2-(2'-Hydroxyphenyl)benzimidazole and its Nitrogen Substituted Analogues Toward Metal Ions
4.1. Absorption Spectra
4.1.1. Alkali and Alkaline Metal Ions
HPBI shows in increase in absorbance on increasing concentrations of Na+, Mg2+ and Ca2+ ions, but shows a decrease in the case of Ba2+. However, no shift is observed and the changes in the spectral shape are also not significant. As a representation, Figure 4.1 shows the increase in absorbance of HPBI in increasing concentration of Na+. HPIP-b in presence of Mg2+, Ca2+ and Ba2+ ions and HPIP-c in presence of Na+, Ca2+ and Ba2+ ions also show similar trend in the absorption spectra.
0.00 0.05 0.10 0.15 0.20 0.25
250 280 310 340 370 400
Wavelength (nm)
Absorbance
0 µΜ 0 µΜ 0 µΜ 0 µΜ 345 µΜ 345 µΜ 345 µΜ 345 µΜ
Figure 4.1. Absorption spectra of HPBI in presence of Na+.
However, in the presence of Na+ ion, HPIP-b shows a stepwise change in the absorption spectra as the concentration of Na+ is increased from 0 to 9.7 µM as is shown in Figure 4.2. For initial addition of Na+ ion to HPIP-b solution, the absorbance increases until the concentration of Na+ reaches 5.2 µM (1:1 molar ratio, Figure 4.2a). Above this
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concentration ([Na+ > 5.2 µM), a new band appears at 354 nm while the band maximum at 322 nm decreases and an isosbestic point is observed at 338 nm (Figure 4.2b). The new band reaches the maximum when [Na+] = 6.0 µM. Further increase in concentration from 6.0 µM to 7.1 µM decreases the absorbance of 354 nm band and increases that of 322 nm band with an isosbestic point at 338 nm (Figure 4.2c). Beyond 7.1 µM, the 354 nm band increases again while the 322 nm band decreases (Figure 4.2d). Above 9.7 µM of Na+, no significant change in the spectra is observed.
0 0.04 0.08 0.12 0.16
270 290 310 330 350 370
Wavelength (nm)
Absorbance
0 µΜ0 µΜ0 µΜ 0 µΜ 5.2 µΜ 5.2 µΜ 5.2 µΜ 5.2 µΜ
0 0.04 0.08 0.12 0.16
270 290 310 330 350 370
Wavelength (nm)
Absorbance
5.2 µΜ 5.2 µΜ 5.2 µΜ 5.2 µΜ 6.0 µΜ 6.0 µΜ 6.0 µΜ 6.0 µΜ
0 0.04 0.08 0.12 0.16
270 290 310 330 350 370
Wavelength (nm)
Absorbance
7.1 µΜ 7.1 µΜ7.1 µΜ 7.1 µΜ 6.0 µΜ 6.0 µΜ6.0 µΜ 6.0 µΜ
0 0.04 0.08 0.12 0.16
270 290 310 330 350 370
Wavelength (nm)
Absorbance
7.3 µΜ 7.3 µΜ 7.3 µΜ 7.3 µΜ 9.7 µΜ 9.7 µΜ 9.7 µΜ 9.7 µΜ
Figure 4.2. Stepwise change in the absorption spectra of HPIP-b at (a) 0-5.2 µM, (b) 5.2-6.0 µM, (c) 6.0-7.3 µM, and (d) 7.3-9.7 µM of Na+.
0 0.03 0.06 0.09 0.12
250 280 310 340 370 400
Wavelength (nm)
Absorbance
0 µΜ0 µΜ0 µΜ 0 µΜ 303 µΜ 303 µΜ303 µΜ 303 µΜ
Figure 4.3. Absorption spectra of HPIP-c in presence of Mg2+.
(a) (b)
(c) (d)
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Figure 4.3 shows the effect of Mg2+ ion on HPIP-c. On increasing the concentration of Mg2+, the absorbance at onset increases while that of the main band at 315 decreases.
However, unlike in the earlier case (interaction of Na+ ion with HPIP-b) the spectral changes are not significant.
4.1.2. Ni2+, Cu2+ and Zn2+ Metal Ions
Figure 4.4 depicts the absorption spectra of HPIP-b at different concentrations of Zn2+ ion. The absorbance of the main band decreases and a new band starts to appear at the red side of the absorption spectra (~ 360 nm) and the spectral changes reach saturation at higher concentrations and a quasi isosbestic point is also observed. Similar behaviour is also observed in the other two molecules (Figures not shown), but the saturation point for the interaction of Zn2+ with HPBI and HPIP-b is reached at 500 µM and 460 µM, respectively.
However, for HPIP-c the saturation point is reached at 8 µM which is ~ 1.5 equivalent. This shows the sensitivity of HPIP-c toward Zn2+ is nearly 60 times higher than that of its analogues.
0.00 0.04 0.08 0.12 0.16
285 305 325 345 365 385 405 Wavelength (nm)
Absorbance
0 µΜ 0 µΜ 0 µΜ 0 µΜ 460 µΜ 460 µΜ 460 µΜ 460 µΜ
Figure 4.4. UV-Visible spectra of HPIP-b in presence of Zn2+.
The effect of Cu2+ ions on the absorption spectra of all three fluorophores is nearly similar to that of Zn2+ up to ~ 10 µM (i.e. ~ 2 equivalents). That is, a decrease in absorbance of the main band with a formation of new band on the red side band. The isosbestic points are observed at 334 nm, 342 nm and 334 nm in the absorption spectra of HPBI, HPIP-b and HPIP-c, respectively. Figure 4.5 shows the absorption spectra of HPBI in presence of Cu2+
ion as representative plot. On increasing the concentration until 13 µM, the absorbance of the shoulder at 355-370 nm increases (Figure 4.5a). On the other side of the isosbestic point, the
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absorbances of the vibronic bands at 315-323 nm decrease. Above this concentration of Cu2+
ion the isosbestic points disappear and the absorbances increase with increasing concentration of Cu2+ ion, but the increases in small and it is saturated at ~ 350 µM (Figure 4.5b).
0.00 0.05 0.10 0.15 0.20 0.25
250 280 310 340 370 400
Wavelength (nm)
Absorbance
0 µΜ 0 µΜ 0 µΜ 0 µΜ 13 µΜ 13 µΜ 13 µΜ 13 µΜ
0.00 0.05 0.10 0.15 0.20
250 280 310 340 370 400
Wavelength (nm)
Absorbance
13 µΜ 13 µΜ13 µΜ 13 µΜ 345 µΜ 345 µΜ345 µΜ 345 µΜ
Figure 4.5. UV-Visible spectra of HPBI in presence of Cu2+ (a) from 0 µM to 13 µM, and (b)
≥ 13 µM.
0.00 0.03 0.06 0.09 0.12
250 280 310 340 370 400 430
Wavelength (nm)
Absorbance
0 µΜ 0 µΜ 0 µΜ 0 µΜ 82 µΜ 82 µΜ 82 µΜ 82 µΜ
Figure 4.6. Absorption spectra of HPIP-c in presence of Ni2+. (a)
(b)
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0.00 0.03 0.06 0.09 0.12 0.15
280 300 320 340 360 380 400 420 Wavelength (nm)
Absorbance
0 µΜ0 µΜ0 µΜ 0 µΜ 4 µΜ4 µΜ4 µΜ 4 µΜ
0.00 0.03 0.06 0.09 0.12 0.15
280 300 320 340 360 380 400 420 Wavelength (nm)
Absorbance
4.3 4.3 4.3
4.3 - 8.78.78.78.7 µΜ µΜ µΜ µΜ
0.00 0.03 0.06 0.09 0.12 0.15
280 300 320 340 360 380 400 420 Wavelength (nm)
Absorbance
8.7 µΜ 8.7 µΜ 8.7 µΜ 8.7 µΜ 130 µΜ 130 µΜ 130 µΜ 130 µΜ
Figure 4.7. Stepwise change in absorbance of HPIP-b in presence of Ni2+ in the range (a) 0 - 4 µM, (b) 4.3 - 8.7 µM, and (c) 8.7 - 130 µM.
The effect of Ni2+ on the absorption spectra of HPBI and HPIP-c is also similar to that of Zn2+ ion. Figure 4.6 shows the representative spectra of HPIP-c in presence of Ni2+ ion.
With increase in Ni2+ ion concentration, the absorbances of the vibronic bands at 315-330 nm decrease. On the red side, a new red band appears at 360-370 nm whose absorbance increases
(a)
(b)
(c)
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with Ni2+ ion concentration. The spectra of HPBI and HPIP-c reach the saturation point at 55 µM and 82 µM, respectively.
However, HPIP-b shows stepwise change in the absorption spectra upon interaction with Ni2+ ion (Figures 4.7). The effect is quite similar to that of Na+ ion on HPIP-b. A new band appears at 380 nm and its absorbance increases while those of vibronic bands at blue side decrease with increase in Ni2+ ion concentration up to 4 µM (~ 1 equivalent) and an isosbestic point is observed at ~ 345 nm (Figure 4.7a). From 4 µM to 4.3 µM, the band at 380 nm disappears rapidly and the spectra remain constant till 8.7 µM (Figure 4.7b). With further increase in Ni2+ ion amount, a new band starts to appear at ~ 360 nm and the absorbances of the vibronic bands at 296 nm and 321 nm decrease. The changes continue till the saturation point (130 µM). A quasi-isosbestic point is observed at ~ 334 nm (Figure 4.7c).