The design and synthesis of polycyclic conjugated hydrocarbons via the incorporation of bifunctional Diels-Alder building blocks

HYDROCARBONS VIA THE INCORPORATION OF BIFUNCTIONAL DIELS-ALDER BUILDING BLOCKS

by

Sarah P. Luppino

B.A. magna cum laude

Chemistry and Italian Bowdoin College, 2010

SUBMITTED TO THE DEPARTMENT OF CHEMISTRY

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN CHEMISTRY

AT THE

MASSACHUSETTS INSTITUTE OF TECHNOLOGY

MAMZ~aI&STI91ES OF TOHNQ V

JUL;

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LIBRARIES

C 2017 Massachusetts Institute of Technology, 2017. All rights reserved.

Signature of Author:

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Certified by: Accepted by: epartme&f Xf Chemistry ""4 J , May 17,.,2017

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Robert W. Field Haslam and Dewey Professor of Chemistry Chairman, Departmental Committee on Graduate Students

Committee of the Department of Chemistry as follows

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Professor Timothy F. Jamison __

Department of Chemistry Thesis Committee Chairman

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Professor Timothy M. Swager

Department of Chemistry U

Thesis Supervisor

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Professor Jeremiah A. Johnson

Department of Chemistry Committee Member

5-BY

SARAH P. LUPPINO

SUBMITTED TO THE DEPARTMENT OF CHEMISTRY

ON MAY 22, 2017

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN CHEMISTRY

ABSTRACT

In Chapter 1, we discuss the synthetic progress, challenges, and applications of polycyclic conjugated hydrocarbons (PCHs). In particular, we explore synthetic strategies to access polycyclic aromatic hydrocarbons and nonbenzenoid oligoacenes, and also discuss synthetic efforts to stabilize, solubilize and tune the properties of these classes of molecules.

In Chapter 2, we describe the synthesis and characterization of seven new linearly conjugated ladder compounds of the phenylene-containing oligoacene (POA) molecule class.

Each derivative incorporates a fused four-membered-ring linkage in the acene-like backbone. Crystal packing, spectroscopic and electrochemical properties of the molecules are described. In Chapter 3, we describe the synthesis of a set of naphthazarin-containing polycyclic conjugated hydrocarbons, which were accessed through sequential Diels-Alder reactions on a tautomerized naphthazarin core. Iptycene and POA motifs were incorporated into the synthesis of these compounds. We discuss their complexation with BF₂ to achieve full conjugation across the molecular backbone, and the potential these compounds demonstrate for naphthazarin's utility in the synthesis of novel organic electronic materials.

In Chapter 4, we describe the synthetic progress towards a thirteen-aromatic-ring extended POA. Two different synthetic approaches were employed, and we discuss the pros and cons of each approach to achieve the final target.

In Chapter 5, we discuss a convergent synthesis of a heptiptycene PCH, its crystal stacking properties, photophysical characterizations, and its effectiveness as a selector molecule in chemiresistive sensing with single-walled carbon nanotubes (SWCNTs).

Thesis Supervisor: Timothy M. Swager

Title: John D. MacArthur Professor of Chemistry

-7-CHAPTER 1

This chapter was written by the author.

CHAPTER 2

This chapter was carried out by the author. The author thanks Dr. Peter Miler and Dr. Jonathan Becker for collecting and solving X-ray structure data. The author also thanks Dr. Dan Congreve and Joel Jean for helpful discussions about semiconductor applications.

CHAPTER 3

This chapter was a collaborative effort between the author, Dr. Cagatay Dengiz, Dr. Gregory D. Gutierrez and Professor Timothy M. Swager. Dr. Gutierrez carried out preliminary experiments on the iptycene synthesis. The author and Dr. Dengiz together carried out the syntheses, characterization, and writing of the manuscript. Dr. Dengiz performed the TD-DFT calculations. The author thanks Dr. Dengiz for many helpful discussions. The authors thanks Dr. Peter MUller and Dr. Jonathan Becker for collecting and solving X-ray structure data.

CHAPTER 4

This chapter was carried out by the author. The author thanks Dr. Rebecca R. Parkhurst for her supportive consultation on the flash vacuum pyrolysis set-up. The author also thanks Dr. Baltasar Bonillo for his helpful discussions in the early part of this project, his support, and guidance on aryne chemistries and acene synthesis.

CHAPTER 5

This chapter was carried out by the author. The author thanks Dr. Sophie F. Liu for help with the chemiresistive sensor experiments, Dr. Georgios Markopoulos for his generous donation of the N6-Trimer and discussions on charge complex formation, Joseph M. Azzarelli for his help in aqueous sensing, and Tony Wu for his absolute quantum yield measurement. The author also thanks Dr. Peter MUller for collecting and solving X-ray structure data.

-9-Title Page ... I Signature Page ... 3 D edication ... 5 A bstract ... 7 Respective Contributions ... 9 Table of Contents ... I I List of Figures ... 15 List of Schem es ... 23 List of Tables ... 25 List of A bbreviations ... 27 CHAPTER1 INTRODUCTION: POLYCYCLIC CONJUGATED HYDROCARBONS ... 29

1. 1 Introduction ... 30

1.2 Polycyclic A rom atic Hydrocarbons (PA H s) ... 31

1.2.1 A cene Stability Challenges ... 32

1.3 N onbenzenoid Oligoarenes ... 34

1.3.1 [N ]Phenylenes ... 35

1.3.2 Phenylene-Containing Oligoacenes ... 35

1.4 References ... 38

CHAPTER 2 DIFFERENTIALLY SUBSTITUTED PHENYLENE-CONTAINING OLIGOACENES ... 41

2.1 Introduction ... 42

2.2 Synthesis ... 43

2.3 Characterization ... 45

2.4 Conclusion ... 48

2.5 A cknow ledgem ents ... 48

2.6 Experim ental D etails ... 49

2.6.1 G eneral M ethods and M aterials ... 49

2.6.2 Synthetic Procedures ... 51

-2.8 Appendix for Chapter 2 ... 64

2.8.1 Electrochem istry ... 64

2.8.2 UV/vis and Fluorescence Spectra... 66

2.8.3 X -Ray Crystallography ... 68

2.8.4 Additional Characterizations ... 71

2.8.5 N M R Spectra ... 72

2.8.6 Cartesian Coordinates and Energies for Computed Structures... 82

CHAPTER 3 NAPHTHAZARIN-POLYCYCLIC CONJUGATED HYDROCARBONS AND IPTYCENES ... 91

3.1 Introduction... 92

3.2 Results and Discussion ... 94

3.2.1 Synthesis of N aphthazarin Triptycene Derivatives ... 94

3.2.2 Synthesis of N aphthazarin Pentiptycene Derivatives ... 95

3.2.3 Synthesis of Naphthazarin Phenylene-Containing-Oligoacene Derivatives ... 96

3.2.4 Spectroscopic Properties of Mono- and Double-Adducts ... 101

3.2.5 Investigation of BF2-Com plexation... 103

3.3 Conclusion ... 105

3.4 Experim ental Section... 106

3.4.1 General M ethods and M aterials... 106

3.4.2 Synthetic Procedures... 107

3.5 Acknow ledgem ents... 120

3.6 References... 121

3.7 Appendix for Chapter 3 ... 124

3.7.1 X -Ray Crystallography ... 124

3.7.2 UV/vis and Fluorescence Spectra... 129

3.7.3 N M R Spectra ... 147

TOWARD A 13-AROMATIC-RING PHENYLENE-CONTAINING OLIGOACENE ... 173

4.1 Introduction... 174

4.2 Synthetic Strategy to A ccess Extended POA s... 175

4.3 Results and D iscussion ... 176

4.3.1 Synthetic Route I: Bis-D iene Approach... 176

4.3.2 Synthetic Route II: Bisaryne Approach... 180

4.4 Experim ental Details... 184

4.4.1 G eneral M ethods and M aterials... 184

4.4.2 Synthetic Procedures... 185

4.5 References... 194

4.6 A ppendix for Chapter 4 ... 196

4.6.1 N M R Spectra ... 196

CHAPTER 5 A C ONVERGENT H EPTIPTYCENE SYNTHESIS ... 209

5.1 Introduction... 210

5.2 Synthesis of H eptiptycene 3 ... 211

5.2.1 Triple-Aryne [4+2] Cycloaddition on a Triphenylene Precursor ... 211

5.2.2 Metal-Mediated Trimerization of a Triptycene Precursor ... 212

5.3 Characterization of H eptiptycene 3 ... 216

5.3.1 Photophysical Characterization ... 216

5.3.2 X -Ray Crystallography ... 217

5.4 Sensing and A nalyte Binding Studies... 218

5.5 Experim ental Details... 221

5.5.1 G eneral M ethods and M aterials... 221

5.5.2 Synthetic Procedures... 222

5.5.3 X -Ray Crystallography ... 225

5.6 References... 226

5.7 A ppendix for Chapter 5 ... 228

5.7.1 N M R Spectra ... 228

Figure 1.1. Polycyclic aromatic hydrocarbons, a subset of polycyclic conjugated

hydrocarbons, as structural fragments of graphene. Depicted: pentacene (red), phenalene (blue), perylene (light green), phenanthrene (dark green), triphenylene (p u rp le)...3 0 Figure 1.2 Example oligoarene classes: (a) acenes, (b) [N]phenylenes, (c)

phenylene-containing oligoacenes (PO A s). ... 31 Figure 1.3 From naphthalene to hexacene, stability decreases while electronic properties

are improved. Each molecule contains only one Clar sextet (bolded). ... 32

Figure 1.4 A selection of functionalized pentacene molecules and their half-lives as a

measure of stability. Half-lives were determined for 2.0 x 10-4 M solutions in CH2Cl2 at 25 'C and exposed to ambient light and air.2 ... 34

Figure 1.5 Selection of published structures 9,12,34,35 incorporating at least one short

acene unit linked by four-membered ring linkages to other acene or benzene units... 36

Figure 2.1 (a) Absorbance and fluorescence spectra of 4a in CHCl3. (b) Absorbance

spectra of derivatives 4a-e, g (solution, CHCl₃). Note: Photophysical analysis of 4f is

complicated as a result of rapid photochemical degradation over the course of the UV-vis measurement. See Appendix for complete spectroscopic data. (c) Comparison of solution-state and solid-state fluorescence (left 4a, right 4e). ... 46

Figure 2.2 X-ray crystal packing. Top: crystal packing, Middle: end view, and Bottom:

side view : (a) 4a (b) 4c (c) 4e... 48

Figure 2.3 Electrochemical Properties of compounds 4a-g and derivative previous

4

synthesized by our group. ... 64

Figure 2.4 Cyclic Voltammetry (Note: organized by increasing Ex potential.)

Performed in a 0.1 M solution of TBAPF6 in CH2Cl2, Pt button electrode, scan rate 100 mV/s, ferrocene as an external standard. All CVs were performed under a flow

-g lov eb ox. ... 6 5

Figure 2.5 Absorbance 4a Xmax = 441 11M (6(441) = 22381, 6(416) = 14719), Xem = 446

im, Pem = 0.03, 11 = 0.4 ns (ci = 0.91), T2 = 6.4 ns (a2 = 0.09); 4b kmax = 428 nm

(-(428)= 15329, 8(403)= 11062), Xem = 434 nm, (Pem = 0.54, T= 6.5 ns (c = 1.00); 4c

kmax = 417 17M (8(417) = 1106*, 8(394) =919*), kem = 422 nm, Pem = 0.36, -r = 9.7 ns

(a = 1.00); 4d kmax = 420 rnm (8(420) = 10791, 8(397)= _{7800), kem = 425 nm, (Pem =}

0.03, 11 = 0.6 ns (cc = 0.81), T2 = 7.4 ns (a2 = 0.19); 4e kmax = 441 im (6(441) =

10913, 8(402) = 8999), Xem = 443 nm, em = 0.37, -l = 8.7 ns (a = 0.88), T2 = 2838

ns (a2 = 0.12); 4f Xmax = 448 nm, kem = 452 nm, 9em = 0.08, T, = 4.9 ns (c = 0.18),

T2 = 8.5 ns (a2 = 0.82); 4g max = 426 im (8(426) = 11997, 8(415) = 8760), kem = 435

un, (Pem = 0.39, -= 5.0 ns (a = 1.00). ... 66

Figure 2.6 Absorbance and Fluorescence spectra for compounds 4b-g (Note: 4a can be

found in Figure 2.1). All measurements were taken in CHCl3... ... ... . . . 67

Figure 2.7 Red, solution state emission in CHCl3, Blue, film emission (spincoated) (a)

4e (b) 4a (c) 4d (d) 4b. ... 68 Figure 2.8 X-ray crystal structure of TIPS-derivative 4a. Hydrogens are omitted for

clarity. X-ray quality crystals were grown via slow vapor diffusion of hexanes into a

chloroform solution. (a) Anisotropic thermal ellipsoids set at 50% probability. (b) Another view of crystal packing of 4a. Interplanar packing distance was on the order

of 7.2 A, and torsional angles around the four-membered ring were 1.90 and 4.20 ... 69

Figure 2.9 X-ray crystal structure of tetrafluoro-derivative 4c. Hydrogens are omitted

for clarity. X-ray quality crystals were grown via slow evaporation of a chloroform solution. (a) Anisotropic thermal ellipsoids set at 50% probability. (b) Another view of crystal packing of 4c. Interplanar 7r- a packing distance was measured as 3.6-3.7

A,

Figure 2.10 X-ray crystal structure anthracene-containing 4e. Hydrogens are omitted

Another view of crystal packing of 4e. The distance between the two molecule planes was 4.37

A,

an d 0 .3 )... 7 0 Figure 2.11 DSC of derivatives 4a, 4b, 4d, 4e. ... 71 Figure 2.12 Thermal stability of a selection of the synthesized compounds as

demonstrated by thermogravimetric analysis (TGA) of compounds 4a, 4b, 4d, 4e... 71

Figure 3.1 Three of the naphthazarin-containing target molecules synthesized in this

stu d y . ... 9 3

Figure 3.2 Three X-ray crystal structures of iptycenes 6b, 7a, and 8a... 95

Figure 3.3 X-ray crystal structures of POAs 10, 11, and 12... 98

Figure 3.4 UV/vis spectra of selected compounds la (red line), 2a (black line), 3 (green

line), and 7a (blue line), in CH2Cl2 at 298 K ... 102

Figure 3.5 'H NMR spectra (400 MHz) of compounds la to BF2-la in CDCl3 at 298 K

dem onstrating BF2-com plexation... 104

Figure 3.6 UV/vis spectra of compound la (black line) and BF2-1a (blue line) in

C H 2C l2 at 298 K . ... 105

Figure 3.7 ORTEP plot of 6b, arbitrary numbering, H-atoms are omitted for clarity.

Atomic displacement parameters are drawn at 50% probability level. Selected bond length (A): C25-C14 1.503(4), C14-C15 1.403(4), C14-C13 1.394(4), C13-C12

1.385(4), C12-C11 1.512(4), CIl-ClO 1.569(4), C10-C9 1.517(4), C9-C8 1.456(4),

C8-C7 1.396(4), C7-C6 1.397(5), C6-C5 1.359(5), C7-03 1.353(4), C9-04

1.242(3), C l-C IO 1.547(4), C3-C8 1.424(4). ... 124

Figure 3.8 ORTEP plot of 7a, arbitrary numbering, H-atoms are omitted for clarity.

Atomic displacement parameters are drawn at 50% probability level. Selected bond length (A): C31-C32 1.3858(16), C32-C33 1.3994(17), C33-C34 1.3870(18),

C34-C35 1.3998(17), C34-C35-C36 1.3890(16), C36-C12 1.5289(15), C12-C11 1.5240(15), CI1-CO 1.4100(15), C1O-04 1.3455(13), C1O-C9 1.4050(16), C9-C4 1.4240(16), C9-C8 1.4632(16), C8-03 1.2419(15), C8-C7 1.4778(17), C7-C6 1.3387(18)... 125

-Atomic displacement parameters are drawn at 50% probability level. Selected bond length (A): C29-C28 1.388(2), C28-C27 1.396(2), C27-C26 1.387(2), C26-C25 1.522(2), C25-C5 1.528(2), C5-C6 1.385(2), C5-C4 1.398(2), C4-C3 1.406(2), C3-C2 1.457(2), C3-C2-C1 1.511(2), Cl-C1O 1.549(2), C1-C18 1.571(2), C18-C17 1.515(2), C17-C16 1.386(2), C16-C15 1.396(2), C15-C14 1.386(2), C4-02 1.3485(18), C 2-0 1 1.2415(18). ... 125 Figure 3.10. ORTEP plot of 10, arbitrary numbering, H-atoms are omitted for clarity.

Atomic displacement parameters are drawn at 50% probability level. Selected bond length (A): C5-C6 1.386(2), C6-C7 1.405(2), C7-C8 1.3774(18), C3-C8 1.3957(19), C8-C9 1.5276(18), 01-C9 1.4584(15), C9-C1O 1.5486(18), Ci-CO 1.5790(19), ClO-ClI 1.5232(18), C11-C24 1.328(2), C11-C12 1.4932(19), C12-C13 1.534(3), C12-C13-C22 1.535(4), C12-C13-C14 1.485(3), C14-C15 1.470(2), C15-C20 1.414(2), C15-C16 1.401(2), C16-C17 1.404(2), C17-C18 1.361(3), C14-02 1.2314(19), C 16-0 3 1.354(2). ... 126 Figure 3.11. ORTEP plot of 11, arbitrary numbering, H-atoms are omitted for clarity.

Atomic displacement parameters are drawn at 50% probability level. Selected bond length (A): C5-C6 1.3875(16), C6-C7 1.4026(15), C7-C8 1.3814(13), C3-C8

1.3956(13), C8-C9 1.5230(13), O1-C9 1.4567(11), C9-C1O 1.5573(12), CI-CO

1.5824(13), C1O-C1 1.5181(13), C11-C24 1.3317(12), C11-C12 1.4814(13),

C12-C13 1.5105(13), C12-C13-C22 1.3585(13), C12-C13-C14 1.4830(13), C14-C15 1.4573(13),

C15-C20 1.4156(14), C15-C16 1.3956(13), C16-C17 1.4119(15), C17-C18

1.3628(17), C14-02 1.2415(12), C16-03 1.3437(14)... 127

Figure 3.12. ORTEP plot of 12, arbitrary numbering, H-atoms are omitted for clarity.

Atomic displacement parameters are drawn at 50% probability level. Selected bond length (A): C5-C6 1.389(2), C6-C7 1.401(2), C7-C8 1.383(2), C3-C8 1.394(2),

C8-C9 1.524(2), 01-C9 1.4557(17), C9-C1O 1.5486(19), Cl-Ci 1.5818(19),

CIO-C1 15246(19), C11-C24 1.392(2), Cll-C12 1.381(2), C12-C13 1.4093(19), C13-C22 1.4095(19), C13-C14 1.480(2), C14-C15 1.459(2), C20 1.421(2),

C15-1.3 4 0 6 (18). ... 12 8

Figure 3.13. UV/Vis absorption spectra of la (red line), 6a (black line), 7a (blue line), and 8a (green line) in CH₂Cl₂ at 298 K... 129

Figure 3.14. UV/Vis absorption spectra of lb (red line), 6b (black line), 7b (blue line), and 8b (green line) in CH2Cl2 at 298 K... 129 Figure 3.15. UV/Vis absorption spectra of lc (red line), 6c (black line), 7c (blue line),

and 8c (green line) in CH2Cl2 at 298 K. ... 130 Figure 3.16. UV/Vis absorption spectra of 6d (black line), 7d (blue line), and 8d (green

line) in C H 2C l2 at 298 K ... 130 Figure 3.17. UV/Vis absorption spectra of 6a (black line), 6b (blue line), 6c (green

line), and 6d (red line) in CH2Cl2 at 298 K. ... 131 Figure 3.18. UV/Vis absorption spectra of 7a (black line), 7b (blue line), 7c (green

line), and 7d (red line) in CH2Cl2 at 298 K. ... 131 Figure 3.19. UV/Vis absorption spectra of 8a (black line), 8b (blue line), 8c (green

line), and 8d (red line) in CH₂Cl2 at 298 K. ... 132 Figure 3.20. UV/Vis absorption spectra of la (black line), lb (blue line), and ic (green

line) in C H ₂C l2 at 298 K ... 132 Figure 3.21. UV/Vis absorption spectra of of 2a (black line), 2b (blue line), 10 (orange

line), 11 (green line), and 12 (red line) in CH2Cl2 at 298 K... 133 Figure 3.22. UV/Vis absorption spectra of 2a (black line), 3 (green line), 7a (blue line),

12 (orange), and 15 (red line) in CH2Cl2 at 298 K. ... 133 Figure 3.23. UV/Vis absorption spectra of 12 (black line), 13 (green line), and 15 (red

line) in C H ₂C l2 at 298 K ... 134 Figure 3.24. Normalized absorbance (blue) and emission (red) spectra of 6a in CH2Cl2

at 2 9 8 K . ... 13 4 Figure 3.25. Normalized absorbance (blue) and emission (red) spectra of 6b in CH2Cl2

at 2 9 8 K . ... 13 5

Figure 3.26. Normalized absorbance (blue) and emission (red) spectra of 6c in CH₂C2

at 2 9 8 K . ... 13 5

-at 2 9 8 K . ... 13 6

Figure 3.28. Normalized absorbance (blue) and emission (red) spectra of 8a in CH2Cl2

at 2 9 8 K . ... 13 6

Figure 3.29. Normalized absorbance (blue) and emission (red) spectra of 8b in CH2C 2

at 2 9 8 K . ... 13 7

Figure 3.30. Normalized absorbance (blue) and emission (red) spectra of 8c in CH2Cl2

at 2 9 8 K . ... 13 7

Figure 3.31. Normalized absorbance (blue) and emission (red) spectra of 8d in CH2Cl2

at 2 9 8 K . ... 13 8

Figure 3.32. Normalized absorbance (blue) and emission (red) spectra of 10 in CH2Cl2

at 2 9 8 K . ... 13 8

Figure 3.33. Normalized absorbance (blue) and emission (red) spectra of 11 in CH2Cl2

at 2 9 8 K . ... 139

Figure 3.34. Normalized absorbance (blue) and emission (red) spectra of 12 in CH2Cl2

at 2 9 8 K . ... 13 9

Figure 3.35. Normalized absorbance (blue) and emission (red) spectra of 2a in CH2Cl2

at 2 9 8 K . ... 14 0

Figure 3.36. Normalized absorbance (blue) and emission (red) spectra of 2b in CH2Cl2

at 2 9 8 K . ... 14 0

Figure 3.37. Normalized absorbance (blue) and emission (red) spectra of 13 in CH2Cl2

at 2 9 8 K . ... 14 1

Figure 3.38. Normalized absorbance (blue) and emission (red) spectra of 15 in CH2Cl2

at 2 9 8 K . ... 14 1

Figure 3.39. Normalized absorbance (blue) and emission (red) spectra of 3 in CH2Cl2 at

2 9 8 K . ... 14 2

Figure 3.40. Normalized absorption spectra of la (black line) and la-BF2 (blue line) in

C H 2C l2 at 298 K . ... 142

Figure 3.41. Normalized absorption spectra of lb (black line) and lb-BF2 (blue line) in

C H ₂C l₂ at 298 K . ... 143

Figure 3.43. Normalized absorption spectra of la-BF2 (green line), lb-BF2 (black line), and lc-BF2 (blue line) in CH2Cl2 at 298 K . ... 144

Figure 3.44. Normalized absorption spectra of 2a (black line) and 2a-BF2 (blue line) in C H 2C l2 at 298 K . ... 144

Figure 3.45. Normalized absorption spectra of 3 (black line) and 3-BF2 (blue line) in C H 2C l2 at 298 K . ... 145

Figure 3.46. Normalized absorption spectra of 2a-BF2 (black line) and 3-BF2 (blue line) in C H 2C l2 at 298 K . ... 145

Figure 3.47. Normalized absorption spectrum of id in CH2Cl2 at 298 K. ... 146

Figure 3.48 M ALDI-TOF spectrum of 14... 171

Figure 3.49 MALDI-TOF spectrum of 14 (expanded). ... 171

Figure 4.1 (a) acenes, (b) [N]phenylenes, and (c) POAs... 174

Figure 4.2 Extended POAs in the literature.6 POAs are labeled such that [N,N,N]POA reveals number of aromatic rings (N) in each acene unit, and "," denotes the cyclobutadiene linkages... 175

Figure 5.1 (a) Structures of triphenylene 1 and target heptiptycene 3. (b) Schematic showing the potential 7t-7t binding interaction of an analyte, benzene, with 3, and (c) the binding of a triptycene molecule with 3 via six possible CH-71 interactions... 211

Figure 5.2 Schematic showing coordination modes of the two catalyst systems explored: (a) nickel- and (b) palladium -mediated catalysis. ... 215

Figure 5.3 Absorption (solid) and fluorescence (dashed) spectra in chloroform. Triphenylene (1) in red, Xmax= 260 nm, Xem = 353 nm, 3 in blue, Xmax = 284 nm, ke = 383 nm . Dem = 0.40. ... 2 17 Figure 5.4 X-ray structures of 3. (a) Side view of a staggered dimer of 3, (b) top view of dimer 3, (c) packing of dimers showing disordered chloroform molecules... 218

Figure 5.5 Schematic demonstrating sensing protocol and setup.2_{' A mixture of} SWCNTs and selector molecule 3 was dropcast between gold electrodes. The change in resistance in response to VOC analytes was measured using a potentiostat... 219

Schem e 1.1 Decomposition routes of pentacene... 33

Scheme 2.1 Reagents and conditions: (a) aryne generation for 2a, n-BuLi, PhMe, -40 0C

to r.t.; for 2b-e, isopentyl nitrite, 1,4-dioxane/PhMe, 100 0C; (b) DDQ, benzene, 75

0_{C; (c) p-TsOH, Ac}

20, 100 0C; (d) Na2S204, KOt-Bu, Mel, 2:1 MeOH-THF (e)

PhSH, Cul (10 mol %), proline (10 mol %), K₃PO4, DMF. ... 43

Scheme 3.1 Synthesis of naphthazarin triptycenes 7a-d and pentiptycenes la-d. ... 94 Scheme 3.2 Targeted synthesis of naphthazarin POA mono-adducts 2a-b... 97

Scheme 3.3 Targeted synthesis of POA double-adduct 14. ... 99

Schem e 3.4 Targeted synthesis of iptycene-POA 3. ... 100

Scheme 4.1 Synthetic strategies I and II employed to access target compound 2.

Conversion of 1,5-hexadiyne to 3,4-bismethylenecyclobutene 1 by flash vacuum pyrolysis is show n in the inset. ... 176 Scheme 4.2 Conversion of tetrabromobenzenes 6a-f to diepoxyanthracenes 7a-f for

access to double-diene building blocks 4... 177 Scheme 4.3 Mechanistic description of the conversion from dienophiles 7 to

bis-dienes 4 using two equivalents of DPT, followed by two equivalents of 1. Reactions are run in PhM e at r.t. ... 179 Scheme 4.4 Proof-of-concept reactions of bis-diene 4d with potential dienophiles... 180

Scheme 4.5 Synthesis of two different end-unit dienes. ... 181

Schem e 4.6 Attempted synthesis of [3,3,3]POA 15... 182

Scheme 4.7 Attempted synthesis of final [2,3,3,3,2]POA 2. ... 183

Schem e 5.1 The triple-aryne route to access 3.5... 212

Scheme 5.2 Synthetic scheme to achieve target heptiptycene 3 (one set of conditions

also produced tetramer 5) using a metal-mediated approach to cyclotrimerization... 212

Scheme 5.3 Scheme showing the two highest yielding synthetic approaches to 3... 214

-Table 1.1 Tabulation of spectroscopic properties of a selection of acenes,

[N]phenylenes, and [N]POAs. Eg is optical bandgap, except for Entry 2, which was

determined electrochemically. a Ref 9, b Ref 40, c Ref 20... 37

Table 2.1 Photophysical data for compounds 4a-g. asolution state measurement

(C H C l₃)...4 6

Table 2.2 Electrochemical Properties of compounds 4a-g. aPerformed in a 0.1 M

solution of TBAPF6 in CH2Cl2, Pt button electrode, scan rate 100 mV/s, ferrocene

as an external standard. bHOMO levels were determined from Eox and used in conjunction with the optical band gap (Eg (opt)) to determine LUMO levels. Determined from koset in CHC13.dCalculations performed using B3LYP/6-3 1 G* *. .... 64

Table 3.1 Photophysical characterization of emissive compounds synthesized. All m easurem ents were performed in CH2Cl2. ... . . . 103

Table 3.2 Tabulated photophysical data. All measurements taken in CH2Cl2 at 298 K... 146

Table 4.1 Conversion of tetrabromobenzenes 6a-f to diepoxyanthracenes 7a-f for

access to bis-diene building blocks 4. Furan was used in excess in all scenarios

(> I0 equ iv .)...177 Table 5.1. Conditions for cyclotrimerization attempts to form 3 (selected examples).

Note: *tetramer 5 also isolated in 5% yield (Entry 3). o-DBB

(ortho-dibromobenzene), bpy (2,2'-bypyridine), dppp

(1,3-bis(diphenylphosphino)propane), dba (dibenzylideneacetone), PPh3 (triphenylphosphine), phen (phenanthroline), COD (cyclooctadiene). ... 213

-2D Ac₂0 BMCB bpy br. COD d DART dba DCM dd DDQ DMF DMPU dppp DPT DSC ESI FC GC-MS HOMO HRMS i-PrMgCl KOt-Bu LUMO m MALDI-TOF MeCN Mel MeOH n-BuLi NMR o-DCB OFET OLED OPV ORTEP two dimensional acetic anhydride bismethylenecyclobutene 2,2'-bypyridine broad peak (NMR) cyclooctadiene doublet (NMR)

Direct Analysis in Real Time dibenzylideneacetone dichloromethane doublet of doublets (NMR) 2,3-dichloro-5,6-dicyano-1,4-benzoquinone dimethylformamide 1,3-Dimethyl-3,4,5,6-tetrahydro-2-pyrimidinone 1,3-bis(diphenylphosphino)propane 3,6-di-2-pyridyl- 1,2,4,5-tetrazine differential scanning calorimetry electrospray ionization

flash column chromatography

gas chromatography-mass spectrometry highest occupied molecular orbital high-resolution mass spectra isopropyl magnesium chloride potassium tert-butoxide

lowest unoccupied molecular orbital multiplet (NMR)

matrix-assisted laser desorption/ionization -time of flight

acetonitrile iodomethane methanol n-butyl lithium

nuclear magnetic resonance ortho-dichlorobenzene organic field-effect transistor organic light-emitting diode organic photovoltaic

Oak Ridge Thermal-Ellipsoid Plot Program

-p-TsOH para-toluene sulfonic acid PAH polycyclic aromatic hydrocarbon

PCH polycyclic conjugated hydrocarbon phen phenanthroline

PhH benzene PhMe toluene

PhSH thiophenol

POA phenylene-containing oliogacene PPh3 triphenylphosphine

ppm parts per million

PPTS pyridinium p-toluenesulfonate r.t. room temperature

Rf retention factor (chromatography)

RFID radio frequency identification s singlet (NMR)

SI supporting information

SWCNT single-walled carbon nanotube t triplet (NMR)

TBAPF6 tetrabutylammonium hexafluorophosphate TD-DFT time-dependent density functional theory

TGA thermogravimetric analysis

THF tetrahydrofuran

TIPS triisopropyl silyl

TLC thin layer chromatography

TMS trimethylsilyl

UV ultraviolet VIS visible

VOC volatile organic compound

CHAPTER 1

Introduction: Polycyclic Conjugated Hydrocarbons

-Polycyclic Conjugated Hydrocarbons

1.1 Introduction

Organic electronic materials have come to the forefront of materials science as candidates for next-generation electronic devices. In particular, they represent a viable route to achieving flexible and printed electronics,1 semi-transparent electronics such as organic photovoltaic (OPV) windows,2 3 _{and light-weight devices like radio frequency identification} tags (RFID) or portable OPVs that could enable improved deployment to underserved geographic regions.4 Current silicon-based technologies are typically heavier, opaque, and unable to be solution-processed. These challenges faced by silicon technologies open the door for organic materials to gain a foothold in the future electronics market.",2,4-6

Figure 1.1. Polycyclic aromatic hydrocarbons, a subset of polycyclic conjugated

hydrocarbons, as structural fragments of graphene. Depicted: pentacene (red), phenalene (blue), perylene (light green), phenanthrene (dark green), triphenylene (purple).

Polycyclic conjugated hydrocarbons (PCHs) represent some of the top-performing organic small molecule semiconductors. ',3'5' PCHs are organic ladder molecules or oligomers that consist of fused rings, forming the skeleton of a fully conjugated, two-dimensional (2D) molecular backbone.

Polycyclic aromatic hydrocarbons (PAHs) are a widely researched subset of PCHs that are structural fragments of nanographene,''8 _{and consist of multiple fused aromatic rings} (Figure 1.1). PCHs also include conjugated molecular systems containing units of nonbenzenoid and or antiaromatic character, such as [N]phenylenes or phenylene-containing

oligoacenes (Figure 1.2).912 These molecules are less explored as charge transporting materials, but their syntheses may present new routes to access and tune desired molecular properties and stability due to their unique molecular architectures.

(a) (b) (C)

n -n x yn

Figure 1.2 Example oligoarene classes: (a) acenes, (b) [N]phenylenes, (c)

phenylene-containing oligoacenes (POAs).

1.2 Polycyclic Aromatic Hydrocarbons (PAHs)

Though PAHs may be best known as carcinogenic pollutants formed in the combustion processes for transportation and manufacturing,1 3 _{or for their abundant presence} in the interstellar medium of galaxies,14 _{they have also captured the interest of the} semiconducting community. Acenes, the most extended class of PAHs, have gained attention as potential candidates for organic optoelectronic and electronic applications. In particular, their rigid molecular backbone, increasing charge mobilities,7

,'5 decreasing molecular

-31-reorganization energy,16 and narrowing band widths'7 _{are all factors that contribute to the} semiconductor potential of extended acenes.

1.2.1 Acene Stability Challenges

Though extended conjugation in acenes brings about many desirable and improved electronic properties, there are some challenges. One significant hurdle for this class of

PAHs is that their extended conjugation also results in poor stability, which impedes their widespread utility in devices (Figure 1.3).7 This decrease in stability can be explained by Clar's sextet rule.18 _{In the acene series depicted in Figure 1.3, only one aromatic R-sextet, or} benzene-like moiety, can be drawn per acene structure. In larger PAHs, the overall aromaticity and inherent stability of the molecule increases where more disjoint sextets can be drawn. Extended acenes, however, are plagued by a low number of Clar sextets, which

translates to low aromaticity and poor stability.7',10"'8 1 9

stability

desirable electronic properties

/ \ / \

/ \1

/

\/

Figure 1.3 From naphthalene to hexacene, stability decreases while electronic properties are

The decreased aromaticity in extended acenes results in an increased reactivity of the central acene ring, which can participate in cycloaddition reactions as a reactive diene. Breaking the extended conjugation across the acene backbone is therefore a thermodynamically favorable process, as the resultant product generates two separate entities, increasing the sextet character of the overall molecule (Scheme 1.1).

dimerize

0 102

Scheme 1.1 Decomposition routes of pentacene.

Acene decomposition typically occurs in one of two ways. Most reactive at its central ring, it either dimerizes to form a butterfly decomposition product via a [4+4] cycloaddition reaction, or reacts with molecular oxygen in a [4+2] cycloaddition reaction, subsequently oxidizing to the quinone product (Scheme 1.1). Photodegradation typically occurs as Type II photosensitized oxidation, either via electron transfer or through singlet oxygen (102)

sensitization pathways, yielding a Diels-Alder cycloadduct product.20 _{One of the best} defenses to protect against dimerization is steric resistance, while susceptibility to oxidation and dimerization are both mitigated through electronic effects, such as appending alkynes, thioethers, or other HOMO-lowering substituents to reduce the reactivity of the central diene.5 20-2 3

-Various synthetic strategies have been employed not only to improve acene stability, but also to tune and improve their electronic properties and solubility in organic solvents. Some of the key approaches are through the incorporation of heteroatoms in the acene backbone5 2_4-2 7 _{or by appending various substituents}5_,20 _{to stabilize the acene core or} solubilize the molecule for improved processibility (Figure 1.4).

TIPS N NN NN NZ NZ NZ NQ NZ NZ + N IPSU 1 2 3 4

t1/2= 3.7 min t1/2 = 8.5 min t1/2 = 520 min t1/2 = 1140 min

Figure 1.4 A selection of functionalized pentacene molecules and their half-lives as a

measure of stability. Half-lives were determined for 2.0 x 10-4 M solutions in CH2Cl2 at 25

'C and exposed to ambient light and air.20

Most notably, triisopropylsilyl (TIPS)-pentacene (compound 3 in Figure 1.4) has emerged as a promising candidate for processable small molecule semiconductors in organic

field effect transistors (OFETs), and has achieved mobilities of up to 4.6 cm2_/(V-s).12 8

However, as the field advances and looks toward the development of ever-higher performing materials, the development of new PCH structures may be necessary. While functionalized higher-order acenes represent one possible synthetic strategy,5,7,19,20,22 their predicted instability complicates the possibility of effectively expanding this series much further.19,2 1,29

1.3 Nonbenzenoid Oligoarenes

Oligoarenes that contain fused non-aromatic rings such as the [N]phenylene class (Figure 1.2b) have appeared less frequently in the semiconducting literature,'0 _{but may}

present a unique access route to generating organic semiconductors with high carrier mobilities, molecular stability and solution processibility.

1.3.1 [N]Phenylenes

The [N]phenylene class of PCHs is similar to acenes, but structurally alternates between formally antiaromatic four-membered rings and benzene units (Figure 1.2b). [N]Phenylenes can be linear, helical, zig-zag, or branched, and are readily accessed by a [2+2+2] synthetic strategy, popularized by Vollhardt and collaborators.30 [N]Phenylenes are not only fascinating due to their interesting molecular architecture; electronically, their formally antiaromatic four-membered ring units effectively localize the 71-electron conjugation to the benzene units. This can be observed structurally in the alternation of bond lengths. For instance, theoretical calculations show the bonds bridging the two benzene units together to converge at a length of 1.511

A

1 This is the same

value observed experimentally for [3]phenylene, and reveals strong single-bond charater.3 2 _In extended [N]phenylenes, where N > 2, the central benzene units have been shown to behave as two ir-allyl halves,3 2 _{further demonstrating localization of the electrons far from the} potential antiaromaticity of the four-membered rings. Despite this localization, we do see communication across the formally-antiaromatic rings through experimental and theoretical evidence of narrowing band gaps in extended derivatives.30,3 3

1.3.2 Phenylene-Containing Oligoacenes

Phenylene-containing oligoacenes (POAs) make up a family of PCH compounds first pioneered by the Swager Group in 2012,9 with similar derivatives containing just one acene unit having appeared previously in the literature (Figure 1.5). 12,4,35 Similar to the [N]phenylenes, POAs consist of acene units (e.g. naphthalene, anthracene) linked together

-via formally antiaromatic cyclobutadiene units. The goal of clipping the acene into smaller

units and sewing them together with the four-membered ring linkages is a method to mediate the extent of conjugation across the molecular backbone for improved stability. Localization induced by the cyclobutadiene units limits the conjugation across the full chromophore backbone, and the shorter acene units enable the access of molecules with an increased number of Clar sextets for better molecular stability.9 1 8

TMSIV ₁ M S Ph TIPS Ph Ph TIPS

Willis and coworkers. _{TMS A}

N T MS

po I T

I I P

ON _{TIPS TPPh[2POA}

n = n1 (3,3,3]POA TP 23PA TP

CN Yang and cowirkers.

McOmie and moworkers. _{Swager and woworkers}

Figure 1.5 Selection of published structures9, 12,34,35 incorporating at least one short acene

unit linked by four-membered ring linkages to other acene or benzene units.

Trends observed in the photophysical properties of POAs help explore how their molecular structure impacts the complete delocalization that occurs across an acene backbone (Table 1.1). As the TIPS-acene series grows by two rings, from anthracene to pentacene, we see a decrease in band gap by nearly 1 eV (entries 1-3). The band gaps of POAs, by contrast, narrow at a much slower rate (entries 4-6). In particular, extending the POA series by two benzene rings (entry 5 to entry 6, see Figure 1.5 for structures) results in a minor decrease of only 0.06 eV. Because of the localization effect observed in POAs,9 _they offer a more controlled synthetic route to access larger chromophores of improved stability and precise band gap tuning. Control over organic semiconductor band gaps is particularly important for applications in organic light-emitting diodes (OLEDs) and organic photovoltaics (OPVs).3 Additionally, acenes have been observed to undergo a phenomenon

called singlet fission, an energetically favorable process by which two excitons, or electron-hole pairs, can be harvested from one photon.3_6-39 _{With singlet fission materials, quantum} efficiencies in solar cells have the potential to double in magnitude.3 8 _{Optimal fission} materials require precise tuning of the band gap and triplet energy levels, a process to which POAs may be amenable.

Entry Compound N (# benzene rings) total # rings lowest E X. Eg (eV)

1 TIPS-anthracenea 3 3 442 2.74 2 TIPS-tetraceneb 4 4 534 2.30 3 TIPS-pentacene' 5 5 643 1.81 4 [2,3]POA" 5 6 464 2.57 5 12,3,2]POAa 7 9 500 2.43 6 [3,3,3]POAS 9 11 502 2.37

Table 1.1 Tabulation of spectroscopic properties of a selection of acenes, [Niphenylenes, and [N]POAs. Eg is optical bandgap, except for Entry 2, which was determined

electrochemically. a Ref 9, b Ref40 c Ref 20

One aim of the research presented in this thesis is to access new POAs to achieve rigid molecular structures of extended conjugation and strong solubility that can achieve high carrier mobilities but with improved stability. In Chapters 2, 3 and 4, the author will discuss efforts to grow this class of compounds to encompass new derivatives and explore the effects of functionality on the resultant electronics of the systems.

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CHAPTER 2

Differentially Substituted Phenylene-Containing

Oligoacenes

Parts of this chapter were reprinted with permission from Luppino, S. P.; Swager, T. M. "Differentially Substituted Phenylene-Containing Oligoacene Derivatives."

2017, 28, 323-326.

-41

2.1 Introduction

Acenes are leading small molecule semiconductors, and consist of a rigid

molecular backbone of linearly annulated benzene rings.1,2 Their rigidity and extended conjugation promote favorable 7t-electron delocalization and high fluorescence with minimal non-radiative losses, which are key attributes for high-performance organic

optoelectronics and electronics.1,2 _{A major challenge confronting the use of acenes in}

organic electronics is their susceptibility to photodegradation with extended conjugation lengths. Decomposition typically occurs by reaction with molecular oxygen at the central ring, which reacts to form an endoperoxide, then ultimately yields a final quinone

product.3

One strategy to simultaneously preserve the desirable electronic features of longer acenes and increase their stability is to reduce the extent of conjugation across the

backbone via integration of formally antiaromatic cyclobutadiene units.4 _{This strategy}

seeks to capitalize on the known localization effect observed in [N]phenylenes, a class of

polycyclic conjugated hydrocarbons with alternating benzene and cyclobutadiene units.5

Select examples in the literature, 6-8 as well as previous work from our laboratory,4 have

demonstrated multiple synthetic methods to achieve fully conjugated ladder systems

incorporating fused four-membered-rings. 4,6,9-11

In the present work, we aim to demonstrate the versatility of exodiene 14 as a building block for the construction of phenylene-containing oligoacenes, and explore our ability to tune the resultant molecular properties by installing varying functionality along the backbone. We herein describe the synthesis of seven new ladder molecules (4a-g) comprised of naphthalene and anthracene units fused by a four-membered ring linkage

(Scheme 2.1). The photophysical and electrochemical properties of these molecules are investigated to understand the extent of 2-conjugation across their annulated cores, and X-ray analysis is examined to probe their crystal packing preferences.

Ph Ph 13 Ph R1 + 2o-d (,RR ~y ~R3 RI41 3 RI Ph Ph R Ph R Br 2 H₂N* P2 1 Br R 3 HOOC R3

3b Al = CTIPS. R2 =R3 = Br (50' .yield) 4a R = CCTIPS, R2 R3 = Br (84% yield)

3P =H. R= 3 =OMe (4% yeld) 4bR I= H, R2 = 3 =OMe (72% yield)

3o R = R2= 3 =F (20% yeld) 4c Pl1=R2 =3 = F(69%r)lad) _ __2a 2 b-d 3dI' R2=H, 3= Br(49%yield) 4d R =R2=H, R3=Br (87%yield) 2

2b-4g = 2 = RH = SPh (80% yield) Ph R4 Ph R4 HO 1 + 20(a). (b) I + 2. a 2 Ph 0 Ph R4 Ph R4 (d) 3e R4 H (44% yield) 4e P4 =H (99% yield) 3fR4=OMe(51% yield) 41R4=OMe (63% yield)

Ph 0

2f4

Sche eI Reagents and co di n (a) aryne generation for2a n-Bul, PhMe, -48 -C to r.: for 2b-e, isopenty! nitrite, 100 OC; (b) DDO, benzene, 75 OC (c) p-TsOH, Ac2O, 100 OC:

(d) Na2SPO. KOt-Bu, Mel, 2:1 MeOH-THF (a) PhSH, Cut (10 mo %). prohine (10 mol%). K3P04 DME.

Scheme 2.1 Reagents and conditions: (a) aryne generation for 2a, n-BuLi, PhMe, -40 C to r.t.; for 2b-e, isopentyl nitrite, 1,4-dioxane/PhMe, 100 C; (b) DDQ, benzene, 75 C; (c) p-TsOH, Ac20, 100 0C; (d) Na2S204, KOt-Bu, Mel, 2:1 MeOH-THF (e) PhSH, Cul

(10 mol %), proline (10 mol %), K3PO4, DMF.

2.2 Synthesis

Exodiene 1 was synthesized in two steps according to a literature procedure,4,12-18 and was systematically reacted with an array of different dienophiles 2a-e and 1,4-naphthoquinone to furnish compounds 3a-f (Scheme 2.1). These compounds represent the outer molecular frameworks for the targeted products. Fully conjugated products 4a-g were obtained through a final dehydration step under acidic conditions.

The majority of the dienophiles in this study were aryne intermediates, and were generated in situ by one of two ways: treatment of a dibromobenzene derivative (2a) with n-butyllithium, or treatment of an anthranilic acid precursor (2b-e) with isopentyl nitrite.

A milder dienophile, 1,4-naphthoquinone (2f), was used4 to access final derivative 4f.

-Exodiene 1 underwent a Diels-Alder cycloaddition with o-dibromobenzene derivative 2a using one equivalent of n-butyllithium to furnish compound 3a in 50% yield. We also observed the formation of a byproduct that arose from a bis-aryne intermediate. The resulting double-Diels-Alder product was obtained in 20% yield, and is a compound reported previously by our group.4 This suggests the dibromo-monoadduct 3a may be more susceptible to lithiation than the tetrabromobenzene starting material 2a due to the higher solubility of 3a.19

The reaction of diene 1 with a selection of anthranilic acid derivatives 2b-d or 3-amino-2-naphthoic acid 2e yields compounds 3b-e in moderate yields. Reaction of 1 with 3,4,5,6-tetrafluoroanthranilic acid 2c is the lowest yielding transformation. This is likely due to the high reactivity of the very electrophilic tetrafluoroaryne intermediate that is generated in situ. 4-Bromoanthranilic acid 2d and 3-amino-2-naphthoic acid 2e fare better, with yields closer to 50%, whereas the electron-donating

4,5-dimethoxyanthranilic acid 2b fares best at 64% yield. Quinone 2f can be accessed using a three-step procedure previously published by our group.4 Methylation of 2f under reducing conditions furnishes 3f in 51% yield, though this compound and the fully unsaturated 3f proved susceptible to reoxidation during purification.

All six compounds were then exposed to acidic conditions to yield the dehydrated

products 4b-f with yields of 63-99%.20 Monobrominated 4d was converted to the corresponding thioether 4g in 80% yield using a copper catalyzed cross-coupling approach.

2.3 Characterization

Well-defined vibrational manifolds are observed within the absorption and emission spectra of the synthesized materials, thereby demonstrating the inherent rigidity of their core ladder structures (see example spectra of 4a in Figure 2.1(a)). The Stokes shifts of these rigid materials were small, and ranged from 2 to 8 nm. Emission quantum yields of the blue fluorescent materials ranged from PDem = 0.03 for the

bromide-substituted compounds 4a and 4d to Dem = 0.30 to 0.55 for those without heavy atoms (Table 2.1). This observation can be explained by the heavy-atom effect in 4a and 4d, where intersystem crossing dominates and nonradiative processes become faster than the triplet emission.2 1

The photophysical properties of 4a-g reveal trends related to the substitution pattern of the derivatives. For example, in the overlaid absorption spectra shown in Figure 2.1(b), the electron-rich derivatives 4b and 4g are slightly more red-shifted than the electron-deficient 4c and 4d. The derivatives with extended conjugation, whether by acetylene units as in 4a or an additional annulated ring as in 4e, red-shift the observed absorption and emission even further. The overall range of the shifts, however, is small: for example, electron deficient 4c absorbs at 417 nm, whereas the systems with the more extended conjugation, 4a and 4e, absorb at 441 nm. This suggests that incorporation of the cyclobutadiene linkage is an effective means to reduce the extent of conjugation across the molecular backbone, and enable fine-tuning of the band-gap and luminescent properties within a smaller window. When compared to derivatives of the acene class, a systematic bathochromic shift of approximately 100 nm is typically observed with as we extend conjugation by an additional ring: Xmax shifts from 286 nm for naphthalene, to 375

-nm for anthracene, to 477 -nm for tetracene. 22 Electrochemical measurements confirm that these compounds have large band-gaps, on the order of 2.68 - 2.86 eV (Table 2.2).

01 'a .E 0 z z 1 0n 0 0 0 .10 350 400 450 500 550 600 Wavelength (nm)

4- 4a: Fl (film) 4e:gl (film)

0 Z0 400 500 600 400 500 600 Wavelength (nmr) (b) 0.8 .0 .0 0 .0 z 0.6 0.4 0.2 0 375 400 425 Wavelength (nm) 450 475

Figure 2.1 (a) Absorbance and fluorescence spectra of 4a in CHCl3. (b) Absorbance spectra of derivatives 4a-e, g (solution, CHCl3). Note: Photophysical analysis of 4f is complicated as a result of rapid photochemical degradation over the course of the UV-vis measurement. See Appendix for complete spectroscopic

solution-state and solid-state fluorescence (left 4a, right 4e).

Compound 4a 4b 4c 4d 4e 4f 4g kmax,a nm 441 428 417 420 441 448 426 Xema nm 446 434 422 425 443 452 435 <Dem 0.03 0.54 0.36 0.03 0.37 0.08 0.39 data. (c) Comparison of T," ans (a) 0.4 (0.91), 6.4 (0.09) 6.5 9.7 0.6 (0.81), 7.4 (0.19) 8.7 (0.88), 2838 (0.12) 4.9 (0.18), 8.5 (0.82) 5.0

Table 2.1 Photophysical data for compounds 4a-g. asolution state measurement (CHCl3). (a) (c) - 4aAb - 4a Fl -4a 4b -4c -4d -4e -4g

Photophysical studies of thin films revealed a red-shift of 7 nm for the triisopropylsilylethynyl (TIPS)-substituted derivative 4a as compared to a 43 nm red-shift for 4e (Figure 2.1(c)). Red-shifts of 15 nm were observed for compounds 4b and 4d. These findings suggest differences in the extent of solid-state aggregation according to degree of functional group bulkiness. For instance, we see that the bare anthracene cores of 4e aggregate more significantly than 4a, which is decorated with two bulky TIPS groups. This feature is likely due to the breakup of 7-7c interactions by the large TIPS

groups of 4a and the lack thereof in 4e.

Analysis by X-ray crystallography was used as another means to examine solid-state aggregation. Comparing the crystal packing preferences of 4a to those of 4c and 4e,

we again see a difference in aggregation and packing (Figure 2.2). Switching from the TIPS-acetylene derivatized product 4a, which is too bulky to enable efficient stacking,4 to a much barer core structure as in 4c or 4e enables the molecules to stack in much closer proximity, which is key for effective charge transport in organic electronics.'

Interestingly, the crystal structures of derivatives 4a and 4c show a curvature to the backbone, demonstrating some degree of flexibility. This is contrary to the planarity observed in larger phenylene-containing oligoacene derivatives, such as 4e and those previously synthesized by our laboratory,4 but is consistent with trends seen among the [N]phenylene class.4'11 _{The curvature observed is likely to arise from the crystal packing} forces exerted on the backbone structures.

-(a) (b) (c)

Figure 2.2. X-ray crystal packing. Top: crystal packing, Middle: end view, and Bottom: side view: (a) 4a (b) 4c (c) 4e.

2.4 Conclusion

In conclusion, we have reported the synthesis of seven new ladder molecules. We are currently investigating the subsequent incorporation of these derivatives and higher-order analogs into field-effect transistors to assess their promise as high-mobility organic electronic materials.

2.5 Acknowledgements

This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. 1122374, and National Science Foundation

2.6 Experimental Details

2.6.1 General Methods and Materials

All reactions were carried out under argon using standard Schlenk technique

unless otherwise noted. All solvents were of ACS reagent grade or better. Toluene was passed through a solvent purification system via columns of activated alumina, stored over 3

A

1 4, 2a20 _{and 2g}4 _{were prepared according to the procedures described previously by our} group.

'H NMR (400 MHz), 13C NMR (101 MHz, proton-decoupled) and 9F NMR (376 MHz, proton-decoupled) NMR spectra were acquired in CDCl3 on a Bruker Avance III HD Spectrometer. Chemical shifts (6) are reported in parts per million (ppm) and referenced to residual NMR CDCl3 peaks. (CDCl3: 6 7.26 ppm for 'H, 77.16 for 13C). No

internal standard was used for 19F experiments.

High-resolution mass spectra (HRMS) were obtained at the MIT Department of

Chemistry Instrumentation Facility with either electrospray (ESI) or Direct Analysis in Real Time (DART) as the ionization technique.

Ultraviolet-visible absorption spectra were measured with an Agilent Cary 4000 UV/vis spectrophotometer and corrected for background signal with a solvent-filled