Development of new precursors for immobilizing dyes onto silica surfaces

Article abstract of DOI:10.1016/j.dyepig.2014.04.002

A series of new precursors for immobilization of dyes onto silica surface have been synthesized and characterized. Diazobenzene-derived dyes were synthesized by coupling salicylaldehyde and a diazotized aniline derivative. Condensation of the resultant carbaldehyde functionalized dyes with 3-aminopropylsilatrane afforded diazo-azomethine functionalized compounds containing silatranyl moiety, which were characterized using spectroscopic techniques, elemental analysis and single-crystal X-ray diffraction. Substituent dependent variation in electronic properties of silatranyl dyes was studied and the capability of immobilization of silatranyl substituted dyes was tested by synthesizing silica nanoparticles coated with immobilized dye. The derivatization of silica was confirmed by spectrophotometric, thermogravimetric and microscopic studies. The functionalized silica exhibit uniformly dispersed spherical particles of 170 nm diameter with an estimated approximate 8 nm coating of dye. As the silatranyl substituted dyes can be isolated in crystalline forms, they proved to be better precursors for immobilization in terms of purity and uniform distribution onto silica surfaces. The modified silica surfaces possess binding sites for capturing copper ions.

Full text of DOI:10.1016/j.dyepig.2014.04.002

Accepted Manuscript
Development of new precursors for immobilizing dyes onto silica surfaces
Ruchi Mutneja, Raghubir Singh, Varinder Kaur, Jörg Wagler, Edwin Kroke
PII:
S0143-7208(14)00122-3
10.1016/j.dyepig.2014.04.002
DYPI 4330
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 9 January 2014
Revised Date: 26 March 2014
Accepted Date: 1 April 2014
Please cite this article as: Mutneja R, Singh R, Kaur V, Wagler J, Kroke E, Development of new
precursors for immobilizing dyes onto silica surfaces, Dyes and Pigments (2014), doi: 10.1016/
j.dyepig.2014.04.002.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Development of new precursors for immobilizing dyes onto silica surfaces
Ruchi Mutnejaa, Raghubir Singhb*, Varinder Kaura*, Jörg Waglerc and Edwin Krokec
aDepartment of Chemistry, Panjab University, Chandigarh – 160 014, India
bDAV College, Sector 10, Chandigarh – 160 010, India
cInstitut für Anorganische Chemie, Technische Universität Bergakademie
Freiberg – 09596 Freiberg, Germany
Abstract
A series of new precursors for immobilization of dyes onto silica surface have been
synthesized and characterized. Diazobenzene-derived dyes were synthesized by coupling
salicylaldehyde and a diazotized aniline derivative. Condensation of the resultant
carbaldehyde functionalized dyes with 3-aminopropylsilatrane afforded diazo-azomethine
functionalized compounds containing silatranyl moiety, which were characterized using
spectroscopic techniques, elemental analysis and single-crystal X-ray diffraction. Substituent
dependent variation in electronic properties of silatranyl dyes was studied and the capability
of immobilization of silatranyl substituted dyes was tested by synthesizing silica
nanoparticles coated with immobilized dye. The derivatization of silica was confirmed by
spectrophotometric, thermogravimetric and microscopic studies. The functionalized silica
exhibit uniformly dispersed spherical particles of 170 nm diameter with an estimated
approximate 8 nm coating of dye. As the silatranyl substituted dyes can be isolated in
crystalline forms, they proved to be better precursors for immobilization in terms of purity
and uniform distribution onto silica surfaces. The modified silica surfaces possess binding
sites for capturing copper ions.
Key words
Diazobenzene-derived dyes, silica nanoparticles, 3-aminopropylsilatrane, silatranyl,
silatranes
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Introduction
Azo dyes have received much attention because of their wide application in the field
of dyeing textile fibre, biomedical studies, advanced application in organic synthesis and high
technology areas such as laser, liquid crystalline displays, electro-optical devices and ink-jet
printers etc. [1-4]. Silica supported dyes have been utilized for catalytic purposes, dye
sensitized solar cells [5,6] and biomedical purposes [7]. Dyes immobilized on silica surface
enhance the stability of pigments and reduce their toxicity in printer’s ink [8,9]. In addition,
dye functionalized silanes have been proved better linkers for dye sensitized solar cells
[10,11]. Keeping in mind the importance of dye immobilized on silica surfaces, an attempt is
made to develop some new precursors for immobilization of dyes on silica surfaces.
The primary requirement for immobilization of a desired functionality onto silica
surface is an adequate precursor with capability of controlled hydrolysis and condensation.
Generally, alkyltrialkoxysilanes with suitable functionality had been utilized as precursors for
the modification of silica surfaces [12-22]. In the previous reports, two main strategies have
been described for the functionalization of silica: 1) derivatization of precursor to introduce
the desired functionality followed by immobilization onto silica surface, or 2) by
immobilizing precursor onto silica surface followed by derivatization to link the desired
functionality with the precursor [23,24]. An illustration for the modification of silica via
strategies 1) and 2) is given in Scheme 1. Functionalization via both strategies may lead to
heterogeneous distribution of functional sites on silica surface. In the first strategy, this
problem may be associated with difficulties to isolate the derivatized alkyltrialkoxysilanes
(which contain bulky groups) in pure form. These compounds are generally oils and can be
purified by vacuum distillation at higher temperature, which may lead to partial
decomposition of attached functionalities. Hence, use of precursors without purification may
lead to non-uniform distribution of functionalities due to the presence of impurities. In the
second strategy, when precursors are immobilized in pure form followed by derivatization
with suitable reagent, silica with mixed functionalities is achieved due to incomplete
derivatization of the parent functional group immobilized on silica. Generally, 3-
aminopropyltriethoxysilane is used as a precursor for developing functionalized silica
surfaces and allows for wide applications in the field of catalysis and metal ion sensing [25-
27].
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Moreover, polymerization of alkyltrialkoxysilanes is relatively fast as they are very
susceptible to hydrolysis, so it is difficult to get desired thickness and morphology of
coatings. Thus, it is interesting to use silatranyl group as an organosilicate precursor, because
silatrane Si−O bonds are more stable towards hydrolysis as compared to those in related
triethoxysilanes [28]. Therefore, silatranyl derivatives may be useful synthons to generate dye
functionalized silica surfaces. Previously we reported some silatranyl derivatives with useful
functionalities [29-32]. In the present work, silatranyl moiety is introduced into different dyes
to develop new precursors for the development of dye functionalized silica. Syntheses of the
silatranyl derivatives of the dyes were explored via azomethine bond formation between dye
and 3-aminopropylsilatrane. One of the silatranyl derivatives of dyes was further tested for
the development of dye functionalized silica nanoparticles.
2. Experimental
2.1. Materials
All the solvents were purchased commercially, dried before use and stored under
nitrogen atmosphere. Salicylaldehyde (Acros), p-anisidine (Acros), 2-methylaniline (Sd-fine),
aniline (Merck), 4-methylaniline (Sd-fine), 4-ethylaniline (Acros), sodium carbonate
(Merck), sodium nitrite (Qualigens), 3-aminopropyl(triethoxy)silane (Aldrich) and
triethanolamine (Merck) were used as suc h without any purification. 3-aminopropylsilatrane
was synthesized from 3-aminopropyltriethoxysilane and triethanolamine as reported in
literature [29].
2.2 Physical measurements
Infrared spectra were routinely obtained on Thermo scientific NICOLET IS50 FT-IR
and Perkin Elmer RX-I FT IR Spectrophotometer. Mass spectral measurements (ESI source
with capillary voltage 2500 V) were carried out on a VG Analytical (70-S) spectrometer. C,
H, N elemental microanalyses were obtained on a FLASH-2000 organic elemental analyzer.
The solution NMR spectra were recorded at 25°C on a Bruker Avance II FT NMR (AL 400
MHz) spectrometer (1H, 13C). Chemical shifts in ppm are reported relative to
tetramethylsilane (TMS). Single-crystal X-ray structure analyses were carried out on a Stoe
IPDS-2T diffractometer using Mo Kα-radiation (λ= 0.71073 Å). The structures were solved
3

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by direct methods (SHELXS-97) and refined with full-matrix least-squares method
(refinement of F2 against all reflections with SHELXL-97). Electronic spectral measurements
were carried out using JASCO V-530 double beam spectrophotometer in the range 250-600
nm. The morphology and size of silica nanoparticles was investigated by using transmission
electron microscopy (TEM) at 80 kV (Hitachi H-7500).
2.3 Synthesis of Azo dyes
Azo dyes were synthesized according to a well known method reported in literature
[33]. That is, aniline or its derivatives (25.0 mmol) were dissolved in conc. hydrochloric acid
(6.0 mL) and heated to form a clear solution. Upon cooling the solution in an ice/NaCl bath a
solution of sodium nitrite (28.0 mmol) in water (10 mL) was added drop-wise keeping the
temperature of the reaction mixture in the range 0-5 °C. The contents were stirred for 30 min
at 0 °C followed by the drop-wise addition of salicylaldehyde (25.0 mmol). Sodium
carbonate (9.0 g) dissolved in water (75 mL) was added to the resulting mixture and stirred
for 2 h at 0-5 °C. The orange precipitate thus formed was filtered and recrystallized from
ethanol.
2.3.1 2-Hydroxy-5-phenylazobenzaldehyde (1a)
1
Yield: 73%. M.p.: 120-125 °C. IR (cm-1): 1568 ν (-N=N-), 1637 ν (C=O), 3186 ν (O-H). H
NMR (CDCl3, 300 MHz, ppm): δ 7.05 (d, 1H11), 7.40 (t, 2H3,5 ), 7.46 (tt, 1H4), 7.81 (dd,
2H2,6), 8.08 (dd, 1H12), 8.11(d, 1H8), 9.97 (s, 1H, CHO), 11.21 (s, 1H, OH). 13C NMR (75.57
MHz, CDCl3): δ (ppm) 118.5 (C3), 120.3 (C8,12), 122.7 (C1), 122.8 (C4), 129.0 (C6), 129.2
(C9,11), 130.6 (C10), 145.8 (C5), 152.3 (C7), 162.3 (C2), 196.5 (CHO).
2.3.2 2-Hydroxy-5-p-tolylazobenzaldehyde (1b)
1
Yield: 71%. M.p.: 155-160 °C. IR (cm-1): 1567 ν (-N=N-), 1646 ν (C=O), 3325 ν (O-H). H
NMR (CDCl3, 300 MHz, ppm): δ 2.09 (s, 3H, Ar−CH3), 6.81 (d, 1H11), 7.28 (d, 2H3,5 ), 7.69
(dd, 2H2,6), 7.72 (dd, 1H12), 8.06 (d, 1H8), 9.95 (s, 1H, CHO), 11.07 (s, 1H, OH). 13C NMR
(75.57 MHz, CDCl3): δ (ppm) 120.5 (C3), 122.3 (C8,12), 126.7 (C1), 128.3 (C6), 129.55 (C9,11),
129.63 (C4), 140.2 (C10), 143.5 (C5), 150.8 (C7), 164.1(C2), 195.6 (CHO).
2.3.3 5-(4-Ethylphenylazo)-2-hydroxybenzaldehyde (1c)
4

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1
Yield: 75%. M.p.: 120-122 °C. IR (cm-1): 1578 ν (-N=N-), 1656 ν (C=O), 3182 ν (O-H). H
NMR (CDCl3, 300 MHz, ppm): δ 1.25 (t, 3H, Ar−CCH3), 2.67 (q, 2H, Ar−CH2C), 7.01 (d,
1H11), 7.23 (d, 2H3,5 ), 7.72 (d, 2H2,6), 8.0 (dd, 1H12), 8.11 (d, 1H8), 9.96 (s, 1H, CHO), 11.19
(s, 1H, OH). 13C NMR (75.57 MHz, CDCl3): δ (ppm) 120.5 (C3), 122.3 (C8,12), 122.9 (C1),
128.6 (C6), 129.0 (C9,11), 129.1 (C4), 146.0 (C10), 147.9 (C5), 150.6 (C7), 164.8 (C2), 196.6
(CHO).
2.3.4 2-Hydroxy-5-(4-methoxyphenylazo)benzaldehyde (1d)
1
Yield: 71%. M.p.: 125-128 °C. IR (cm-1): 1597 ν (-N=N-), 1655 ν (C=O), 3785 ν (O-H), H
NMR (CDCl3, 300 MHz, ppm): δ 3.83 (s, 3H, OCH3), 6.91(d, 1H11), 7.00 (d, 2H3,5 ), 7.79 (d,
13
2H2,6), 7.77 (dd, 1H12), 8.06 (d, 1H8), 9.96 (s, 1H, CHO), 11.15 (s, 1H, OH). C NMR
(75.57 MHz, CDCl3): δ (ppm) 114.5 (C8,12), 118.4 (C3), 120.3 (C1), 124.7 (C9,11), 128.7 (C6),
130.5 (C4), 145.9 (C5), 146.6 (C7), 162.1 (C10), 163.2 (C2), 196.60 (CHO).
2.3.5 2-Hydroxy-5-o-tolylazobenzaldehyde (1e)
Yield: 76%. M.p.: 120-123 °C. IR (cm-1): 1580 ν (−N=N−), 1651 ν (C=O), 3300 ν (O−H) . 1H
NMR (CDCl3, 300 MHz, ppm): δ 2.66 (s, 3H, Ar-CH3), 7.01 (d, 1H11), 7.18 - 7.24 (m, 3H3,4,5
13
), 7.79 (d, 1H6), 8.05 (dd, 1H12), 8.08 (d, 1H8), 9.97 (s, 1H, CHO), 11.20 (s, 1H, OH). C
NMR (75.57 MHz, CDCl3): δ (ppm) 119.8 (C3), 136.0 (C8,12), 125.6 (C1), 128.46 (C6), 130.8
(C9,11), 129.5 (C4), 137.8 (C10), 144.7 (C5), 151.3 (C7), 168.1 (C2), 195.3 (CHO).
2.4 Linking silatranyl moiety with dyes
3-Aminopropylsilatrane (2.15 mmol, 0.50 g) in toluene (15 mL) was added to a
single-neck round bottom flask fitted with Dean-Stark trap. Azo dye (2.01 mmol) dissolved
in toluene was added to 3-aminopropylsilatrane drop-wise using a syringe. The contents were
refluxed and water produced during the reaction was removed by azeotropic distillation with
toluene. The contents were cooled to room temperature and toluene was removed under
vacuum to obtain a solid product. The orange solid was washed with diethyl ether, filtered
and dried. Crystals for X-ray crystallography were obtained at room temperature in
DCM/hexane mixture by slow evaporation method.
2.4.1 4-Phenylazo-2-{[3-(2,8,9-trioxa-5-aza-1-silabicyclo[3.3.3]undec-1-yl)propylimino]me
thyl}-phenol (2a)
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Yield 74%. M.p.:150-155 °C. Anal. Calcd for C22H32N4O4Si: C, 61.51; H, 6.88; N, 11.96;
Found: C, 59.83; H, 6.63; N, 12.00. IR (cm-1): 580 m (υ Si←N), 717 s, 762 s (υs Si–O), 844 m,
877 w (υ C–N), 903 m (υs NC3), 934 m (υ C–C), 986 w, 1021 s, (υas NC3), 1098 vs (υas Si–O),
1128 vs (υ C–O), 1179 m (τ CH2O), 1279 m (ω CH2O), 1372 m, 1413 w (δ CH3C), 1476 (υ
1
C=C), 1568 (–N=N–), 1658 vs (υ C=N), 2876 s, 3039 (υs CH2), 3186 b (N–H---O). H NMR
(300 MHz, CDCl3): δ (ppm) 0.36 (m, 2H, SiCH2), 1.75 (m, 2H, CCH2C), 2.71 (t, 6H, NCH2C,
J = 5.8 Hz), 3.54 (t, 2H, CCH2N), 3.66 (t, 6H, OCH2, J = 5.8 Hz), 6.48 (d, H11), 7.00-7.38 (m,
3H3,4,5 ), 7.40 (dd, 2H2,6), 7.51 (dd, 1H12), 7.52 (d, 1H8), 7.91 (s, 1H, N=CH), 13.90 (s, 1H,
OH), 13C NMR (75.57 MHz, CDCl3): δ (ppm) 13.32 (SiCH2), 26.48 (CCH2C), 51.36
(CCH2N), 57.84 (NCH2), 60.73 (OCH2), 120.5 (C3), 122.4 (C1), 122.8 (C8), 128.2 (C12), 129.0
(C6), 129.1 (C4), 130.1 (C9) 130.2 (C11), 131.1 (C10), 143.8 (C5), 152.7 (C7), 164.7 (C=N),
165.9 (C2). MS (% relative abundance of m/z assignment): 174 (16.8, N(CH2CH2O)3Si)+, 233.2
+
{8.07, N(OCH2CH2)3Si(CH2)3NH3 }, 441.2 (100, M + H)+, 463.4 (6.60, M + Na)+, 464.2
(8.81, M + H + Na)+.
2.4.2 4-p-Tolylazo-2-{[3-(2,8,9-trioxa-5-aza-1-silabicyclo[3.3.3]undec-1-yl)propylimino]met
hyl}phenol (2b)
Yield: 77%. M.p.:125-128°C. Anal. Calcd for C23H30N4O4Si: C, 60.77; H, 6.65; N, 12.32;
Found: C, 58.67; H, 6.43; N, 12.49. IR (cm-1): 582 m (υ Si←N), 710 s, 768 s (υs Si–O), 794
m, 866 w (υ C–N), 910 m (υs NC3), 940 m (υ C–C), 986 w, 1021 s, (υas NC3), 1088 vs (υas Si–
O), 1124 vs (υ C–O), 1176 m (τ CH2O), 1225 m (ω CH2O), 1352 m, 1413 w (δ CH3C), 1481
1
(υ C=C), 1580 (−N=N−), 1630 vs (υ C=N), 2876 s, 2962 (υs CH2), 3325 b (N–H---O). H
NMR (300 MHz, CDCl3): δ (ppm) 0.36 (m, 2H, SiCH2), 1.74 (m, 2H, CCH2C), 2.35 (s, 3H,
Ar−CH3), 2.69 (t, 6H, NCH2C, J = 5.7), 3.53 (t, 2H, CCH2N), 3.65 (t, 6H, OCH2, J = 5.7 Hz),
6.98 (d, 1H11), 7.30 (d, 2H3,5 ), 7.72 (dd, 2H2,6), 7.92 (dd, 1H12), 7.98 (d, 1H8), 8.27 (s, 1H,
13
N=CH), 14.45 (s, 1H, OH). C NMR (75.57 MHz, CDCl3): δ (ppm) 13.30 (SiCH2), 21.60
(Ar−CH3), 26.48 (CCH2C), 51.32 (CCH2N), 57.79 (NCH2), 60.99 (OCH2), 116.1 (C3), 119.3
(C1), 122.6 (C8, C12), 126.7 (C6), 128.3 (C4), 129.6 (C9,11), 139.9 (C10), 144.2 (C5), 150.9 (C7),
163.9 (N=CH), 168.04 (C2). MS (% relative abundance of m/z assignment): 174 (15.5,
N(CH2CH2O)3Si)+),
192
{3.75,
N(CH2CH2O)3Si)+
+
NH3},
233
{4.89,
+
N(CH2CH2O)3Si(CH2)3NH3 }, 455.1 (100, M + H)+, 478.1 (5.83, M + Na)+.
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2.4.3 4-(4-Ethylphenylazo)-2-{[3-(2,8,9-trioxa-5-aza-1-silabicyclo[3.3.3]undec-1-yl)propyl
imino]methyl}phenol (2c)
Yield: 73%. M.p.:135-138°C. Anal. Calcd for C24H32N4O4Si: C, 61.51; H, 6.88; N, 11.96;
Found: C, 62.70; H, 6.76; N, 10.92. IR (cm-1): 585 m (υ Si←N), 710 s, 762 s (υs Si–O), 783
m, 866 w (υ C–N), 910 m (υs NC3), 940 m (υ C–C), 986 w, 1021 s, (υas NC3), 1088 vs (υas Si–
O), 1124 vs (υ C–O), 1176 m (τ CH2O), 1225 m (ω CH2O), 1352 m, 1413 w (δ CH3C), 1439
1
(υ C=C), 1598 (–N=N–), 1634 vs (υ C=N), 2864 s, 2974 (υs CH2), 3392 b (N–H---O). H
NMR (300 MHz, CDCl3): δ (ppm) 0.47 (m, 2H, SiCH2), 1.27 (t, 3H, Ar−C−CH3) 1.84 (m,
2H, CCH2C), 2.71 (q, 2H, Ar−CH2−C), 2.78 (t, 6H, NCH2C, J = 4.35 Hz), 3.59 (t, 2H,
CCH2N), 3.75 (t, 6H, OCH2, J = 4.35 Hz), 6.91 (d, 1H11), 7.26 (d, 2H3,5 ), 7.76 (d, 2H2,6), 7.80
(dd, 1H12), 7.94 (d, 1H8), 8.25 (1H, N=CH), 14.60 (s, 1H, OH). 13C NMR (75.57 MHz,
CDCl3): δ (ppm) 13.26 (SiCH2), 15.73 (Ar−CH2CH3), 26.51 (CCH2C), 28.9 (Ar-CH2CH3),
51.45 (CCH2N), 57.90 (NCH2), 61.39 (OCH2), 116.3 (C3), 120.3 (C8,12), 122.4 (C1), 128.3
(C6), 128.4 (C4), 129.4 (C9,11), 143.8 (C10), 146.7 (C7), 151.0 (C5), 163.2 (N=CH), 164.7
(C2),. MS (% relative abundance of m/z assignment): 174 (17.22, N(CH2CH2O)3Si)+), 233
+
{15.1, N(CH2CH2O)3Si(CH2)3NH3 }, 266.3 {4.59, Si(OCH2CH2)N(CH2)3N=CH + Na}, 469.4
(100, M + H)+, 491.5 (1.59, M + Na)+, 492.5 (0.49, M + H + Na)+.
2.4.4 4-(4-Methoxyphenylazo)-2-{[3-(2,8,9-trioxa-5-aza-1-silabicyclo[3.3.3]undec-1-yl)pro
pyimino]methyl}phenol (2d)
Yield: 78%. M.p.:160-163°C. Anal. Calcd for C23H30N4O5Si: C, 58.70; H, 6.43; N,
11.91; Found: C, 56.25; H, 6.07; N, 10.55. IR ( cm-1): 586 m (υ Si←N), 721 s, 771 s (υs Si–
O), 840 m, 877 w (υ C–N), 908 m (υs NC3), 937 m (υ C–C), 987 w, 1028 s, (υas NC3), 1089
vs (υas Si–O), 1129.42 vs (υ C–O), 1181 m (τ CH2O), 1225 m (ω CH2O), 1352 m, 1413 w (δ
CH3C), 1461 (υ C=C), 1595 (−N=N−), 1628 vs (υ C=N), 2866 s, 2929 (υs CH2), 3342 b (N–
H---O). 1H NMR (300 MHz, CDCl3): δ (ppm); δ 0.35 (m, 2H, SiCH2), 1.74 (m, 2H, CCH2C),
2.69 (t, 6H, NCH2C, J = 5.7 Hz), 3.53 (t, 2H, CCH2N), 3.65 (t, 6H, OCH2, J = 5.7 Hz), 3.80
(s, 3H, OCH3), 6.84 (d, 1H11), 7.23 (d, 2H3,5 ), 7.75 (d, 2H2,6), 7.79 (dd, 1H12), 7.82 (d, 1H8),
13
8.28 (N=CH), 11.15 (s, 1H, OH), C NMR (75.57 MHz, CDCl3): δ (ppm) 13.34 (SiCH2),
26.52 (CCH2C), 51.32 (CCH2N), 57.79 (NCH2), 55.37 (Ar−OCH3), 61.21 (OCH2), 114.1
(C9,11), 117.7 (C3), 119.1 (C1), 124.3 (C8,12), 126.5 (C4), 127.8 (C6), 163.9 (C10), 147.7 (C7),
144.8 (C5), 163.9 (N=CH), 167.41 (C2). MS (% relative abundance of m/z assignment):
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150{3.47, NH(CH2CH2OH)3}, 174 (10.68, N(CH2CH2O)3Si)+), 233 {12.02,
+
N(CH2CH2O)3Si(CH2)3NH3 }, 471.2 (100, M + H)+, 493.2 (3.83, M + Na)+.
2.4.5 4-o-Tolylazo-2-{[3-(2,8,9-trioxa-5-aza-1-silabicyclo[3.3.3]undec-1-yl)-propyliminome
thyl}phenol (2e)
Yield: 80%. M.p.:125-128°C. Anal. Calcd for C23H30N4O4Si: C, 60.77; H, 6.65; N, 12.32;
Found: C, 59.12; H, 6.28; N, 11.77. IR (cm-1): 583 m (υ Si←N), 717 s, 762 s (υs Si–O), 835
m, 877 w (υ C–N), 904 m (υs NC3), 934 m (υ C–C), 1013 w, 1052 s, (υas NC3), 1098 vs (υas
Si–O), 1128 vs (υ C–O), 1179 m (τ CH2O), 1274 m (ω CH2O), 1372 m, 1479 w (δ CH3C),
1479 (υ C=C), 1590 (–N=N–), 1633 vs (υ C=N), 2863 s, 2925 (υs CH2), 3300 b (N–H---O).
1H NMR (300 MHz, CDCl3): δ (ppm) 0.37 (m, 2H, SiCH2), 1.75 (m, 2H, CCH2C), 2.63 (s,
3H, Ar−CH3), 2.70 (t, 6H, NCH2C, J = 5.7 Hz), 3.59 (t, 2H, CCH2N), 3.75 (t, 6H, OCH2, J =
5.7 Hz), 6.84 (d, 1H11), 7.11 - 7.47 (m, 3H3,4,5 ), 7.52 (d, 1H6), 7.83 (dd, 1H12), 7.86 (d, 1H8),
13
8.28 ( N=CH), 14.20 (s, 1H, OH), C NMR (75.57 MHz, CDCl3) δ (ppm) 13.36 (SiCH2),
17.72 (Ar-CH3), 26.51 (CCH2C), 51.32 (CCH2N), 57.83 (NCH2), 60.68 (OCH2), 115.5 (C3),
116.1 (C8), 117.3 (C1), 119.7 (C6), 126.35 (C9), 126.4 (C11), 126.5 (C4), 129.2 (C10), 129.9
(C12), 137.2 (C7), 131.1 (C5), 163.9 (N=CH), 168.75 (C2). MS (% relative abundance of m/z
assignment): 174 {13.08, Si(OCH2CH2)3N+}, 233.2 {61.95, N(CH2CH2O)3Si(CH2)3NH3 },
+
455.2 (100, M + H)+, 477.2 (28.49, M + Na)+, 478.3 (7.62, M + H + Na)+.
2.5 Synthesis of dye coated silica nanoparticles (D-SNP)
Silatranyl functionalized dye compound 2d was used to test immobilization onto
silica surface using a modified Stöber method [34]. Dye functionalized silica nanoparticles
(D-SNP) were prepared by stirring 1.5 mL of tetraethylorthosilicate and 3 mL of aqueous
25% ammonia solution in 45 mL of ethanol at room temperature. When the solution turned
milky (after ca. 1 h), silatranyl functionalized dye (2d, 0.25 g, 5.3 mmol) was added to the
above solution and stirred overnight. The particles formed were centrifuged and washed 4-5
times with anhydrous ethanol followed by washing with dichloromethane to remove
physically adsorbed dye. In addition, silica nanoparticles (SNPs) without the addition of dye
were prepared following same procedure but without adding dye compound (2d). The
obtained silica particles (SNPs and D-SNPs) were characterized by IR, TEM and TGA
analysis to confirm immobilization of dye onto silica surface.
8

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2.6 Metal binding studies
Binding capability of D-SNPs for coordinating metal ions was studied by UV-Vis
spectrophotometry. Two solutions containing 5 mL of copper(II) chloride solution (0.5 mmol
L-1) in methanol were stirred with 0.1 g of SNPs and D-SNPs in separate flasks for 30 min.
The solutions were filtered and the absorbance of filtrate was recorded
spectrophotometrically. It was compared with absorbance of 0.5 mmol L-1 copper(II) chloride
solution.
3. Results and Discussion
3.1 Synthesis
Substituted anilines were diazotized with sodium nitrite in the presence of
hydrochloric acid followed by coupling with salicylaldehyde to yield azo dyes (1a-1e). To
introduce the active moiety for immobilization, azo dyes were reacted with 3-
aminopropylsilatrane to form their silatranyl derivatives (2a-2e), which are diazo-azomethine
dyes (Scheme 2). Compounds 2a-2e were stable at room temperature and were isolated as
solids, which were purified by recrystallization from a mixture of dichloromethane (DCM)
and hexane. As a representative example, compound 2d was utilized further for the
development of dye functionalized silica nanoparticles using Stöber’s method, which
involves addition of silatranyl derivative of dye during the nucleation of silica nanoparticles.
Silatranyl derivatives of dyes offer advantages in the purification of dyes as well as
immobilization via formation of Si-O bonds. Silatranyl moiety also acts as hydrolysis
retarding agent, which regulates the hydrolysis and condensation processes very efficiently
due to steric and chelate effects of the tricyclic cage. It also results in the formation of
uniform coating onto silica surface. Apart from this, steric effects are observed when dyes are
directly reacted with active silanol groups on silica surface, which retards the immobilization
due to restriction of incoming molecules. Therefore, insertion of the propyl chain as a linking
group between silica surface and dyes facilitates tethering of bulky dye molecules by
reducing steric hindrance at the silica surface. The distribution of functionalities on the silica
surface is also uniform as the precursor is modified before immobilization and isolated in
pure form. In addition, Schiff base linkage in the modified surface extends the coordination
sites and may act as active centre for extracting metal ions.
9

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Azo-dye immobilization using precursor 2d is illustrated in Scheme 3. Herein, two
types of silica nanoparticles were developed in the absence of 2d and in the presence of dye
precursor using the same method to correlate the parameters. Dye functionalized silica
nanoparticles were obtained by one pot synthesis following the Stöber Method. It involves
the formation of silica nanoparticles with the simultaneous addition of 2d on the appearance
of turbidity after nucleation of silica obtained by hydrolysis/condensation of TEOS. The –OH
groups formed on the surface of silica nanoparticles react with silatranyl moiety to release
ethanolamine, which also helps in the dispersion of particles. The modifier binds with silica
surface through covalent bonding.
3.2 Characterization of silatranyl dyes
Both dyes 1a-1e and their Schiff bases 2a-2e exhibited broad bands in the IR region
3200-3400 cm-1 and 1434-1495 cm-1 due to OH stretching and N=N stretching vibration,
respectively. Absence of C=O absorption and appearance of absorption band in the region of
1600-1620 cm-1 indicates the formation of diazo-azomethine derivatives of dyes. NMR (1H
and 13C) spectra of silatranyl derivatives of dyes support the formulation of synthesized
compounds. The characteristic peaks of silatranyl moieties appeared in the range of 2.69-2.78
and 3.65-3.75 ppm due to NCH2 and OCH2 groups, respectively. The mass spectra of diazo-
azomethine compounds exhibited corresponding molecular ion peaks. Besides these peaks,
all compounds show peaks at m/e 493.2, 478.1, 491.5, 477.2, 463.4 due to (M+Na)+. A most
common fragmentation of silatranyl ring by the cleavage of Si-CH2 bond caused a peak at
m/e 174 and cleavage of azomethine linkage forms aminopropylsilatranyl cation observed at
m/e 233 in all compounds.
3.2.1 X-Ray Crystallography
Molecular structures of compounds 2a-2e are shown in Fig. 1-5 along with the atomic
numbering scheme. X-ray crystallographic parameters, selected bond angles, and bond
distances are given in Tables 1-2. In case of 2b-2d, molecules are packed in such a way that
long axial chains of two molecules approach towards each other but their dye scaffolds turned
away producing antenna like projections. Packing in 2e is found to be different as the
molecules form a V-shaped structure in which dye part approach towards each other to form
a cone while silatranyl rings form terminal ends of cone. These cones are further arranged
antiparallel to each other in a lattice (Fig. S1- S4).
10

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3.2.2 Thermogravimetric analysis
Thermogravimetric (TGA) curves of compounds 2a-2e were recorded to study their
thermal stability (Fig. 6). All the complexes were heated from 25 to 1000 °C. The compounds
showed a similar pattern on heating due to loss of complete skeleton in the form of ethanol,
methanol, nitrogen dioxide, carbon dioxide and water. The thermogravimetric curve for SNP and
D-SNP are also shown, which revealed the immmobalization of dye on SNPs.
The tricyclic cage of silatranyl moiety may be lost as ethanol up to 250° C, which is
clearly indicated on the TGA graph of each compound. The rest of the molecule is
decomposed after crossing 300 °C with the evolution of common gases leaving only silica at
the end. Silica nano particles with immobilized dye showed only 22% loss, which revealed
the rest part of silica nanoparticles is composed of siloxane and remained unaffected even at
1000° C.
3.2.3 Electronic absorption Spectra
The electronic spectra of azo dyes (1a-1e) and their diazo-azomethine derivatives (2a-
2e) were recorded in acetonitrile at room temperature (Fig. S5). Spectra of compounds 2a-2e
show absorption bands in the region 270-300 nm and 344-385 nm with a shoulder in the
region 420-440 nm. The bands are broad, which is quite common in the systems having –OH
group opposite to N=N and C=N in aromatic skeletons. Broadness of bands and appearance
of shoulders in the spectra may be attributed to the formation of azo-hydrazone-enaminone
tautomers, which exist in equilibrium in solution phase (Fig. S6). The UV-Visible spectra of
diazo-azomethine derivatives show slight bathochromic shift in lower energy transition
relative to corresponding azo dyes, which may be explained on the basis of extended
conjugation system and intramolecular hydrogen bonding of C=N and OH group.
Solvatochromism behavior of all dyes after the introduction of silatranyl moiety was studied,
which revealed slight change in the electronic transitions with the variation in polarity of
solvent used.
This indicates shifts in transitions are independent upon the polarity of solvent and
variation in band is due to change in the equilibrium between three tautomers. All solvents
except THF show high capability to accept the proton of OH-group and results in the
formation of imino-hydrazone and azo-enaminone tautomers. Therefore, assignment of bands
into π-π* transition in benzenoid system and π-π* transition in conjugated system (azo,
azomethine and aryl rings) is difficult.
11

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3.3 Characterization of silica nanoparticles functionalized with dyes
The FT-IR spectra of precipitated silica nanoparticles and dye coated silica
nanoparticles are shown in Fig. 7 Absorption peaks at 469, 801, 954 and 1099 cm−1
correspond to the bending vibration, symmetric stretching vibration, asymmetric stretching
vibration of Si-O-Si and bending vibration of Si-OH, respectively. The absorption bands at
1638 and 3428 cm−1 correspond to bending and stretching vibration of OH. At the same time,
the absorption bands at 1400 and 1501 cm−1 correspond to the stretching vibration of diazo
group, which demonstrates coating of dye onto silica surface.
TEM images of spherical and uniformly coated silica nanoparticles (SNPs) are shown
in Fig. 8. The size of the silica nanoparticles prepared without the addition of modifier lies in
the range of 160-170 nm. The average size of dye functionalized silica nanoparticles is
calculated around 180 nm indicating the coating of modifier on the silica surface without
affecting the size of particles to much extent. The images revealed smooth surface of silica
after immobilization of dyes, which may be due to uniform distribution of modifier on silica
surface. TEM images of SNPs after immobilization of diazo-azomethine dyes clearly indicate
increased particle size, which is interpreted to be due to a uniform layer with an estimated
thickness of 8 nm coated on the surface of SNPs. The smoothness in coating is obtained due
to silatranyl group as modifier because triethanolamine anions released during the hydrolysis
prevents the formation of aggregates on silica surfaces. This particular advantage of silatranes
for developing smooth surfaces has also been reported in AFM applications [35].
4. Metal binding studies
Dye coated silica nanoparticles (D-SNPs) carrying binding sites for metal ions may
act as metal capturing surfaces. These binding sites are found to be capable of coordinating
copper ions from the solution more efficiently relative to uncoated silica nanoparticles
(SNPs). Both the SNPs and D-SNPs were stirred with copper(II) chloride solution to study
the sorption of copper ions. After filtration, UV-Vis spectra of filtrates obtained from both
solutions were recorded. The band with absorption maximum at 270 nm for copper chloride
solution diminishes as it is stirred with D-SNPs whereas absorption remains same after
stirring with SNPs. It clearly indicates high affinity of D-SNP for the sorption of copper due
to the presence of diazo and azomethine moieties (Fig. 9).
12

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5. Conclusion
Silatranyl moieties are introduced in azo dye structures by a simple and efficient
method via azomethine linkage. The derivatization of dyes makes them immobilizable on
silica surfaces directly through Si-O covalent bonding. Monodispersed, spherical silica
nanoparticles can be isolated by one pot synthesis as suggested by Stöber. The method is
highly efficient and results in the formation of smoothly coated silica nanoparticles. It may be
attributed to direct interaction of dyes with silanol groups during the formation of silica
nanoparticles. The long propyl chain in the modifier also helps in reducing the steric
hindrance at silica surface, which may result from direct interaction with bulky dye
substituents. The surfaces can be used for analytical and catalytic application due the
presence of metal coordination sites.
Appendix A. Supplementary material
CCDC 968725-968729 contain the supplementary crystallographic data for 2a-2e. These data
can be obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgement
Authors are thankful to UGC, New Delhi for providing financial support under UGC Start Up
Scheme (No. F.20-1/2012(BSR)/20-9(12)/2012(BSR)
13

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Scheme and Figure Captions
Scheme 1
Scheme 2:
Modification of silica
General procedure for the introduction of silatranyl group in diazo
azomethine dyes
Scheme 3:
Fig. 1
Immobilization of silica nanoparticles with diazo-azomethine dyes
Molecular structure of 2a in the crystal structure of its DCM solvate.
Ellipsoids are shown at the 40% probability level. Most of the C-bound H
atoms were omitted and selected atoms are labelled for clarity.
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Molecular structure of 2b in the crystal. Ellipsoids are shown at the 40%
probability level. C-bound H atoms were omitted and selected atoms are
labelled for clarity.
Molecular structure of 2c in the crystal. Ellipsoids are shown at the 40%
probability level. C-bound H atoms were omitted and selected atoms are
labelled for clarity.
Molecular structure of 2d in the crystal. Ellipsoids are shown at the 40%
probability level. C-bound H atoms were omitted and selected atoms are
labelled for clarity.
Molecular structure of 2e in the crystal. Ellipsoids are shown at the 40%
probability level. C-bound H atoms were omitted and selected atoms are
labelled for clarity.
Fig.6
Fig. 7
Fig. 8
Thermogravimetric curves for 2a-2e as well as for SNP and D-SNP
IR spectra of (A) Silica nanoparticles (B) Dye modified silica nanoparticles
Transmission Electron Micrograph of Silica nanoparticles at different
magnification (A-C). Dye modified silica nanoparticles showing coating of
dye at different magnifications (D-F)
Fig. 9
UV-Vis absorbance spectra of 0.5 mmol L-1 copper(II) chloride solution in
methanol (a) before binding (b) after stirring with 0.10 g of D-SNP in 5 mL of
this solution (c) after stirring with 0.10 g of SNP in 5 mL of this solution

ACCEPTED MANUSCRIPT
Table 1 X-ray crystal data and structure refinement of 2a-2e.
(2a)2(CH2Cl2)0.7 2b
2c
2d
2e
Empirical formula
C44.7H57.4Cl1.4N8O8S C23H30N4O4Si
i2
C24H32N4O4Si
C23H30N4O5Si
C23H30N4O4Si
Formula weight
T (K)
λ (Ǻ)
Crystal system, space
group
940.60
150(2)
0.71073
Triclinic
P-1
454.60
180(2)
0.71073
Monoclinic
P21/c
468.63
200(2)
0.71073
Monoclinic
P21/c
470.60
180(2)
0.71073
Monoclinic
P21/c
454.60
180(2)
0.71073
Monoclinic
P21/c
Unit cell dimensions
a (Ǻ)
11.2221(8)
18.3768(10)
18.8325(11)
16.1912(8)
7.2149(4)
b (Ǻ)
c (Ǻ)
12.9533(11)
17.2325(14)
7.7832(3)
18.1366(11)
7.9000(2)
18.2846(10)
7.8419(6)
18.2830(10)
29.7236(15)
10.9266(6)
α (°)
β (°)
70.994(6)
77.271(6)
86.757(6)
2309.9(3)
2
1.352
0.220
994.7
0.15 x 0.10 x 0.02
90
90
90
92.036(4)
90
2319.9(2)
4
1.347
0.144
1000
90
115.992(4)
90
2331.7(2)
4
1.295
0.138
968
117.352(4)
90
2416.2(2)
4
1.288
0.135
1000
0.60 x 0.55 x 0.18
101.510(4)
90
2296.1(2)
4
1.315
0.140
968
γ (°)
V (Ǻ3)
Z
ρcalc(Mg/m3)
µMokµα(mm-1)
F (000)
Crystal size (mm)
0.45 x 0.40 x 0.08
0.30 x 0.15 x 0.07 0.35 x 0.20 x 0.15
Theta range for data
collection
2.4 to 25.0 deg.
2.5 to 27.0 deg.
2.4 to 28.0 deg.
2.5 to 25.0 deg.
2.3 to 27.0 deg.
Limiting indices
-13≤ h≤ 13,
-15≤ k≤ 15,
-20≤ l≤ 20
15879
-23≤ h≤ 23,
-9≤ k≤ 9,
-23≤ l≤ 23
20574
-24≤ h≤ 24
-8≤ k≤ 10
-24≤ l≤ 22
19107
-19≤ h≤ 19
-9≤ k≤ 9
-21≤ l≤ 20
17859
-9≤ h≤ 8,
-37≤ k≤ 37,
-13≤ l≤ 13
20906
Reflections collected

ACCEPTED MANUSCRIPT
Unique
8033 [R(int) =
0.0733]
25.0/ 98.7
5079 [R(int) =
0.0305]
27.0/ 99.9
5811 [R(int) =
0.0217]
28.0/ 99.6
4019 1
27.97/ 98.5
None
5000 [R(int) =
0.0330]
27.0/ 99.9
θ max
(°)/Completeness(%)
Absorption correction
Integration
Integration
Integration
Integration
Max. and min.
transmission
0.9953 and 0.9577
0.9957 and 0.9606
0.9861and 0.9229
-
0.9894 and 0.9139
Refinement method
Full-matrix least-
squares on F2
Full-matrix least-
squares on F2
Full-matrix least-
squares on F2
Full-matrix least- Full-matrix least-
squares on F2
4019/ 6/311
squares on F2
5000/3/324
Data / restraints /
parameters
8033/44/665
5079/13/332
5811/9/346
GoF on F2
1.030
1.031
1.035
1.060
1.022
R [I>2σ(I)]
R1 = 0.0606, wR2 = R1 = 0.0366, wR2 = R1 = .0366, wR2 =
0.1148 0.0911 0.1002
R1 = 0.1413, wR2 = R1 = 0.0536, wR2 = R1 = 0.0435, wR2 = R1 = 0.0914, wR2 R1 = 0.0492, wR2
R1 = 0.0644, wR2 R1 = 0.0367, wR2
= 0.1737 = 0.0891
R (all data)
0.1380
0.0989
0.0425
= 0.1993
= 0.0951
Largest diff. peak and
0.283 and -0.306
0.279 and -0.233
0.286 and -0.271
0.315 and -0.330
0.328 and -0.226
hole (eǺ-3)
1
This crystal structure was determined from a twin. Refinement was carried out with a HKLF5 data set with merged equivalents.
Thus, no R(int) is reported. The batch scale factor for the twin populations refined to 0.471(1).

ACCEPTED MANUSCRIPT
Table 2 Selected bond length (Å) and angles (°) of 2a-2e
Parameter
X-ray crystal data
2a
2b
2c
2d
2e
N(1)-Si
2.163(3)
1.881(4)
1.470(5)
1.677(3)
1.654(3)
1.667(2)
1.524(5)
1.515(5)
117.51(14)
119.16(14)
119.27(14)
97.11(14)
96.80(15)
96.29(15)
83.19(13)
83.23(12)
83.38(12)
179.64(15)
2.1930(12)
1.8831(14)
1.4566(18)
1.6644(10)
1.6651(10)
1.6716(11)
1.531(2)
2.1729(10)
1.8870(12)
1.4593(16)
1.6656(9)
1.6656(9)
1.6769(9)
1.5295(17)
1.5272(17)
118.78(5)
118.78(5)
117.91(5)
97.40(5)
2.192(2)
1.884(2)
1.464(3)
1.664(2)
1.673(2)
1.663(2)
1.527(3)
1.524(3)
117.21(10)
119.11(11)
118.64(10)
97.67(10)
96.97(10)
97.88(9)
82.68(8)
82.18(8)
82.61(8)
179.15(11)
2.177(8)
1.8778(15)
1.4645(19)
1.6598(11)
1.6629(11)
1.6670(11)
1.526(2)
1.513(2)
118.74(6)
117.97(6)
118.35(6)
98.09(6)
97.17(7)
97.03(6)
83.21(7)
81.88(7)
82.62(7)
178.66(8)
C(7)-Si
C(9)-N(2)
O(1)-Si
O(2)-Si
O(3)-Si
C(7)-C(8)
C(8)-C(9)
1.5285(19)
119.06(6)
118.59(6)
117.42(6)
97.67(5)
O(1)-Si-O(2)
O(1)-Si-O(3)
O(2)-Si-O(3)
C(7)-Si-O(1)
C(7)-Si-O(2)
C(7)-Si-O(3)
N(1)-Si-O(1)
N(1)-Si-O(2)
N(1)-Si-O(3)
N(1)-Si(1)-C(7)
96.99(6)
96.96(5)
97.63(6)
96.98(5)
82.71(5)
83.32(4)
82.61(5)
82.82(4)
82.40(5)
96.98(5)
179.55(6)
179.26(5)

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