Alberto Moscatelli COLUMBIA UNIVERSITY

Transcription

1 Part I. Interactions between Singlet ( ) Oxygen and Nitroxides as Seen by Electron Paramagnetic Resonance Methods: Quenching, Chemically Induced Electron Spin Polarization, and Applications in Oximetry. Part II. Photolysis of Dibenzylketones Sorbed in MFI Zeolite in the Presence of Spectator Molecules: Cage Effect, Kinetics and External Surface Sites Characterization. Alberto Moscatelli Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Graduate School of Arts and Sciences COLUMBIA UNIVERSITY 008

2 008 Alberto moscatelli All Rights Reserved

3 ABSTRACT PART I. INTERACTIONS BETWEEN SINGLET ( ) OXYGEN AND NITROXIDES AS SEEN BY ELECTRON PARAMAGNETIC RESONANCE METHODS: QUENCHING CHEMICALLY INDUCED ELECTRON SPIN POLARIZATION, AND APPLICATIONS IN OXIMETRY. PART II: PHOTOLYSIS OF DIBENZYLKETONES SORBED IN MFI ZEOLITE IN THE PRESENCE OF SPECTATOR MOLECULES: CAGE EFFECT, KINETICS AND EXTERNAL SURFACE SITES CHARACTERIZATION. Alberto Moscatelli In Part I, the lowest singlet excited state of the oxygen molecule, O ( ) is studied by Electron Paramagnetic Resonance (EPR) spectroscopy. In solution, deactivation of O ( ) by nitroxide molecules produces Chemically Induced Electron Spin Polarization (CIDEP). The process is followed in the microsecond time scale by time-resolved EPR. Spin polarization efficiencies per quenching event are found to be dependant on the molecular structure of the nitroxide, although only the nitroxide moiety is responsible for the spin deactivation of O ( ). Where possible, magnetic measurements are coupled with optical methods in order to corroborate results. O ( ) was generated by both photosensitization and photodecomposition of endoperoxide molecules. CIDEP generated in both cases is compared. The decay of the CIDEP signal is controlled by the O ( ) lifetime, providing a sensitive method to measure oxygen concentration. Moreover, the validity of the classical

4 method of oximetry by EPR is extended to oxygen concentrations up to about 0 times larger than those currently used. In the gas phase, O ( ) generated both by microwave discharge and photosensitization with naphthalene, is detected. Generation efficiencies are precisely determined, as both O ( ) and the ground state of the oxygen molecule, O ( 3 Σ), can be detected under the same conditions. Line widths of both species are measured by varying temperature and pressure and a comparison in terms of spin relaxation mechanisms is attempted. Both O ( ) and O ( 3 Σ) lines are followed over time. Typical lifetime values of O ( ) are found to be between 00 and 400 ms. Observed quenching rate constants are measured by varying pressure and temperature. In Part II, the nitroxide spin probing technique is used to characterize the external surface of MFI zeolites by continuous-wave EPR. Strong binding sites are titrated by a direct method and the energy of interaction for a series of dibenzylketones is measured using the Langmuir isotherm. Photolysis in the presence of spectator molecules, such as water or pyridine, displaces dibenzylketones from the internal sites to the external surface.

5 TABLE OF CONTENTS ABSTRACT...III ACKNOWLEDGMENTS... XIV CHAPTER. THE RADICAL TRIPLET PAIR MECHANISM FOR NITROXIDE SPIN POLARIZATION INDUCED BY DEACTIVATION OF ELECTRONICALLY EXCITED MOLECULES..... INTRODUCTION AND HISTORICAL PERSPECTIVE..... THE RADICAL TRIPLET PAIR MECHANISM OF RADICAL SPIN POLARIZATION The Spin States The Static Interactions and Energy Levels The Time Dependent Hamiltonian: Zero Field Splitting Interaction Phenomenological Description of the Radical Triplet Pair Spin Polarization Mechanism: Quartet-Precursor and Doublet Precursor Dynamic Model for the Radical Triplet Pair Mechanism EXPERIMENTAL SECTION Materials and sample preparation procedure Time-resolved EPR experiments CHEMICALLY INDUCED DYNAMIC ELECTRON POLARIZATION (CIDEP) GENERATED THROUGH THE INTERACTION BETWEEN NITROXIDE RADICALS AND SINGLET ( ) MOLECULAR OXYGEN Introduction and Goals a. Decay Kinetics of Spin Polarization Signal: Modified Bloch Equations and their simulation b. The case of Singlet ( ) Oxygen Quenching Results and Discussion...39 i

6 .4.3. Conclusions CHEMICALLY INDUCED DYNAMIC ELECTRON POLARIZATION (CIDEP) GENERATED THROUGH THE INTERACTION BETWEEN NITROXIDE RADICALS AND ENDOPEROXIDES Introduction and Goals Results Discussion Conclusions BIBLIOGRAPHY AND NOTES...60 CHAPTER. INTERACTION BETWEEN NITROXIDES AND TRIPLET MOLECULAR OXYGEN AS SEEN BY EPR SPECTROSCOPY INTRODUCTION Description of Spin Exchange and its Spectroscopic Manifestation in EPR EXPERIMENTAL SECTION Materials and Sample Preparation Instrumentation and parameters Simulation Strategy of EPR spectra A SEARCH FOR A REVERSIBLE COMPLEX Introduction Results Discussion a. Complexes of Molecular Oxygen with Organic Molecules b. Application of the RTPM Theoretical Models to the case of Singlet Oxygen deactivation c. Possible Structures for the Supramolecular Complex between Oxygen and Nitroxides Conclusions...8 ii

7 .4. OXIMETRY OF OXYGEN SUPERSATURATED SOLUTIONS USING NITROXIDES AS EPR PROBE Introduction Results and Discussion Conclusions BIBLIOGRAPHY AND NOTES...94 CHAPTER 3. TIME-RESOLVED EPR STUDY OF SINGLET ( ) OXYGEN IN THE GAS PHASE GENERATED BY PHOTOSENSITIZATION WITH NAPHTHALENE DERIVATIVES INTRODUCTION Motivations and Goals Paramagnetism of ( ) Oxygen Hund s Cases for Oxygen ( 3 Σ) and Oxygen ( ) Literature Survey of Singlet Oxygen Studies in the Gas Phase by EPR EXPERIMENTAL SECTION Experiments with Microwave Discharge Experiments with Photosensitization of Naphthalene Vapors Technical Limitations RESULTS AND DISCUSSION O ( ) Generated by Photosensitization with Naphthalene in Stationary Conditions O ( ) Generated by Photosensitization with Napthalene and by Microwave Discharge with Flowing Oxygen Experimental O ( 3 Σ) and O ( ) Line Width Measurements Time-Resolved Measurements on O ( ) Generated by Photosensitization with Naphthalene vapors in Flowing O System...7 iii

8 Quenching Mechanisms for O ( ) Generated by Photosensitization with Naphthalene in Flowing O CONCLUSIONS BIBLIOGRAPHY...40 GENERAL CONCLUSIONS AND POSSIBLE FUTURE DIRECTIONS OF THE INVESTIGATION...4 CHAPTER 4. PHOTOLYSIS OF DIBENZYLKETONES SORBED IN MFI ZEOLITE IN THE PRESENCE OF SPECTATOR MOLECULES: CAGE EFFECT, KINETICS AND EXTERNAL SURFACE SITES CHARACTERIZATION INTRODUCTION EPR of Nitroxides for Spin Labeling Methods EXPERIMENTAL SECTION Materials EPR analysis with nitroxide spin probes FT-IR measurements Photolysis of DBK s and cage effect measurement Langmuir isotherms RESULTS AND DISCUSSION Photolysis of DBK@MFI: cage effect and kinetics Photolysis of o-medbk@mfi: kinetics Titration of the strong binding sites on the external surface of MFI zeolite by EPR spectroscopy FT-IR Analysis: three peaks for three types of interactions Langmuir Isotherm...75 iv

9 4.4. CONCLUSIONS BIBLIOGRAPHY AND NOTES...8 APPENDIX A. INSTRUMENTAL CONNECTIONS AND DETAILS FOR THE ACQUISITION OF THE D-TR-EPR SURFACES...84 APPENDIX B. SIMPLE MATLAB ROUTINES FOR HANDLING THE LABVIEW OUTPUT FILES (EXTENSION: SUR)...90 APPENDIX C. MATLAB PROGRAM FOR THE SIMULATION OF THE SPIN POLARIZED SIGNAL EVOLUTION OVER TIME...94 v

10 LIST OF FIGURES Figure.. Schematic representation of the radical excited state (singlet or triplet) pair encounter in non viscous solutions at millimolar concentrations....3 Figure.. (a) Schematic representation of population distribution between the two electronic spin states of a radical species in thermal equilibrium; (b) and (c) situation in the case of spin polarized radical...4 Figure.3. Vector representation of the radical spin states (S = /)...7 Figure.4. Vector representation of the triplet spin states (S = )...8 Figure.5. One-dimension representation of the quartet and doublet spin states as vector sum of the spin angular momenta of the three unpaired electrons of the radical triplet pair...0 Figure.6. Energy diagram of the radical triplet spin states under the effect of the Zeeman interaction ony...3 Figure.7. Static energy diagram for the radical triplet pair as function of pair separation....6 Figure.8. Energy levels of a cylindrically symmetric triplet state at zero field...8 Figure.9. Vectorial representation of the ZFS interaction during the radical triplet pair separation in the case of QP-RTPM (Paragraph..4)...0 Figure.0. Schematic representation of the steps of the quartet precursor RTPM that give rise to emissive EPR lines...3 Figure.. Schematic representation of the steps of the quartet precursor RTPM that give rise to absorptive EPR lines....4 Figure.. Crossing region in the radical triplet energy surface...7 Figure.3. (a) Emissive signal generated by the quartet precursor RTPM after quenching of the anthracene triplet excited state; (b) Absorptive signal generated by the doublet precursor RTPM after deactivating the singlet excited state of 9,0-dimethylanthracene to its excited triplet state...33 Figure.4. Typical absorptive signal that results from the deactivation of singlet oxygen by nitroxide...34 Figure.5. Example of the outcome of the algorithm written in MATLAB to simulate the signal evolution of spin polarized nitroxides vi

11 Figure.6. (a) tr-epr surface of a solution of anthracene and TEMPO in air saturated solution, with its relative signal decay kinetics. (b) tr-epr surface of the same solution in Ar saturated solution, with its relative decay kinetics Figure.7. cw-epr spectra of nitroxide in (a) Ar saturated, (b) air saturated and (c) O saturated conditions Figure.8. Superposition of tr-epr and tr-phosphorescence decays in (a) h 6 -benzene and (b) d 6 - benzene...4 Figure.9. (a) Spectra obtained integrating between to µs of a solution containing 4 N and 5 N nitroxides in solution. (b) tr-epr surface in air saturated solution...44 Figure.0. (a) CIDEP induced by photolysis of DMODPA-O in the presence of 4-oxo-TEMPO. (b) Time evolution. (c) Spectral slices just at the maximum of spin polarized signal and after 89 ms....5 Figure.. (a) CIDEP induced by photolysis of DMODPA in the presence of 4-oxo-TEMPO. (b) Time evolution. (c) Spectral slices at different times: polarization inverts from absorptive to emissive Figure.. Absorption (blue trace) and emission spectra (at 77 K) of (a) DMODPA (red trace) and (b) DMODPA-O (green trace)...54 Figure.3. (a) Triplet spectrum of DMPDMO-O and (b) phosphorescence decay kinetics...55 Figure.4. a) Time evolution of the tr-epr signal recorded after irradiation of DMDPA-O in the presence of 4-oxo-TEMPO in toluene. b) Spectrum extracted between to 5 µs after the laser pulse. c) Spectrum extracted between 8 to µs after the laser pulse Figure.. EPR spectra of di-t-butyl nitroxide in ethanol at (a) 0-4 M, (b) 0 - M, (c) 0 - M and (d) pure liquid nitroxide Figure.. Time evolution of the normalized double integrated EPR signal of 4-oxo-TEMPO in benzene at 330 K Figure.4. UV-Vis spectra of a 0.6 mm TEMPO solution (a) in f 6 -BTHF and (b) in hexane...75 vii

12 Figure.5. Effect of complex in the radical triplet pair surface crossing. (a) Quartetdoublet surface crossing is passed through almost completely non adiabatically; (b) Quartet-doublet surface crossing is passed through adiabatically...78 Figure.6. EPR spectra of 4-oxo-TEMPO (0.6 mm) in benzene at 330 K at different times Figure.7. (a) UV-vis spectra of benzene solutions containing DMN-O. (b) Difference between the absorbance at 9 nm and at infinite time and the absorbance at time t versus time...87 Figure.8. (a) H L extracted from spectral simulation versus time at three different concentrations of DMN-O at 330 K; (b) H L extracted from spectral simulation versus time at different temperatures at [DMN-O ] = 30 mm. (c) Correlation between H L and oxygen concentration for the sample at 330 K and [DMN-O ] = 30 mm...89 Figure.9. Plot of the experimental parameter ( H*) versus O concentration...9 Figure.0. Plot of H L versus oxygen concentration for the pressurized experiment in benzene...9 Figure 3.. Steps involved in the deactivation of singlet oxygen ( O ) by nitroxide (TEMPO) with subsequent production of spin polarized nitroxide (TEMPO ) Figure 3.. Expected time evolution of the EPR lines of the singlet oxygen signal. In (a) the case of absorptive spin polarized signal is represented, whereas in (b) the case of emissive spin polarized signal is represented...98 Figure 3.3. (a) Electronic configuration of molecular oxyge;. (b) Probability density of the π* orbitals; (c) Possible electronic configurations of molecular oxygen...00 Figure 3.4. (a) Scheme of the Hund s case a) applied to O ( ). (b) Energy levels corresponding to the first rotational quantum number...0 Figure 3.5. (a) Scheme of the Hund s case b) applied to O ( 3 Σ). (b) Example of the Zeeman levels for the rotational level R =...03 Figure 3.6. EPR spectrum of O ( 3 Σ) in the gas phase at 00 mtorr of pressure Figure 3.7.(a) First EPR spectrum of O ( ) present in literature, 97. (b) last O ( ) EPR spectrum present in literature, viii

13 Figure 3.8. (a) Photograph of the microwave discharge cavity. (b) Photograph of the EPR cavity during photoexcitation with a lamp and a quartz rod...0 Figure 3.9. Replacement of Figure Figure 3.0. Spectra of a sample containing gaseous O and naphthalene vapor in stationary conditions obtained in the absence (a) and in the presence (b) of photo excitation of naphthalene....4 Figure 3.. Spectra of O ( ) generated by microwave discharge (a) and by photosensitization with naphthalene (b) in a flowing O system...7 Figure 3.. Left: cw-epr spectra recorded setting the temperature of naphthalene flask at (a) 98 K and (b) 33 K at constant oxygen pressure of 00 mtorr...0 Figure 3.3. Line widths measured for the selected O ( 3 Σ) line (high field line of Figure 3.b) as function of O pressure in flowing conditions....3 Figure 3.4. Line widths of both O ( 3 Σ) and O ( ) lines as function of the O pressure, in flowing condition and in presence of light irradiation...5 Figure 3.5. tr-epr D-spectrum of the third line of O ( ) in gas phase...8 Figure 3.6. Time evolution of both positive and negative part of the first derivative absorption O ( ) signals....9 Figure 3.7. (a) EPR spectrum of O ( ) along with one line of O ( 3 Σ). (b) Evolution over time of the selected O ( 3 Σ) line.c) Superposition of the positive part of the third O ( ) line and the positive part of the O ( 3 Σ), multiplied by - for convenience...3 Figure 3.8. Signal decay of the O ( 3 Σ) selected line with different time constants (tc) set at the instrument....3 Figure 3.9. O ( ) decay signals at T = 98 K at different pressures of O Figure 3.0. Values of t determined by fitting the decay profile of O ( ) with a bi-exponential equation at two different temperatures...36 Figure 4.. MFI zeolite framework and kinetic diameter of p-medbk and o-medbk Figure 4.. Schematic representation of photolysis of DBK s@mfi zeolite....5 ix

14 Figure 4.3. Time line of an X-EPR spectrometer Figure 4.4. Variation of the line shape with the decrease of the rotational correlation time of motion, τ C Figure 4.5. Plot of the hyperfine coupling constant a( 4 N) as a function of the polarity of different solvents...56 Figure 4.6. Schematic representation of the home made apparatus used for loading zeolites with spectator solvent molecules...60 Figure 4.7. Plot of % cage effect versus % water loading for ZSM-5 zeolite with different Si/Al ratio...6 Figure 4.8. Plot of % cage effect versus % pyridine loading for ZSM-5 zeolite with different Si/Al ratio..63 Figure 4.9.EPR spectra after photolysis of (a) % loading DBK@MFI (Si/Al = 0) and (b) % loading of o-medbk@mfi (Si/Al = 0) with the respective signal evolution over time...64 Figure 4.0. EPR spectrum after irradiation of % DBK@MFI (Si/Al = 0) in the presence of (a) diphenylethane, (b) benzene and (c) pyridine at different loadings Figure 4.. EPR spectrum after irradiation of % o-medbk@mfi (Si/Al = 0) in the presence of p- xylene and pyridine Figure DPA-TEMPO EPR spectra at different loadings, with relative simulation, and schematic representation of the loading of the external surface of zeolite in the light of the model described in the text Figure 4.3. (a) Behavior of the EPR linewidth and (b) of the spin-spin exchange frequency versus 4-DPA- TEMPO surface loading...7 Figure 4.4. Critical loading of nitroxide 4-DPA-TEMPO versus external surface area for different monodisperse silicalites....7 Figure 4.5. FT-IR spectum of o-medbk@mfi at different loadings...75 Figure 4.6. Langmuir isotherm for 4-oxo-TEMPO and TEMPO adsorbed onto silicalite S x

15 LIST OF SCHEMES Scheme.. Reactions involved in the CIDEP generation of nitroxides (T) by RTPM during deactivation of singlet and triplet excited states....3 Scheme.. Reactions involved in the CIDEP generation of nitroxides by RTPM during deactivation of singlet ( ) oxygen...34 Scheme.3. Photochemistry of endoperoxide Scheme.4. Scheme of the reactions involved in the photodecomposition of endoperoxide to produce singlet oxygen and its subsequent deactivation by 4-oxo-TEMPO resulting in spin polarization...50 Scheme.5. Scheme of the photodecomposition of,4-dimethyl-,4-diphenyl-,4-peroxyanthracene (DMDPA-O )...58 Scheme.. Schematic representation of the steps involved in the deactivation of singlet oxygen by nitroxides...64 Scheme.. Spin exchange mechanism...65 Scheme.3. Thermal decomposition of DMN-O endoperoxide...70 Scheme.4. Kinetics of the production of supersaturated solution and subsequent air equilibration from thermal decomposition of DMN-O...84 Scheme 3.. Setup for the EPR detection of O ( ) generated by microwave discharge Scheme 3.. Setup for the EPR detection of O ( ) generated by photosensitization with naphthalene vapors...0 Scheme 4.. Reaction scheme for the photolysis of dibenzyl ketones Scheme 4.. Limit structure of a nitroxide moiety...56 xi

16 LIST OF CHARTS Chart.. Structure of nitroxides used in the present work...35 Chart 4.. Structures of the ketones used in the present study Chart 4.. Structure of the nitroxide probes used in the present work...68 xii

17 LIST OF TABLES Table.. Zero Field Splitting Parameters for some selected molecules....8 Table.. Sign of spin polarized signal....7 Table.3. Quenching rate constants of singlet oxygen by nitroxides ( ) and relative spin polarization efficiencies...43 NO k q Table.. Loss of nitroxide signal and oxygen concentration for the thermal decomposition in benzene of DMN-O at different times Table.. Normalized double integration intensity varying the nitroxide concentration in benzene Table.3. Kinetic parameters used to simulate the curves in Figure.8a, b Table 3.. O ( ) generation efficiencies measured in different experiments....8 Table 3.. O ( ) generation efficiencies upon photosensitization with naphthalene in O flowing conditions measured at different temperatures...0 Table 3.3. Observed and calculated intensities corresponding to the four O ( ) EPR transitions... Table 3.4. Line widths of O ( 3 Σ) line in the presence of naphthalene vapors at T = 38 K in the absence and in the presence of light irradiation...5 Table 3.5. Fitting parameters for the decays of Figure Table 4.. Percentage of water absorbed in the MFI zeolites, evaluated by cage effect (C.E.) measurements...6 Table 4.. Critical loading values (c*) and corresponding average probe distance for different MFI zeolites...7 Table 4.3. Langmuir isotherm fitting parameters...76 xiii

18 ACKNOWLEDGMENTS I would like to acknowledge my advisor, Professor Nicholas J. Turro, for his continuous encouragement throughout the years spent in his group. His fascinating view of chemistry; his passion for science and his lively cleverness will stand as an outstanding example of life for me. Professor Turro has not just been a scientific advisor; he has also been a friend, to which joyfully converse about anything, and a listener, to which faithfully address personal issues. I am also particularly grateful to him for the precious network of scientific contacts particularly the many authorities in the electron paramagnetic resonance field, whom I have had the opportunity to meet during these years. I am enormously thankful to Sandy Turro for her constant smiles, her graceful manners, and the positive attitude unceasingly spread throughout the group members and throughout the years. I would not have been taken part in the Ph.D. program at Columbia University if Professor M. Francesca Ottaviani would not have encouraged me to pursue it. She has always believed in my potential even, and especially during, tough times. Therefore, despite the ups and downs that have been inevitably present in these years, I feel profoundly indebted towards her for encouraging me to be part of such a wonderful experience. A special thank you goes to all the people I have had the opportunity and the luck to work with. First and foremost, Dr. Steffen Jockusch and Dr. Xuegong Lei, Professor Turro s right and left arms, respectively. They have taught me everything I know on how to perform physical chemistry experiments, take care of instrumentation, deal with external companies for supply materials, and also taught countless wet lab tips. Then, Dr. Marco Ruzzi and Dr. Elena Sartori, xiv

19 exceptional scientists and invaluable friends from whom I learnt the time-resolved EPR technique, as well as some basics of Mathematica, Matlab and Labview. I am also very grateful to Dr. Zhiqiang Liu for introducing me into the group at the very beginning of my program. I cannot neglect the importance of external collaborations I have had the possibility to be part of. Especially fruitful has been the one with Professor P. Somasundaran and Dr. Puspendu Deo, with whom I was fortunate to publish two papers on polymer-surfactant systems. The measurements performed with Dr. Luigi Zecca and Dr. Fabio Zucca on the neuromelanins of baboon and human brain has been another significant collaboration of these almost five years. Finally, I wish to address a very special thank you to all the people that have come and gone to and from the Turro group. Each and every one of them has honored me with a piece of their friendship. Many other people have contributed to a pleasant stay at Columbia University, but rather than mentioning all of them, I will keep them in my heart. With God s blessing, everyone will surely be present with their own idiosyncrasies in my accounts of the events between that May of 003 and that January of 008! xv

20 Part I. Interactions between Singlet ( ) Oxygen and Nitroxides as Seen by Electron Paramagnetic Resonance Methods: Quenching, Chemically Induced Electron Spin Polarization, and Applications in Oximetry.

21 Chapter. The Radical Triplet Pair Mechanism for Nitroxide Spin Polarization Induced by Deactivation of Electronically Excited Molecules.. Introduction and Historical Perspective Deactivation of electronically excited organic molecules by a quencher generally produces a non-thermal population of the electronic, vibrational and rotational states of the latter. However, whereas the studying of decaying vibrational or rotational nonthermal states is complicated by their very short lifetimes (typically in the picoseconds time scale), the study of deactivation from electronically excited states (usually from picoseconds to milliseconds) has led to the determination of the quenching rate constants. These are a very useful kinetic parameter but they do not carry mechanistic information about the quenching process at a molecular level. Paramagnetic molecules, such as nitroxides, facilitate the electronic deactivation of excited molecules by inducing the spin-forbidden intersystem crossing (isc) transition in the encounter pair; in turn, deactivation produces a non-thermal electron spin population distribution in the paramagnetic molecule, which carries detailed information about the collisional quenching event (Figure.). While following the electronic deactivation in real time is a spectroscopic challenge, due to these events occurring on the ultra fast time scale of the order of picoseconds or faster, pieces of information contained in the spin energy levels of the paramagnetic quencher are locked into a time scale of microseconds, the latter

22 3 being the typical time scale for electron spin relaxation processes. Therefore, an indirect, but useful way to study the electronic deactivation of excited singlet and triplet molecules is by means of time-resolved electron paramagnetic resonance (tr-epr). Moreover, considering that encounter rates in non-viscous solutions, that is in typical organic solvents, are about 0 7 s - in the millimolar concentration range typical in photochemical experiments, the measurement of vibrational or rotational relaxation events are too fast to provide a complete description of the whole encounter event in solution. In contrast, the electron spin relaxation time scale is long enough for the spin system to bear pieces of information of the entire pair dynamics: encounter, deactivation and separation (Figure.). Encounter Deactivation Separation A* + R A* R A + (R ) k 0 7 s - Figure.. Schematic representation of the radical excited state (singlet or triplet) pair encounter in non viscous solutions at millimolar concentrations. A* represents an electronically excited state molecule, R a radical quencher and (R ) represents the spin polarized radical. On the minus side, however, lies the fact that tr-epr is not a very sensitive technique for the following reasons: (i) the weak magnetic dipole transition between the α and the β spin states which is typically two to three orders of magnitude less intense than electric dipole transitions, and (ii) the fact that the energy gap between the two spin levels, under a typical 0.34 T of magnetic field strength, is so small (0.3 cm - or J) that the population of these levels at room temperature is practically the same. However, the deactivation process schematically represented in Figure.,

23 4 produces (R ), a spin polarized paramagnetic species in which the population of its spin energy level is far from thermal equilibrium (Figure.). Since the intensity of the EPR spectra is proportional to the difference in the spin population, this enhancement enables the performance of tr-epr measurements. a) b) c) α α α β β β R (R ) Figure.. (a) Schematic representation of population distribution between the two electronic spin states of a radical species in thermal equilibrium; (b) and (c) situation in the case of spin polarized radical. When the β spins are more populated than the α spins the radical is polarized in absorption (b), vice versa it is polarized in emission (c). The study of electron and nuclear spin polarization effects induced by chemical reactions, a field called Chemically Induced Dynamic Electron and Nuclear Polarization (CIDEP and CIDNP respectively), has led to the understanding and the prediction of the outcome of chemical reactions by changing magnetic interactions between the reagents or applying an external magnetic field. 3, 4 Historically, early observations of magnetic effects of chemical reactions date back to the 960s, 5 in which anomalous EPR lines from hydrogen atoms were assigned to an instrumental effect, and their explanation came with the introduction of the chemical physics models of the radical pair mechanism (RPM). 6, 7 Later, in the 970s, the discovery that triplet states can also induce spin polarization 8, 9 lead to the development of the model of the triplet mechanism (TM). 8, 0 The mechanism under study in this thesis, the radical triplet pair mechanism (RTPM) was discovered in the 990s, and derives from the deactivation of an excited state by a radical species.

24 Following is described a detailed account of the chemical physics model developed to explain the mechanism. 5.. The Radical Triplet Pair Mechanism of Radical Spin Polarization To describe the Radical Triplet Pair Mechanism (RTPM) both spin and chemical dynamics between the species involved need to be considered. As stated by its name, polarization derives from the interaction between a radical (spin quantum number S R = /) and a triplet state (S T = ). In order to limit the number of variable in the play and in accordance with the literature of the field, it was decided to use a stable radical, and in particular nitroxides, to study the RTPM polarization, thus eliminating reactive radical chemical kinetics from the system dynamics. In order to explain the RTPM polarization it is useful to first describe the static, qualitative picture with the spin functions involved and the Hamiltonian of interest (Paragraph.. to..3). Then, a static description on how the spin dynamics and the chemical dynamics concomitantly produce spin polarization is presented (Paragraph..). Finally, the key dynamic features in light of the current model presented in the literature, will be described further in order to draw a more complete description of the quenching mechanisms involved in the deactivation of excited singlet and triplet states by radical species (Paragraph..5). This will help to link the observed tr-epr spectra and time decay signals to the chemical description of the system at the molecular level.

25 6... The Spin States A radical is a chemical species with an unpaired electron. Therefore its spin quantum number S R = /, where the sub R stands for radical. This originates two possible spin states that differ by their spin magnetic quantum number, so called because the two states are always degenerate, unless a magnetic field is present. The two values of the spin magnetic quantum number for a radical are m SR = ± / and for this reason a species with one unpaired electron is called doublet. The length of the vector representing the spin angular momentum in space is [ ( S +) ] h S and lies at a direction such that its projection along one arbitrary chosen axis (conventionally the z-axis) is m S h. However, for the commutation relationships among the angular momentum components, it is not possible to specify the projection of S along the other two axes (x and y) and thus the spin system is represented as a cone. The vector representation of a doublet spin state is drawn to scale in Figure.3 (lengths in units of h ). A useful inclusive representation of these spin states is indicating the total spin quantum number and the spin magnetic quantum number, (i.e.: S R, m SR or simply m SR ) so that for a doublet we have the two states: + α β (.)

26 7 z z θ θ α Figure.3. Vector representation of the radical spin states (S = /). The length of the spin magnetic moment is , its projection along the z-axis, which corresponds to the magnetic quantum o number, is 0.5 and angle θ is arccos( 3) Although this figure is accurate for both S and S z, for the higher spin states, such as triplets and quartets, such one-dimension representation will be appropriate only for S z, as a fully accurate picture will be more cumbersome in the description of the spin system of the present thesis. β The same reasoning applies to the triplet state. In this case, there are two unpaired electrons, obviously in different orbitals, that make the total spin quantum number of the molecule S T =, where the sub T stands for triplet. Therefore, the spin magnetic quantum numbers can have values m ST = 0, ± and for this reason such a species is called triplet. It results convenient to express the triplet eigenstates as a combination of the spins states of the two unpaired electrons, α and β. The three spin states in a triplet are: T T T + 0 = + = α α = 0 = = = β β ( α β + β α ) (.) The vectorial representation of a triplet state, as the result of the combination of the spin angular momentum of the two unpaired electrons, is schematically drawn in Figure.4. In this one-dimensional representation, the spin magnetic quantum number of the triplet spin (in orange) is accurate, but its length and its angle with respect to the z-axis are not; nonetheless this simplified representation will be useful in discussing the conversion between spin states, like in the case of doublet-quartet mixing discussed on Paragraph..3.

27 8 + = α α T + α α T 0 0 = + = β β β = T Figure.4. Vector representation of the triplet spin states (S = ). The triplet states are constructed by a simple vector sum from the spin of the two unpaired electrons, which form the triplet, according to the wave functions of Equation.. In this one-dimension representation the length of the spin magnetic quantum number of the triplet spin is accurate, but its length and its angle with respect to the z-axis are not. β When a doublet and a triplet come together and interact, there are 3 = 6 possible combinations of spin states. Their complete representation is obtained indicating both the two total spin quantum numbers and the two spin magnetic quantum numbers of the radical and the triplet taking part in the radical triplet system (i.e.: S, m ; S, m R SR T ST or simply m ; m ). 3 The six spin states in a radical triplet system are: SR ST + ; + ; + + ;0 ;0 + ; ; (.3) However, depending on the type of interactions involved, such a representation is not always the best one. An alternative, completely equivalent way to represent the states, which arises from the addition of two angular momenta, is to specify both the total angular momentum of the involved species, along with the total angular momentum of the coupled system with its spin magnetic quantum number, that is S, S ; S, m, or R T S simply S, ms. 4 According to the rules of the addition of two angular momenta, namely the Clebsch-Gordan series, the encounter of a doublet and a triplet produces two new

28 3 states, a quartet ( S = S R + ST = ) and a doublet ( S = S R + ST = 9 ). The former have 4 possible values for the spin magnetic quantum number, = ± 3/ and ± /, whereas m SQ the latter has two possible values, m SD = ± /. As is evident, there are six possible combinations, in accordance with the previous representation. Explicitly these six states are: 3 3, 3 ±,, ± and, ±. If we take the initial radical and triplet spin functions as the basis set for the representation of the spin wave functions, then the four quartet spin states and the two doublet state of Equation.3 are linear combination of the initial basis-set (Equations. and.). The explicit expression of the quartet wave function in term of this basis-set is: 3, ± 3 3, ± Q Q ± 3 ± = = ± ± 3 0 ± + 3 ± m (.4a) and for the doublet:, D = 0 ± + ± m (.4b) 3 3 ± ± The explicit derivation of these states can be made using the raising and lowering operators and the common rules for addition of angular momenta. An alternative representation of the quartet and doublet states of Equation.4, is the one in which the basis set is the spins of the three unpaired electrons taking part in the radical triplet pair. The explicit expression can be obtained substituting Equations. into Equations.4 and it is:

29 0 Q Q Q Q = α α α = = 3 3 = β β β R ( α α β + α β α + β α α ) ( β β α + β α β + α β β ) R R R R R R R (.5) D D + = = 6 6 [ ( α α β ) α β α β α α ] R [ ( β β α ) β α β α β β ] R R R R R The vectorial representation of the quartet state and its magnetic sublevels in terms of the Equation.5 is depicted in Figure.5. Q + 3/ = = Q +3 3 α α R α α α R α R Q + / = 3 α α α = Q + β R β β α α α R α R α α + α + β D = / 6 = D + β R β R β α Figure.5. One-dimension representation of the quartet and doublet spin states as vector sum of the spin angular momenta of the three unpaired electrons of the radical triplet pair. The vector sum follows the eigenfunctions of Equation.5 and the common convention by which the minus sign in the wave function corresponds to a horizontal spin flip in the vector model. To construct the spin states Q, Q and 3 D, it is enough to swap the α and the β states: the resulting spin states will be flipped vertically. As already noted, the spin representation in Equation.4 is equivalent to the representation of Equations.3. However, in some cases it is more convenient to use the former and in other cases to use the latter, depending on which interaction is considered.

30 ... The Static Interactions and Energy Levels With no interaction, the six spin states characteristic of the radical triplet pair would be degenerate. However, interactions between spins and with the external magnetic field are active and determine the energy splitting between the six levels, from which it is possible to derive the energy diagram for the radical triplet system. The total Hamiltonian can be divided into a static part, H 0, which determines the energy of the spin states, and a time-dependent perturbation, H(t), which mixes some of these states under certain conditions (Paragraph..3); it can be written: H ˆ = H + H ( ) 0 t (.6) The pertinent static Hamiltonian for the description of the radical triplet pair is: Hˆ Hˆ Zeeman Hˆ 0 = + Exchange (.7) The Zeeman interaction evaluates the effect of the interaction of the external magnetic field (B 0 ) to the spin magnetic moment ( µ = βg e S ) of the molecules present in the sample. Its formal expression is: Hˆ Zeeman + ( g ˆ ˆ RSR gt ST ) B 0 = β (.8) However, some approximations can be made for our system: (i) Tensor representation of the g factor is redundant, because the random tumbling of the molecule in solution (τ C 0 - s) is much faster with respect to the distance between the extremes resonance values due to the g tensor components expressed in unit of time. For instance, for the simplest nitroxide (di-terz-butyl nitroxide) g tensor values are g XX =.088, g YY =.006 and g ZZ =.007 (that is: g ZZ g XX 0-3 ); 5 for an X-band EPR spectrometer operating at about

31 0 9 Hz, the frequency difference would be 0 6 Hz, or, expressed in unit of time, τ 0-6 s, which is 5 orders of magnitude slower than τ C. (ii) Since as seen in point (i), interaction with the external magnetic field is isotropic in our limit of operation, the external magnetic field vector B 0, can be replaced with its only component B z. Consequently, also the spin operator, Ŝ, can be replaced with its component along the applied magnetic field, Ŝ z. (iii) The nature of organic triplet states and nitroxides is such that the g values are very similar; even though sometimes lines might not be centered at the exact same field value. For practical purposes and ease of calculation, we can write gr = g T. Therefore, Hamiltonian in Equation.8 can be replaced with the following: Zeeman R T ( Sˆ z Sˆ z ) B z Hˆ = gβ + (.9) Applying the Hamiltonian of Equation.9 to the spin functions of Equation.4 and knowing that: S ˆ S, m = m S, m (.0) z s s s the following six energy levels are created: ˆ T gβ BzS z, ± = ± gβbz, ± 3 ˆ T 3 3 gβ BzS z, ± = ± gβbz, ± ˆ R gβ BzS z, ± = ± gβbz, ± (.) As may be noted, under the sole Zeeman interaction, two energy levels result twofold degenerate, as the energy depends only on the z component of the spin quantum number, m S, and not on the total spin quantum number, S (Equation.0). Energy levels

32 of the radical triplet system under the effect of the Zeeman interaction alone are schematically reported in Figure.6. 3 Energy β B 0 J 4 g z 3 3, + 3, +,, + 3,,, Figure.6. Energy diagram of the radical triplet spin states under the effect of the Zeeman interaction only; two levels are two-fold degenerate. 3 3, The second term of Hamiltonian.7 is the exchange interaction between two spins. This interaction derives from the general Heitler-London formulation of the formation of chemical bonding. It contains two important contributions: (i) a Coulombic term for the through-space interaction between negatively charged electrons and (ii) a spin dependent component that account for the separation in energy among different total spin multiplicities. Because of point (i), the spin exchange Hamiltonian is dependent upon the separation between spins, r. Its general expression is: ()( r 4Sˆ Sˆ ) H ˆ exchange( r) = J T R 3 + (.) where J(r) is the function for the exponentially decaying exchange interaction: J λ ( r d ) ( r) = J e 0 (.3) where d is the closest possible approach of the pair and J 0 is the maximum value of J, that is J(d). Applying Hamiltonian. to the spin functions of Equation.4 and knowing that:

33 4 ( ) s s m S S S S S S m S S S S S, ;, ˆ ˆ ˆ, ;, ˆ ˆ = (.4) ( ) s s m S S S m S S,, ˆ + = (.5) the following expressions are obtained: () ()( ) () 3, 3,; 3, 3,; ˆ ˆ ˆ 3 3 ± = + ± r J S S S r J r J T R () ()( ) (), 3,;, 3,; ˆ ˆ ˆ 3 3 ± = + ± r J S S S r J r J T R (.6) () ()( ) (),,;,,; ˆ ˆ ˆ 3 3 ± = ± r J S S S r J r J T R As is evident, the energy splitting due to the exchange interaction does not affect spin magnetic sublevels, m S, of a given spin quantum number state. As a result, the four quartet states sublevels are not split apart relatively to each other and this also holds for the two doublet state sublevels. However, the spin exchange interaction does split apart the quartet level from the doublet level, as the radical and the triplet molecules come closer and closer; at r = d, the splitting is J 0. Depending on the sign of J 0, the quartet state may lie above or below in energy with respect to the doublet state. In the most commonly encountered situations the sign of J 0 is negative. This comes from the application of the Pauli principle for which the total wave function, including the spin wave function, must be anti-symmetrical with respect to the exchange of any pair of electrons. Anti-symmetrical wave functions correspond to singlet and doublet spin states, whereas triplet and quartet are symmetrical. Therefore, for the same reason as for which in the majority of cases singlet states lie lower in energy than triplet states in the case of

34 radical pairs, the doublet states lie lower than the quartet state in the case of radical - triplet pairs. We now need to consider the intensity of this interaction. Because of the difficulty of performing the exact calculation of the exchange integral on molecular systems at the closest approaching distance of the pair (r = d), it can only be determined approximately. However, for TEMPO organic triplets, it seems to range between 0 - and 0 - J. 6 An important point is that this value is at least two orders of magnitude smaller than the exchange integral for radical pairs, as radical triplet pairs are not expected to form chemical bonds. Nevertheless, it is fundamental to note that the exchange interaction energy at r = d is much larger than the Zeeman interaction; a condition sometimes referred as strong exchange limit: 5 g β B << J 0 (.7) In such conditions, the energy diagram of a radical triplet pair encounter is sketched in Figure.7 (not to scale, since the separation between the Zeeman levels should be about 00 times smaller than J 0 ). A key feature of this energy diagram is the energy level crossing between quartet and doublet states at different pair distance r and r.

35 6 Q +3 Q + Q Q 3 β B 0 J 4 g e T + + α 0 0 J J T + β ; T + α + 0 T 0 β ; T + α T + + β D + D d r r Distance Figure.7. Static energy diagram for the radical triplet pair as function of pair separation (Energy axis not in scale)...3. The Time Dependent Hamiltonian: Zero Field Splitting Interaction Due to the dipolar interaction between the two unpaired spins of the triplet state molecule, its three magnetic spin energy levels are not degenerate even if the external magnetic field is absent: this is the so-called zero field splitting (ZFS) interaction. The perturbation induced by an electron spin to another electron spin gives rise to a fine splitting, as opposed to the hyperfine splitting which occurs between a nuclear spin and an electron spin. The general expression for the zero field interaction, in terms of total spin S, is: ˆ = S D S (.8) H ZFS

36 7 where D is the spin-spin dipole coupling tensor. However, because the interaction is between two spins of the same molecule, it is possible to rewrite the dipole-dipole interaction in a more convenient form that takes into consideration molecular symmetry and therefore uses the molecular axis tern (X, Y, Z). Skipping the intermediate algebra, 7 the usual form of the zero field interaction as it appears in most EPR literature is: Hˆ ZFS ( S S ) = D SZ S + E X Y (.9) 3 where S X, S Y and S Z are the spin component along the molecular axis, and D and E are the zero field splitting parameters that account for the molecular symmetry as follows: (i) In the general case of orthorhombic symmetry, both D and E are non zero and the energies along the three molecular axis are different from one another. (ii) In the case of cylindrical symmetry, energies along the X and Y axis are the same and E equals 0, thus leaving only a two-fold degeneracy in the zero-field splitting; (iii) In the case of spherical symmetry, both D and E would be zero and there would not be any dipole effect 8. In the case of conjugated organic triplets the E parameter can usually be safely neglected; as reported in Table. for the case of anthracene and naphthalene triplets, E is about an order of magnitude smaller than D. Therefore, we can rewrite the zero-field Hamiltonian.9 as: Hˆ ZFS = D SZ S (.0) 3 Solved for the zero field condition, the Hamiltonian.0 gives rises to the energy level diagram of a cylindrically symmetric molecule as sketched in Figure.8.

37 8 Table.. Zero Field Splitting Parameters for some selected molecules. D ZFS E ZFS cm - J cm - J Naphthalene a) Anthracene a) Oxygen ( 3 Σ) b) a) Reference 9; b) Reference J Energy X, Y D 3 D Z D 3 Figure.8. Energy levels of a cylindrically symmetric triplet state at zero field. The value of D is indicative for organic triplets (Table.). Since the experiments performed are always carried out at high-field, we need to translate the zero-field energy levels into the set of high-field eigenfunctions of Equation.. The wave functions of the triplet state at high field are eigenfunctions of the spin operator in the laboratory tern of axes (x, y, z), along whose z-axis the Zeeman and the exchange interactions are quantized, but they can also be expressed as linear combinations of the eigenfunctions at zero field, along the tern of molecular axes (X, Y, Z). The form of the triplet eigenfunctions in such terms is: + = 0 = Z = ( X + i Y ) ( X i Y ) (.) so that in each case in a wave function there is the 0 spin state, this derives from the Z molecular spin state, and similarly for the ± states. A look at the spin

38 9 eigenfunctions of the radical triplet pair (Equation.4) highlights a hidden mixing mechanism between the quartet and doublet spin states. For instance, the Q and the D + can be expressed with the same combination of eigenstates of the triplet molecular axis, as a result those two states are mixed by the ZFS interaction, as may be observed from Equation.. 3,, + = = 3 3 Z Z ( X i Y ) ( X i Y ) + (.) The ZFS mixing mechanism between the quartet and singlet spin states is fundamentally the same as the singlet-triplet mixing of a radical pair under the effect of a perturbating interaction such as a fluctuating magnetic field from the lattice, or a spin-orbit interaction or an uneven hyperfine interaction. A vectorial description of the quartet-doublet mixing between Q and D + is sketched in Figure.9.

39 0 Q D + Ĥ ZFS Q 3 Ĥ ZFS D + Q 3 Ĥ D ZFS Figure.9. Vectorial representation of the ZFS interaction during the radical triplet pair separation in the case of QP-RTPM (Paragraph..4). Being a purely magnetic interaction, the total spin is not conserved, as it is possible to see from the figure. After the interaction has taken place there is an increase of α spins at the expense of β spins. On the other hand, in the case of DP-RTPM (Paragraph..4), the ZFS interaction takes place in the reverse sense, increasing the β spin population. Overall, the quartet-doublet state mixings through the ZSF interaction are: Q 3 / D+, D+ Q and Q 3 D. The first occurs when the radical and the triplet species are at the distance r, whereas the latter two occur at r (Figure.7). Note that the Zeeman and the exchange interaction cancel at the mixing points r and r, enabling the weak ZFS interaction to mix the states. The crossing region can therefore be described as an avoided crossing. The matrix elements of the perturbation, that give the strength of mixing, derived applying the Hamiltonian.0 to the mixing wave functions, give the following results: Q Q Q 3 3 Hˆ Hˆ Hˆ ZFS ZFS ZFS D D D + + D = 45 4D = 45 D = 5 (.3)

40 Finally, it is important to emphasize that the direct study of triplets by EPR usually requires solid samples at low temperatures either trapped in crystal or glassy matrices. In fact, in the solution phase, the dipolar, zero field splitting, interaction is averaged by rapid tumbling motion, thus reducing all the molecular axes into the laboratory frame. Spectroscopically, as the temperature is raised, this gives rise to an initial line narrowing with a loss of anisotropic details and eventually to the decrease of line intensity up to the complete disappearance of the spectrum, since all three molecular axes eventually become equivalent and equally populated. For this reason, the EPR spectra of the solution phase of the radical triplet pair only carry the radical information directly (a remarkable exception to this behavior is triplet C 60, that gives a sharp line at room temperature)...4. Phenomenological Description of the Radical Triplet Pair Spin Polarization Mechanism: Quartet-Precursor and Doublet Precursor Radical triplet polarization is the result of the action of three consecutive steps: pair encounter, spin selective deactivation and pair separation. A schematic representation of this process is reported in Figure.0. During pair encounter, as the radical and the triplet molecule approach each other, the exchange interaction increases exponentially. The quartet and doublet spin states separate as described in Paragraph..: the six spin states are equally populated at this stage. At r = d, spin selective deactivation to the singlet ground state occurs almost instantaneously as this is a spin allowed transition, through the following process: 3 * ( R T ) ( R S )