prepared
according to Fig. 1. In a typical experiment, a suspension of
5.12 g (20 mmol) of diethyl 2,5-dioxocyclohexane-1,4-dicar-
boxylate (1) in 150 mL of glacial acetic acid was heated to
40 1C followed by addition of 4.76 g (44 mmol) of phenyl-
hydrazine (2). The solution, initially colorless, became yellow
to a dark brown upon addition of phenylhydrazine. The final
dark brown solution was mixed for 2 h, diluted in distilled
water and extracted three times with dichloromethane. After
drying the organic phase with sodium sulphate, the solution
was concentrated and the diethyl 2,5-diaminoterephthalate (3)
was precipitated with cyclohexane and purified by column
chromatography eluted with CHCl3 as the solvent. To a
solution of 252 mg (1 mmol) of 3 in ethyl acetate (10 mL)
was added 800 mL (3.16 mmol) of 3-(triethoxysilyl)propyliso-
cyanate (4) and the solution was heated at reflux for 20 h. The
diethylterephthalate (5), which precipitates into the reaction
mixture, was filtered, washed with hexane and dried at room
temperature. No additional purification was needed.
Diethyl 2,5-diaminoterephthalate (3). Yield 40%; light yellow
needles; m.p. 167–169 1C. Found in the literature: 165–167 1C.38
1H NMR (300 MHz, CDCl3): d (ppm) = 7.20 (s, 2H, H2 and
H5); 4.85 (broad s, 4H, NH2); 4.31 (q, 4H, H8, J = 7.2 Hz); 1.36
(t, 6H, H9, J = 7.2 Hz). FTIR (cmÀ1, KBr): 3463 (nas NH2),
3366 (ns NH2), 2996 (naliphatic C–H), 1681 (n CQO), 1580
(naromatic CQC), 1212 (n C–O–C).
Diethyl 2,5-bis[N,N-(3-triethoxysilyl)propylurea]terephthalate (5).
Yield 70%; m.p. 236–238 1C. Anal. calcd. for C32H58N4O12Si2
(776.36 g molÀ1): C 51.45; H 7.83; N 7.50 (%). Found: C 50.31;
Fig. 1 Synthesis of the terephthalates (3) and (5).
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H 7.70; N 7.49 (%). 1H NMR (300 MHz, CDCl3): d (ppm) =
10.05 (s, 2H, NH); 9.16 (s, 2H, H5 and H2); 5.00 (broad s, 2H,
NH); 4.40 (q, 4H, H8, J = 6.9 Hz); 3.86 (q, 12H, H14, J =
6.9 Hz); 3.40–3.25 (m, 4H, H11); 1.81–1.61 (m, 4H, H12); 1.44
(t, 6H, H9, J = 6.6 Hz); 1.26 (t, 18H, H15, J = 6.3 Hz); 0.71
(t, 4H, H13, J = 7.8 Hz). 13C NMR (75 MHz, CDCl3): d
(ppm) = 168 (C7); 155 (C10); 136 (C3 and C6); 122 (C2 and
C5); 119 (C1 and C4); 62 (C8); 59 (C14); 43 (C11); 23 (C12); 18
(C15); 14 (C9); 8 (C13). FTIR (cmÀ1, KBr): 3295 (n NH), 3092
(naromatic C–H), 2965 (naliphatic C–H), 1708 (nester CQO), 1646
(nurea CQO), 1574 (naromatic CQC), 1102 (n C–O–C).
times with ethanol, acetone, ethyl ether and dried under vacuum
for 6 h to give pale yellow powder (517 mg) namely DPM2.
FTIR (cmÀ1, KBr): 3422 (n SiO–H), 1646 (nester and amide CQO),
1062 (n Si–O–Si).
Results and discussion
Photophysical characterization
Fig. 3 presents the normalized absorption of the fluorescent
dyes in different organic solvents and the relevant photo-
physical data are summarized in Table 1. The terephthalates
3 and 5 show a main absorption band (labs) located at around
435 nm and 386 nm, respectively. The molar extinction
coefficient values (emax) are in agreement with 1pp* transitions.
The large difference in the location of the absorption maxima
between these dyes (B50 nm) could not be related only to the
amino group derivatization, which leads to changes in the
electronic distribution in the molecule. The red-shifted bands
in the absorption spectra of dye 3 with respect to dye 5 can
also be related to the formation of intramolecular charge
transfer (ICT) state even in the ground-state due to the strong
electron-donor amino groups (–NH2) and moderate electron-
withdrawing carbonyl groups (–CQO) present in the benzenic
ring. Thus, spectroscopic bands are drastically affected by the
solvent nature.
Synthesis of the hybrid materials. The hydrolysis-polycon-
densation of 5 allowed the formation of polysilsesquioxanes
prepared in the absence (DPM 1) or in the presence (DPM 2)
of TEOS using HCl as catalyst (Fig. 2).
General procedure for the synthesis of xerogel (DPM1). To a
solution of 156 mg (0.21 mmol) of 5 and THF (1.4 mL) was
added 0.083 mL of the catalyst HCl (0.5 M) and water
(1.4 mL) and the mixture was kept under stirring at 60 1C
for 4 days. The obtained solid (116 mg), which precipitates in
solution, was washed several times with ethanol, acetone, ethyl
ether and dried under vacuum for 6 h to give pale yellow
powder namely DPM1. FTIR (cmÀ1, KBr): 3320 (n SiO–H),
2980 (naliphatic C–H), 1716 (nester CQO), 1665 (namide CQO),
1100 (n Si–O–Si).
At this point it is worth mentioning that the terephthalate
core was chosen as a building block for preparing hybrid
materials not only by the two amino moieties, which allowed
us to obtain bis-silylated materials, but also due to the ability
to lead proton transfer in the excited state (ESIPT).40
Concerning this phototautomerism in the excited state, it is
well understood that the main conditions for the ESIPT to
occur are the presence of an intramolecular hydrogen bond
(IHB) between the H atom of the donor group (–OH, –NH2
or –NHR) and the acceptor (–NQ or carbonyl) (intrinsic
General procedure for the synthesis of xerogel (DPM2). To a
solution of 200 mg of 5 (0.27 mmol) and 1.58 g (7.58 mmol) of
TEOS in DMSO (3.66 mL) was added 0.172 mL of water and
0.14 mL of the catalyst HCl (0.5 M). The mixture was kept
under static conditions at room temperature (25 1C). After
ageing the gel for fifteen days, it was powdered, washed several
Fig. 3 Normalized absorption spectra of 3 (top) and 5 (bottom) in
different organic solvents [10À5 M].
Fig. 2 Preparation of materials DPM1 and DPM2.
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Table 1 UV-Vis absorption, fluorescence emission and excitation
data of the dyes 3 and 5, where labs is the absorption maxima, lex is
the excitation maxima and lem is the fluorescence emission maxima in
nanometers, e is the molar extinction coefficient in 104 L molÀ1 cmÀ1
,
Ffl is the fluorescence quantum yield and DlST is the Stokes’ shift in
nm/cmÀ1
Dye
Solvent
labs (e)
lex
lem
DlST
Ffl
3
1,4-Dioxane
THF
Ethanol
MeCN
1,4-Dioxane
THF
Ethanol
DMF
423 (0.60)
448 (4.42)
433 (6.10)
437 (6.75)
393 (0.72)
379 (2.73)
385 (1.30)
388 (5.60)
452
465
446
451
408
411
382
408
488
547
564
546
471
469
474
473
65/3149
99/4040
131/5364
109/4568
78/4214
90/5063
89/4877
85/4632
0.80a
0.63a
0.35a
0.50a
0.85b
0.66b
0.73b
0.77b
5
a
Fluorescein (labs = 429 nm) as the fluorescence quantum yield
b
standard. Anthracene (labs = 375 nm) as the fluorescence quantum
yield standard.
Fig. 5 Normalized fluorescence emission and excitation spectra of 3
(top) and 5 (bottom) in different organic solvents [10À5 M]. The
pictures of the respective dyes in THF solutions and under UV
radiation (365 nm) were also presented.
locally excited (LE) state, may have the lowest energy. In this
way, the role of the solvent polarity is not only to lower the
energy of the excited state due to general solvent effects, but
also to govern which state has the lowest energy.49–51
Dye 3 in 1,4-dioxane presents a broad blue-shifted band
located at 488 nm with a small Stokes shift (3149 cmÀ1), which
can be related to the locally excited state. In this less polar
solvent, the ICT state is not favored, and the band position is
similar to that observed to dye 5, which is not practically
affected by the polarity of the solvent. The significative
difference in the emission maxima between 1,4-dioxane and
acetonitrile provides important evidence for some charge
transfer character in the excited species, which can probably
be related with the amino and carbonyl groups at the ortho-
positions of the terephthalate core.41 An observation at the
ICT (B536 nm) and the locally excited emissions (488 nm)
give two distinct spectra, with excitation maxima at around
452 nm and 427 nm, respectively (Fig. S13 and S14, ESIw).
This photophysical feature indicates that the fluorescence
emission band originates from at least two different species
in the ground state and the absorption spectrum of this dye is a
mixture of these species in the ground state.
Fig. 4 Intramolecular hydrogen bond sites in the terephthalate core.
intramolecular transfers) in the ground state.39 The terephtha-
lates 3 and 5 present two donor centers (–NH2 or –NHR,
respectively) which can form IHB with the acceptor center
(–CQO) (Fig. 4). The photophysical characteristics of ESIPT
dyes is mainly based on the large Stokes’ shift (higher than
6000 cmÀ1 40
and readily backward proton transfer in the
)
ground state after fluorescence emission, although recent
papers have been discussing the ESIPT mechanism with
smaller Stokes’s shift.41,42
The fluorescence emission and excitation spectra of the
terephthalates 3 and 5 are shown in Fig. 5. The dye 3 presents
one main emission band located at 488–564 nm depending on
the solvent, with no overlap of the absorption and fluorescence
spectra, suggesting that photo-excitation induced large struc-
tural changes26 or charge redistribution in the excited state.43
The ability to form an intramolecular hydrogen bond, as well
as the significative difference between absorption and emission
maxima, suggest that the large structural change could be
related to the ESIPT mechanism. However, the fluorescence
quantum yields are quite far from the values usually observed
to ESIPT dyes.44–48
The dye 5 presents a main emission band located at 470 nm
with no solvatochromic effect and a relative large Stokes’ shift
(4214–5063 cmÀ1). These results indicate that the derivatiza-
tion of the amino group to the ureido one seems to play a
major role in the photophysics of these dyes, where the
substitution of the strongly electron-donor –NH2 moiety by
the ureido reduces the electron-donor capacity of the nitrogen
atom due to the electron-withdrawing nature of the carbonyl
group that ICT cannot occur. Therefore, emission bands
practically are unaffected by the solvents (typical for p–p*
transitions). It is worth mentioning that although the fluores-
cence emission is less affected by the solvent polarity, since the
ICT state no longer takes place in the excited state, the Stokes’
shift remains higher and could even be related to an ESIPT
mechanism;41,42 however, one more time the high values for
Additionally, although the emission maxima shift with the
solvent, it could not be observed a tendency concerning the
increase of the solvent polarity/emission maxima location.
Such a solvatochromic effect in the emission maxima of 3
can be associated with changes in the excited state charge’s
distribution as compared to that in the ground state. In more
polar aprotic media, the species with charge separation (ICT
state) may become the lowest energy state. In a less polar
solvent the species without charge separation, the so-called
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Fig. 7 Normalized UV-Vis diffuse reflectance (left) and fluorescence
emission (right) of the fluorescent materials. The UV-Vis and fluorescence
emission of dye 5 in 1,4-dioxane is also presented for comparison. The
picture of the DPM2 after gelation and under UV radiation (365 nm) was
also presented.
Table 2 Photophysical properties measured using a front face solid
sample holder for packed powder samples of DPM1 and DPM2,
where labs is the absorption maxima, lem is the fluorescence emission
maxima in nanometers, Ffl is the fluorescence quantum yield and DlST
is the Stokes’ shift in nm/cmÀ1
a
b
c
Ffl
Material
labs
lem
DlST
DPM1
DPM2
390
394
504
471
114/5799
77/4149
0.02
0.15
Fig. 6 Fluorescence response functions of dye 3 (black) and dye 5
(red) in 1,4-dioxane at room temperature. The LE and the ICT decays
for the dye 3 are analyzed simultaneously (global analysis) to fit a dual
exponential decay function. Weighted residual is also presented.
a
Absorption maxima were measured using an integrating sphere.
Emission maxima upon irradiation at absorption (lexc = 390 nm).
The relative fluorescence quantum yield was determined with sodium
b
c
salicylate (lexc = 370 nm).
the fluorescence quantum yields clearly indicate that the
emission is probably related to a locally excited state in spite
of the lower values expected for the ESIPT tautomer,44–47
although the mechanism of the energy loss in the excited state
still remains unknown. The full width at half the maximum
height (fwhm) of the fluorescence band of dye 5 is practically
invariant in all studied solvents suggesting that not much
change is occurring in the geometry of the molecule on
excitation to the excited state.39
obtaining an environment to the fluorescent dye 5, which is
quite similar to the solution. On the other hand, the self-
condensed material DPM1 presents a fluorescence emission
maximum red-shifted by 33 nm if compared with DPM2,
which can be probably attributed to p–p stacking between
the terephthalate cores. The obtained materials DPM1 and
DPM2 exhibit a weak yellow emission in a solid state and
present low fluorescence quantum yields when compared
with the precursor 5 in solution. Moreover the fluorescence
quantum yield for DPM1 is significantly lower than the
obtained for DPM2. The excited state deactivation is presumably
due to nonradiative decay between the fluorophore with
the inorganic network in spite of the radiative pathways.
Additionally, according to recently published work concerning
the terephthalate derivatives, the quantum yields of pure
powder dyes in a solid state were measured and compared to
the dyes in solution and the authors found a decrease of this
property in the solid state, unless the dyes exhibit an aggregation-
induced emission (AIE).24,26
The time resolved fluorescence emission was performed to
the dyes 3 and 5 in 1,4-dioxane (Fig. 6), where a single
exponential decay could be observed to the dye 5 (9.29 ns).
On the other hand, the experimental results for dye 3 could be
fitted by a dual exponential decay, where a fast fluorescence
decay (0.046 ns), which could be related to the LE state,
followed by a slower one (15.06 ns) could be measured. The
relevant data from this experiment are presented in Table S3
(ESIw).
In Fig. 7 are presented the photophysical study in the solid
state of the obtained materials. The relevant data of DPM1
and DPM2 are summarized in Table 2. In the solid state, the
fluorescent materials present absorption maxima located at
around 390 nm, which are quite similar to those obtained to
the silsesquioxane 5 in solution, indicating that the electronic
structure of the fluorescent precursor was not significantly
perturbed in the ground state after its self-condensation
(DPM1) or co-condensation with TEOS (DPM2). However,
the fluorescence emissions display notable differences between
the final materials. The co-condensation with TEOS allowed in
It is worth mentioning that broader fluorescence emission
curves (fwhm 4 100 nm)13,15 were described for similar silica
hybrid materials, which could be related to two emission
components. Since the materials prepared in this work present
fwhm B80 nm, the contribution of oxygen-related defects in
the siliceous skeleton and electron–hole recombination in the
urea groups cannot be discharged for the fluorescence
emission.
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FTIR characterization
components at 1564 cmÀ1 (fwhm = 49 cmÀ1) and 1545 cmÀ1
(fwhm = 50 cmÀ1), suggesting the formation of hydrogen-
bonded aggregates with two distinct degrees of order. Other-
wise, for DPM2 only one component at 1551 cmÀ1 (fwhm =
56 cmÀ1) was done by curve-fitting of the amide II band,
suggesting that in this material, the co-condensation of dye 5
with TEOS leads to a distancing of N–H and CQO groups
and therefore, the probability to form hydrogen-bonds
diminishes.
The FTIR spectral features of the materials DPM1 and
DPM2, as well as the silsesquioxane 5 are shown in Fig. 8.
Concerning the dye 5 and the self-condensed material DPM1,
both spectra showed bands due to ureido (3300 cmÀ1), propyl
(2970, 2920 and 2880 cmÀ1), and the carbonyl groups
(1700 and 1640 cmÀ1 to ester and amide respectively).
Additionally, the DPM1 displayed stretching bands of the
siloxane network (Si–OH at 3400 cmÀ1 and Si–O–Si at 1100
and 1070 cmÀ1). On the other hand, the DPM2 shows only the
bands related to the inorganic network, as expected.
The amide I band of DPM1 was decomposed into four
components situated at 1712 (fwhm = 48 cmÀ1), 1694
(fwhm = 53 cmÀ1), 1663 (fwhm = 28 cmÀ1) and 1646 cmÀ1
(fwhm = 35 cmÀ1). In the case of the DPM2 material the
amide I was decomposed into three components at 1700
(fwhm = 45 cmÀ1), 1663 (fwhm = 27 cmÀ1) and 1636 cmÀ1
(fwhm = 49 cmÀ1). The 1712–1694 cmÀ1 interval is attributed
to free CQO groups and the band at 1663 cmÀ1 is assigned to
the absorption of hydrogen-bonded CQO groups in disordered
amide–amide aggregates. Finally, the bands at 1646 and
1636 cmÀ1 for DPM1 and DPM2, respectively, are ascribed
to the absorption of CQO groups included in significantly
more ordered hydrogen-bonded amideÀamide associations.
It is worth mentioning that these spectroscopic results of
DPM1 suggest that it does not occur at an effective condensa-
tion of the dye 5. The results from thermogravimetric analysis
(Fig. S15, ESIw) corroborate with this affirmation, where a
similar decomposition profile could be observed between dye 5
and DPM1, as well as a higher decomposition temperature to
the organic portion in the hybrid material (310 1C) in spite of
the dye 5 (267 1C).
The FTIR spectra of DPM1 and DPM2 (Fig. 8) were used
for the determination of the so-called amide I and amide II bands,
as already presented in the literature.13 The 1800–1500 cmÀ1
interval is dominated by a large amide I band at around
1660 cmÀ1 and a sharp amide II band centered at 1550 cmÀ1
(Fig. 9). For both materials, the amide II band, which is mainly
associated with the N–H in-plane bending vibration,13 is more
intense than the amide I band. For DPM1, the curve-fitting
performed for the amide II band was resolved into two individual
X-ray characterization
In Fig. 10 is depicted the X-ray diffraction patterns of dye 5. It
can be observed two main patterns located in the scattering
vector regions 2.44 and 7.38 nmÀ1. The intense peak at
2.44 nmÀ1 (d = 2.57 nm) can be related to the C–C distance
between the carbon atoms from the ethoxy groups bounded to
the silicon atoms (ca. 2.68 nm from theoretical calculations see
ESIw). On the other hand, the attribution of the less intense
peak at 7.38 nmÀ1 (d = 0.85 nm) is not so clear, since two
different sites in the structure could fit with the experimental
Fig. 8 The FTIR spectra of (a) dye 5, (b) DPM1 and (c) DPM2.
Fig. 9 Curve-fitting results in the amide I and amide II regions of the
hybrid materials DPM1 and DPM2.
Fig. 10 X-ray diffraction patterns of the silsesquioxane 5.
c
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data: (i) the distance between the oxygen atoms from the two
ester (ca. 0.74 nm) or (ii) the distance between the two
carbonyl groups from the urea moieties (ca. 0.76 nm). It could
not be possible to obtain X-ray diffraction patterns related to
organization in nanometric level from the materials. A broad
and weak peak located at 3.961 and 6.241 for the materials
DPM1 and DPM2 could be observed, respectively (Fig. S9
and S10, ESIw). The observed X-ray patterns indicate
an absence of organization in the material, although the
silsesquioxane 5 presents hydrogen bond formation ability,
the ester groups play a fundamental role in the precursor
organization does not allow specific interactions leading to
organization at the nanometric level.
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Conclusions
The present study leads to the following conclusions: (i) a
fluorescent terephthalate derivative was successfully obtained
and derivatized in order to obtain a silsesquioxane precursor
to prepare fluorescent organic–inorganic hybrid materials;
(ii) the dye 3 shows typical absorption in the UV-Vis region
and a fluorescence band in the yellow region due to ICT states,
which are very dependent on the solvent polarity due to the
strong electron-donor amino groups and moderate electron-
withdrawing carbonyl groups present in the benzenic ring;
(iii) the substitution of the strongly electron-donor amino
group by the –NH–CQO– group the electron-donor capacity
is reduced due to the electron-withdrawing nature of the
carbonyl group prevents the formation of ICT state. There-
fore, emission bands practically are unaffected by the solvent
polarity; (iv) the dye 3 presents one main emission band
located at a blue-yellow region with no overlap of the absorp-
tion and fluorescence spectra, suggesting that photo-excitation
induced large charge redistribution in the excited state, which
can be related to an ICT state; (v) the new fluorescent hybrid
materials show absorption maxima located at around 390 nm,
indicating that the electronic structure of the fluorescent
precursor was not significantly perturbed in the ground state
after the material preparation. The DPM2 presents a photo-
physical behavior quite similar to the precursor in solution.
The DPM1 presents fluorescence emission red-shifted by
33 nm in spite of DPM2, which can be probably attributed
to p–p stacking between the terephthalate cores. Any evidence
of auto-organization could be observed in these materials.
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Acknowledgements
We are grateful for financial support and scholarships from
the Brazilian agencies CNPq and CAPES and Instituto
Nacional de Inovac
¸ ao em Diagnosticos para a Sau´ de Pu´ blica
´
(INDI-Sau´ de).
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