Medium
3.2. Effect of nitrogen substitution on anion sensitivity and deprotonation of HPBI
3.2.5 Existence of cis-H bond complex and different anions of HPIP-b and HPIP-c The main absorption bands of the imidazopyridines derivatives correspond to their cis-
enol conformer.72, 118, 120 The bathochromic shift in the main absorption band of HPIP-b upon addition of fluoride ion is similar to that observed in the absorption spectra of HPBI upon addition of fluoride due to the formation of cis-H-bond complex. The vibrational structure is also much resolved in the presence of fluoride. These changes indicate a stronger hydrogen bonding interaction between the cis-enol and the added anion, and it is labeled as cis-H-bond complex. On the other hand, the absence of similar shift in the main absorption band of HPIP-c clearly indicates a weaker interaction between HPIP-c and the added anions in the ground state.
In presence of fluoride, when excited at 320 nm, the normal and the tautomer emissions of HPIP-b disappear to form a single emission spectrum at 455 nm. The new band is red shifted as compare to the tautomer emission and blue shifted as compare to
Figure 3.2.11. 1H NMR spectra of HPIP-c at different fluoride concentration in DMSO-d6.
the normal emission. This is consistent with the formation of the anion by intermolecular proton transfer of the phenolic proton.117 The excitation spectrum corresponding to the 455 nm emission matches with the absorption spectra of the cis-H- bond complex (Figure 3.2.5.B). Upon excitation, due to enhancement in the acidity of the phenolic group, the cis-H-bond complex donates the proton to the nearby hydrogen bonded fluoride ion to generate the anion in the excited state. Since, the cis-H-bond complex can produce the cis-anion but not the trans-anion, the 455 emission should correspond to the emission from the cis-anion. This is similar to the emission from the cis-anion, observed upon excitation of cis-H-bond complex of HPBI in presence of fluoride ion (Section 3.1.5). The theoretically predicted emission energy of the cis-anion (Table 3.2.1) is also in agreement with the experimental emission energy. The calculated absorption maximum for the cis-anion is 410 nm. A very low intensity at the longer wavelength region of the excitation spectra monitored at 455 nm suggests that the cis- anion is present in very small quantity in the ground state (Figure 3.2.5.B). However, excitation at 370 nm indicates that at very low fluoride concentration (0.4 µM) HPIP-b yields a different species which emits at 440 nm. The species can be assigned to the trans- anion. The theoretical calculations also predicted such blue shift in the emission maximum of the trans-anion compared to that of the cis-anion (Table 3.2.1.). The agreement of calculated emission energy of the trans-anion with the experimental value substantiates the assignment. The biexponential decay observed at 410 nm ratifies the presence of an additional ~ 410 nm blue shifted emission with 2.0 ns lifetime at higher concentration of fluoride. This indicates the formation of dianion. As the second deprotonation takes place at the azole ‘NH’ of the cis-anion, the emission band is blue shifted.8 The excitation spectrum corresponded to the dianionic species has a band maxima at 365 nm (Figure 3.2.5.B). It explains the little enhancement in the absorbance at 365 nm at very high anion concentration (Figure 3.2.1.). On the other hand, HPIP-b exhibits trans-anion emissions up to 1.2 µM of acetate concentration. The cis-anion emission appears at higher acetate concentration. The dissimilarity in the interactions of fluoride and acetate with the fluorophore is due to the difference in their relative strengths. Fluoride is a stronger base than acetate. Under isolated condition, the trans- anion is more stable than the cis-anion due the intramolecular hydrogen bond between the phenolate ion and the ‘NH’ proton (Chart 3.2.1.). Therefore, as mentioned in the previous section, the existence of trans- or cis- anion depends on this intramolecular hydrogen bond. If the external anion cannot interact with the ‘NH’ proton efficiently,
Intermolecular Proton Transfer of HPBI and its Nitrogenous Analogues in Polar Aprotic Medium
the unstable cis anion rotates to form the trans-anion. Due to higher basicity, fluoride effectively stabilizes the ‘NH’ proton even at low concentration. This prevents the formation of intramolecular hydrogen bond by rotation. But comparatively weaker (lower strength) acetate anion is inefficient to stabilize the ‘NH’ proton in sufficient extent at lower concentration. As a consequence, the trans-anion of HPIP-b is observed at very low fluoride concentration and low acetate concentration. At higher anion concentration, the ‘NH’ proton is stabilized by the external anion, and the cis-anion is detected.
Conversely, HPIP-c has weaker interaction even with fluoride. At low concentration of fluoride and acetate, the excitation at longer wavelength results in the 427 nm emission. The high concentration yields the 438 nm emission. The 427 nm and 438 nm
Scheme 3.2.1. Formation of different anions of (A) HPIP-b and (B) HPIP-c in the ground and excited state in presence of fluoride and acetate in acetonitrile.
emissions from HPIP-c should correspond to the trans-anionic and cis-anionic forms, respectively. This assignment is based on the relative shift of the absorption and emission spectra of cis- and trans- anion. The calculated emission energies also agree well with the experimental values (Table 3.2.1). Accordingly, the excitation spectral maximum at 368 nm which is obtained at low fluoride concentration can be attributed to the trans-anion. The main band of the excitation spectra at high fluoride concentration matches with the absorption spectrum of the weak cis-H-bond complex. This further confirms that the anion that formed at higher concentration is the cis-anion and therefore the 438 nm emission corresponds to cis-anion. Simultaneously, a low intensity at longer wavelength region indicates the existence of negligible amount of the cis-anion in ground state. The second 1.2 ns fluorescence lifetime obtained at very high concentration is due to the dianion.
The transformations of HPIP-b and HPIP-c in presence of different anions are summarized in Scheme 3.2.1. The absorption spectra and the excitation spectra suggest the presence of very small amount of cis- or trans- anion in the ground state. However, in the excited state the emissions are observed from the anions. As mentioned earlier, this is due to the enhanced acidity in the excited state, which facilitates the intermolecular proton transfer. In the excited state, depending on the concentration of fluoride and acetate, HPIP-b and HPIP-c exists as cis- and trans-anion.
The NMR titration of HPIP-b and HPIP-c with fluoride anion also reveals the formation of cis-H-bond complex. The observed upfield shift of proton peaks upon addition of anion indicates the shielding effect due to the presence of negatively charged ions near the molecule. The downfield shift of the H-6’ proton peak suggests a through space interaction between this proton and the fluoride ion which is accumulated near the ‘NH’ proton.114 Such shifts were also observed when HPBI forms cis-H-bond complex (Figure 3.1.9). The negative charge accumulates on the oxygen atom either due to the hydrogen bonding interaction of the negatively charged ion or by the formation of anionic form. This charge generates a high shielding effect. The extent of this effects on H-3’ (ortho to hydroxyl group) and H-5’ (para to hydroxyl group) must be different.
It should depend on the strength of the negative charge generated on the oxygen atom.
Hence, the peaks of these protons (H-3’ & H-5’) which were close to each other before the interaction with the external anion, started to move apart from each other after the interaction. The separation between the peaks of H-3’ and H-5’ are 0.040 0.005 for nitrogen substituted molecules (Figure 3.2.9. and 3.2.10.). Whereas, it was 0.085 ppm
Intermolecular Proton Transfer of HPBI and its Nitrogenous Analogues in Polar Aprotic Medium
for HPBI upon deprotonation of ‘OH’ proton (Figure 3.1.9). The large difference observed in the NMR spectrum of HPBI in presence of fluoride was the result of accumulation of more negative charge due to the deprotonation of ‘OH’ proton. Since, the anion formation is less in HPIP-b and HPIP-c, this difference is small. The initially observed downfield shift in the peak of H-6’ in cis-H-bond complex was shifted upfield considerably upon formation of anion in HPBI (Figure 3.1.9). Such an upfield shift in the H-6’ proton peak at high concentration of fluoride is negligible for HPIP-b and HPIP-c. This further suggests that the formation of anion in the ground state is less in nitrogen substituted analogs.