Fluorescence and Phosphorescence Spectroscopy. Physicochemical Principles and Practice PDF
Preview Fluorescence and Phosphorescence Spectroscopy. Physicochemical Principles and Practice
Other Titles of Interest BAKER and BETTERIDGE Photoelectron Spectroscopy: Chemical and Analytical Aspects BECKEY Field Ionization Mass Spectrometry DAMANY et al. Some Aspects of Vacuum Ultraviolet Radiation Physics ELWELL and GIDLEY Atomic Absorption Spectroscopy EMSLEY and LINDON NMR Spectroscopy using Liquid Crystal Solvents JACKMAN and STERNHELL Applications of Nuclear Magnetic Resonance Spectroscopy in Organic Chemistry 2nd edition SCHEINMANN An Introduction to Spectroscopic Methods for the Identification of Organic Compounds (2 vols.) SOBELMAN An Introduction to the Theory of Atomic Spectra FLUORESCENCE AND PHOSPHORESCENCE SPECTROSCOPY : PHYSICOCHEMICAL PRINCIPLES AND PRACTICE by STEPHEN G. SCHULMAN, Ph.D. Department of Pharmaceutical Chemistry College of Pharmacy, University of Florida PERGAMON PRESS OXFORD · NEW YORK · TORONTO · SYDNEY · PARIS · FRANKFURT U.K. Pergamon Press Ltd., Headington Hill Hall, Oxford OX3 OBW, England U.S.A. Pergamon Press Inc., Maxwell House, Fairview Park, Elmsford, New York 10523, U.S.A. CANADA Pergamon of Canada Ltd., 75 The East Mall, Toronto, Ontario, Canada AUSTRALIA Pergamon Press (Aust.) Pty. Ltd., P.O. Box 544, Potts Point, N.S.W. 2011, Australia FRANCE Pergamon Press SARL, 24 rue des Ecoles, 75240 Paris, Cedex 05, France FEDERAL REPUBLIC Pergamon Press GmbH, 6242 Kronberg-Taunus, OF GERMANY Pferdstrasse 1, Federal Republic of Germany Copyright © 1977 S. G. Schulman All Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers First edition 1977 Reprinted 1979 Library of Congress Cataloging in Publication Data Schulman, Stephen Gregory. Fluorescence and phosphorescence spectroscopy. (International series in analytical chemistry; v. 59) 1. Fluorescence spectroscopy. 2. Phosphorescence spectroscopy. 1. Title. QD96.F56S38 1976 543'.085 75-46591 ISBN 008-020499-6 In order to make this volume available as economically ana rapidly as possible the author's typescript has been reproducea in its original form. This method unfortunately has its typo- graphical limitations but it is hoped that they in no way distract the reader. Printed in Great Britain by A. Wheaton & Co. Ltd, Exeter To Joke and Christina for their love, encouragement, tolerance and infinite patience. PREFACE The past two decades have witnessed the emergence of fluorescence and phosphorescence spectroscopy to become among the most useful of tools in experimental biology and chemistry. The availability of low to moderate cost commercial instrumen- tation, in recent years, has made these analytical methods accessible to virtually all laboratories. No other instrumental methods available at comparable cost, can equal or surpass fluorimetry and phosphorimetry in analytical sensitivity. Con- -9 centrations of luminescing materials as low as 10 molar are routinely determined. This aspect is particularly desirable in the biomédical sciences where low concentrations of drugs, metabolites and toxins in blood serum and urine must routinely be monitored. Although there are many excellent reference works currently available on luminescence spectroscopy, many of these tend to be either theoretically oriented, with heavy emphasis on quantum mechanical interpretation of spectra of simple molecules as they relate to molecular electronic energy states, or instru- mentally oriented, with emphasis on electronic and optical aspects of instrument design. Most practitioners of fluores- cence and phosphorescence spectroscopy, currently, are either analytical chemists or biologically oriented scientists with limited background in quantum mechanics, electronics, optics, and higher mathematics in general. This group of experimenters is usually constrained to the use of commercially available instrumentation in which there is no great deal of variability in fundamental design from one manufacturer to another. More- over, the luminescing molecules of interest to this group are often extremely complicated drugs and metabolites which do not lend themselves to detailed understanding by rigorous quantum mechanical treatments. vii viii Preface Yet it is important that the limits of instrumental capa- bility and at least a qualitative picture of the relationships between molecular electronic structure, environmental inter- actions and luminescence spectra can be understood by all practitioners in order to maximize sensitivity, selectivity, interpretation and overall reliability of data taken in fluores- cence and phosphorescence spectral measurements. This book then, is written with the analytical chemist and biological scientist in mind and represents an attempt to make the in- strumental, and especially the structural and environmental aspects of luminescence spectra intelligible to the reader with a general college background in chemistry and physics. The author expresses his gratitude to Ms. Carolyn B. Gran- tham, Dr. A.C. Capomacchia, Dr. D.V. Naik, Mr. R.J. Sturgeon, and Mr. Peter F. Eisenhardt for their invaluable comments and assistance with the preparation of this manuscript. Gainesville, Florida Stephen G. Schulman July, 1976 CHAPTER 1 PHOTOPHYSICAL PROCESSES IN ISOLATED MOLECULES Fluorescence and phosphorescence occur in molecules as a result of and subsequent to a series of physical phenomena, normally beginning with the absorption of light. These phe- nomena as well as fluorescence and phosphorescence are derived from the electromagnetic nature of light, the details of molec- ular structure (especially molecular electronic structure) and the nature of the environment of the luminescent molecule. It should be fairly obvious that an appreciation of these phenom- ena is necessary for the understanding of the relationships between molecular structure and luminescence spectroscopy to chemical and biological problems. This section then, will deal with molecular electronic structure, the interactions of light with charged particles in molecules and the events that follow the absorption of light in molecules. For the sake of simplic- ity, these arguments will deal only with isolated molecules. The effects of environmental interactions of molecules on electronic spectra will be considered in a later chapter. Molecular Electronic Structure The interactions between atomic electrons to form chemical bonds are due to the valence electrons and orbitals comprising the partially filled outer shells of atoms. The electrons occupying the filled inner shells of atoms which belong to molecules are localized upon the atoms from which they origi- nated and contribute only very weakly, through their repulsive properties, to molecular electronic structure. Because it is the valence electrons which are responsible for the electronic spectra of molecules in the visible and ultraviolet regions of the spectrum as well as for chemical reactivity, our considera- tion of molecular electronic structure will be confined to 1 2 S. G. Schulman those features which arise from the valence electrons and Or- bitals of molecules. A chemical bond or occupied molecular orbital may be thought to originate from the overlap of occupied atomic Or- bitals. The geometry of the overlap is used to classify the type of chemical bonding, and the filling of the molecular orbital is governed by the Pauli exclusion principle (i.e. a maximum of two electrons can occupy one orbital). Some of the various types of molecular orbitals are described below. g-3onds The overlap of two atomic orbitals along the line joining the nuclei of the bonded atoms results in a σ-bond. A σ-bond can accommodate two electrons, in accordance with the Pauli exclusion principle. The distribution of charge in a σ-bond is strongly localized between the two bonded atoms. Although each atom participating in the σ-bond contributes one atomic orbital to the formation of the σ-orbital, the two electrons occupying the σ-orbital may originate, one from each atom or both from the same atom. In the former case, the σ-bond is called a covalent bond, and in the latter case, it is called a coordinate covalent bond. If the two atoms joined by the covalent bond exert unequal attractions upon the electron pair comprising the σ-bond, the electron pair will spend more time near one atom than the other. In this case, the more strongly attracting atom is said to be more electronegative than the other, and the bond is said to be a polar covalent bond. Because the electronic charge in a σ- bond is localized along the line between two atoms, electronic repulsion and the exclusion principle prevent the formation of more than one σ-bond between any two atoms in a molecule. Electrons engaged in σ-bonding are usually bound very tightly by the molecule. Consequently, a great deal of energy is re- quired to promote these electrons to vacant molecular orbitals. Photophysical Processes in Isolated Molecules 3 This means that molecular electronic spectra involving σ- electron transitions occur well into the vacuum ultraviolet and are not of interest in conventional luminescence spectroscopy which is concerned with the general region between the near ultraviolet and the near infrared (i.e. 200-1000 nm). π-Bonds The overlap of two atomic orbitals at right angles to the line joining the nuclei of the bonded atoms is said to result in a π-bond. π-Bonding is a weaker interaction than σ-bonding and consequently, usually occurs secondarily to σ-bonding. The formation of π-bonds always involves atomic p or d orbitals, but never s orbitals. In a π-bond, the distribution of elec- tronic charge is concentrated above and below the plane con- taining the σ-bond axis. While σ-electrons are strongly localized between the atoms they bind, π-electrons, not being concentrated immediately between the parent atoms, are freer to move within the molecule and are frequently distributed over several atoms. If several atoms are σ-bonded in series, and each has a p or d orbital with the proper spatial orientation to form a π-bond with the others, rather than an alternating series of localized two atom π-bonds being formed, a set of π- orbitals are formed which are spread over the entire series of atoms. These π-orbitals are said to be delocalized. In some cyclic organic molecules, π-delocalization extends over the entire molecule. These compounds are said to be aromatic and are the molecules of primary interest in fluorescence and phosphorescence spectroscopy. Because π-electrons are not concentrated between the bonded atoms, they are not as tightly bound as σ-electrons. Hence, their electronic spectra occur at lower frequencies than do σ- electron spectra. For molecules containing isolated π-bonds, the transitions involving π-electrons are still in the vacuum ultraviolet or at the limit of the near ultraviolet. Molecules containing delocalized π-electrons usually have π-electron 4 S. G. Schulman spectra in the near ultraviolet while the superdelocalized π- systems, the aromatic molecules, have π-electron spectra which range from the near ultraviolet for small molecules, to the near infrared for large ones· Nonbonded Electrons With the exceptions of most transition metal complexes and a very few stable free radicals, enough σ- and π-bonds will encompass any atom in a molecule, to engage in bonding any electrons which were unpaired in the valence shell of the iso- lated atom. In all atoms of the periodic table which have more than four electrons in the valence shell (e.g. nitrogen), how- ever, there are electrons in the valence shell which are already paired. The orbitals occupied by these electrons are already filled, although the valence shell, in the isolated atom, is not complete. These electrons are unavailable for conventional covalent bonding and yet have energies comparable to other electrons in the same shell. Consequently, they are called nonbonding or n-electrons. When the atom is engaged in bonding, the valence shell electrons involved in σ-bonding will drop in energy, well below the energy of the n-electrons. The energy of the electrons involved in π-bonding (if any) will usually drop below the energy of the n-electrons but not quite as much as in the case of the σ-electrons. Because the n-electrons are higher in energy than either the σ- or π-electrons, they must be considered as potential contributors to the spectral fea- tures of molecules possessing them. While the n-electrons originate from pure atomic orbitals, the difference in repul- sion experienced by these electrons as a result of the differ- ence in electronic environment of a molecular atom compared with that of an isolated atom infer that the n-orbitals will have more of the properties of localized molecular orbitals than of atomic orbitals. This is indeed found to be the case. The bonding geometries of atoms in molecules are influenced at least as strongly by the number of nonbonded electron pairs as by the number of bonded electron pairs around them.
The list of books you might like
