In Situ Absorption and Fluorescence Studies

IN SITU ABSORPTION AND FLUORESCENCE STUDIES OF ELECTROGENERATED SPECIES A Thesis Submitted for the Degree of Doctor of Philosophy a of the University of London and for the Diploma an.d 3 Membership of Imperial College of Science and Technology Cameron William =or McLeod B.Sc. September 1977 Department of Chemistry Imperial College of Science & Technology London S, ,7 ABSTRACT The thesis describes analytical studies which are concerned with the measurement, at electrode surfaces, of highly absorbing/fluorescent species electrogenerated (invariably under controlled potential conditions) from in- - organic and organic precursors. Introductory chapters outline research proposals, include a review of solution spectroelectrochemical studies and provide details- of instrumentation and cell design. Chapter 3 is concerned with metal ion investigations, particularly cadmium. In situ monitoring during electrolysis indicated that highly absorbing cadmium hydroxide particles were produced when the solution layer at the working electrode became alkaline. The reduction reactions of dissolved oxygen and platinum surface oxide and the hydrogen evolution reaction accounted for hydroxide formation. In chapters 4 and 5 model compounds, a-tolidine and ferrocyanide, were used to illustrate the analytical value of the spectroelectrochemical approach (in stationary and flowing solutions) and to check cell performance. The electro-initiation of fluorogenic reactions utilising a gold micromesh cell is described in chapter 6. Morphine, heroin (after hydrolysis), laudanosine, reserpine, thioguanine and homovanillic acid were successfully examined and analysis of pharmaceutical products containing morphine and reserpine was performed. The penultimate chapter discusses absorption and fluorescence studies of some polycyclic aromatic hydrocarbons in acetonitrile. Finally, a general conclusion and suggestions for future work are presented. 2 ACKNONLEDGEMEUTS I am grateful to Professor T.S. West for giving me the opportunity and encouragement to perform this research; his suggestions and comments during the programme were invaluable. thanks also go to fellow research workers) J.F. Tyson, H.B. Sadeghi, A.J.C.L. Hogarth and S. Cooke for helpful discussion. The latter part of research was conducted at the Department of Chemistry, UniversiLy of Aberdeen and it gives me pleasure to acknowledge the co—operation and assistance of R.A. Chalmers and I.L. Karr. Thanks are also due to staff at Imperial College and Aberdeen University and in particular to R. Cobley and A. Barclay for the construction of cells and to S. Black for drawing and photographic services. Financial support from the Chemical Society/ Analytical Division is acknowledged. C 0 II P E N P S Page Abstract 2 Acialowledgements 3 CHAPTER 1 General Introduction 7 1.1. Introduction 7 1.2. Literature Review of Spectroelectrochemistry 10 CHAPTER 2 Equipment 23 2.1. Spectroelectrochemical Instrumentation 23 2.2. Spectroelectrochemical Cells 30 2.3. Miscellaneous Equipment 39 CHAPTER 3 Studies with Metal Ions 40 3.1. Introduction 4.0 3.2. Preliminary Studies 43 3.3. Absorption Monitoring during Controlled Potential Electrolysis 60 3.4. Absorption Studies of Chemically Prepared Metal Hydroxide Solutions 82 3.5. Conclusions 98 CHAPTER 4 Studies with Potassium Ferrocyanide and 99 Introductory Remarks 99 4.2. Experimental Details 100 4 Page 4.3. Potassium Ferrocyanide 101 4.4. (r=Toliaine 116 4.5. Conclusions 122 CHAPTER 5 Studies in Flowing Solution 123 5.1, Introduction 123 5.2. Experimental Procedure 125 5.3. Oxidation of Ferrocyanide 126 5.4. Oxidation of 01-Tolidine 128 5.5. Reduction of Dissolved Oxygen 134 5.6. Reduction of Methyl Viologen 133 5.7. Conclusions 144 CHAPTER 6 Aqueous Fluorescence Studies 145 6.1. Introductory Remarks 145 6.2. Morphine 147 6.3. Laudanosine 184 6.4, Reserpine 189 6.5. Thioguanine 195 6.6. Homovanillic Acid 200 6.7. Instrumental Sensitivity 205 6.8. Conclusions 207 CHAPTER 7 Studies with Polycyclic Aromatic Hydrocarbons 209 7.1. Introduction 209 5 Page 7.2. Experimental Details 212 7.3. Reduction Experiments 215 7.4. Oxidation Experiments 222 7.5. Conclusions 225 CHAPTER 8 General Conclusion and Recommendations 226 REFERENCES' 228 ERRATUM cunoli cr-me_thil,ilictArtr\ 0.,(vt.tne. should. 0-t-DULci-i-cle. 6 CHAPTER 1 General introduction 1.1. Introduction The techniques of ultraviolet-visible absorption and fluorescence spectroscopy play an important role in the field of instrumental analysis providing a means for the rapid determination of organic and inorganic constituents in samples from diverse origins. In an attempt to extend their -range of application, the possibility of coupling spectroscopic and electrochemical methods and utilising in situ absorption and fluorescence monitoring of electrogenerated species for analytical purposes was investigated. The spectroelectrochemical technique has already been established over the last decade as a powerful tool-for elucidation and - 1/2 characterisation studies of electrode reaction mechanisms but it has only recently been shown to have analytical potentia13-7 Combined methods of analysi- are already well-established, none more so than gas chromatography-mass spectroscopy, and it is hoped that spectroelectrochemistry can eventually occupy a position of similar standing. The basis of the approach is simply that a redox reaction alters the atomic/molecular structure of the reactant producing a new species. Providing the species is stable, the resulting change in spectroscopic properties may provide analytically important information: changes in absorption and fluorescence spectra assist in the characterisation of the parent compound while increased molar extinction coefficients and fluorescence yields improve detection limits for trace determinations. The spectroelectrochemical technique, in effect, provides a convenient and reproducible method for performing selective chromogenic/fluorogenic reactions with simultaneous in situ absorption/fluorescence measurement. Although the above comments apply equally to inorganic and organic systems, studies with the latter group are potentially more rewarding due primarily to the large number of electroactive organic structures which exhibit characteristic absorption and possible fluorescence* In addition, organic electrode reactions seldom involve only electron transfer in contrast - to inorganic ions. The initially formed radical ion species are generally highly reactive and subsequent follow-up chemical reactions occur; the lifetimes of the reactive intermediates can, however, usually be prolonged by suitable choice of non-aqueous solvent. The possibility therefore exists, with organic systems, of spectrally monitoring not only changes in the oxidation state of the parent compound, but also monitoring the formation of new chemical compounds. To investigate the proposals, instrumentation was built to accomodate spectroelectrochemical cells utilising working electrode materials based on carbon, platinum and gold. Studies were performed in quiescent and flowing solutions where mass transport was controlled by diffusion and convective diffusion respectively. Initial investigations • were concerned with heavy metal ions, particularly cadmium, and the reduction reactions of dissolved oxygen and platinum surface oxide were of particular importance-. Model compounds, 8 potassium ferrocyanide and a-tolidine, were used to check the monitoring performance of the cells and to illustrate the analytical value of the combined approach. Fluorescence research was directed at analytically important compounds including alkaloids, pharmaceutical products and polycyclic aromatic hydrocarbons. The results for morphine using a • gold micromesh cell were particularly promising. Before the experiments are discussed a short review of spectroelectrochemistry is presented. 9 1.2. Literature Review of SpectroelectrochemisLa 1.2.1. Introductory remarks The combination of electrochemical and spectroscopic methods is a subject in which considerable research has been done in the last decade. Coupling the appropriate spectroscopic technique to the electrochemical experiment has enabled significant progress to be made in a number of areas notably in electrode surface characterisation studies and in the field of electrochemical reaction mechanisms. Extensive 1 2 and authoratative reviews by Kuwana and Winograd 7 which deal primarily with the latter subject are available while a series 8 of papers edited by Muller and which appear in one volume are concerned with the former subject. Electron spin resonance(ESR) coupled studies are reviewed by Ad.ams9and 10 also more recently by McKinney . The main features of spectroelectrochemical methods are presented by referring individually to the principal optical techniques and to considerations such as equations, cells, electrodes, instrumentation and applications. Since this thesis is concerned with ultraviolet—visible monitoring in the solution phase, techniques used primarily for surface studies such as ellipsometry, holography and microscopy are not considered and the reader should refer to the volume edited by Muller8; an exception is the specular reflection technique. ESR work is also not considered, but studies in the infrared are included. 10 1.2.2. Transmission spectroscopy In situ spectral monitoring by transmission spectre- scopy provides the simplest route for relating solution spectroscopic changes to electrochemical parameters. The basis of the approach is

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