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Title of Thesis

Zahida Khalid
Institute/University/Department Details
University of Karachi/Department of Chemistry
Number of Pages
Keywords (Extracted from title, table of contents and abstract of thesis)
methylene blue, photochemical reduction, urea, allylurea, phenyhurea, thiourea, allylthiourea, photo-polymerisation, quantum yield

The present investigation reveals the study of photochemical reduction of methylene blue by some organic reductants. For this purpose urea, allylurea, phenyhurea, thiourea and allylthiourea were chosen as suitable reducing materials, because their homologues and benzene ring substituted derivatives are well characterized and are prepared easily in a fairly good crystalline state. The reduction was carried out in 50% v/v aqueous 2-propanol. The photochemical reactions were characterized by using special optical arrangements for the irradiation of deaerated test solutions.

As a starting point, the photoreduction of methylene blue was carried out in presence of various reducing agents at different acidities. Acidities of the solutions were determined by Hammett acidity function (Ho). The best selected acidities range found to be from 4.06 to 7.59. The quantum yield calculated at various acidities, decreases as the acidity values increases. The quantum yield of the reaction varies directly with the increase in the concentration of the reductants and temperatures. Morover, it was found that quantum yield for the reaction was independent of the concentration of the methylene blue. The results and experimental findings obtained were also interpreted in terms of comparative reaction mechanism associated with reported work, in terms of structure and concentration of reducing agents. Ratios of the rate constants were calculated by using various relations in proposed mechanism. However tests were also put forwarded to verify the values of the rate constants of various steps. From the present investigation it has become more certain that the quantum yield is essentially controlled by two equilibria.

i. The triplet state of methylene blue with the proton and the protonated triplet state of methylene blue. MBH+T + H+ _______ MBH++ 2T H

ii. The protonated triplet state of methylene blue with reductants, and associated complex of methylene blue. MBH++2T + AH2 _______ MBH++2T + AH2

Since the quantum yield is directly proportional to the rate constant and varies with temperature, Arrhenius and Eyring relationships were used to calculate activation parameters such as energy of activation (Ea), free energy change of activation (∆G#), enthalpy change (∆H#), and entropy change (∆S#), of activation

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S. No. Chapter Title of the Chapters Page Size (KB)
1 0 Contents
110.75 KB
2 1 Introduction 3-4
104.93 KB
  1.1 Introduction to Photochemistry 3
  1.2 Importance of Potochemistry 4
  1.3 Classification and Designations of Dyes 10
  1.4 Application of Dye 13
3 2 Literature Survey 15-48
283.97 KB
  2.1 Introduction 15
  2.2 Variables States of Methylene Blue 16
  2.3 Absorption Studies of Methylene Blue 18
  2.4 Reduction States of Methylene Blue 22
  2.5 Photophysics of Methylene Blue in Water 22
  2.6 Reduction Process of the Dye 23
  2.7 Reduction of Methylene Blue in Absence of Reducing Agent 27
  2.8 Photochemical Reduction of Methylene Blue by Organic Reductants 28
  2.9 Photochemical Reductions of Thiazine Dyes 30
  2.10 Electro Redction of Dye by Complexes 34
  2.11 Pulse Radiolsis /Laser Photolysis 38
  2.12 Electron Transfer Reactions 43
  2.13 Photo-Polymerisation of Methylene Blue 45
  2.14 Photo-Bleeching of Methylene Blue 47
  2.15 Aim of the present work
4 3 Experimental 49-65
191.67 KB
  3.1 Introduction to Experimental Aspects 49
  3.2 Experimental Setup 54
  3.3 Measurement of Light Intensity 62
  3.4 Measurement of Acidity of Solution 63
  3.5 Measurement of Extinction Co-Efficient Methylene Blue 64
  3.6 Experimental procedure 65
5 4 Results 67-11
436.17 KB
  4.1 Absortion Spectra of Methylene Blue 67
  4.2 Calculation of Quantum Yield 68
  4.3 Determination of Quantum Yield 75
  4.4 Presentation of Experimental Data 78
  4.5 Influence of Acidity on the Quantum Yield of Photoreduction of Methylene Blue in Aqueous 2-Propanol 79
  4.6 Effect of Concentration of Reducing Agents of the Quantum Yield of Photo-Reduction of Methylene Blue in Aqueous 2-Propanol 90
  4.7 Effect of Concentration of Methylene Blue on the Quantum Yield of Photo-Reduction in Aqueous 2-Propanol 99
  4.7 Effect of Variation of Temperature on The Quantum Yield of Photo-Reduction in Aqueous 2-Propanol 103
6 5 Discussion 117-158
689.08 KB
  5.1 Introduction 117
  5.2 Qualitative Characteristics of Methylene Blue by Spectral Analysis 118
  5.3 Variation in Absorption Spectra of Methylene Blue with the Acidity 119
  5.4 Spectral Studies in the Presence of Reductants 120
  5.5 Influence of Acidity on Quantum Yield 122
  5.6 Influence of Concentration of Reductants on Quantum Yield 123
  5.7 Effect of Temperature on the Quantum Yield 124
  5.8 General Mechanism Proposed for the Photo-Reduction of Methylene Blue By Urea/ Thiourea and their Derivatives 132
  5.9 Justification of Proposed Mechanism 147
  5.10 Conclusion 154
  5.11 Comparison with Reported work 156
  5.12 Future Perspectives 159-160
  5.13 Figures 161-190
  5.14 Symbols Used in Text
  5.15 References 191-205