Polymorph 1.29a Polymorph 1.29b Figure 1.11: Conformational polymorphs 1.29a and 1.29b
1.5 Quinone and their derivatives as sensors for anions and cations
1.5.1: Quinone and their derivatives in anion sensing
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N N
N N
O
O
N N
N N
O
O Zn
Zn H
H
Polymorph 1.29a Polymorph 1.29b
fluoride ion. Such hydrogen bond formation had affected the electronic properties of the chromophores, resulting in a charge-transfer transition.76-77 Fluoride ions interacted with the receptors more strongly than other halides due to its higher electronegativity and smaller size.78-79
O
O N
OH R
R = Br (1.30a), Me(1.30b), NO2 (1.30c)
(a) (b)
Figure 1.12: (a)Anthraquinone based receptor and (b) Change in absorption of receptor 1.30a on addition of 0-10 equiv. of tetrabutylammonium fluoride in dimethylsulphoxide.
There are other also examples on related anthracene-9,10-dione derived quaternary ammonium salt-based chromogenic and fluorescent chemosensors for fluoride ions.80-82 For example, chemosensors 1.31 and 1.32 possesses quaternary ammonium cation and N- H group. They exhibit absorption and emission changes with fluoride ions.
O
O NH H3CN
H3C
R +
R = H, X = Cl R = NO2, X = Br
X- O
O NH H3CN
H3C+Cl-
NH N CH3
CH3 Cl- +
1.31 1.32
Figure 1.13: Anthraquinone based receptors.
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Chemosensor 1.31 on excitation at 450 nm emits at 580 nm. No significant response to other anions such as chloride, bromide, iodide, nitrate, acetate, sulphate, phosphateand perchlorate were observed with the receptor 1.31. The changes in the fluorescence emission of the 1.31 and 1.32 on addition of fluoride ions are shown in figure 1.14b.
(a) (b)
Figure 1.14: (a) Fluorescence emission of compounds (a) 1.31 and (b) 1.32 on addition of fluoride ions.
Addition of fluoride ions the fluorescence intensity at 580 nm was gradually decreased and a new blue shifted fluorescence emission at two new emission bands at 505 and 540 nm appeared. But compound 1.32 on addition of fluoride ions shows quenching of emission at 580 nm.
6-(4-Ethylphenoxy)-5,12-naphthacenequinone (1.33) is photochromic.83-84 UV-visible absorption spectra of compound 1.33 in acetonitrile changes on irradiation by 365 nm radiation. The spectral changes show characteristic peaks in 400-550 nm region, which corresponds to quinoidal form of phenoxynaphthacenequinone. Peak intensities increase with increase of irradiation time. Gradual addition of cyanide ions to a solution of 1.33 while UV irradiation is in operation, brings about significant change in the absorption spectra in a selective as well as sensitive manner over the other anions. The color of acetonitrile solution of 1.33 was changed from yellow to pale brown. This spectral change helps to detect cyanide anion at a concentration as low as 18.7 µM.
O
O O Et
1.33
(a) (b)
Figure 1.15: (a) UV-vis spectra of 1.33 and (b) Changes in absorption of 1.33 when irradiated by 365 nm in the presence of different anions.
A film of poly-(2-hydroxyethylmethacrylate) doped with 1,2-diaminoanthraquinone is used as colorimetric sensing material for detection of nitric oxide and nitrite ion.85-86 In this colorimetric sensing process chemical transformation on the diamine groups takes place to form heterocyclic compound shown in figure 1.16. The heterocyclic compound formation is very specific by nitric acid in presence of oxygen or with nitrite ions and an acid in aqeous medium.
O
O
NH2
NH2 NO + H2O / O2 NO2- + H2O / H+
O
O
N HN N
1.34 1.35
Figure 1.16: Reaction in colorimetric detections of nitric oxide and nitrate anion.
1,4-Di-(2-aminoethylamino)anthraquinone urea or thiourea are used todistinguish isomers of dicarboxylate anions.87 Selective colour changes of receptors (1.36a-b) in presence of maleate and fumarate anions in dimethylsulphoxide/water were observed by naked eye.
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O O
NH HN
NH HN
HN
X X
R NH
R
X = S, R = p-NO2-naphthyl X = O, R = P-NO2-phenyl
(1.36a) (1.36b)
(a) (b)
Figure 1.17: (a) Urea/thiourea based anthraquinone receptors for dicarboxylic acids and (b) Changes of UV-visible absorptions of 1.36a in dimethylsulphoxide on the addition of malonate anion.
When concentration of maleate was increased, a new absorption band at 480 nm was formed, whose intensity was substantially enhanced on increase in concentration of maleate anions. In contrast, a similar experiment with fumarate salt showed no significant change in UV-visible spectra. Different colors observed with maleate and fumarate was related regiochemistry of the receptor. Maleate anion has a cis configuration perfectly fit between the thiourea parts of receptor inducing a conformation change in the receptor as shown in figure 1.18.
O
O HN
HN N N
S
H H O2N
N S N
O2N H H
O O
O O
Figure 1.18: Binding of 1.36a with maleate anion.
In contrast, fumarate anion has a trans geometry which cannot form 1:1 host-guest complex to hold two thiourea units from same host molecule. Receptor 1.36b is a urea
based molecule changes color from blue to green with maleate and blue to pink color with fumarate respectively. Hydrogen bond host-guest complex affects electronic properties of the chromophore of parent host molecule which results in the observed color changes.
N N
N N NHHN
O
O
O
O
HN HN
N N NH
NH O
O
Ts
Ts
1.37 1.38 1.39
Figure 1.19: Anthraquinone based and diazaanthraquinone based anion receptors.
To make efficient receptors for ions anthraquinone derivatives are modified by connecting additional heterocyclic rings or by incorporating hetero atoms to anthraquinone skeleton. Examples of such anion receptors are shown in figure 1.19. UV- visible spectroscopy and visual inspection of solutions of 1.37-1.39 after addition of an anion such as fluoride, cyanide, acetate or pyrophosphate ions show dramatic changes in color.88 Based on the redox properties of such compounds coupled with fluorescence properties of the oxidized and reduced states reversible switch is developed. Fluorescence emission study indicated that N-dansylcarbazoloquinone shows highly reversible increase and decrease in emission relating a molecular switching property. By adding sodium borohydride to a solution of compound 1.40 emissions can be generated and allowing aerial oxidation of compound 1.41 the quenching of emission can be achieved as illustrated in Scheme 1.7.89
O
O NH
NH S O O
N CH3
CH3
OH
NH
NH S O O
N CH3
CH3 NaBH4
MeOH, RT [O]
1.40 1.41
OH
Fluorescence OFF Fluorescence ON Scheme 1.7: Fluorescence switch off by redox reaction.
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In this example it was shown that the fluorescence emission of 1.40 on excitation at 336 nm of the dansyl group was totally quenched but could be instantly generated after reduction of the quinone to dihydroxy compound by chemical reduction of the quinone 1.41 with sodium borohydride. These are some of the examples of quinonic derivates which act as receptors for anions. They behave differently in different pH, redox conditions as well as photochemical condition. Thus there is a large scope to develop new quinoidal scaffolds for detections of anions and also understand their selectivity towards anion binding.