Tunable Luminescent Boron Complexes A Thesis Submitted to the faculty of the Graduate School of the University of Minnesota By Christian Lawrence Toonstra In partial fulfillment of the requirements for the degree of Master of Science Dr. Paul Kiprof, Advisor July, 2011 © Christian Lawrence Toonstra Acknowledgements I am indebted to Dr. Paul Kiprof for his knowledge and support over the last two years. I am also grateful to Dr. Steven Berry and Melanie Halverson for their help in X-ray crystallography, as well as Dr. Ahmed Heikal for his assistance in measuring quantum yield. I also want to thank Dr. Alan Oyler for his help in obtaining the APCI-MS data. Finally, I greatly appreciate the support from the department, especially Randall Helander. i Abstract The use of BF2 adducts has a long and important history in the field of auxochromic dyes, most notably in the sundry applications of the BODIPY family of dyes. Previous work in our laboratory on phenyl borinic acid yielded good emissive properties including quantum yield. BF2 was chosen due to the potential increase in the luminescence intensity of the adducts as compared to phenyl borinic acid derivatives. Simple azole based ligands were chosen due to their flexibility in color tuning. The azole N,O-type motif was found to be amenable to formation of adducts with BF2 as the oxygen readily formed a relatively stable boron ester type bond, while the non-bonding lone pair of electrons on the nitrogen is donated to the boron. Color tuning in this family of ligands is attainable through extension of the π system, or through auxochromic heteroatoms. The characterization of products included NMR, LC-APCI-MS, and X-ray crystallography. The luminescence data was collected using fluorimetry. The emissive nature of the complexes was probed using computational techniques. The TD-DFT data obtained from these computational studies was compared to the absorbance data that was obtained. The current findings as well as short-term future plans will be presented. ii Overview Beyond the appeal of advancements in display clarity and resolution, the development of organic light emitting diodes (OLEDs) represents a further step toward progression in the development of highly-tuned luminescent devices that are likewise energy efficient. This paper presents new research into a family of boron based OLEDs, having interesting properties that make them potentially useful in the development of OLEDs. The scaffold chosen was a tunable azole-based ligand. This thesis is separated into three chapters covering current, relevant research into boron-based OLEDs., rationalization of the photophysical properties of these new complexes, and experimental information. A brief overview of boron and it’s role in OLEDs with respect to the development of the boron compounds presented here is discussed in chapter one. Characterization was completed through a combination of NMR, X-ray crystallography, HR-APCI-MS, UV-vis, Fluorimetry, and Computational TD-DFT, as means to probe both the physical and optical properties of the compounds, this is discussed in the second chapter. The final chapter, along with the appendix, outlines the experimental details of this project. iii TABLE OF CONTENTS List of Tables v List of Figures vi List of Abbreviations viii Chapter 1 i Background of Boron Chemistry i Application of boron to OLEDs vi Overview of OLEDs ix New Directions for OLEDs Design xxiii Chapter 2 xxix New directions for luminescent boron adducts xxix Instrumentation xxxi Optical Data (UV-vis and Fluorimetry) xxxi Nodal Plane Theory xxxvi NMR xl HR-APCI-MS xlii X-ray Crystallography xliii Theoretical Chemistry xlv Chapter 3 lxix Instrumentation lxix Materials lxx Synthesis lxxi References lxxviii Appendix lxxxvi iv List of Tables Scheme 1 Ligand synthesis xxvii Scheme 2 Adduct synthesis xxx Table 1 Comparison of emission data xxxiii Table 2 Summarized optical properties of all of the compounds xxxv Table 3 Comparison of Stoke’s shift values xxxix Scheme 3 Possible products of the reaction of phenyl boronic acid and 10- Hydroxybenzo[h]quinoline lxiii Table 4 Raw data for quantum yield calculations cxiix v List of Figures Fig. 1 Boron fragmentation iii Fig. 2 Examples of medically useful boron derivatives. iv Fig. 3 ELF isosurface plots of boron-halogen complexes. v Fig. 4 Examples of boron-based ETLs and HTLs. iix Fig. 5 Cross Section of a typical OLED x Fig. 6 Charge “hop” mechanism x Fig. 7 Examples of the physical basis of OLED operation xii Fig. 8 Methods of energy transfer xiv Fig. 9 OLED operating mechanism xvi Fig. 10 Synthetic methods for BODIPY xiix Fig. 11 Examples of 2-pyridyl adducts and BORAZAN xx Fig. 12 Examples of quinolato-type adducts xxi Fig. 13 Examples of three-coordinate organoboron compounds xxii Fig. 14 Examples of benzoboroxole adducts xxv Fig. 15 Emission comparison xxxii Fig. 16 Overlay of the emission spectra of the azole adducts xxxiii Fig. 17 Stoke’s shift comparison xxxvi Fig. 18 Nodal plane theory diagram xxxiix Fig. 19 MS diagram xliii Fig. 20 Crystal structure of BF2(1,2-HNBT) xliii Fig. 21 Crystal packing unit cell of BF2(1,2-HNBT) xlv Fig. 22 Molecular orbital plots of BF2(HPBO) and BF2(HPBT) xlvii vi Fig. 23 Molecular orbital plots of BF2(1,2-HNBO) and BF2(1,2-HNBT) xlvii Fig. 24 Molecular orbital plots of BF2(2,3-HNBO) and BF2(2,3-HNBT) xlviii Fig. 25 Calculated band gap energies liii Fig. 26 VMOdes plots liv Fig. 27 Possible substitution points on the azole and phenyl scaffold lx Fig. 28 Calculated excitation spectrum of EDG substituted azole derivatives lxi Fig. 29 VMOdes and molecular orbital plot of BF2(HPBI) lxv Fig. 30 Calculated excitation spectrum of anthracene derivatives lxviii Fig. 31-82 Raw NMR data lxxxvi Fig. 83-87 Raw HR-MS data cxii Fig. 88-94 Overlay of experimental excitation and emission data cxv Fig. 93-99 Experimental excitation versus calculated excitation spectrum cxviii vii List of Abbreviations LET LINEAR ENERGY TRANSFER BNCT BORON NEUTRON CAPTURE THERAPY ELF ELECTRON LOCALIZATION FUNCTION OLEDS ORGANIC LIGHT EMITTING DIODES LCD LIQUID CRYSTAL DISPLAY ITO INDIUM TIN OXIDE EML EMISSIVE MATERIALS LAYER ETL ELECTRON TRANSPORT LAYER HTL HOLE TRANSPORT LAYER EWG ELECTRON WITHDRAWING GROUP EDG ELECTRON DONATING GROUP SMOLEDS SMALL MOLECULE ORGANIC LIGHT EMITTING DIODE PHOLEDS PHOSPHORESCENT ORGANIC LIGHT EMITTING DIODE LUMO LOWEST UNOCCUPIED MOLECULAR ORBITAL HOMO HIGHEST OCCUPIED MOLECULAR ORBITAL BODIPY BORON-DI-PYRROMETHENE BORAZAN LITERATURE TRADEMARK ALQ3 ALUMINUM-TRISQUINOLATO DDQ 2,3-DICHLORO-5,6-DICYANO-1,4-BENZOQUINONE PLEDS POLYMERIC LIGHT EMITTING DIODE IR INFRARED DCM DICHLOROMETHANE DMSO DIMETHYL SULFOXIDE DMF N,N-DIMETHYL FORMAMIDE THF TETRAHYDROFURANS APCI ATMOSPHERIC PRESSURE CHEMICAL IONIZATION HR-MS HIGH-RESOLUTION MASS SPECTROSCOPY UV-VIS ULTRA VIOLET-VISIBLE UV ULTRA VIOLET TD-DFT TIME DEPENDENT- DENSITY FUNCTIONAL THEORY viii QY QUANTUM YIELD ESIPT EXCITED STATE INTRAMOLECULAR PROTON TRANSFER PET PHOTOINDUCED ELECTRON TRANSFER NMR NUCLEAR MAGNETIC RESONANCE PPM PARTS PER MILLION PCM POLARIZABLE CONTINUUM MODEL VMODES VIRTUAL MOLECULAR ORBITAL DESCRIPTION PROGRAM FMO FRONTIER MOLECULAR ORBITAL DRE DEWAR RESONANCE ENERGY HBQ BENZOHYDROXY QUINOLINE HQ HYDROXY QUINOLINE HPBO HYDROXYPHENYL BENZOXAZOLE HPBT HYDROXYPHENYL BENZOTHIAZOLE HPBI HYDROXYPHENYL BENZIMIDAZOLE HNBO HYDROXYNAPHTHYL BENZOXAZOLE HNBT HYDROXYNAPHTHYL BENZOTHIAZOLE HNBI HYDROXYNAPHTHYL BENZIMIDAZOLE DPA DIPHENYL ANTHRACENE HABO HYDROXYANTHRYL BENZOXAZOLE HABT HYDROXYANTHRYL BENZOTHIAZOLE ix Chapter 1 Boron Chemistry and its Applications The sundry uses of Boron have made this metalloid a popular topic of study since its isolation in 1808 by Gay-Lussac and Thenard. It was finally recognized as an element in 1924 by Jöns Jakob Berzelius.1 The history of boron dates back as far as the sixteenth century, then called tinkal in Arabia, where it was used to assist melting processes. One of the interesting features of boron compounds is their lack of an octet in many species as well as their Lewis acidity. The first major contribution to the chemistry of the boron-nitrogen bond came in 1926 when Stock and Pohland reacted diborane with ammonia, forming borazine, known colloquially as the “inorganic benzene.”1 Boron is by no means a major constituent in the earth’s crust, accounting for a mere 3 parts per million. This is astonishingly low given the variety of uses for which boron is requisite, especially the glass industry. When obtained as a crude ore, boron is most often found in the form of tourmaline, which is ~10% boron within aluminosilicate.2 The vast majority of boron deposits are in Turkey, while much of the remainder is in California in the United States. Boron has features that are similar to both of its neighbors, carbon and the metalloids, specifically silicon. The unique feature of boron is that, though it maintains some characteristics with its neighbors, it is chemically distinct. For example, group 13 elements, except boron, are all easily ionizable, allowing their cations to play an important role in their chemistry. The energy of ionization is too high to play a role in boron chemistry. This allows researchers to use boron in many applications both organic and inorganic. Boron is known to exist in at least five allotropes, illustrating the similarity to carbon.3 1 Part of the unique nature of boron lies in its valency prediction. Monovalent boron would have a ground state with only one unpaired electron in the p-orbital. However, the promotion of the s2p1 ground state to the hybridized s1p2 state is energetically small. The promotion requires much less energy compared to the amount of energy that is released by the formation of three covalent bonds, this is the reason why monovalent boron is not able to be isolated, except at high temperatures. The three singly-occupied orbitals in the sp2 state account for the formation of trigonal planar covalent boron complexes.