Organo-functionalized trimethoxysilanes featuring thioester linkage: Synthetic and UV–Vis spectral investigations

Article abstract of DOI:10.1016/j.jorganchem.2016.02.013

The current work reveals a series of new organo-functionalized trimethoxysilanes (OfTMS) linked via a 3C tether to the thioester group along with the inclusion of versatile aromatic and heteroaromatic sequences. The synthetic procedure implicates the one-pot thioesterification reaction of the precursor carboxylic acids (1a-r) with 3-mercaptopropyltrimethoxysilane (MPTMS), stimulated by 1,1′-carbonyldiimidazole (CDI). The OfTMS have been attentively characterized by elemental analysis, infrared and [¹H, ¹³C] NMR spectroscopic techniques. The UV–Vis absorption behaviour demonstrates that the alkoxysilanes possess high sensitivity to the changes caused in the environment on account of different substitutions. Furthermore, the solvent effect on the absorption spectra has been scrutinized and quantified using the Kamlet-Taft approach. Importantly, the fabricated stable alkoxysilanes can be aspired for advance applications in the field of material science.

Full text of DOI:10.1016/j.jorganchem.2016.02.013

Accepted Manuscript
Organo-functionalized trimethoxysilanes featuring thioester linkage: Synthetic and
UV-Vis spectral investigations
Gurjaspreet Singh, Sunita Rani, Amandeep Saroa, Aanchal Arora
PII:
S0022-328X(16)30046-8
10.1016/j.jorganchem.2016.02.013
JOM 19397
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 27 November 2015
Revised Date: 5 February 2016
Accepted Date: 8 February 2016
Please cite this article as: G. Singh, S. Rani, A. Saroa, A. Arora, Organo-functionalized
trimethoxysilanes featuring thioester linkage: Synthetic and UV-Vis spectral investigations, Journal of
Organometallic Chemistry (2016), doi: 10.1016/j.jorganchem.2016.02.013.
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ACCEPTED MANUSCRIPT
Graphical abstract

ACCEPTED MANUSCRIPT
Organo-functionalized trimethoxysilanes featuring thioester linkage: Synthetic and UV-Vis
spectral investigations
Gurjaspreet Singh*, Sunita Rani, Amandeep Saroa, Aanchal Arora
Department of Chemistry, Panjab University, Chandigarh, 160014, India
*Corresponding Author
Dr. Gurjaspreet Singh
Assistant Professor,
Department of Chemistry, Panjab University, Chandigarh
India
+91-0172-2534428
Email: gjpsingh@pu.ac.in
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Abstract
The current work reveals a series of new organo-functionalized trimethoxysilanes (OfTMS)
linked via a 3 C tether to the thioester group along with the inclusion of versatile aromatic
and heteroaromatic sequences. The synthetic procedure implicates the one-pot
thioesterification reaction of the precursor carboxylic acids (1a-r) with 3-
mercaptopropyltrimethoxysilane (MPTMS), stimulated by 1,1’-carbonyldiimidazole (CDI).
The OfTMS have been attentively characterized by elemental analysis, infrared and [1H, 13C]
NMR spectroscopic techniques. The UV-Vis absorption behaviour demonstrates that the
alkoxysilanes possess high sensitivity to the changes caused in the environment on account of
different substitutions. Furthermore, the solvent effect on the absorption spectra has been
scrutinized and quantified using the Kamlet-Taft approach. Importantly, the fabricated stable
alkoxysilanes can be aspired for advance applications in the field of material science.
Key words: Organosilanes, Thioester, Solvatochromism, Kamlet-Taft equation
1. Introduction
Organically modified silanes, Y-(CH2)3-SiX3 consist of the hydrolysable silyl bonds with
alkoxy, amine, acyloxy, glycidyloxy or halogen units (X) and a substituent Y which provides
structural malleability for the explicit functioning.[1,2] The organosilanes represent a fiercely
developing branch of synthetic chemistry having the utmost importance in organic synthesis,
metal complexes, catalysis, drug designing, polymers, analyte sensing and HPLC packing.[3]
The assimilation of fascinating organic fragments with the trialkoxysilyl moiety is grabbing
even more attention. This is because of their potential to react with both inorganic and
organic substrates, adhesives, as well as other alkoxysilane derivatives to form multifarious
organic-inorganic hybrids, dominant in nanomaterial and polymer science.[4] So far in the
field of siliceous materials, alkoxysilanes have been the most common platform for the
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functionalization of metal oxide surface by siloxane bonding, along with an ease of handling
and smooth reaction governance abilities.[5]
The exploitation of smaller organic transformations that employ simple synthetic
steps to provide maximum structural diversity within a single molecular framework is a deep-
rooted area in the organosilicon chemistry.[6] However, more discreet measurements are
required during the synthesis of organosilanes so that the organic moiety cannot interfere
with the susceptible silyl part.[7] Conventional approaches to synthesize alkoxysilanes involve
cross-coupling, hydrosilylation, transmetalation and replacement reactions.[8] The expensive
metals involved, limited functional group tolerance, lower product yields, tedious isolation
and purification techniques were the major shortcomings of these methodologies.
On the other hand, carboxylic acids are ubiquitous in the form of natural products,
transition metal complexes, pharmaceutics and polymers etc.[9] The carboxyl group and its
derivatized forms, for instance amide, ester, anhydride, carbamate etc. have been
incorporated as an organic linker to the trialkoxysilyl moiety.[7,10] To-date, the blending of
thioester unit with the silyl fragment is rarely reported and that too has been achieved by the
complicated synthetic pathways.[11] The thioesters are biologically important motifs having
vital roles in living cells, explicitly as intermediate for acyl group transfer reaction in
metabolic processes and in the biosynthesis of polyketides and nonribosomal polypeptides.[12]
Not only pharmacologically, these mono S-analogues of oxyesters are actively involved in
organic synthesis such as asymmetric addition, aldol and Michael reactions etc.[13] Keeping
the above perspectives in mind and extending our research interest on the alkoxysilanes[14]
,
we have now designed an efficacious synthetic route in which atmospheric reaction
conditions are employed. The rationale behind the synthesis of an array of compounds is to
have a better visualization of the effect of incorporating different aromatic rings (3a-i) as well
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as the electronic nature of the substituent attached at the phenyl ring (3j-r) on the UV-Vis
absorption profiles and solvatochromic properties.
The paper is organized as follows: Firstly, the synthetic procedure and
characterization of new OfTMS bearing thioester linkage are described. Secondly, we have
discussed the UV-Vis spectra of the silanes and then continued with the investigation on the
solvatochromic behaviour of these compounds where substitution effect is clearly observed.
Finally in the end, an appropriate conclusion is presented to summarize the paper.
2. Experimental
2.1. Materials and equipments
All carboxylic acids (Aldrich), MPTMS (Avra) and CDI (Avra) were used directly as
1
received. The H and 13C NMR spectra were recorded on a JEOL (AL 300 MHz)
spectrometer using CDCl3 as an internal reference and chemical shifts were reported relative
to tetramethylsilane. Electronic spectral measurements were carried out using a JASCO V-
530 UV-Vis spectrophotometer. The infrared spectra were recorded as neat spectra on a
Thermo Scientific NICOLET IS50 spectrophotometer. Elemental analysis was obtained on
Perkin Elmer Model 2400 CHNS elemental analyzer and Thermo Scientific Flash 2000
organic elemental analyzer.
2.2. General procedure for the synthesis of OfTMS (3a-r)
1 equiv. of the respective carboxylic acid (1a-r) and 1.20 equiv. of CDI were mixed in THF
(20 ml) at room temperature in a round bottomed flask. The reaction mixture was heated to
reflux for 1 h while evolution of the by-product carbon dioxide gas started within 10-15 min.
After cooling to room temperature, the addition of MPTMS (1 equiv.) was carried out and
heating was continued for 8 h. The reaction mixture was diluted with water and the aqueous
layer was extracted with dichloromethane (15 ml). The combined organic layer was washed
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with brine (25%) and dried over anhydrous Na2SO4. The mixture was concentrated in vacuo
to furnish highly viscous oil as the desired product.
2.2.1. S-3-(trimethoxysilyl)propyl benzothioate (3a)
The quantities used were as follows: 1a (0.50 g, 4.09 mmol), CDI (0.80 g, 4.91 mmol),
MPTMS (0.80 ml, 4.08 mmol). Anal. Calcd. for C13H20O4SSi: C, 51.97; H, 6.71; S, 10.67.
Found: C, 51.91; H, 6.67; S, 10.59. IR (neat, cm-1): 1648 (v C═O), 1075 (v Si‒O), 684 (v
1
C‒S). H NMR (300 MHz, CDCl3, 25 °C): δ = 0.67 (m, 2H, SiCH2), 1.69 (m, 2H, CCH2C),
2.99 (t, 2H, CH2S, J = 7.3 Hz), 3.47 (s, 3H, OCH3), 7.33 (t, 2H, H3,5, J = 8.1 Hz), 7.43 (d, 1H,
H4, J = 8.3 Hz), 7.86 (d, 2H, H2,6, J = 8.0 Hz). 13C NMR (75 MHz, CDCl3, 25 °C): δ = 8.64
(SiCH2), 20.33 (CCH2C), 31.44 (CH2S), 50.09 (OCH3), 127.03 (C2,6), 128.27 (C3,5), 132.87
(C4), 137.19 (C1), 191.02 (C═O).
2.2.2. S-3-(trimethoxysilyl)propyl pyridine-2-carbothioate (3b)
The quantities used were as follows: 1b (0.50 g, 4.06 mmol), CDI (0.79 g, 4.88 mmol),
MPTMS (0.80 ml, 4.08 mmol). Anal. Calcd. for C12H19NO4SSi: C, 47.81; H, 6.35; N, 4.65;
S, 10.64. Found: C, 47.78; H, 6.33; N, 4.60; S, 10.59. IR (neat, cm-1): 1653 (v C═O), 1088 (v
1
Si‒O), 688 (v C‒S). H NMR (300 MHz, CDCl3, 25 °C): δ = 0.56 (m, 2H, SiCH2), 1.62 (m,
2H, CCH2C), 3.06 (t, 2H, CH2S, J = 7.3 Hz), 3.49 (s, 9H, OCH3), 7.22 (m, 1H, H4), 8.04-8.11
13
(m, 1H, H3), 8.52 (d, 1H, H2, J = 4.7 Hz), 8.92 (s, 1H, H5). C NMR (75 MHz, CDCl3, 25
°C): δ = 10.37 (SiCH2), 18.48 (CCH2C), 37.98 (CH2S), 57.66 (OCH3), 123.29 (C2), 131.43
(C4), 135.42 (C3), 147.88 (C5), 151.49 (C1), 190.69 (C═O).
2.2.3. S-3-(trimethoxysilyl)propyl pyridine-3-carbothioate (3c)
The quantities used were as follows: 1c (0.50 g, 4.06 mmol), CDI (0.79 g, 4.88 mmol),
MPTMS (0.80 ml, 4.08 mmol). Anal. Calcd. for C12H19NO4SSi: C, 47.81; H, 6.35; N, 4.65;
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S, 10.64. Found: C, 47.72; H, 6.28; N, 4.60; S, 10.59. IR (neat, cm-1): 1655 (v C═O), 1084 (v
1
Si‒O), 680 (v C‒S). H NMR (300 MHz, CDCl3, 25 °C): δ = 0.58 (m, 2H, SiCH2), 1.64 (m,
2H, CCH2C), 2.97 (t, 2H, CH2S, J = 7.3 Hz), 3.56 (s, 9H, OCH3), 7.24 (t, 1H, H4, J = 5.5 Hz),
13
8.09 (t, 1H, H3, J = 5.7 Hz), 8.54 (d, 1H, H5, J = 4.7 Hz), 8.94 (s, 1H, H1). C NMR (75
MHz, CDCl3, 25 °C): δ = 8.84 (SiCH2), 17.63 (CCH2C), 31.26 (CH2S), 50.25 (OCH3),
120.24 (C4), 127.32 (C2), 136.82 (C3), 149.04 (C5), 152.32 (C1), 193.07 (C═O).
2.2.4. S-3-(trimethoxysilyl)propyl pyridine-4-carbothioate (3d)
The quantities used were as follows: 1d (0.50 g, 4.06 mmol), CDI (0.79 g, 4.88 mmol),
MPTMS (0.80 ml, 4.08 mmol). Anal. Calcd. for C12H19NO4SSi: C, 47.81; H, 6.35; N, 4.65;
S, 10.64. Found: C, 47.78; H, 6.39; N, 4.60; S, 10.59. IR (neat, cm-1): 1657 (v C═O), 1090 (v
1
Si‒O), 683 (v C‒S). H NMR (300 MHz, CDCl3, 25 °C): δ = 0.59 (m, 2H, SiCH2), 1.65 (m,
2H, CCH2C), 2.86 (t, 2H, CH2S, J = 7.3 Hz), 3.51 (s, 9H, OCH3), 7.61 (d, 2H, H2,5, J = 5.8
13
Hz), 8.56 (d, 2H, H3,4, J = 5.8 Hz). C NMR (75 MHz, CDCl3, 25 °C): δ = 10.35 (SiCH2),
15.30 (CCH2C), 37.20 (CH2S), 60.94 (OCH3), 150.44 (C1), 165.64 (C3,4), 191.59 (C═O).
2.2.5. S-3-(trimethoxysilyl)propyl pyrazine-2-carbothioate (3e)
The quantities used were as follows: 1e (0.50 g, 4.03 mmol), CDI (0.78 g, 4.83 mmol),
MPTMS (0.79 ml, 4.03 mmol). Anal. Calcd. for C11H18N2O4SSi: C, 43.69; H, 6.00; N, 9.26;
S, 10.60. Found: C, 43.62; H, 5.96; N, 9.29; S, 10.53. IR (neat, cm-1): 1659 (v C═O), 1087 (v
1
Si‒O), 680 (v C‒S). H NMR (300 MHz, CDCl3, 25 °C): δ = 0.69 (m, 2H, SiCH2), 1.74 (m,
2H, CCH2C), 2.99 (t, 2H, CH2S, J = 7.2 Hz), 3.48 (s, 9H, OCH3), 8.56 (m, 1H, H4), 8.71 (m,
1H, H3), 9.06 (m, 1H, H2). 13C NMR (75 MHz, CDCl3, 25 °C): δ = 10.63 (SiCH2), 20.92
(CCH2C), 32.31 (CH2S), 51.26 (OCH3), 141.94 (C2,3), 143.65 (C4), 148.32 (C1), 192.62
(C═O).
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2.2.6. S,S-bis(3-(trimethoxysilyl)propyl) pyridine-2,3-bis(carbothioate) (3f)
The quantities used were as follows: 1f (0.50 g, 2.99 mmol), CDI (1.16 g, 7.18 mmol),
MPTMS (1.17 ml, 5.97 mmol). Anal. Calcd. for C19H33NO8S2Si2: C, 43.57; H, 6.35; N, 2.67;
S, 12.24. Found: C, 43.51; H, 6.19; N, 2.62; S, 12.20. IR (neat, cm-1): 1665 (v C═O), 1075 (v
1
Si‒O), 689 (v C‒S). H NMR (300 MHz, CDCl3, 25 °C): δ = 0.63 (m, 4H, SiCH2), 1.62 (m,
4H, CCH2C), 2.85 (t, 2H, CH2S, J = 7.3 Hz), 3.05 (t, 2H, CH2S, J = 7.3 Hz), 3.43 (s, 9H,
OCH3), 3.47 (s, 9H, OCH3), 7.57 (t, 1H, H4, J = 8.1 Hz), 7.77 (d, 1H, H5, J = 9.1 Hz), 8.79
(d, 1H, H3, J = 9.1 Hz). 13C NMR (75 MHz, CDCl3, 25 °C): δ = 8.90 (SiCH2), 15.98
(CCH2C), 33.35 (CH2S), 50.51 (OCH3), 122.28 (C2), 126.96 (C4), 151.55 (C3), 155.16 (C1,5),
190.74 (C═O), 191.91 (C═O).
2.2.7. S,S-bis(3-(trimethoxysilyl)propyl) pyridine-2,6-bis(carbothioate) (3g)
The quantities used were as follows: 1g (0.50 g, 2.99 mmol), CDI (1.16 g, 7.18 mmol),
MPTMS (1.17 ml, 5.97 mmol). Anal. Calcd. for C19H33NO8S2Si2: C, 43.57; H, 6.35; N, 2.67;
S, 12.24. Found: C, 43.49; H, 6.30; N, 2.63; S, 12.18. IR (neat, cm-1): 1650 (v C═O), 1070 (v
1
Si‒O), 678 (v C‒S). H NMR (300 MHz, CDCl3, 25 °C): δ = 0.59 (m, 4H, SiCH2), 1.77 (m,
4H, CCH2C), 3.03 (t, 4H, CH2S, J = 7.4 Hz), 3.46 (s, 18H, OCH3), 7.96 (t, 1H, H3, J = 9.1
13
Hz), 8.26 (d, 2H, H2,4, J = 7.9 Hz). C NMR (75 MHz, CDCl3, 25 °C): δ = 10.27 (SiCH2),
16.85 (CCH2C), 38.10 (CH2S), 50.77 (OCH3), 125.21 (C3), 139.21 (C2,4), 149.57 (C1,5),
192.32 (C═O).
2.2.8. S-3-(trimethoxysilyl)propyl furan-2-carbothioate (3h)
The quantities used were as follows: 1h (0.50 g, 4.46 mmol), CDI (0.87 g, 5.37 mmol),
MPTMS (0.87 ml, 4.44 mmol). Anal. Calcd. for C11H18O5SSi: C, 45.49; H, 6.25; S, 11.04.
Found: C, 45.42; H, 6.16; S, 11.00. IR (neat, cm-1): 1667 (v C═O), 1085 (v Si‒O), 689 (v
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1
C‒S). H NMR (300 MHz, CDCl3, 25 °C): δ = 0.59 (m, 2H, SiCH2), 1.61 (m, 2H, CCH2C),
2.90 (t, 2H, CH2S, J = 7.2 Hz), 3.38 (s, 9H, OCH3), 7.38 (t, 1H, H3, J = 4.5 Hz), 7.47 (d, 1H,
H2, J = 4.6 Hz), 7.71 (d, 1H, H4, J = 4.6 Hz). 13C NMR (75 MHz, CDCl3, 25 °C): δ = 10.16
(SiCH2), 15.92 (CCH2C), 37.43 (CH2S), 59.78 (OCH3), 113.48 (C3), 129.22 (C2), 146.18
(C4), 152.12 (C1), 176.72 (C═O).
2.2.9. S-3-(trimethoxysilyl)propyl thiophene-2-carbothioate (3i)
The quantities used were as follows: 1i (0.50 g, 3.91 mmol), CDI (0.76 g, 4.69 mmol),
MPTMS (0.76 ml, 3.90 mmol). Anal. Calcd. for C11H18O4S2Si: C, 43.11; H, 5.92; S, 20.93.
Found: C, 43.02; H, 5.84; S, 20.88. IR (neat, cm-1): 1675 (v C═O), 1078 (v Si‒O), 670 (v
1
C‒S). H NMR (300 MHz, CDCl3, 25 °C): δ = 0.68 (m, 2H, SiCH2), 1.73 (m, 2H, CCH2C),
3.00 (t, 2H, CH2S, J = 7.3 Hz), 3.49 (s, 9H, OCH3), 7.02 (t, 1H, H3, J = 4.5 Hz), 7.51 (d, 1H,
13
H2, J = 5.0 Hz), 7.69 (d, 1H, H4, J = 3.7 Hz). C NMR (75 MHz, CDCl3, 25 °C): δ = 9.08
(SiCH2), 16.75 (CCH2C), 36.28 (CH2S), 56.79 (OCH3), 127.69 (C3), 128.52 (C2), 130.03
(C4), 140.33 (C1), 181.34 (C═O).
2.2.10. S-(3-(trimethoxysilyl)propyl) 2-(phenylthio)ethanethioate (3j)
The quantities used were as follows: 1j (0.50 g, 2.98 mmol), CDI (0.58 g, 3.57 mmol),
MPTMS (0.58 ml, 2.98 mmol). Anal. Calcd. for C14H22O4S2Si: C, 48.52; H, 6.40; S, 18.51.
Found: C, 48.24; H, 6.19; N, S, 18.31. IR (neat, cm-1): 1645 (v C═O), 1099 (v Si‒O), 689 (v
1
C‒S). H NMR (300 MHz, CDCl3, 25 °C): δ = 0.60 (m, 2H, SiCH2), 1.57 (m, 2H, CCH2C),
2.80 (t, 2H, CH2S, J = 7.3 Hz), 3.45 (s, 9H, OCH3), 3.70 (s, 2H, SCH2), 7.11-7.29 (m, 5H,
Ar-H). 13C NMR (75 MHz, CDCl3, 25 °C): δ = 10.44 (SiCH2), 15.88 (CCH2C), 35.93
(CH2S), 51.63 (SCH2), 58.26 (OCH3), 123.89 (C4), 125.84 (C2,6), 128.93 (C3,5), 136.02 (C1),
190.12 (C═O).
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2.2.11. S-3-(trimethoxysilyl)propyl 2-methylbenzothioate (3k)
The quantities used were as follows: 1k (0.50 g, 3.67 mmol), CDI (0.71 g, 4.40 mmol),
MPTMS (0.72 ml, 3.67 mmol). Anal. Calcd. for C14H22O4SSi: C, 53.47; H, 7.05; S, 10.20.
Found: C, 53.39; H, 5.87; S, 10.70. IR (neat, cm-1): 1655 (v C═O), 1078 (v Si‒O), 684 (v
1
C‒S). H NMR (300 MHz, CDCl3, 25 °C): δ = 0.47 (m, 2H, SiCH2), 1.68 (m, 2H, CCH2C),
2.39 (s, 3H, CH3), 3.02 (t, 2H, CH2S, J = 7.3 Hz), 3.45 (s, 3H, OCH3), 7.25-8.67 (m, 4H, Ar-
13
H). C NMR (75 MHz, CDCl3, 25 °C): δ = 10.53 (SiCH2), 15.52 (CCH2C), 17.04 (CH3),
36.88 (CH2S), 58.19 (OCH3), 126.87 (C5), 127.61 (C6), 129.82 (C3), 133.36 (C2,4), 135.46
(C1), 191.19 (C═O).
2.2.12. S-3-(trimethoxysilyl)propyl 4-methoxybenzothioate (3l)
The quantities used were as follows: 1l (0.50 g, 3.28 mmol), CDI (0.64 g, 3.95 mmol),
MPTMS (0.64 ml, 3.28 mmol). Anal. Calcd. for C14H22O5SSi: C, 50.88; H, 6.71; S, 9.70.
Found: C, 50.77; H, 6.65; S, 9.63. IR (neat, cm-1): 1665 (v C═O), 1078 (v Si‒O), 676 (v C‒S).
1H NMR (300 MHz, CDCl3, 25 °C): δ = 0.70 (m, 2H, SiCH2), 1.70 (m, 2H, CCH2C), 2.98 (t,
2H, CH2S, J = 7.3 Hz), 3.49 (s, 3H, OCH3), 3.79 (s, 3H, OCH3), 6.82 (d, 2H, H3,5, J = 8.6
13
Hz), 7.85 (d, 2H, H2,6, J = 8.6 Hz). C NMR (75 MHz, CDCl3, 25 °C): δ = 8.86 (SiCH2),
16.88 (CCH2C), 31.60 (CH2S), 50.45 (OCH3), 55.27 (OCH3), 113.72 (C3,5), 129.49 (C2,6),
134.88 (C1), 163.74 (C4), 189.89 (C═O).
2.2.13. S-3-(trimethoxysilyl)propyl 2-hydroxybenzothioate (3m)
The quantities used were as follows: 1m (0.50 g, 3.62 mmol), CDI (0.70 g, 4.32 mmol),
MPTMS (0.71 ml, 3.62 mmol). Anal. Calcd. for C13H20O5SSi: C, 49.34; H, 6.37; S, 10.13.
Found: C, 49.29; H, 6.33; S, 10.06. IR (neat, cm-1): 1657 (v C═O), 1075 (v Si‒O), 680 (v
1
C‒S). H NMR (300 MHz, CDCl3, 25 °C): δ = 0.73 (m, 2H, SiCH2), 1.78 (m, 2H, CCH2C),
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3.06 (t, 2H, CH2S, J = 7.3 Hz), 3.54 (s, 3H, OCH3), 6.91 (d, 1H, H3, J = 5.6 Hz), 7.40 (t, 1H,
H5, J = 5.8 Hz), 7.81 (d, 1H, H4, J = 5.9 Hz), 8.13 (s, 1H, H6). 13C NMR (75 MHz, CDCl3, 25
°C): δ = 8.93 (SiCH2), 16.67 (CCH2C), 31.43 (CH2S), 50.43 (OCH3), 118.36 (C3), 119.06
(C1), 128.96 (C5), 131.02 (C6), 135.68 (C4), 159.91 (C2), 191.82 (C═O).
2.2.14. S-(3-(trimethoxysilyl)propyl) 2-mercaptobenzothioate (3n)
The quantities used were as follows: 1n (0.50 g, 3.25 mmol), CDI (0.63 g, 3.89 mmol),
MPTMS (0.64 ml, 3.26 mmol). Anal. Calcd. for C13H20O4S2Si: C, 46.96; H, 6.06; S, 19.29.
Found: C, 46.29; H, 6.33; S, 10.06. IR (neat, cm-1): 1653 (v C═O), 1078 (v Si‒O), 667 (v
1
C‒S). H NMR (300 MHz, CDCl3, 25 °C): δ = 0.37 (m, 2H, SiCH2), 1.64 (m, 2H, CCH2C),
2.99 (t, 2H, CH2S, J = 7.3 Hz), 3.58 (s, 3H, OCH3), 7.05 (s, 1H, H5), 7.46 (s, 2H, H3,4), 8.18
13
(s, 1H, H6). C NMR (75 MHz, CDCl3, 25 °C): δ = 10.45 (SiCH2), 15.74 (CCH2C), 35.36
(CH2S), 51.36 (OCH3), 127.40 (C5), 127.94 (C2), 128.15 (C4), 129.01 (C6), 132.19 (C3),
136.71 (C1), 191.57 (C═O).
2.2.15. S-3-(trimethoxysilyl)propyl 2-aminobenzothioate (3o)
The quantities used were as follows: 1o (0.50 g, 3.64 mmol), CDI (0.71 g, 4.37 mmol),
MPTMS (0.71 ml, 3.62 mmol). Anal. Calcd. for C13H21NO4SSi: C, 49.50; H, 6.71; N, 4.44;
S, 10.16. Found: C, 49.39; H, 6.66; N, 4.40; S, 10.09. IR (neat, cm-1): 1658 (v C═O), 1078 (v
1
Si‒O), 684 (v C‒S). H NMR (300 MHz, CDCl3, 25 °C): δ = 0.66 (m, 2H, SiCH2), 1.68 (m,
2H, CCH2C), 3.11 (t, 2H, CH2S, J = 7.3 Hz), 3.60 (s, 3H, OCH3), 6.58 (d, 1H, H3, J = 5.6
Hz), 6.93 (s, 1H, H5), 7.42 (t, 1H, H4, J = 5.8 Hz), 7.90 (d, 1H, H6, J = 5.9 Hz). 13C NMR (75
MHz, CDCl3, 25 °C): δ = 10.37 (SiCH2), 15.94 (CCH2C), 36.74 (CH2S), 51.03 (OCH3),
113.69 (C3), 119.78 (C1), 123.29 (C5), 131.52 (C6), 135.42 (C4), 147.88 (C2), 191.88 (C═O).
2.2.16. S-3-(trimethoxysilyl)propyl 2-nitrobenzothioate (3p)
10

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The quantities used were as follows: 1p (0.50 g, 2.99 mmol), CDI (0.58 g, 3.48 mmol),
MPTMS (0.59 ml, 3.00 mmol). Anal. Calcd. for C13H19NO6SSi: C, 45.20; H, 5.54; N, 4.05;
S, 9.28. Found: C, 45.14; H, 5.45; N, 4.01; S, 9.24. IR (neat, cm-1): 1660 (v C═O), 1078 (v
1
Si‒O), 685 (v C‒S). H NMR (300 MHz, CDCl3, 25 °C): δ = 0.26 (m, 2H, SiCH2), 1.55 (m,
2H, CCH2C), 2.86 (t, 2H, CH2S, J = 7.3 Hz), 3.51 (s, 3H, OCH3), 7.10-7.43 (m, 4H, Ar-H).
13C NMR (75 MHz, CDCl3, 25 °C): δ = 10.25 (SiCH2), 16.23 (CCH2C), 33.83 (CH2S), 57.74
(OCH3), 124.51 (C3), 128.96 (C6), 131.31 (C1,4), 135.33 (C2,5), 191.98 (C═O).
2.2.17. S-3-(trimethoxysilyl)propyl 2-chlorobenzothioate (3q)
The quantities used were as follows: 1q (0.50 g, 3.19 mmol), CDI (0.62 g, 3.83 mmol),
MPTMS (0.63 ml, 3.20 mmol). Anal. Calcd. for C13H19ClO4SSi: C, 46.62; H, 5.72; S, 9.57.
Found: C, 46.25; H, 5.57; S, 9.36. IR (neat, cm-1): 1655 (v C═O), 1086 (v Si‒O), 678 (v C‒S).
1H NMR (300 MHz, CDCl3, 25 °C): δ = 0.68 (m, 2H, SiCH2), 1.71 (m, 2H, CCH2C), 2.99 (t,
2H, CH2S, J = 7.3 Hz), 3.47 (s, 3H, OCH3), 7.20 (t, 1H, H5, J = 5.9), 7.31 (m, 2H, H3,4), 7.52
(d, 1H, H6, J = 5.8). 13C NMR (75 MHz, CDCl3, 25 °C): δ = 8.49 (SiCH2), 17.45 (CCH2C),
32.31 (CH2S), 50.14 (OCH3), 126.42 (C5), 128.91 (C6), 130.61 (C3), 131.81 (C1), 137.73
(C4), 191.17 (C═O).
2.2.18. S-3-(trimethoxysilyl)propyl biphenyl-4-carbothioate (3r)
The quantities used were as follows: 1r (0.50 g, 2.52 mmol), CDI (0.49 g, 3.03 mmol),
MPTMS (0.49 ml, 2.50 mmol). Anal. Calcd. for C19H24O4SSi: C, 60.61; H, 6.42; S, 7.46.
Found: C, 60.56; H, 6.38; S, 7.43. IR (neat, cm-1): 1651 (v C═O), 1083 (v Si‒O), 684 (v C‒S).
1H NMR (300 MHz, CDCl3, 25 °C): δ = 0.68 (m, 2H, SiCH2), 1.72 (m, 2H, CCH2C), 3.00 (t,
2H, CH2S, J = 7.3 Hz), 3.45 (s, 3H, OCH3), 7.22-7.31 (m, 3H, H9,10,11), 7.45-7.53 (m, 4H,
H3,5,8,12), 7.91 (d, 2H, H2,6, J = 8.3 Hz). 13C NMR (75 MHz, CDCl3, 25 °C): δ = 9.41 (SiCH2),
11

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15.53 (CCH2C), 36.35 (CH2S), 57.49 (OCH3), 126.95 (C10), 127.10 (C8), 127.57 (C12),
127.78 (C3,5), 128.79 (C2,6), 133.59 (C9,11), 140.16 (C1,7), 143.92 (C4), 190.79 (C═O).
3. Results and discussion
The synthetic route to access alkoxysilanes is outlined in Scheme 1.
O
O
Si
N
N
CDI
=
MPTMS
N
=
,
HS
O
NH HN
O
O
O
O
O
N
Ar
Si
O
Ar
1aꢀr
OH
Ar
N
S
O
2aꢀr
intermediates
3aꢀr
CO2
1.
2.
HN
HN
N
Scheme 1: Synthesis of thioester allied organosilanes (3a-r)
Table 1 Thioester allied organosilanes (3a-r)
12

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3.1 Synthesis
As the organosilanes turned out to be moisture sensitive; it was preferred traditionally to
functionalize the organic fragment with silyl group in the later step of synthesis. For this
reason, it is highly enviable to apply those reactions which can afford the alkoxysilane with
an easy pathway that can be performed under normal atmospheric conditions without any
chromatographic separation or distillation approach. Recently we have reported an adept
methodology for the preparation of heteroaromatic thioester based organosilatranes.[15] The
same synthetic formula is found to be pertinent for the generation of the present silanes. A
one-pot, three-component protocol involving the CDI mediated thioesterification of
carboxylic acid with MPTMS has been employed as a synthetic guideline here (Scheme 1).
The acyl carboxy substituted imidazole is first formed during the reaction of carboxylic acid
with CDI, which combines rapidly with the imidazole unit to form respective acyl imidazole
13

ACCEPTED MANUSCRIPT
(2a-r). The in-situ coupling of these acyl imidazole units with MPTMS imparts OfTMS
derivatives (Table 1). The merit of this protocol was tested by varying the substrate and all
reactions were executed using optimized reaction conditions (Table 2, entry 10). The
highlight of this route is that all silanes are easily generated without utilizing inert atmosphere
and the trimethoxysilyl group can be successfully integrated with each challenging substrate.
3.1.1. Optimization of reaction conditions
To optimize the best suitable path for stitching of silyl moiety with various aromatic units via
thioester linkage, different reaction conditions were attempted. Out of the numerous protocols
for preparing thioester, the one involving carboxylic acid and alcohol condensation have
emerged as the smartest and simplest method (Scheme 2). Subsequently, the thioesters can be
[15]
prepared via acylation of thiol
with carboxylic acid (route a), or by the reaction of
[16]
thiocarboxylic acid with alcohol
(route b). However, in our case due to the non-
availability of both the ingredients required for route b, this could not be attempted.
Scheme 2: Possible routes of synthesis of thioester from carboxylic acid
Following route a, we initiated our work using benzoic acid (1a) and MPTMS as
substrates to optimize the reaction conditions. In the first place, the blank reaction i.e. without
any coupling reagent or catalyst was tried. The reaction did not proceed at all even after
keeping for a long duration, implying that some additional driving force has to be applied as a
high energy barrier exists between the two components. Following this, a well known
bronsted acid catalyst for the esterification reaction viz. conc. H2SO4 was added to the
14

ACCEPTED MANUSCRIPT
reaction mixture.[17] This time the reaction commenced but with a very low yield of the
expected product. The possible explanation for this can be the hydrolysis of the product
silane under acidic condition.[18] In the next case, 1a was pre-activated with thionyl chloride
forming its highly reactive derivatized form i.e. acid chloride[19] (Scheme 3), which was then
coupled with MPTMS. Though we were successful in the generation of the desired product,
but there were some serious and unavoidable shortcomings of this methodology, which
include handling of fuming and moisture sensitive thionyl chloride, acyl chloride and the
liberation of toxic HCl gas. Besides, an additional base had to be used in this reaction to
absorb the by-product HCl gas (Scheme 3). To overcome these limitations, another reagent
was sought out. Out of the commercially available coupling reagents[19] like DCC, CDI,
DEAD etc, we have opted for the CDI mediated coupling of acid with MPTMS since CDI is
economically viable and generates cleaner reaction with easily removable innocuous by-
products (Scheme 1). When started with carboxylic acid:CDI in 1:1 ratio, only 40% product
yield was achieved. Although thiol can also be activated with CDI but thiolysis via this
method required prolonged heating along with an extra nucleophile such as sodium ethoxide
to catalyse the reaction on account of its weaker acidic character.[20] Consequently, this path
was eradicated and the reaction was further attempted with acid and CDI in different ratios
(Table 2, entries 5-10). The thiol source was added only after the complete activation of the
acid (ca 1 h, monitored by TLC). Using acid and CDI in 1:1.2 ratios, a significant increment
in the product yield was observed.
15

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Table 2 Optimization of reaction conditions for the synthesis of compound 3a
Catalyst/
Amount
Reaction
Reaction
Yield
(%)b
0
Entry
Solvent
coupling agent employeda
conditions duration (h)
1
2
None
H2SO4
SOCl2
CDI
-
Toluene
110 °C, st
110 °C, st
40 °C, st
110 °C, st
110 °C, st
110 °C, st
110 °C, st
110 °C, st
110 °C, st
65 °C, st
40 °C, st
60 °C, st
10
8
0.1 equiv. Toluene
20
3
10 equiv.
1 equiv.
DCM
5
72
4
Toluene
12
12
8
40
5
CDI
1.1 equiv. Toluene
1.2 equiv. Toluene
1.3 equiv. Toluene
1.4 equiv. Toluene
1.5 equiv. Toluene
56
6
CDI
79
7
CDI
8
78
8
CDI
8
79
9
CDI
8
79
10
11
12
CDI
CDI
1.2 equiv.
1.2 equiv.
1.2 equiv.
THF
DCM
CHCl3
8
85
8
75
CDI
8
72
a: the amount used is w.r.t. carboxylic acid (1a), b: determined by 1H NMR analysis of the
sample. The bold values specify the best optimized condition for the reaction.
By changing the ratios further, any augment in the yield was not obtained (entries 6-
8). The solvent medium inspection revealed that THF is a better choice amongst the tested
solvents (entries 9-11). On the whole, we can state that the coupling of carboxylic acid with
MPTMS via CDI mediation is an attractive route to assemble silyl unit with the aromatic
thioester.
16

ACCEPTED MANUSCRIPT
Scheme 3: Synthesis of OfTMS derivatives via acid chloride
3.2. Characterization of OfTMS (3a-r)
The structures and purity of all the products were inferred from their infrared, NMR (1H and
13C), and elemental analysis data. In the FT-IR spectra, absence of S-H (MPTMS), O-H
(carboxylic acids) absorption bands and the occurrence of strong bands at 1648-1675 cm-1
due to C=O stretching vibrations verify the coupling of carboxylic acid with MPTMS. The
bands at 1070-1099 cm-1, typical of the Si-O asymmetric vibrations confirm the product
formation.
1H NMR spectra have demonstrated the symbolic peaks of the silyl fragment
corresponding to the Si-OCH3 group in the range of 3.43-3.79 ppm. In the spectra of all
compounds, signal due to CH2S protons shifts its position from δ = 2.55 ppm (MPTMS) to δ
= 2.80-3.11 ppm (3a-r). This shift confirms the acylation of thiol group to form thioesters. In
13C NMR spectra, the peak belonging to C=O group of thioester linkage in the region 176.72-
193.07 ppm validate the product formation. Even though we have employed non-inert
conditions for the synthesis and used water for the work up, the spectroscopic studies have
revealed that only non-hydrolysed species are present in the product, corroborating the
stability of trimethoxysilanes modified with aromatic thioester.
3.3. UV-Vis spectral study
The molecular frameworks with various substitutions possess atypical properties
when exposed to UV-Vis radiations.[21] Therefore, we intend to undertake the tailoring of
different aromatic moieties with the organosilicon derivatives. The chemical structure,
electronic character and the position of the aromatic ring with respect to the thioester group
are expected to alter the absorption band definitely, if not largely for the different
alkoxysilanes. The UV-Vis absorption analyses of the compounds (3a-r) were carried out at a
17

ACCEPTED MANUSCRIPT
molar concentration 3x10-5 M in chloroform (Fig. 1). The OfTMS derivatives acquire major
absorption bands in the region of 249-349 nm with very high values of extinction coefficients
(εmax), which are ascribed to the π-π* electronic transitions in these conjugate systems and the
salient features of the absorption peaks are presented in Table 3.
Fig. 1. UV-Vis spectra of compounds (3a-r) in chloroform
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Table 3 Observed yields, UV-Vis absorption parameters for compounds (3a-r) and the %
contribution of the solvatochromic parameters
% contribution of
εmax × 104
Compound
ID
Synthetic
yield (%)
solvatochromic
parameters
Substrate
λmax (nm)
(Lmol-1cm-1)
Pπ
Pα
Pβ
Ph
o-Py
85
83
80
81
82
86
84
84
87
84
81
79
85
72
82
80
84
83
272
265
265
265
263
284
296
283
287
249
280
282
309
315
348
265
274
290
11.49
12.33
12.30
12.29
10.88
11.15
13.40
9.27
25.38 30.08 44.53
13.99 55.47 30.54
3a
3b
3c
3d
3e
3f
m-Py
33.88 62.99
3.13
p-Py
21.53 45.57 32.89
23.24 48.36 28.40
17.52 50.66 31.82
30.94 37.95 31.11
pyza
2,3-py
2,6-py
furan
3g
3h
3i
35.68 59.62
4.66
thiophene
Ph-SCH2
Ph-Me
Ph-OMe
Ph-OH
Ph-SH
Ph-NH2
Ph-NO2
Ph-Cl
9.58
23.31 14.55 62.13
-
10.15
10.55
10.78
11.95
12.35
13.98
11.17
11.67
12.35
3j
59.69
3.28
37.03
3k
3l
38.27 22.99 38.73
10.75 41.67 47.58
24.50 30.54 44.96
3m
3n
3o
3p
3q
3r
55.67 36.74
-
7.58
32.40 37.54 30.06
18.13 49.48 32.39
Ph-Ph
19

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The spectral characteristics of any chromophore depend on the electronic environment
and degree of conjugation existing in it. Now considering the carbonyl group in our system,
its one side is occupied by the silyl substituted thiopropyl chain, which imposes only weak
electron releasing effect and can be surmised to have a little effect on the π-π* transition of
carbonyl bond. It is only the delocalized aromatic rings attached to the carbonyl group due to
which the λmax values are observed at higher values than the unsubstituted derivative.[22] For
instance compound 3j, where the carbonyl group is not directly bonded to the aromatic ring
but connected alternatively via CH2S linker, shows the absorption band at a low λmax value
i.e. 249 nm (Fig. 1a). While unsubstituted benzene ring allied silane 3a absorbs at 272 nm,
this accentuates the conjugation influence on the UV-Vis absorption profile. The π1-
conjugation corresponding to C=O group combines synergistically with π2-conjugation
arising from the attached aromatic moiety, leading to the higher λmax values (Fig. 2a). For a
better vision about the substituent effect on the absorption behaviour, it seems necessary to
categorize the silanes into two series: first one enclosing various heterocyclic rings as the
binding unit, while the second series is integrated with differently substituted phenyl ring
analogues. Further, the silane 3a carrying unsubstituted phenyl ring has been chosen as the
reference compound for comparative analysis.
20

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Π1- conjugation
proton accepting site
O
a)
R
S
X
Π2- conjugation
b)
c)
Fig. 2. (a) Defined π-electronic units present in the aromatic thioesters; (b) mesomeric
structures of substituted thioesters with electron-donating; (c) with electron-accepting
substituent
The replacement of CH group with the N atom at the aromatic ring (3b-d), instigates a
blue shift in the absorption spectra owing to the electron withdrawing effect of the ring N
atom. The electron density reduction leads to more energy gap between HOMO and LUMO,
which result in lower λmax values.[23] The absorption profiles of the pyridine ring derivatized
silanes (3b-d) have the same features (Fig. 1b) and it is worth mentioning here that the
positional isomeric effect on the λmax value is not observed in these compounds. The
incorporation of the 2nd N atom into the ring in compound 3e further decreases λmax values as
depicted in Fig. 1b. The disubstituted pyridine ring derivatives 3f and 3g possess higher λmax
values than the corresponding mono derivatives (3b-d) owing to the bonus conjugation
imposed by the 2nd thioester group (Fig. 1c). However, the five-membered ring systems
behaved differently than we expected. On the basis of conjugation length, both 3h and 3i
should absorb UV radiations at lower λmax values than previously discussed six-membered
21

ACCEPTED MANUSCRIPT
derivatives, while these compounds have shown much higher absorptions (Fig. 1c). We infer
that this phenomenon is caused by the difference in the overall electronic influence of the
aromatic ring on the carbonyl group. Both furan and thiophene rings are electron releasing
heterocycles[24] whereas pyridine and pyrazine are electron withdrawing in nature, which is
responsible for the higher λmax values in 3h and 3i.
Moving over to the second series of silanes, where the core phenyl ring featuring
electron donor and acceptor unit of varying electron density allow a systematic fine tuning
with the UV-Vis absorptions. The electron releasing power of the affixed substituents is in
the order: Me < OMe < OH < SH < NH2. Exactly same increasing order has been observed in
the λmax values as shown in Fig. 1d. Here the non-bonding electrons of the substituent also
contribute towards the mobility of π-electrons into the aromatic ring through resonance and
consequently shifting of absorption maxima towards higher wavelength region occurs.
Switching over to the electron withdrawing substituent, the effect of NO2 group (3p)
is coherent with the theoretical consideration and an expected blue shift of 7 nm is observed
(Table 3). An assessment of the absorption profile of 3q (Fig. 1e) reveals that the results are
contradictory here. This can be explicated on the basis of electron releasing mesomeric effect
of Cl atom which induces higher λmax value than could be proposed for an electronegative
substituent, though still lower than the electron releasing substituents.[14c] The extended
conjugation bestowed by the additional phenyl ring in the aromatic sequence (3r) leads to 18
nm red-shifted absorption profile as compared to 3a. Briefly we can say that the π-electron
density shift from the aromatic unit (resulting from the ring itself, its position or the
substituent attached to it) influences the π-delocalization around C=O group. For instance, in
the resonance structures (Fig. 2b), the additional electrons at the ring generate an increased
charge density at the carbonyl group, while with electro withdrawing substituents, a decrease
22

ACCEPTED MANUSCRIPT
in electron density occurs (Fig. 2c), which is responsible for the variation of absorption
maxima values in these compounds.
3.4. Solvatochromism
Solvent-dependent absorption or solvatochromism, a term coined by Hantzsch, explores the
local polarity and solvent interactions with a chromophore.[25] Owing to the solute-solvent
interactions, an influence on the electronic absorption can be observed in terms of changes in
the shape, spectral maxima position and intensity as well. Solvatochromism is generated by
the differential solvation of the ground and the first excited states of the solute molecules
which further depend upon the solute structure, solvent nature and solute-solvent interactions.
The general trend observed with changing solvent polarity is as follows: if with increasing
solvent polarity, the ground state of the solute is better stabilized than the electronically
excited state, the negative solvatochromism persists leading to a blue shift in the absorption
maxima. Conversely, if stabilization of the excited state is better than the ground state, the
positive solvatochromism will prevail i.e. a red shifted profile will be observed with an
increase in solvent polarity. The system which undergoes π-π* transitions on the absorption
of UV light generally has more polar electronically excited state compared to the ground state
and hence the energy of the excited state is more affected by the dipole-dipole interactions
with the solvent molecules.[26] A legitimate corollary of these interactions leads to the
conclusion that the energy gap for π-π* transitions falls causing the red shifting in absorption
maxima. The specific chemical interactions, for instance, the possible H-bonding with the
solvent molecule also affect the absorption behaviour since H-bonding leads to an overall
alteration in the electronic density distribution.[27]
Herein we have embarked on a comprehensive study of solute-solvent interactions
with a series of solvents of different polarity and H-bonding attributes. To the best of our
23

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knowledge such a study on the organosilicon derivatives has not been conducted so far. To
start with, the concentration-dependent UV-Vis spectroscopic study of all the compounds was
carried out which showed no apparent shift in the UV-Vis absorption maxima wavelength.
This implies that these compounds do not self-aggregate and hence can be investigated with
regard to their solvatochromic properties.[28] For achieving a correlation of the solvent effect
on the solute molecules, a large number of scales have been meticulously developed and
utilized.[29] In the present work, we have explored the empirical solvatochromic scale
developed by Kamlet and Taft.[30] The linear regression approach articulated by Kamlet and
Taft is one of the most ambitious and successful quantitative treatment for the estimation of
diverse intermolecular solute-solvent interactions by means of a multi parameter equation:
vmax = vmax,0 + sπ* + aα + bβ
̅
̅
where vmax is the UV-Vis absorption maximum wavelength of a solute in a particular solvent;
̅
vmax,0 is the value of this property for the same solute in a hypothetical solvent for which π* =
̅
α = β = 0. π* is an index of the solvent dipolarity/polarizability, α is a measure of the solvent
hydrogen-bond donor (HBD) capacity and β is a measure of the solvent hydrogen-bond
acceptor (HBA) capacity. The regression coefficients s, a and b are the respective
susceptibility constants. These analytical parameters are important as their magnitude and
sign are the direct measurement of the type of solvatochromism the solute molecules exhibit.
We have used seven aprotic solvents tetrahydrofuran (THF), acetonitrile (ACN), N,N-
dimethylformamide (DMF), ethylacetate (EtOAc), chloroform (CHCl3), dichloromethane
(DCM), dichloroethane (DCE) and six protic solvents viz. methanol (MeOH), ethanol
(EtOH), n-propanol (PrOH), n-butanol (BuOH), iso-propanol (iPrOH), iso-butanol (iBuOH)
of varying polaritiy (Table 4) for studying the solvatochromic properties of the silanes by
observing their absorption spectral changes.
24

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Table 4 Physical properties and Kamlet-Taft parameters of the employed solvents[31]
Solvent
MeOH
EtOH
PrOH
BuOH
iPrOH
iBuOH
CHCl3
ACN
ε
n
α
β
π*
32.7
24.5
20.5
17.6
17.9
22.0
4.81
37.5
36.7
8.93
10.36
7.58
6.02
1.33
1.36
1.38
1.39
1.38
1.39
1.44
1.34
1.43
1.42
1.44
1.41
1.37
0.98
0.86
0.84
0.84
0.76
0.82
0.20
0.19
0.00
0.13
0
0.66
0.75
0.90
0.84
0.84
0.80
0.10
0.40
0.69
0.10
0.10
0.11
0.45
0.68
0.54
0.52
0.47
0.48
0.69
0.69
0.75
0.88
0.82
0.81
0.54
0.55
DMF
DCM
DCE
THF
0
EtOAc
0
ε: relative permittivity; n: refractive index; α: the solvent’s HBD acidity; β: the solvent’s
HBA basicity; π*: the solvent’s dipolarity/polarizability.
The absorption studies of the non-conjugated derivative (3j) could not be observed in
all solvents because of its lower λmax value as compared to the cut-off values of most of the
solvents, whereas compound 3p was partly soluble or insoluble in the majority of them.
Therefore, these compounds could not be analysed for their solvatochromic behaviour. The
absorption pattern of compound 3r has been represented in Fig. 3a for protic solvents and in
Fig. 3b for aprotic solvents, while the absorption data of all compounds in different solvents
has been depicted in Table 5.
25

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3.0
1.0
0.8
0.6
0.4
0.2
0.0
CHCl3
ACN
DMF
DCM
DCE
b)
MeOH
EtOH
PrOH
a)
2.5
2.0
BuOH
iPrOH
1.5
THF
EtOAc
iBuOH
1.0
0.5
0.0
260 280 300 320 340 360 380 400
260 280 300 320 340 360 380 400
Wavelength (nm)
Wavelength (nm)
Fig. 3. UV-Vis spectra of compound 3r in (a) protic solvents; (b) aprotic solvents
Table 5 UV spectral data vmax (103 cm-1) of aromatic thioester substituted silanes
MeOH EtOH PrOH BuOH iPrOH iBuOH CHCl3 ACN DMF DCM DCE THF EtOAc
ID
3a
3b
3c
3d
3e
3f
36.81 36.84 36.80 36.87 36.87 36.87 36.76 36.93
37.98 37.88 37.84 37.88 37.84 37.84 37.73 37.59
38.17 38.17 36.80 38.17 38.17 38.17 37.77 37.95
37.30 36.90 36.90 36.90 36.90 36.90 37.78 36.76
37.31 37.31 37.31 37.31 37.31 37.31 37.95 38.96
b
b
b
b
b
36.86 36.82 36.72 37.09
37.59 37.59 37.59 37.88
37.73 37.73 37.95 37.73
37.48 37.48 36.48 36.28
38.4 38.4 38.95 38.90
35.99 35.75 35.75 35.75 35.84 35.99 35.09 35.21 35.21 35.15 35.15 35.15 35.21
34.48 34.90 34.90 34.90 34.90 34.90 33.80 34.16 34.16 33.60 33.60 33.69 34.08
36.17 35.95 35.40 35.49 35.49 35.33 35.25 34.83 34.35 35.28 34.67 34.78 34.33
3g
3h
3i
35.59 35.59
a
35.27 35.46 35.46 34.83 33.59 33.05 34.78 34.78 34.45 34.35
36.36 36.06 35.94 35.94 36.06 35.99 35.65 36.23 36.87 35.95 35.88 35.44 35.88
35.59 35.46 35.46 35.46 35.46 35.46 35.59 35.59 37.04 35.59 35.33 35.20 35.97
31.65 32.05 32.80 32.89 32.89 32.55 32.36 32.80 34.49 32.35 32.47 32.75 34.15
31.54 32.05 31.85 31.85 31.85 31.78 31.74 32.26 34.13 31.95 31.73 31.38 31.95
30.12 29.95 29.70 29.85 29.85 29.65 28.67 28.83 28.16 28.43 28.76 28.72 29.35
3k
3l
3m
3n
3o
3q
37.17 36.76
a
36.49 36.49
a
37.03 36.63 35.21 35.08 35.08
a
36.63
26

ACCEPTED MANUSCRIPT
34.56 34.01 34.01 34.01 34.01 34.24 34.45 34.08 32.45 34.16 34.26 34.46 33.29
3r
a: insoluble in the given solvent; b: solvent has higher cut off value
The data in Table 6 shows that a few of the derivatives with Ph, pyrazine, Ph-Me, Ph-
OMe and Ph-SH substituent have +ve s values hinting that they have more polar ground state
than the corresponding excited state and thereby these compounds possess negative
solvatochromism. It is worth pointing out that in these compounds the increased polarity of
the solvents leads to a hypsochromic shift in the absorption profiles, while all other
derivatives have shown the positive solvatochromism, usual of π-π* transitions.
Table 6 vo, s, a and b (in cm-1) and correlation coefficients (R2) obtained from the Kamlet-
Taft fitting of the absorption data
Compound ID Substituent vo (103 cm-1) s (103 cm-1) a (103 cm-1) b (103 cm-1)
R2
Ph
o-Py
36.45
37.71
37.94
38.04
37.76
34.51
35.91
35.38
33.83
34.19
34.23
32.85
30.48
0.485
-0.147
-0.227
-0.521
1.143
-0.685
-1.781
-0.468
-0.495
2.021
1.668
-0.748
1.668
-0.575
0.583
0.422
1.719
2.379
1.981
2.185
0.781
0.309
-0.111
-1.002
-2.898
-2.079
0.851
-0.321
-0.021
-1.088
1.397
-1.244
-1.791
0.061
1.319
1.254
1.688
3.309
3.060
0.910
0.981
0.870
0.938
0.921
0.943
0.905
0.945
0.966
0.950
0.893
0.922
0.932
3a
3b
3c
3d
3e
3f
m-Py
p-Py
pyrazine
2,3-py
2,6-py
furan
3g
3h
3i
thiophene
Ph-Me
Ph-OMe
Ph-OH
Ph-SH
3k
3l
3m
3n
27

ACCEPTED MANUSCRIPT
Ph-NH2
Ph-Cl
29.70
35.15
34.97
-1.689
-1.498
-0.749
1.114
1.735
-1.766
0.230
-1.389
-2.574
0.948
0.825
0.932
3o
3q
3r
Ph-Ph
Meanwhile, if the coefficients a and b have negative signs, the bathochromic shifts are
expected with increasing solvent HBD and HBA capabilities, respectively. In the silanes
belonging to series 2 i.e. with substituted phenyl ring derivatives, the parameter a has -ve
sign, suggestive of the bathochromic shift with increase in solvent hydrogen-bond donor
acidity. Such a shift arises on account of H-bonding interactions of the HBD solvents with O
of C=O group. Whereas in silanes of series 1 which is composed of heterocyclic analogues,
the already existing C=O interaction competes with the additional interaction of heteroatom
with HBD solvent. This is illustrated by the +ve sign of parameter a which means that the
interaction of heteroatom with the HBD solvent is so strong that it outweighs the interaction
with the C=O group and therefore these silanes have shown hypsochromic shift with
increasing solvent hydrogen-bond donor acidity.[24] Moreover, the largest value of coefficient
a is observed in silane 3e containing pyrazine ring which is in accord with its structure as this
molecule has the maximum tendency to form H-bonds with the HBD solvent owing to the
presence of two N atoms in the aromatic ring.
Additionally, some of the silanes with Ar-OH, Ar-SH, Ar-NH2 substituents contain H-
bond donor sites and hence can interact with the H-bond acceptor solvents. The maximum
value of parameter b is found in case of silane with OH substituent on the phenyl ring which
is attributed to its capability of forming strong H-bonding with the HBA solvent.[32] An
exceptional behaviour is shown by the Ar-NH2 substituent as it is having the least value of
the coefficient b. This anomaly might be due to the involvement of NH2 group in the intra-
molecular H-bonding with the O atom of the thioester group.[33]
28