Excited-State Proton Transfer and Conformational Relaxation of 2-(4'-Pyridyl)benzimidazole in Nafion Films.

The effect of annealing on the acidity and water uptake of Nafion films has been studied by using the acidity sensing fluorophore 2-(4'-pyridyl)benzimidazole (4PBI). The difference in acidity and the microenvironment of the fluorophore in annealed and nonannealed films is brought out in this study. The annealed film is found to have less water uptake than nonannealed films. The amount of water uptake increases upon acid treatment of the films, as all the steady-state and time-resolved behaviour of the molecule in nonannealed films is restored. These observations are rationalised by the formation of anhydrides upon annealing and their hydrolysis to sulfonic acid groups upon acid treatment. Interestingly, the acidity of annealed films is found to be even less than that of Na+ exchanged films, indicating that annealing removes more protons from the Nafion films than cation exchange can.

Interactions of a few azole derivatives with a transport protein: role of heteroatoms.

The interaction of a few azole derivatives, 2-(4'-N,N-dimethylaminophenyl)benzimidazole, 2-(4'-N,N-dimethylaminophenyl)benzoxazole, 2-(4'-N,N-dimethylaminophenyl)oxazolo[4,5-b]pyridine with bovine serum albumin (BSA) were examined by absorption and fluorescence spectroscopy. The results were compared with the previously studied imidazopyridine derivative 2-(4'-N,N-dimethylaminophenyl)imidazo[4,5-b]pyridine. Displacement studies were carried out with site selective probes to locate the binding site of these ligands. The spectral shifts and the binding constant vary depending on the nature of the ligand. The fluorescence intensity of both oxazole derivatives 2-(4'-N,N-dimethylaminophenyl)benzoxazole and 2-(4'-N,N-dimethylaminophenyl) oxazolo[4,5-b]pyridine increases substantially in the presence of BSA, whereas the intensity of 2-(4'-N,N-dimethylaminophenyl)benzimidazole decreases. However, hypsochromic shift is observed in presence of BSA. The results obtained from the docking studies are also in good agreement with the experimental results. The location and orientation of binding depend upon the nature of the ligand. The studies revealed that apart from hydrophobic interaction, hydrogen bonding also plays a vital role in the molecular binding. Oxazoles have higher binding affinity than imidazoles and substitution of extra nitrogen further increases the binding affinity.

Photoinduced intramolecular charge transfer in trans-2-[4'-(N,N-dimethylamino)styryl]imidazo[4,5-b]pyridine: effect of introducing a C[double bond, length as m-dash]C double bond.

The spectral characteristics of trans-2-[4'-(N,N-dimethylamino)styryl]imidazo[4,5-b]pyridine (t-DMASIP-b) have been investigated using absorption and fluorescence techniques, and compared with 2-(4'-N,N-dimethylamino)imidazo[4,5-b]pyridine (DMAPIP-b). The study reveals that introduction of a C[double bond, length as m-dash]C double bond strongly perturbs the photophysics of the system. Unlike DMAPIP-b, t-DMASIP-b emits a single emission in aprotic and protic solvents. The emission occurs from the locally excited state in nonpolar solvents and from a planar intramolecular charge transfer (PICT) state in polar solvents. Multiple linear regression analysis suggests that among the different solvent parameters, the dipolar interaction contributes more to the stabilization of the system in both the ground and excited states. Theoretical calculations suggest that, unlike in DMAPIP-b, proton coupled twisted intramolecular charge transfer (TICT) emission does not occur in t-DMASIP-b. The higher quantum yield obtained in the viscous solvent glycerol is attributed to the restriction of the twisting of the olefinic bond. The photoirradiation of t-DMASIP-b shows that isomerization takes place in all solvents, including viscous glycerol. The theoretically simulated potential energy surface shows that isomerization occurs via a phantom state, which is a nonradiative process. The rise in temperature favors the photoisomerization, thus, the fluorescence quantum yield decreases. The prototropic study indicates that, unlike in DMAPIP-b, the protonation takes place at different places to form the monocations.

Specific site binding of metal ions on the intramolecular charge transfer fluorophore in micelles.

The binding interactions of Cu(2+), an essential trace metal ion, and Cd(2+), a deleterious metal ion, with 2-(4'-N,N-dimethylaminophenyl)oxazolo[4,5-b]pyridine (DMAPOP) were studied in acetonitrile and sodium dodecyl sulphate (SDS). The studies show that the surfactant can formulate the metal ions to bind at a specific binding site. In acetonitrile, the Cu(2+) binds at the dimethylamino nitrogen and pyridine nitrogen to form two different types of complexes with DMAPOP. On the other hand, Cd(2+) coordinates through the dimethylamino nitrogen and oxazole nitrogen to form two different types of complexes with DMAPOP in acetonitrile. But SDS effectively controls the binding site and both metal ions bind at the same ring nitrogen in SDS. The binding affinities of both metal ions with DMAPOP vary with the concentration of SDS.

Double proton transfer induced twisted intramolecular charge transfer emission in 2-(4'-N,N-dimethylaminophenyl)imidazo[4,5-b]pyridine.

The spectral characteristics of N,N-dimethyl-4-(4-methyl-4H-imidazo[4,5-b]pyridin-2-yl)benzenamine (PyN-Me), 1-methyl-2-(4'-(N,N-dimethylaminophenyl)imidazo[4,5-b]pyridine (ImNH-Me), and 2-phenylimidazo[4,5-b]pyridine (PIP) are investigated to understand the mechanism of protic solvent induced dual fluorescence of 2-(4'-N,N-dimethylaminophenyl)imidazo[4,5-b]pyridine (DMAPIP-b). No dual emission is observed from PyN-Me where pyridyl nitrogen blocked from hydrogen bonding with protic solvents confirms the importance of hydrogen bonding of protic solvents with the pyridyl nitrogen in dual emission of DMAPIP-b. Like DMAPIP-b, ImNH-Me also exhibits weak emission and has a shorter fluorescence lifetime in methanol. However, single emission is observed from ImNH-Me in all solvents including protic solvents. This suggests that the imidazole >NH hydrogen also plays a role in the dual emission process. The longer wavelength emission of DMAPIP-b in water increases with increase in pH of the solution owing to deprotonation of the imidazole >NH group. On the basis of these results, the mechanism for the dual emission of DMAPIP-b is proposed.

Alkyl chain dependent interactions of ligands with bovine serum albumin.

The interactions of 2-(4'-N,N-dimethylamino)phenylimidazo[4,5-b]pyridine (DMAPIP-b), an inhibitor for aurora kinase and its long chain alkylated derivatives (1 and 2, Chart 1) with bovine serum albumin (BSA) are investigated. Both the effect of BSA on the spectral characteristics of the ligands and the effect of ligands on the spectral characteristics of BSA are studied. The fluorescence of all the three molecules increases substantially in the presence of protein. The alkylated molecules can sense BSA at lower concentration than DMAPIP-b. In the presence of BSA a 45 fold increase is observed in the fluorescent intensity of pyridine nitrogen alkylated molecule (1). The binding sites and the interactions of ligands depend on the presence and the position of alkyl chain. DMAPIP-b and molecule 1 quench the intrinsic fluorescence of BSA by energy transfer. On the other hand molecule 2 induces conformation change that leads to quenching of intrinsic fluorescence of BSA. Docking studies are also performed to support the experimental results.

Photoisomerization of trans-2-[4'-(dimethylamino)styryl]benzothiazole.

Photoreaction of trans-2-[4'-(dimethylamino)styryl]benzothiazole (t-DMASBT) under direct irradiation has been investigated in dioxane, chloroform, methanol and glycerol to understand the mechanism of photoisomerization. Contrary to an earlier report, isomerization takes place in all these solvents including glycerol. The results show that restriction on photoisomerization leads to the increase in fluorescence quantum yield in glycerol. The results are consistent with the theoretically simulated potential energy surface reported earlier using time-dependent density functional theory (TDDFT) calculations. DFT calculations on cis isomers under isolated condition have suggested that cis-B conformer is more stable than cis-A conformer due to hydrogen-bonding interaction. In the ground state, cis-DMASBT is predominantly present as cis-B. The fluorescence spectra of the irradiated t-DMASBT suggested that photoisomerization follows not the adiabatic path as proposed by Saha et al., but the nonadiabatic path.

Photophysical study of 2-(4'-N,N-dimethylaminophenyl)oxazolo[4,5-b]pyridine in different solvents and at various pH.

The spectral characteristics of 2-(4'-N,N-dimethylaminophenyl)oxazolo[4,5-b]pyridine (DMAPOP) have been investigated in solvents of different polarity and hydrogen bonding capacity. Unlike its imidazole analogue, DMAPOP emits single emission in both aprotic and protic solvents and the hydrogen-bond induced TICT emission is not observed in any protic solvents. The solvent effect on both absorption and the emission spectral data are analyzed by multiple parametric regression analysis. The prototropic studies reveal that two kinds of monocations are formed by protonation of pyridine nitrogen (MC1) and the dimethylamino nitrogen (MC3) in both ground and excited states. However three kinds of dications are formed by protonation of pyridine and oxazole nitrogens (DC1), pyridine and dimethylamino nitrogens (DC2), and dimethylamino and oxazole nitrogens (DC3).

The role of hydrogen bonding in excited state intramolecular charge transfer.

Intramolecular charge transfer (ICT) that occurs upon photoexcitation of molecules is a vital process in nature and it has ample applications in chemistry and biology. The ICT process of the excited molecules is affected by several environmental factors including polarity, viscosity and hydrogen bonding. The effect of polarity and viscosity on the ICT processes is well understood. But, despite the fact that hydrogen bonding significantly influences the ICT process, the specific role of hydrogen bonding in the formation and stabilization of the ICT state is not unambiguously established. Some literature reports predicted that the hydrogen bonding of the solvent with a donor promotes the formation of a twisted intramolecular charge transfer (TICT) state. Some other reports stated that it inhibits the formation of the TICT state. Alternatively, it was proposed that the hydrogen bonding of the solvent with an acceptor favors the TICT state. It is also observed that a dynamic equilibrium is established between the free and the hydrogen bonded ICT states. This perspective focuses on the specific role played by hydrogen bonding of the solvent with the donor and the acceptor, and by proton transfer in the ICT process. The utility of such influence in molecular recognition and anion sensing is discussed with a few recent literature examples in the end.