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ABSTRACT

ZHANG, JINYUAN. Study of Bridge Effects on Electronic Coupling of Donor- Bridge-Acceptor Biradical Complexes. (Under the direction of Dr. David Shultz).

A series of Donor-Bridge-Acceptor (D-B-A) (D: S=1/2 ortho-semiquinonate, SQ; A: S=1/2 nitronlynitroxide, NN) biradical complexes featuring different bridges were synthesized to serve as ground state analogues of charge separated excited states as well as molecular analogs of single-molecule break junction devices. The aim is to elucidate bridge-mediated electronic structure contributions to electronic coupling.

The study of biradicals with sterically hindered para-phenylene bridges and an “Aviram-Ratner” (bicycle[2.2.2]octane) bridge allowed for an experimentally- determined evaluation of torsionally dependent (π) and torsionally independent (σ) contributions to the electronic and exchange couplings at parity of donor, acceptor and donor-acceptor distance. The torsional dependence was illustrated using a 3-dimensional, “Ramachandran-type” plot that related D-B torsion and B-A torsions to both electronic and exchange couplings.

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Study of Bridge Effects on Electronic Coupling of Donor-Bridge-Acceptor Biradical Complexes

by Jinyuan Zhang

A dissertation submitted to the Graduate Faculty of North Carolina State University

in partial fulfillment of the requirements for the Degree of

Doctor of Philosophy

Chemistry

Raleigh, North Carolina

2016

APPROVED BY:

________________________ ________________________

David Shultz Elon Ison

Committee Chair

________________________ ________________________

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BIOGRAPHY

Jinyuan was born on June 30th, 1988 in Kaiyuan, Yunnan Province, China. He was curious about various natural and scientific phenomena since childhood. After entrance into junior high school, he developed my interest in chemistry and started to set up his own lab at home to do basic chemistry experiments. Thus, he, without hesitation, chose chemistry as his major when he entered Xiamen University in 2006. Xiamen University has the most beautiful campus in China and it was on that campus he first met his wife Jiazhen Song. In 2010, he had a chance to get in the Key Laboratory for Modern Biochemistry of Fujian Province where he conducted my undergraduate project on asymmetric synthesis of a complicated natural product stemofoline under Prof. Peiqiang Huang’s guidance.

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ACKNOWLEDGEMENT

First of all, I would like to express my sincere gratitude to my advisor Prof. David Shultz for the continuous support of my entire Ph.D study. His guidance helped me in both the research leading to and the writing of this dissertation.

Secondly, I would thank Prof. Martin Kirk and his group for collaborating with us. Also I would like to thank my committee members: Prof. Elon Ison, Prof. Walter Weare and Prof. Joshua Pierce, for their insightful comments and encouragement that helped me overcome difficulties encountered during my Ph.D studies.

My sincere thank also goes to the Shultz Group members: Dan, “Tich,” Dr. Wang and “Shaker” for their help during my research and their effort to make the lab an interesting place. I also thank our staff crystallographer, Roger Sommer as well as Lukasz Wojtas (U. South Florida) for crystal structures, and Prof. Nathaniel Finney for synthetic advice.

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TABLE OF CONTENTS

LIST OF TABLES………...vi

LIST OF FIGURES……….………vii

LIST OF SCHEMES………..…….xii

LIST OF ABBREVIATIONS………...…………....xiv

I. General Introduction, Background and Theory………....1

I.1. Introduction of Electron Transfer in Donor-Bridge-Acceptor Systems…………...1

I.2. How to Measure Electronic Coupling……….2

I.2.1. Photoinduced Electron Transfer Reactions in D-B-A Systems……….2

I.2.2. Single Molecule Conductance from STM-BJ Experiment………...….4

I.2.3 Magnetic Exchange Coupling in D-B-A Biradical Complexes ………….…5

I.2. Introduction to Exchange Coupling in Biradicals………...….7

I.2.1. Definition of Exchange Coupling Parameter J………..7

I.2.2. Measurement of Exchange Coupling JDA in D-B-A Biradical Complexes…8 I.3. Valence Bond Configuration Interaction Method………..………11

I.4. Introduction to D-B-A Biradical Complexes in the Shultz Group……….14

References………....20

II. Determining the Conformational Landscape of σ and π Coupling Using para- Phenylene and “Aviram–Ratner” Bridges………..………...…24

II.1. Introduction………...…24

II.1.1. Introduction to Torsional Dependence on Electronic Coupling …………..24

II.1.2. Introduction to “Aviram-Ratner” Bridge: Bicyclo[2.2.2]octane Bridge.... 26

II.1.3. Introduction to Spin Polarization of σ-Framework………...….27

II.1.4. Target Molecules and Expectation………...…29

II.2. Results and Discussion of Completed Work……….31

II.2.1. Synthesis of Biradicals with Methyl Substituted Phenylene Bridges…….31

II.2.2. Synthesis of Biradical with Bicyclo[2.2.2]octane Bridge………..…33

II.2.3. Structural and Magnetometric Study of Torsional Dependence Effect ….41 II.2.4. Spectroscopic and Theoretical Study of Torsional Dependence Effect…..45

II.2.5. Torsional Independence of Bicyclo[2.2.2]octane Bridge……….51

II.3. Experimental Section……….……....55

References………77

III. Donor-Bridge-Acceptor Biradicals as Models of Single Molecule Devices: Determination of Bridge Rectification Ratios……….………80

III.1. Introduction………..……80

III.1.1. Introduction to Molecular Electronics……….…..…80

III.1.2. Target Molecules and Expectations………...…84

III.2. Results and Discussion of Completed Work………88

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LIST OF TABLES

Table II-1. Different Friedel-Crafts reaction conditions and results………36 Table II-2. Measured exchange coupling (JDA) and calculated electronic coupling

(HDA) for SQ-NN and SQ-Bridge-NN biradicals………...……50

Table II-E1. Crystallographic details of 1-BCO, 1-PhMe2 and 1-Me2Ph….………59

Table II-E2. Select Torsion Angles for Complexes 1-Ph, 1-MePh, 1-PhMe, 1-pXylyl, 1-PhMe2, 1-Me2Ph and 1-PhMe4………60 Table III-1. Comparison of different bromination conditions and the results…..…89 Table III-2. Comparison of different Suzuki reaction conditions and the results…91

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LIST OF FIGURES

Figure I-1. (A) Photoinduced electron transfer (PET) reaction. (B) Orbital diagram

perspective of PET reaction………...…2

Figure I-2. Jablonski diagram of PET reaction in D-B-A system………..……3

Figure I-3. D-B-A system used for PET experiments by Wasielewski.………4

Figure I-4. Cartoon of typical STM molecular junction measurement.………5

Figure I-5. Three steps of STM-BJ experiment to measure molecular conductance…5 Figure I-6. Architecture of D-B-A biradical complexes in the Shultz Group………...6

Figure I-7. Cartoon suggesting the utility of D-B-A biradical electronic structure to elucidate molecular structure-property relationships………..…7

Figure I-8. Theoretical χparaT vs T plots with 500 cm-1 ≤ JDA ≥ 500 cm-1 in system of 2 unpired electrons……….10

Figure I-9. Frontier Orbital diagram showing π interactions of SQ-NN………12

Figure I-10. Simplified MO diagram showed non-zero interaction between LUMONN and SOMOSQ (left). VBCI model of SQ-NN (right)………..13

Figure I-11. SQ-Ph-NN (left). Frontier Orbital diagram of π interaction (right)…...14

Figure I-12. Cross conjugated SQ-mPh-NN and linearly conjugated SQ-pPh-NN...15

Figure I-13. Linearly- and cross conjugated thiophene bridged biradicals…………15

Figure I-14. Biradicals with bridge of different length to study distance dependence effect………16

Figure I-15. Biradicals with torsional bridges and σ-only bridge to study torsional dependence effect………..16

Figure I-16. Biradicals with thiophene bridge and sterically hindered thiophene bridges…..………17

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Figure I-18. Biradicals with asymmetric Donor-Acceptor Bridges: SQ-D-A-NN (left) and SQ-A-D-NN (right)………..…..…18 Figure I-19. Biradicals with asymmetric thiophene-pyridine bridges……….18 Figure I-20. Biradicals with isomeric pyridine bridges………...19 Figure II-1. Computational study on torsional dependence by Albinsson……..……24 Figure II-2. Harriman’s D-B-A system with to study torsional dependence on electronic coupling………...…25 Figure II-3. Mayor’s compounds with different inter-bridge torsions with conductance measurements plot ……….….25 Figure II-4. donor-σ-bridge-acceptor rectifier proposed by Aviram and Ratner……26 Figure II-5. Matsuda’s study on exchange interaction in bis(nitronylnitroxide) radicals………...……27 Figure II-6. Structure of a methyl radical………28 Figure II-7. Two possible alignments of spin in methyl radical, parallel spin (left) and antiparallel spin (right). ………...…28 Figure II-8. Interaction of two π spins via different numbers of C-C bond……..…28 Figure II-9. Designed biradicals with steric phenylene bridge and BCO bridge. .….29 Figure II-10. Bridge Molecule for 1-PhMe2 and 1-Me2Ph………..……31

Figure II-11. Experimental and simulated EPR spectra of 1-PhMe2 (left) and thermal

ellipsoid plots of 1-PhMe2 (right)……….33

Figure II-12. Experimental and simulated EPR spectra of 1-Me2Ph (left) and thermal

ellipsoid plots of 1-Me2Ph (right)……….………36

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Figure II-16. photoinduced electron transfer (PET) experiments of benzo-annulated bicyclo[2.2.2]octanes by Wasielewski and coworkers……….44 Figure II-17. Magnetic susceptibility χT vs. T plot of 1-Ph, 1-MePh, 1-PhMe,

1-Xylyl, 1-PhMe2, 1-Me2Ph and 1-PhMe4.………..…44

Figure II-18. Magnetic susceptibility curve of 1-BCO, 1-Me2Ph and 1-PhMe4.……45

Figure II-19. Electronic absorption spectra of analogue (SQ), analogue (NN) and the SQ-NN biradical complex (SQ-NN)………46 Figure II-20. (A) Stack plot of electronic absorption spectra of all biradical complexes with steric phenylene bridges. (B) Electronic absorption spectra of 1-Ph, 1-MePh and 1-Me2Ph………47 Figure II-21. (A) Electronic absorption spectra of 1-PhMe, 1-Xylyl and 1-PhMe2 (B)

Electronic absorption spectra of 1-PhMe4 and 1-BCO………48

Figure II-22. Exchange coupling versus cosine square of the ϕSQ-B and ϕB-NN torsion

angles for the para-phenylene bridged complexes………49 Figure II-24. Electronic coupling versus cosine of the ϕSQ-B and ϕB-NN torsion angles

for the para-phenylene bridged complexes………50 Figure II-25. e-set orbitals in C3vsymmetryBCO bridge, (above) Interaction between

e-set orbitals of BCO bridge and p orbitals of radicals (below)……52

Figure II-26. σ only spin polarization mechanism for BCO bridge……….53

Figure II-18. PET experiment of D-B-A molecules with different bridges by Wasielewski………54

Figure II-28. Shultz Group’s 1-BCO, 1-Ph, 1-m-Ph and their exchange coupling parameters………..…………..…………..…………...……54 Figure II-E1. (A) Magnetometry of 1-Ph, 1-MePh, 1-PhMe, 1-pXylyl, 1-PhMe4,

1-Me2Ph and 1-PhMe2.(B) Magnetometry of 1-BCO..…………..…55

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Figure III-3. Molecular switch made by Launay to study light -controlled intervalence charge transfer………82 Figure III-4. Molecular rectifier proposed by Aviram and Ratner (left).7 D-B-A

molecule is mimicking the diode in the circuit (right). ………83 Figure III-5. P-N junction in a solid-state rectifier.………83 Figure III-6. Energy level diagram of D--σ-A+ and D+-σ-A- (left). D-σ-A molecule was placed between metal electrodes to act as rectifier (right). ………84 Figure III-7. Real molecular rectifier made by Metzger.………85 Figure III-8 Target Molecules: SQ-D-A-NN (left) and SQ-A-D-NN (right) ………86 Figure III-9. Decrease NN LUMO and increase SQ SOMO would have greater orbital mixing that stabilizes the triplet ground stat………86 Figure III-10. Biradicals with asymmetric thiophene-pyridine bridges………87 Figure II-11. SQ-T-P-NN and SQ-P-T-NN are served as analogues of rectifier in circuit. ………88 Figure III-12. Line-drawing of donor-acceptor bridge (left) and acceptor-donor bridge (right)………89 Figure III-13. Crude IR of Suzuki reaction and IR of D-A bridge 1a ………92 Figure III-14. IR spectra of Crude cyclization mixture and pure 3.19 …………..…94 Figure III-15. IR spectrum of Cat-D-A-NN 3.21. ………..…96 Figure III-16. Line-drawing of T-P Bridge 3.22a (left) P-T Bridge 3.22b (right).….98 Figure III-17. Experimental and simulated EPR spectra of SQ-T-P-NN (left) and

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Figure III-20. Magnetic susceptibility χT vs. T plot of SQ-T-P-NN (blue) and SQ-P-T-NN (red) with fit parameters.………105 Figure III-21. Overall electronic coupling and separated electronic coupling…107 Figure III-22. Electronic absorption spectra for SQ-T-P-NN and SQ-P-T-NN……108 Figure III-23. SQ-T-P-NN, SQ-P-T-NN and their parent molecules.………109

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LIST OF SCHEMES

Scheme II-1. Completed Synthesis of 1-PhMe2 and 1-Me2Ph.………..…31

Scheme II-2. Improved synthetic method for bridge 2.2a.……….…32

Scheme II-3. Synthesis of bridge 2.2b.………...32

Scheme II-4. Proposed synthetic route of 1-BCO biradical………34

Scheme II-5. Synthesis of BCO bridge 2.10.………..…35

Scheme II-6. Attempts at Friedel-Crafts reaction in different conditions. ………….36

Scheme II-7. Synthesis of Br-BCO-acetal 2.26 from 2.10. ………37

Scheme II-8. Preparation of activated Rekie Magnesium………..……37

Scheme II-9. Attempts at Grignard reagent formation on 2.26..………37

Scheme II-11. New synthetic route for 1-BCO.………..…38

Scheme II-12. Completed synthesis of 1-BCO.………..40

Scheme III-1. Proposed synthetic route of SQ-D(A)-A(D)-NN biradical…………..88

Scheme III-2. Synthesis of 2-formyl-4-methoxybenzonitrile.………89

Scheme III-3. Failure of bromination at the para position of aldehyde………89

Scheme III-4. Alternative synthetic route to D-A bridge 3.1a………90

Scheme III-5. Comparison of Sommelet reaction with pyridine N-oxide oxidation in the final step of donor-acceptor bridge. ………..…90

Scheme III-6. Suzuki cross coupling between 1 and D-A bridge 3.1a. ………….…91

Scheme III-7. Mechanism of CHO-CN cyclization under basic conditions. …….…92

Scheme III-8. Protection of aldehyde and Suzuki coupling with protected aldehyde 1c followed by deprotection. ………93

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Scheme III-9. Proposed intramolecular cyclization of o-cyano-benzaldehyde in MeOH……….………95

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LIST OF ABBREVIATIONS

ACN: acetonitrile AcOH: acetic acid AFM : antiferromagnetic AO : atomic orbital

BHA: 2,3-dimethyl-2,3-bis(hydroxyamino)butane BPO : benzoyl peroxide

Cat: catechol

CASSCF: complete active space self consistent field CT : charge transfer

CTC : charge transfer configuration D-B-A : donor-bridge-acceptor DCM : dichloromethane DFT: density functional theory DMF : N,N-dimethylformamide DMSO : dimethyl sulfoxide

EAS : electronic absorption spectroscopy EC : excited configuration

EPR : electron paramagnetic resonance ES : excited state

ET: electron transfer Et2O : diethyl ether

EtOAc : ethyl acetate EtOH: ethanol FM : ferromagnetic

g : conductance

GC : ground configuration GS : ground state

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HBB : Bridge-Bridge electronic coupling constant

HDA : Donor-Acceptor electronic coupling constant

HDB : Donor-Bridge electronic coupling constant

HMO : Hückel molecular orbital HDVV : Heisenberg-Dirac-Van Vleck HMTA: hexamethylenetetramine

HOMO : highest occupied molecular orbital IBX : o-iodoxybenzoic acid

ILCT: intraligand charge transfer IN: imino nitroxide

IR: infrared LA: lewis acid

LUMO : lowest unoccupied molecular orbital MO : molecular orbital

MOM: methoxymethyl MS: mass spectroscopy NBS : N-bromosuccinimide

NMO: N-methylmorpholine N-oxide NMR: nuclear magnetic resonance NN : nitronylnitroxide

PCC : pyridinium chlorochromate PET : photoinduced electron transfer Rf : retention factor

RR : rectification ratio EtOH: ethanol

SOMO : singly occupied molecular orbital SQ : semiquinone

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TLC : thin layer chromatography

VBCI : valence bond configuration interaction

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I. General Introduction, Background and Theory

I.1. Introduction of Electron Transfer in Donor-Bridge-Acceptor Systems

Electron transfer (ET) is a common phenomenon and is the fundamental process of ubiquitous redox reactions. Electron transfer is also a central process in many biological systems1 as well as in molecular devices for solar energy conversion,2,3 photovoltaics,4,5 and furthermore, in the emerging field of molecular electronics.6,7 ET is the key step that propagates the electronic signal between the different components.8 A deep and detailed understanding of ET is desired for interpretation of structure-property relationships of molecules. The key parameter that determines the rate of ET is the electronic coupling matrix element HDA, according to Marcus Theory

(Eq.I-1). 9 While free energy (∆G0) and reorganization energy () are by comparison easily understood, electronic coupling is a bit more elusive. Thus, understanding the electronic coupling HDA is a prerequisite to understanding ET.

          T k G T k H k B B DA ET   

 exp 4

2 0 2

2

h (I-1)

ET in D-B-A systems can happen via different mechanisms.10,11 A superexchange mechanism promotes electron transfer in a single step. In this mechanism, the bridge is used as a medium for electronic coupling via low-lying virtual states, and the redox state of the bridge does not change during ET. Moreover, electronic coupling via superexchange decreases exponentially with distance,12 as expressed in Eq.I-2, where β represents a distance attenuation parameter.

(I-2)

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Donor-bridge-acceptor (D-B-A) molecules are functional units on the molecular scale that promote electron- or energy transfer.13 The D-B-A system is very useful in studying electron ET reactions, it is also relevant, or even has potential application in solar energy conversion14,15 and molecular electronics16 (including gates,12 molecular diodes17,18 and molecular wires19). A variety of D-B-A molecules have been designed and synthesized with regard to their importance of understanding ET process.

I.2. How is Electronic Coupling Measured?

The nature of the electronic coupling in D-B-A systems has become a subject of interest in the last decade due to the importance of electronic coupling in molecular electronics.20 Thus, understanding the electronic structure origins and magnitude of electronic coupling is of great significance. Unfortunately, there is no way to directly measure the electronic coupling matrix element HDA, but there are several ways it

could be calculated.

I.2.1. Photoinduced Electron Transfer Reactions in D-B-A Systems

A typical photoinduced ET (PET) reaction is shown in Figure I-1A. At first, the Donor or the Acceptor is excited by an incoming electromagnetic wave and forms a locally excited state. Then electron transfer occurs, from Donor to Acceptor, and generates a charge-separated excited state. As a result, the Donor is oxidized and the Acceptor is reduced. Figure I-1B showed the orbital diagram of a PET reaction. It is obvious that the product of this reaction, D+-A-, is not only a zwitterion, but it is a biradical as well since it has two unpaired electrons.

Figure I-1. (A) Photoinduced electron transfer (PET) reaction. (B) Orbital diagram perspective of PET reaction.

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D-B-A systems were widely studied by PET experiments21-24 in order to understand detailed mechanisms of ET through the bridge. In PET experiments shown in Figure I-2, charge separation of local excited state D*-B-A gives a singlet charge separated (CS) excited state D+-B-A- which can then undergo charge recombination (CR) and form the ground state D-B-A. With the help of ultrafast spectroscopic methods, one can study the rate constants of charge separation kCS and

charge recombination kCR,12, 25 and finally compute the electronic coupling matrix

element HDA using Marcus theory described in Eq. I-1.

Figure I-2. Jablonski diagram of PET reaction in D-B-A system.

An example of D-B-A system for PET experiments26 was shown in Figure I-3. 3,5-dimethyl-4-(9-anthracenyl) julolidine (DMJ-An) was used as the Donor and naphthalene-1,8:4,5-bis(dicarboximide) (NI) as the Acceptor. In this system, the Bridge 2,7-fluorenone (n = 1-3) (FNn) and p-phenylethynylene (n = 1-3) (PEnP) were

covalently linked to the Donor and the Acceptor, to study the temperature dependence of spin-selective intramolecular charge recombination.

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Since the charge separated excited state D+-B-A- in PET experiments is a biradical, the electronic coupling HDA could be determined from the magnitude of the

singlet-triplet splitting (exchange coupling), 2J. As shown is Figure I-2, if the lifetime of the singlet CS state 1(D+-B-A-) is long enough for an intersystem crossing, the triplet CS 3(D+-B-A-) state would be formed and followed by charge recombination to generate a neutral triplet excited state 3(D-B-A). Using time resolved electron paramagnetic resonance (TREPR) experiments,55 the yield of 3(D-B-A), which is directly correlated with J, could be determined as a function of applied magnetic field strength. This method allows us to calculate HDA is PET experiments besides using

rate constant k measured from transient absorption spectroscopy.

Although PET experiments provide an effective approach to study electron transfer processes, it has some disadvantages. Most importantly, since the charge separated state D+-B-A- is transient, it is impossible to obtain detailed structural information. Moreover, the electronic coupling HDA for the excited state D+-B-A

-biradical is intrinsically low and is in a weak coupling regime, which limits its application on study of slight variation of electronic couplings.

I.2.2. Single Molecule Conductance from STM-BJ Experiment

Single molecule conductance was broadly used to study molecular electronics behaviors such as molecular wires,27 rectifiers,28 quantum interference effect transistor,29 torsional dependence of the bridge on conductance (g = resistance-1),30 distance dependence of the bridge31,32 and aromaticity of the bridge on conductance.33

The conductance g of the bridge was directly correlated to electronic coupling according to Nitzan34-36 and Ratner 37,38 as described in Eq. I-3:

A D DA

G e

g  2  2

h

 (I-3)

Where ΓD and ΓA are the widths of the donor and acceptor energy levels due to their

couplings to the left and right metal leads. GDA is Green's function for the

donor-acceptor interaction which contains electronic couplings, HDA.

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junction (STM-BJ) experiments.39-43 The molecules in the study have a desired bridge in the middle, and end group (typically thiols or amines) on each side that can bind to the tip and the substrate electrodes (typically gold), respectively (Figure I-4).44

Figure I-4. Cartoon of STM measurement, L indicates the linking atom, yellow circles represents gold electrodes.44

The process can be divided into three steps45 shown in Figure I-4: Firstly, a STM tip moves toward the substrate surface until it contacts the molecules bound to the substrate. During the contact period, one or more molecules may bind to the tip via the second end group. Secondly the tip is pulled away from the substrate, and then the molecules break contact with one of the two electrodes individually. Finally, the process is repeated until a large number of molecular junctions are created and the conductance is measured.

Figure I-5. Three steps of STM-BJ experiment to measure molecular conductance.45

I.2.3 Magnetic Exchange Coupling in D-B-A Biradical Complexes

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The Shultz Group uses metal complexes SQ-Bridge-NN biradical system shown in Figure I-6, the donor = S=1/2 ortho-semiquinone, SQ, while the acceptor = S=1/2 nitronylnitroxide, NN. The D-A dyad is connected by a diamagnetic bridge, while the SQ radical anion is ligated to MII having a hydro-tris(3-cumenyl-5-methylpyrazolyl) borate (TpCum,Me) ancillary ligand. In this dissertation, all MII = zinc(II), which is a d10, diamagnetic metal center. Thus, all of the paramagnetism arises from the ligand and not the metal ion. The cagelike TpCum,Me ligand is of sufficient steric bulk enough to magnetically insulated each biradical in the solid state, thereby preventing strong intermolecular interactions that might interfere with analysis of magnetic data. Thus, the observed magnetic exchange couplings describe intramolecular interaction between SQ and NN.

Figure I-6. Architecture of D-B-A biradical complexes in the Shultz Group.

SQ-Bridge-NN heterospin biradicals as a spin source, which are comparatively strongly-coupled biradical systems, and may be thought of as ground-state analogues of charge separated states generated in PET processes. D-B-A biradicals are also effective models of single-molecule-mediated electron transport between biased nanoelectrodes.46,47 The most remarkable thing is, since the D-B-A biradical complexes of Shultz Group are air stable crystals, not transient excited states in PET experiments, we can readily relate detailed structural and spectroscopic information such as the exact torsion angle between the π systems, 48

and real distance- instead of calculated distance- of the bridge,49 to the exchange coupling parameter. This provides us with a deep electronic-structure based understanding of the structure-

Donor: Semiquinone

(SQ)

Diamagnetic Bridge

Acceptor: Nitronylnitroxide

Nitronylnitroxide

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property relationships for electronic coupling, and any process governed by electronic coupline (e.g., electron transfer and electron transport).

In summary, rate constant kDAin PET experiment, single molecule conductance g

and exchange coupling JDA in D-B-A biradical are three major experiment

measurables to calculate and evaluate electronic coupling, HDA. These three

approaches provide different perspective on the nature of electronic coupling, and eventually, they could correllated with each other 30,33 as shown in Figure I-7.

Figure I-7 Cartoon suggesting the utility of D-B-A biradical electronic structure to elucidate molecular structure-property relationships in both PET reactions and conductance experiments with regard to electronic coupling matrix element HDA.

I.2. Introduction to Exchange Coupling in Biradicals I.2.1. Definition of Exchange Coupling Parameter J

When two spins are exchange coupled, a singlet state (S=0) and a triplet state (S=1) are created. The description of this exchange coupling is illustrated by Heisenberg-Dirac-van Vleck (HDVV) Hamiltonian19 (I-4).

b a b a

ab J S S

Hˆ 2 ˆ ˆ

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In this equation, Sˆ and a Sˆ are the spin angular momentum operators. b Jab is

the exchange coupling parameter, which determines the magnitude and sign of interaction between two spins. The total spin operator is SˆtotSˆaSˆb.

a a b a b a

tot S S S S S S

Sˆ2 (ˆ  ˆ )2  ˆ2 ˆ22ˆ ˆ 2 / ) ˆ ˆ ˆ ( ˆ

ˆ 2 2 2

b a tot a

aS S S S

S   

Put equation (I-6) to equation (I-4),

) ˆ ˆ ˆ ( ˆ ˆ 2

ˆ 2 2 2

b a tot b a b a b a

ab J S S J S S S

H    

Eigenvalue of Sˆ2 is S(S+1), therefore, the energy of state could be determined in equation (I-8). )] 1 ( ) 1 ( ) 1 ( [      

Jab Stot Stot Sa Sa Sb Sb

E

The energy of a singlet state (Stot =0) can be described as:

b a b

a ab

s J J J

E 2 3 ) 4 3 4 3 0 ( )] 1 2 1 ( 2 1 ) 1 2 1 ( 2 1 ) 1 0 ( 0 [           

The energy of a triplet state (Stot =1) is:

b a b

a ab

t J J J

E 2 1 ) 4 3 4 3 2 ( )] 1 2 1 ( 2 1 ) 1 2 1 ( 2 1 ) 1 1 ( 1 [           

So, the energy difference between singlet and triplet states is:

b a b a b a t s

st E E J ( J ) J

ΔE 2 2 1 2 3      

By definition, if J >0, the exchange coupling is ferromagnetic because the exchange energy favors parallel spins. If J <0, the exchange coupling is antiferromagnetic because the exchange energy favors antiparallel spins.

I.2.2. Measurement of Exchange Coupling JDA in D-B-A Biradical Complexes

As we discuss above, 2JDA represents the energy gap between singlet and triplet

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function of temperature. Thus, variable-temperature experiments provide approaches for us to measure the energy gap and therefore to calculate the exchange coupling JDA.

First, we are able to obtain JDA value from paramagnetic susceptibility χpara

which is measured by variable-temperature magnetometry. According to Van Vleck's equation,45,46 the magnetic susceptibility χ is described below.

) ( ) ( 2 ) 1 ( ) 0 ( ) 0 ( T k E n B n T k E n B n B n e T k e E N  

 

In this equation, N is the Avogadro's Number, kB is the Boltzmann constant, T is the

temperature, and En is the energy of a state. For a two spin system having a triplet

state (S=1) and singlet state (S=0), we can merge equation (I-8) into Van Vleck's equation (I-12).

      S T k E S T k E B para B S B S e S e S S S T k Ng ) ( ) ( 2 2 ) 1 2 ( ) 1 2 )( 1 ( 2  

In Eq. I-13, N is Avogadro’s number, g is the isotropic Landé constant (2.0023),

β is the Bohr magneton, S is the total spin of a state and ES is the energy of a state.

Substitution gives:             ) 2 ( ) 2 ( 2 2 3 1 6

2 kJT

T k J B para B DA B DA e e T k Ng  

Putting in the constants gives,

            ) 2 ( ) 2 ( 2 3 1 6 2 emuK/mol 0.125 T k J T k J B para B DA B DA e e T k g

From Eq. I-16, we can obtain JDA from a fit to the experimental magnetic

susceptibility χpara.

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Figure I-8 is a stack plot of theoretical χparaT vs. T curve with exchange coupling

500 cm-1 ≤ JDA ≥ 500 cm-1 in a 2 unpaired electron system. From this figure, it is

obvious that the curves of ferromagnetic coupling JDA > 0 is completely different

from antiferromagnetic coupling JDA < 0 curves. If with decreasing temperature,

χparaT increases towards the low temperature limit of 1 emu•K/mol, the electrons are

ferromagnetically coupled. If χparaT decrease with decreasing temperature, tending

towards the low temperature limit of 0 emu•K/mol, the two electrons are antiferromagnetically coupled. We also can see that χparaT = 0.75 emu•K/mol at 300 K

when the two unpaired electrons are uncorrelated, and as temperature increases, both ferromagnetic and antiferromagnetic coupling tend to this value.

Figure I-8. Theoretical χparaT vs T plots with 500 cm-1 ≤ JDA ≥ 500 cm-1 in system of 2

unpaired electrons.

When the exchange coupling is antiferromagnetic, the exchange coupling parameter JDA can also be obtained from variable-temperature electron paramagnetic

resonance (VT-EPR) experiments. In EPR, the intensity of signal is directly proportional to paramagnetic susceptibility χpara given by the Curie law:

T C IEPR para

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 

) ( ) ( ) ( 3 1 3 RT E RT E RT E T S T T s T e e e n n n

T

 

 

Multiply by exp(Es/RT), and alow ET-ES=Δ ETS=2JDA, we can get:

) 2 ( ) 2 ( 3 1 3 ] [ RT J RT J para EPR DA DA e e T C T T C I    

From the equation (I-19) about, if we fit the curve of signal intensity of VT-EPR versus 1/T with the equation above, we are able to obtain the value of JDA. However,

in very strongly ferromagnetically coupled systems, the gap 2JDA is large, which

means the triplet ground state is highly stabilized. As a result, only little change in both the line shape and slope in an IEPR vs. 1/T plot is observed. This method would

only work well in antiferromagnetically coupled biradicals.

I.3. Valence Bond Configuration Interaction Method

We developed a Valence Bond Configuration Interaction (VBCI) model for our SQ-Bridge-NN biradicals.46 VBCI models are widely used in inorganic chemistry, and it retains the simplicity of a VB presentation by expressing the wave function in terms of a minimal number of effective structures that dictate the chemistry of the problem.47 The VBCI model differs from the MO picture by virtue of the fact that it provides a state rather than an orbital description of the system.42 Thus, as a state description, the VBCI model contains parameters that can be evaluated directly with spectroscopic data, allowing for an evaluation of excited-state contributions to the ground state exchange.48

In a D-B-A biradical, ground configuration singlet-triplet splitting results from configuration interaction between the ground- and excited configurations with HDA

being the off-diagonal matrix element that mixes configurations of the same spin multiplicity.45

The key interactions between frontier orbital of semiquinone (SQ) and nitronylnitroxide (NN) are described in Figure I-9.41 In the MO depiction, the singly (I-18)

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occupied molecular orbital (SOMO) on the NN does not interact with SQ because it has a node on the connecting carbon, and thus remains “non-bonding” when connected to SQ. However, the lowest unoccupied molecular orbital (LUMO) of NN has proper symmetry to mix with the SOMO of SQ. This mixing results in the stabilization of the in-phase SQ-NN SOMO. The considerable overlap density between the NN SOMO and the SQ SOMO leads to a substantial exchange integral which gives rise to a strong ferromagnetic coupling.

Figure I-9 Frontier Orbital diagram showing π interactions of SQ-NN.

The SQ-NN frontier MOs are shown again in Figure I-10. According to the electron densities, the SQ-NN bonding orbital is SQ-based, so we could treat it as SQ fragment (SOMOSQ). Likewise, the SQ-NN antibonding orbital is treated as NN

fragment (LUMONN). Based on the electronic coupling matrix elements, We could

know that NN SOMO and SQ SOMO are orthogonal (H11= 0) but NN LUMO and SQ

SOMO are not orthogonal (H12 = 0).

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mix into ground states, and split the singlet and triplet ground levels by 2J. Based on these interactions, the following equation can be obtained that relates the exchange coupling to the electronic coupling:

Figure I-10 Simplified MO diagram showed non-zero interaction between LUMONN

and SOMOSQ (left). VBCI model of SQ-NN (right).

2 0 2

0 2 2 2

K U

K H

J DA

 

In equation I-20, J is the exchange coupling parameter, K0 is an single-site

exchange integral which means the gap between singlet and triplet exited states, U is the mean charge transfer energy from ground state to exited states. HDA is the

electronic coupling matrix element describing the mixing of ground (GC) and excited configurations (CTC).

This equation is significant because it describes the directly proportional relationship between exchange coupling parameter J and electronic coupling matrix element .

2

DA

H J

By measuring K0 and U from electron absorption spectroscopy, and J from EPR

or magnetometry, we are able to get electronic coupling matrix element HDA at high

resolution for any synthetically viable Bridge.

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para-phenylene bridge, we explored the role of the linearly conjugated π-bridge in the

exchange coupling between the donor and the acceptor. We found the para-phenylene bridge would mediate a strong ferromagnetic coupling and thus stabilize the triplet ground state.41 The frontier molecular orbital diagram of SQ, Ph and NN (Figure I-11) gives a good explanation of this result.

Figure I-11. SQ-Ph-NN (left) and its Frontier Orbital diagram of π interaction (right).

In Figure I-11, we used doubly degenerate phenylene LUMO set (e1 and e2) as

the bridge orbitals. Clearly the Ph(e1) orbital strongly interacts with both NN(π*)

LUMO and SQ(π) SOMO, however, Ph(e2) has no interaction with either NN(π*) or

SQ(π) due to improper orbital symmetry, so it becomes nonbonding with respect to these fragment orbitals.

I.4. Introduction to D-B-A Biradical Complexes in the Shultz Group

In the Shultz Group’s biradicals, the same Donor SQ, Acceptor NN and (ZnTpCum,Me)+ are used, thus the only variable is the bridge. This makes it possible for us to systematically study different bridge effects on electronic and exchange coupling using any synthetically viable bridges.

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study “quantum interference” effects49

by comparing the exchange coupling through a cross conjugated π-system and a linearly conjugated π-system SQ-Ph-NN. The results of this study showed SQ-pPh-NN gave a ferromagnetic coupling J = +100 cm-1while SQ-mPh-NN gave antiferromagnetic coupling with a J = -31 cm-1, which implies completely different mechanisms for electronic and exchange coupling. The SQ-mPh-NN with different substitutions were also synthesized to study the effect on the antiferromagnetic coupling. The superexchange pathway for SQ-mPh-NN coupling involves a configuration arising from an Ph-NN (HOMO)→Ph-NN (LUMO) electronic transition.

Figure I-12. Cross conjugated SQ-mPh-NN and linearly conjugated SQ-pPh-NN.

Cross conjugated thiophene bridged biradical complexes were also synthesized by Dr. Stasiw to study the quantum interference effect, but in this case, the thiophene ring is providing an odd-number pathway for spin polarization. A more detailed study of this project is underway (Figure I-13).

Figure I-13. Linearly- and cross conjugated thiophene bridged biradicals.

Since the exchange coupling through the bridge in biradical is via a superexchange mechanism, bridge length would play an important role. We designed the series of molecules shown in Figure I-14 to quantitatively study the bridge length

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effect on the exchange/electronic coupling by inserting different numbers of phenylene and thiophene bridges between the donor and acceptor. Thus, we could obtain the distance attenuation parameter β of phenylene bridge and thiophene bridges, which are a key parameter to electronic coupling HDA in a long distance

superexchange pathway. Understanding the distance-dependence of coupling is also important for the design of molecular electronics components.44 The results of our studies showed that  = 0.39 Å-1 for the p-Ph-bridged series and 0.22 Å-1 for the 2,5-thiophene-bridged series in accordance with findings in more weakly coupled systems.49

Figure I-14. Biradicals with bridge of different length to study distance dependence effect.

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Figure I-15. Biradicals with torsional bridges and σ-only bridge to study torsional dependence effect.

Similarly, as shown in Figure I-16, the study of torsional dependence effect on the electron-rich thiophene bridges is underway. Details of this project are presented in Appendix I.

Figure I-16. Biradicals with thiophene bridge and sterically hindered thiophene bridges.

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Figure I-17. Biradicals with sp3 hybridized σ-bridge (1-BCO) and sp2 hybridized bridge (1-Tc).

Besides the bridge properties we mention above, the electronic nature of the bridge would also play a key role in the exchange coupling between the donor and the acceptor. It would be very interesting to study how the electronic nature of the bridge affects the electronic coupling of the molecule. So we designed a pair of D-B-A biradicals with asymmetric bridges, where electron donor (methoxy) and electron acceptor (cyano) are on the opposite position. It is conceivable that we could strengthen the electronic coupling by increasing the donor ability of the donor, and increasing the acceptor ability of the acceptor. By the same token, we could weaken the electronic coupling by reversing the polarity of the Bridge. Details of this project will be illustrated in Chapter II.

Figure I-18 Biradicals with asymmetric Donor-Acceptor Bridges: SQ-D-A-NN (left) and SQ-A-D-NN (right).

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and pyridine is relative electron poor, the intrinsic dipole exists in this bridge, directing from thiophene to pyridine. By switching the direction of dipole of the bridge, we are able to see the how the dipole direction affect the electronic coupling of the molecule. This effect allowed us to study the rectification ratio of the bridge. Details of this project will be illustrated in Chapter III.

Figure I-19. Biradicals with asymmetric thiophene-pyridine bridges.

Finally, we also designed and synthesized biradicals with pyridine bridges, since the exchange and electronic coupling through the aromatic heterocycle bridge would be interesting as well as relevant to the project above. In particular, the pyridine bridge have two isomers in which nitrogen atom is facing the Donor or the Acceptor, it is very intriguingly to see the difference in electronic coupling due to isomeric effect of the bridge. Details of this project will be illustrated in Chapter III.

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II. Determining the Conformational Landscape of σ and π Coupling Using para- Phenylene and “Aviram–Ratner” Bridges

II.1. Introduction

II.1.1. Introduction to Torsional Dependence on Electronic Coupling

The fact that the magnitude of electronic coupling HDA depends on the nature of

the bridge offers the possibility to modulate HDA by structural, conformation, and

substituents means. One of the key factors relating to the overall electronic coupling is how well the molecular p-orbitals on adjacent units overlap to form a continuous, delocalized π-system. Several experimental and theoretical studies have addressed this issue.1-3

Wasielewski4 showed how torsional motions will primarily affect electronic coupling between adjacent sites, and will impart an angular dependence to Hij (i, j

nearest neighbors) of the form:

Where is the torsional angle and is the electronic coupling between sites i and j at = 0°.

Computational studies of torsional dependence of electronic coupling were reported by Albinsson.5 As shown in Figure II-1, benzene, naphthalene, anthracene and bicyclo[2.2.2] octane were used as bridges to introduce different torsions angles between Donor-Bridge, Bridge-Bridge and Bridge-Acceptor. Cosine dependence of the torsion angles on electronic couplings were discovered in this study.

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Harriman6 and Wenger7 have shown experimentally determined electronic couplings follow a cos(φ) dependence in D-B-A systems. In Harriman’s D-B-A system shown in Figure II-2, different numbers of methylene groups were inserted to control torsion angles as well as lock the confirmation. The electronic coupling HDA

was determined from PET experiments but torsion angles were obtained by calculation.

Figure II-2. Harriman’s D-B-A system with to study torsional dependence on electronic coupling. Electronic coupling is dependent on the cosine of torsion angle.6

Mayor8 and coworkers described the synthesis and structural analysis of a family biphenyl bridged species, shown in Figure II-3, in which the torsion angle φ is fixed by a bridging alkyl chain. Investigation of the single molecule conductance of the series by STM-BJ displays a linear correlation with cos2φ of the inter-ring torsion angle. However, since STM tip inherently allows for more conformational freedom, the torsion angles of molecules anchored on the gold electrode cannot accurately reflect the torsion angle measured from X-ray crystallography.

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These previous studies of bond torsion correlation with electronic coupling, or conductance in D-B-A molecules, have been plotted as HDA vs. cosφ or g vs. cos2φ.

However, when the Donor-Acceptor dyad is asymmetric, the McConnell model9 predicts HDB different than HBA.

1           n DB BB DB BA DB DA H H H H  

If we combine the nearest neighbor torsional dependence with

McConnell model, we can get

1 BB BA

DB) cos(φ ) cos(φ ) φ cos(           n DB BB DB BA DB DA H H H H  

Thus the torsional dependence of electronic coupling is best displayed as a 3-dimensional surface.

II.1.2. Introduction to “Aviram-Ratner” Bridge: Bicyclo[2.2.2]octane.

Over 40 years ago, Aviram and Ratner10 proposed a Donor-σ-acceptor molecule consist of a electron donating π-system and an electron withdrawing π-system separated by a saturated bridge ( Figure III-1). This type of molecule has been studied for decades because of their potential application in organic electronics such as rectifiers.12-15

Figure II-6. donor-σ-bridge-acceptor rectifier proposed by Aviram and Ratner.10

The key role for the rectifying ability of these molecules is the covalent,

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saturated σ-bridge which efficiently separates the donor from the acceptor as it creates a tunneling barrier for ET. If the donor and acceptor were allowed to interact with each other strongly through a π-bridge, the whole molecule will basically remain on the single donor level34 and lose the rectification behavior. Based on the calculated value of electronic coupling and the estimated value of molecular resistance, bicyclo[2.2.2]octane (BCO) is recommended as an excellent “insulator.” 16

Furthermore, the BCO framework holds two bridgehead carbons collinear and opposite to one another at a distance of 2.5 Å.17 At this distance, the direct orbital overlap of bridgehead carbons is very weak, thus the through-space coupling is vanishingly small. ET through σ-bonds is slower than through π-bonds by orders of magnitude. Miller et al.36 claimed that the ET through σ-bridges should be described as long-range electron tunneling.

Using the insulating nature of BCO bridge, Matsuda18 studied the distance dependence of exchange interaction between two phenylene spaced nitronylnitroxide radicals. The exchange interaction direct through phenylene units is too large to be evaluated by EPR, so without BCO bridge, the distance dependence effect could not be observed.

Figure II-7. Matsuda’s study on exchange interaction in bis(nitronylnitroxide) radicals.18

II.1.3 Introduction to Spin Polarization of σ-Frameworks

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hybridized, and the unpaired electron is located on the pz orbital which perpendicular

to the three sp2 hybrid orbitals that comprise the C-H -bonds. Thus, the pz orbital has

a nodal plane that contains the adjacent protons, so there should not be any unpaired spin density at the hydrogen nucleus and hence no hyperfine coupling. However, the fact is we do observe hyperfine coupling of protons. The reason for this contradiction is the spin polarization effect.

When we consider the spin direction of the unpaired electron in pz orbital of and

the electron in the sp2 orbital (σ-bond), two possible alignments of the spin, parallel and antiparallel, could be generated (Figure II-8B). The electron spin on hydrogen should be antiparallel to the spin on sp2 orbital of carbon in order to form a σ-bond. If the parallel and antiparallel alignments are equal in energy, there would be no net electron spin on proton since they cancelled one another. Actually, because of Hund’s rule, the parallel alignment (left) is preferred since it has a lower energy (as per Hund’s rule). The different stability of these two electronic structures would cause one spin to be in excess of the other and hence generate a net spin density on hydrogens. That is the reason we could observe hyperfine coupling of protons in π radicals.

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Simply put, the spin polarization effect is the spin of a π electron polarizes the spin of a σ electron. By the same token, the spin of σ electron also could polarize the spin of π electron. As a result, the spin of a π electron could affect another π electron via σ-bonds. The cartoon showed in Figure II-10 is very persuasive in explaining how spin polarization propagates through σ-bonds. When the two π electrons are connected by odd number of σ-bonds, they tend to be antiparallel spin; when they are connected by even number of σ-bonds, they tend to be parallel spin.

Figure II-9. Interaction of two π spins via different numbers of C-C bond

II.1.4 Target Molecules and Expectations.

We designed a series of D-B-A biradical complexes with methyl substitiuted para-phenylene bridges shown in Figure II-8 that may rotate the SQ or NN out of

plane at different angles to demonstrate at high resolution the torsional dependence of electronic coupling HDA. It is important to notice that there are two bond torsions in

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Figure II-11. Designed biradicals with sterically hindered para-phenylene and BCO bridges.

We anticipate 1-MePh will have a greater φSQ-B than 1-Ph, and 1-Me2Ph twist

the φSQ-B to perpendicular. Similarly, 1-PhMe will have a greater φB-NN and 1-PhMe2

have the greatest φB-NN. 1-Xylyl would slightly rotate the bridge out of plane but in 1-PhMe4, the bridge would be 90° to both SQ and NN.

In 1-PhMe4, since theπ system of the bridge is orthogonal to the π system of SQ

and NN, the π pathway for electronic coupling is turned off. At this point, we also designed the D-B-A biradical complex with a BCO bridge, which is completely lack of π pathway. By the comparison of 1-PhMe4 and 1-BCO, we can verify our

hypothesis that σ pathway will predominate when π conjugation is disrupted.

Moreover, since SQ-B-NN is a heterospin biradical, Donor/Acceptor dyad is asymmetric, electronic coupling between the subunits HDB might be different from HBA

according to McConnell model. Using exact torsion angles φSQ-B and φB-NN from

crystallography and overall electronic coupling HDA calculated from exchange

coupling JDA, we are able to establish the experimentally-determined 3-dimentional

plot of McConnell Model.

II.2 Results and Discussion

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The last two compounds in the steric phenylene series are 1-PhMe2 and 1-Me2Ph,

and they were synthesized using the same synthetic route shown in Scheme II-1, but with different bridge molecules 2a and 2b. (Figure II-10)

Scheme II-1. Completed Synthesis of 1-PhMe2 and 1-Me2Ph.

We can see that the two bridge molecules are just constitutional isomers with two methyl groups facing opposite directions. This is the principle of our molecular design because these two bridges introduce much more steric hindrance to one side than the other.

Figure II-10. Bridge Molecule for 1-PhMe2 and 1-Me2Ph.

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with DMF to give bridge 2.2a.(Scheme II-2)

Scheme II-2. Improved synthetic method for bridge 2.2a.

Bridge 2.2b was synthesized from compound 2.8, the intermediate in the synthesis of bridge 2a. (Scheme II-3) It is remarkable that the second step for both 2.2a and 2.2b are the same, the reason for the different products is that the reaction is kinetic controlled for 2.2a, but thermodynamic controlled for 2.2b.

Scheme II-3. Synthesis of bridge 2.2b.

The synthesis of biradical complexes 1-PhMe2 and 1-Me2Ph started with the

Suzuki coupling of the catechol synthon- compound 1 and bridge 2.2a/b. Then we deprotected the two methoxymethyl groups to get the catechol, condensation cyclized with 2,3-dimethyl-2,3-bis(hydroxyamino)butane (BHA), followed by oxidation with I2 to yield the nitronylnitroxide monoradical. The other side of the molecule was

completed by attaching the catechol to the TpCum,MeZn(OH) then oxidizing with aerial O2 to produce the biradicals 1-PhMe2 and 1-Me2Ph.

The presence of biradical 1-PhMe2 and 1-Me2Ph were determined by electron

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Figure II-11. Experimental and simulated EPR spectra of 1-PhMe2, apparent aN =

3.70 G (left). Thermal ellipsoid plots of 1-PhMe2 (right).

Figure II-12. Experimental and simulated EPR spectra of 1-Me2Ph, apparent aN =

3.65 G (left). Thermal ellipsoid plots of 1-Me2Ph (right).

II.2.2 Synthesis of Biradical with Bicyclo[2.2.2]octane Bridge

Unlike the Shultz Group’s other Donor-Bridge-Acceptor (D-B-A) biradicals, semiquinone-bicyclo[2.2.2]octanyl-nitronylnitroxide SQ-BCO-NN (1-BCO) biradical has a unique structure: a saturated bridge that consists of pure σ bonds. When designing the synthetic route to this molecule, we predicted this step might be very challenging. We noticed that the bond between the BCO bridge and semiquinone is actually a σ bond that connects an sp2

hybridized carbon, to an sp3 hybridized bridgehead carbon. This bond formation is unlikely to be achieved via typical

-1000 0 1000 2000 3000

3450 3460 3470 3480 3490 3500

Exp Sim

Magnetic Field (Gauss)

-2000 -1000 0 1000 2000 3000 4000 5000 6000

3450 3460 3470 3480 3490 3500 EXP SIM

Magnetic Field (Gauss) 1-PhMe2

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palladium catalyzed Suzuki cross coupling which we used to synthesize almost all other biradicals in the past, since Suzuki cross coupling usually requires vinyl, benzyl or aryl on both sides.23 According to some literature,24,25 successful precedence indicated that Br-BCO-CO2Me 2.10 could couple to a phenyl ring by Friedel-Crafts

alkylation, through a bridgehead carbocation intermediate. Therefore, based on this reaction, we designed the synthetic route shown in Scheme II-4.

Scheme II-4 Proposed synthetic route of 1-BCO biradical.

First, Br-BCO-CO2Me 2.10 is to be coupled to 2-t-butylanisole by Friedel-Crafts

alkylation. Then, the ester is reduced to aldehyde by DIBAL-H reduction, followed by methoxy deprotection by BF3·Et2O. The IBX-ascorbic acid method is a new route that

was adopted by the Shultz group recently25, which efficiently converts phenol to catecol in high yield without affecting other functional groups. After that, cyclization with bishydroxylamine (BHA) and oxidation with I2 would yield Cat-BCO-NN. The

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Scheme II-5. Synthesis of BCO bridge 2.10.

Synthesis of BCO bridge molecule 2.10 is known47 (Scheme II-5). Starting with cyclohexane-1,4-dicarboxylic acid 2.17 (mixture of cis- and trans-isomers), reacted with SOCl2 in MeOH to form dimethyl ester 2.18. Then, LDA was used to

deprotonate the α-proton and form an enolate, followed by enolate SN2 attack on

1-bromo-2-chloroethane. This reaction was facilitated by four eq. of HMPA which is a strong Lewis base that binds to Li+ and renders the enolate more nucleophilic. The bicyclo[2.2.2]octane bridged ring was formed by an intramolecular enolate reaction. The most remarkable observation in this transformation is the dramatic change in

1

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Figure II-13. 1H NMR of compound 2.19 (above) and 2.20 (below).

Saponification, followed by acidification to transformed only one of the methyl esters to a carboxylic acid 2.21. Next was a Hunsdiecker reaction: first the acid was turned into the silver carboxylate 2.22, the resulting silver salt contained crystallized water which was removed by heating in vacuo. Finally, the silver salt reacted with Br2

to decarboxylate followed by bromination to give the final product 2.10.

With the BCO bridge molecule 2.10 in hand, we attempted the Friedel-Crafts alkylation reaction that is described in the literature.26,27 However, the reaction did not work for our specific substrates(Scheme II-6), only the isomerized product of the other reactant 2-t-butylanisole was observed. We used different Lewis acid catalysts in different reaction conditions, but none of them gave the desired coupling product. (Table II-1)

Scheme II-6. Attempts at Friedel-Crafts reaction in different conditions.

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Table II-1. Different Friedel-Crafts reaction conditions and results

L.A. Solvent Time Temp. Yield

AlCl3 CS2 16 h r.t. 0 (100% isomerized)

FeCl3 CS2 18 h r.t. 5% coupling, 95% isomerized

ZnCl2 CS2 18 h r.t. No reaction

FeCl3 CS2 19 h -78-0 °C 0 (100% isomerized)

Considering the difficulties encountered during Friedel-Crafts reactions, we tried Grignard reactions.28 Since the ester group is incompatible with Grignard reagents, it is converted to aldehyde and then protected it by the dimethyl acetal to serve as another BCO bridge moiety for coupling reaction (Scheme II-7).

Scheme II-7. Synthesis of Br-BCO-acetal 2.26 from 2.10.

Scheme II-8. Preparation of activated Rekie Magnesium

Scheme III-9. Attempts at Grignard reagent formation on 2.26

Figure

Figure I-3.  D-B-A system used for PET experiments by Wasielewski.26
Figure I-5. Three steps of STM-BJ experiment to measure molecular conductance.45
Figure I-7conductance experiments with regard to electronic coupling matrix element Cartoon suggesting the utility of D-B-A biradical electronic structure to elucidate molecular structure-property relationships in both PET reactions and  HDA
Figure I-9 Frontier Orbital diagram showing π interactions of SQ-NN.
+7

References

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