The work contained in this thesis entitled "Proton transfer and molecular logic functions of a few azole derivatives" is the outcome of the research work carried out by me under the guidance of dr. The thesis concludes with the summary of the current work and the scope of the future work.
Introduction
The thesis deals with different proton transfer processes of some azole derivatives both in the ground state and in the excited state. The fourth and fifth chapters describe the inter- and intra-molecular proton transfer of some azole molecules to produce different types of proton transfer such as different tautomers, anion, cation, zwitter ion, etc.
Materials, Methods and Instrumentation
At the end of the thesis, the construction of the molecular logic gate by perturbing the emission characteristics of fluorophores is shown. The second chapter gives details of the material, methods and instruments used during the thesis.
Chapter 3: Intermolecular Proton Transfer of 2-(2'-hydroxyphenyl) benzimidazole and its Nitrogenous Analogues in Polar Aprotic Medium
Switching between cis- and trans- anions of 2-(2'-hydroxyphenyl)benzimidazole
The key factor in the existence of the cis or trans anion depends on the stabilization of the 'NH' proton. If the external anion forms a strong hydrogen bond with the 'NH' proton of the initially formed cis anion, the phenoxide unit does not rotate to produce the trans anion.
Effect of nitrogen substitution on anion sensitivity and deprotonation of HPBI Nitrogen substitution on the benzene ring of the benzimidazole moiety of HPBI
The dissolution of the external anions in alkaline aqueous medium renders the anion ineffective in stabilizing the 'NH' proton and results in only the trans anion. It has been shown that the sensitivity to the anion and the formation of the anion also depends on the position of the pyridyl nitrogen.
Chapter 4: Intramolecular Proton Transfer and Conformer Specific Aggregation Induced Enhanced Emission of 1,4-bis(2-(2’-
As the pH of the aqueous solution changes, intermolecular proton transfer occurs between BHPBI and the solvent, which results in different anionic and cationic species. On the other hand, when one of the hydroxyl groups is methylated (3-(2'-hydroxyphenyl)-5-(2'- methoxyphenyl)-1H-1,2,4-triazole (MTz)), the molecule exhibits a single emission of the tautomer in soln.
Chapter 6: Construction of Molecular Logic Gates by Perturbing the Emission Characteristics s of a Few Azole Derivatives
Specific site binding of metal ions in presence of anionic micelle and construction of molecular logic gates
Construction of a molecular logic gate and keyboard latch system by inducing intermolecular proton transfer.
Construction of molecular logic gate and keypad lock system by inducing the inter molecular proton transfer
Single fluorophore to address multiple logic gates
Summary and Scope for Future Work
Intramolecular Proton Transfer and Conformer Specific
Chapter 6: Construction of Molecular Logic Gates by Perturbing the Emission Behaviors of a Few Azole Derivatives
List of Abbreviations
List of Charts
The normalized emission spectra of HPIP-c at low and high fluoride concentrations are shown in the inset. Emission spectra of BHPBI (𝑒𝑥 = 290 nm) in the presence of (a) no input, (b) 50 μM fluorine, (c) irradiation, (d) irradiation followed by the addition of 50 μM fluorine addition(s) 50 μM fluoride followed by irradiation.
List of Schemes
List of Tables
Introduction
- Proton transfer
- Intermolecular proton transfer
- Intramolecular proton transfer
- Factors affecting the proton transfer
- Molecular logic gates
- Classification of logic gates
- Implementation of logic gates in molecular scale
- Motivation of the present work
It indicates the transfer of a proton to a water molecule due to the increased acidity of the hydroxyl group in the excited state. In the excited state, the basicity of the pyridine nitrogen (pKa) increases and proton transfer occurs from the imidazole nitrogen to the pyridine nitrogen.
Materials, Methods and Instrumentations
- Introduction
- Materials
- Solvents
- Salts of different metal ions and anions
- Other chemicals
- Sample preparation
- Fluorophore solutions
- Metal ion, anion and pH titration
- Micellar solution
- Aggregates of fluorophores
- Methods
- Quantum yield
- Time-resolved area normalized emission spectra
- Quantum mechanical calculations
- Instruments
- pH meter
- Absorption spectrophotometer
- Steady-state fluorimeter
- Time resolved fluorimeter
- Other instruments
The appropriate amount of salt solution was added to the fixed amount of fluorophore. The emission spectrum reflects different fluorescence intensities as a function of em. the wavelength at which fluorescence is observed) when the excitation wavelength (ex) is fixed. The emission spectrum is distorted by the wavelength dependence of the emission monochromator efficiency and photomultiplier response.
Intermolecular Proton Transfer of 2-(2'- Hydroxyphenyl)benzimidazole and its
Medium
Introduction
The proton transfer efficiency depends on the acidity of the donor and the proton-accepting ability of the base. Since the acidity of the proton donor group is increased in the excited state, the proton transfer is more efficient in the excited state. This chapter demonstrates the formation of anion of HPBI and its nitrogenous analogues by intermolecular proton transfer and the disruption of their existence by chemical stabilization.
Switching between cis- and trans- anions of 2-(2'-hydroxyphenyl) benzimidazole: a molecular rotation perturbed by chemical stabilization
- Quantum chemical calculation
- Interaction of the fluoride anion with HPBI in acetonitrile
- Interaction of hydroxyl anion with HPBI in aprotic solvents
- NMR titrations
- Existence of cis-anion and dianion
- Effect of NaOH
- Interpretation of NMR spectra
- Conclusion
Normalized excitation spectra of HPBI in NaOH saturated acetonitrile (a) 𝑒𝑚 = 460 nm and (b) 𝑒𝑚 = 420 nm together with the absorption spectrum (dashed line) of the solution.). Consequently, unlike in the ground state, the rotation of the cis-anion is not feasible in the excited state (Figure 3.1.1.). Consequently, the stabilization of the 'NH' proton by the anion decreases and this leads to the decrease in the population of stable cis anions.
Effect of nitrogen substitution on anion sensitivity and deprotonation of HPBI
- Absorption nature of the fluorophores in presence of different anions
- Emission nature of different species
- Theoretical calculation
- NMR spectra
- 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-
- Sensitivity and ease of deprotonation
- Conclusion
Instead, an increase in absorbance is observed in the absorption spectra of HPIP-c with increasing fluoride concentration. As the second deprotonation takes place at the azole 'NH' of the cis anion, the emission band is blue shifted.8 The excitation spectrum corresponding to the dianionic species has a band maximum at 365 nm (Figure 3.2.5.B). However, the theoretical calculation predicts a planar geometry for the cis anion of HPIP-b and HPIP-c.
Intramolecular Proton Transfer and Conformer Specific Aggregation Induced
Enhanced Emission of 1,4-bis(2-(2′-
Hydroxyphenyl)benzimidazol-1-ylmethyl)- benzene
Introduction
It is only possible if the ground state equilibrium can be disrupted to end up with a desired conformer. To achieve distinct enol and keto emission, a derivative of HPBI, BHPBI, was selected and investigated.78 BHPBI possesses two HPBI units non-conjugatively linked via a spacer (Chart 4.1). At the end of the chapter, it is shown that by controlling the molecular solubility and modifying the intra/intermolecular H-bonding, different conformers can be separated as different aggregates.
Planar and twisted conformer
Absorption maximum (𝑚𝑎𝑥𝑎𝑏𝑠 , nm), emission maximum (𝑚𝑎𝑥𝑒𝑚 , nm) and excited state lifetime (, ns) of BHPBI solutions in different. 72, 112 The absorption spectra of BHPBI indicate the existence of different ground state conformers which exhibit different absorptions. Instead, the HPBI units of BHPBI possess a nearly planar structure where strong intramolecular hydrogen bonding persists between the azole nitrogen and the phenolic “OH”.
Photoinduced planarization and proton transfer
This indicates that the benzimidazole moiety and the phenolic ring do not have a perfect planar structure. In hydrogen-bonding solvents, the intramolecular hydrogen bond is broken and the phenolic ring undergoes torsional rotation. The results conclude that due to the close proximity of the azole nitrogen and the phenolic 'OH', proton transfer occurs upon photoexcitation to form the amine-keto tautomer.
Triple fluorescence
The relative amplitude of short-lived species is higher when monitored at longer wavelengths. The emission spectra of the visible species emitting fluorescence are resolved by the time-resolved fluorescence method. The emission maxima of the long- and short-lived species in dioxane are 460 nm and 495 nm, respectively.
Prototropic study
The other two lifetimes can be assigned to the cis- and trans-keto tautomer of the unprotonated HPBI unit of BHPBI monocation. The relative population of the species with a lifetime of 0.6 ns increases at 470 nm compared to that at 388 nm. When monitoring at 420 nm, the relative amplitude of the 1.4 ns species increases and that of the 3.8 ns species.
Conformer specific aggregation induced enhanced emission
The complete absence of enol emission and the presence of only tautomer emission can be attributed to the absence of the twisted conformer in these aggregates (in 90% aqueous solution). The lifetimes of enol emission and anion emission in 70% aqueous solution are 0.56 ns and 2.1 ns, respectively. Excitation of the twisted conformer results in enol emission, as observed in other solvents.
Conclusion
The higher solvation energy in 70% water breaks the intramolecular hydrogen bond and produces the twisted conformers. Unlike the nearly planar conformer, in the twisted conformer the azole nitrogen is not in close proximity to accept the proton. The twisted structure produces a normal enol emission while the planar structure produces the tautomer emission.
Introduction
Solvatochromic study of MTz molecules
Absorption maxima (𝑚𝑎𝑥𝑎𝑏𝑠, nm) emission maxima (𝑚𝑎𝑥𝑒𝑚 , nm) and fluorescence lifetime (τ, ns)a for MTz. a relative percentage of different emitting species is given in parentheses. In more polar solvents, due to the overlap of both emissions, biexponential decays are observed in some solvents. The relative amplitude of short-lived species is higher when monitored at shorter wavelengths, and that of long-lived species is higher when monitored at longer wavelengths.
The origin of the different emissions in MTz
Alternatively, proton transfer of the MTz-II conformer can produce a normal keto tautomer. Furthermore, the proton transfer species of MTz-II is more conjugated than that of MTz-I. Based on this, the longer wavelength emission (~550 nm) can be assigned to the keto tautomer of the MTz-II conformer.
Solvatochromic study of Tz molecules
A striking difference between the visible emission fluorescence lifetimes of Tz and MTz is observed. In contrast, we observe a biexponential decay of the visible emission of the Tz molecule in nonpolar and less polar solvents. TRANES were constructed using emission decays recorded at different wavelengths of visible emission in dioxane (Figure 5.5.A).
Pathways to generate different tautomer emission in TZ
It appears that in solution the proton transfer process in Ph-II is not initiated. But it participates in ESIPT after the occurrence of the first proton transfer from the Ph-I unit. The results indicate that in solution two different keto emissions result from successive proton transfer.
Emission from different conformers of Tz
Excitation spectra recorded at different wavelengths of the visible emission Tz in methanol show that the emission of the double tautomer in a polar protic solvent arises from a single precursor/conformer in the ground state (Figure 5.7). As in the non-polar solvent, visible emission from the Tz-I conformer also appears in the polar protic solvent. The excitation spectrum of the shorter wavelength emission band is similar to the excitation spectrum of the single emission band observed in polar protic solvents (Figure 5.7.A).
Annular tautomerism and ESIPT of Tz and MTz
As a result, the intramolecular hydrogen bond between the Ph-II unit and the 'N2' is broken due to the shifting nature of the acidic proton. Due to ring tautomerization, the acidic proton "NH" in MTz-II is shared between "N1" and. In the solid state, due to proton localization, the MTz-II conformer exhibits keto emission in the excited state.
Quantum chemical calculations
The tautomerization prevents MTz-II conformer from forming intramolecular hydrogen bond between 'N2' and phenolic 'OH' proton and prevents the ESIPT in solution. The variation of the ring tautomerism in solid and solution is reported for different azole derivatives.137.
Conclusion
In strong proton-accepting solvents such as DMF and DMSO, Tz forms strong hydrogen-bonding complexes and exhibits anionic emission. Furthermore, the visible emission of Tz in these solvents appears from the Zwitter ionic species, which is deduced from ESIPT.
Construction of Molecular Logic Gates by Perturbing the Emission Behaviors of a Few
Azole Derivatives
Introduction
Three different azole derivatives, DMAPOP, BHPBI and DMAPIP-b (Chart 6.0) were used to construct molecular logic gates in the presence of different inputs. In Section 6.1, the controlled binding of metal ions to a fluorophore in the presence of a micelle is exploited to construct a simple Boolean logic gate. Specific binding of metal ions in the presence of an anionic micelle and construction of molecular logic gates.
Specific site binding of metal ions in presence of anionic micelle and construction of molecular logic gates
- Metal ion binding in micellar medium
- Mode of encapsulation and metal ion binding at selective site
- Construction of molecular logic gates
The addition of metal ions causes a small red shift in the absorption spectrum of DMAPOP. On the other hand, with a gradual increase in Cd2+, a new red-shifted band emerges at 463 nm (Figure 6.1.3.B). The binding constant of Cd2+ with DMAPOP was measured using the Benesi-Hildebrand equation (similar: Again, neither Cd2+ nor Cu2+ generates any hypsochromic shift in the absorption spectra of DMAPOP in the presence of SDS.
Molecular TRANSFER gate
Conclusion
The strong interaction between the anionic head of the SDS micelle and the metal ion ensures that the metal ions bind only through the oxazole nitrogen of DMAPOP. Thus, in the presence of the micellar medium, the metal ion binds with DMAPOP in a specific binding center. The changes in emission characteristics in the presence and absence of the metal ion and the micelle are used to construct INHIBIT, TRANSFER and IN molecular logic gates.
Construction of molecular logic gate and keypad lock system by inducing the inter molecular proton transfer
- Photoirradiation triggered protonation of BHPBI
- Construction of molecular logic gate
- Conclusion
- Interaction of DMAPIP-b with different analyte
- Binary logic gate, full-subtractor and molecular keypad lock
- Ternary system
- Multivalued logic with fuzzy interference system
- Conclusion
Here, the state (high or low value) of a certain input (irradiance) is transferred directly to the output, regardless of the other input. Due to the interference of the DMAPIP-b emission at low mole fraction of [Fe3+], these points were not included in the plot). The emission of the fluorophore can be changed in a controlled manner with predetermined values by the fuzzy logic system.
Summary and Scope for Future Work
Summary
In the second part of the third chapter, anion sensitivity and the nature of deprotonation of HPBI nitrogen analogues in a polar aprotic medium are investigated. The input sequence dependence of the output emission was used to construct a molecular keyboard locking system. In the third section of the sixth chapter, logic gates with different radixes were constructed inside DMAPIP-b.
Scope for future work
The smooth dependence of emission intensities with analyte concentration was used to construct an infinite-valued fuzzy logic system. The fuzzy logic system is further coupled with the neuro-adaptation method to more accurately predict the molecular intensity dependence in the presence of external inputs.
Annexure-A
Annexure-B