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The Synthesis of Molecular Switches

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The Synthesis of Molecular Switches ! ! " ! # $% & ' ()) *&& +,-+./(-, *& /0./+.0-10-.--.) % 23 33 ' 4 5' &6 Abstract According to the famous axiom known as Moore’s Law the number of transistors that can be etched on a given piece of silicon, and therefore the computing power, will double every 18 to 24 months. For the last 40 years Moore’s prediction has held true as computers have grown more and more powerful. However, around 2020 hardware manufac- turers will have reached the physical limits of silicon. A proposed so- lution to this dilemma is molecular electronics. Within this field re- searchers are attempting to develop individual organic molecules and metal complexes that can act as molecular equivalents of electronic components such as diodes, transistors and capacitors. By utilizing molecular electronics to construct the next generation of computers processors with 100,000 times as many components on the same sur- face area could potentially be created. We have synthesized a range of new pyridyl thienopyridine ligands and compared the electrochemical and photophysical properties of their corresponding Ru(II) complexes with that with the Ru(II) com- plexes of a variety of ligands based on 6-thiophen-2-yl-2,2´-bipyridine and 4-thiophen-2-yl-2,2´-bipyridine. While the electrochemistry of the 2+ Ru(II) complexes were similar to that of unsubstituted [Ru(bpy)3] , substantial differences in luminescence lifetimes were found. Our findings show that, due to steric interactions with the auxiliary bipy- ridyl ligands, luminescence is quenched in Ru(II) complexes that in- corporate the 6-thiophen-2-yl-2,2´-bipyridine motif, while it is on par 2+ with the luminescence of [Ru(bpy)3] in the Ru(II) complexes of the pyridyl thienopyridine ligands. The luminescence of the Ru(II) com- plexes based on the 4-thiophen-2-yl-2,2´-bipyridine motif was en- 2+ hanced compared to [Ru(bpy)3] which indicates that complexes of this category are the most favourable for energy/electron-transfer sys- tems. At the core of molecular electronics are the search for molecular ON/OFF switches. We have synthesized a reversible double cyclome- tallated switch based on the Ru(tpy) complex of 3,8-bis-(6-thiophen- 2-yl-pyridin-2-yl)-[4,7]phenanthroline. Upon treatment with acid/base the complex can be switched between the cyclometallated and the S- bonded form. This prototype has potentially three different states which opens the path to systems based on ternary computer logic. Sammanfattning – Swedish abstract Enligt den berömda Moores Lag så kommer det antal transistorer som kan placeras på en bit kisel att dubbleras var 18:e till 24:e månad, d.v.s. processorkapaciteten kommer att dubbleras var 18:e till 24:e månad. De senaste 40 åren har detta påstående varit sant och datorerna har blivit allt kraftfullare. Runt 2020 kommer dock Moores Lag att utsättas för svåra prövningar. Processortillverkarna börjar helt enkelt att nå den fysiska gränsen för hur små transistorer som kan tillverkas av kisel. lösning på detta problem är att gå över till s.k. molekylär- elektronik. Genom att utveckla enskilda molekyler och metallkomplex som kan fungera som dioder, kondensatorer och transistorer kan ut- vecklingen mot allt snabbare och kraftfullare datorer fortsätta. Vi har syntetiserat en ny grupp av pyridyl tienopyridiner och kom- plexbundit dem med ruthenium. De fotofysiska och elektrokemiska egenskaperna för dessa komplex har sedan jämförts med ruthenium- komplex med ligander baserade på 6-tiofen-2-yl-2,2´-bipyridin och 4- tiofen-2-yl-2,2´-bipyridin. De elektrokemiska resultaten visade inga 2+ stora skillnader mellan dessa Ru-komplex och [Ru(bpy)3] . De foto- fysiska mätningarna visade stor skillnad i luminiscenslivslängd. I de komplex vilkas ligander baserades på 6-tiofen-2-yl-2,2´-bipyridin var livslängden för kort för att uppmätas (under 30 ns), vilket förmodligen bero på steriska interaktioner med de övriga koordinerade bipyridylli- 2+ ganderna, medan den hade jämförbar livslängd med [Ru(bpy)3] i de komplex som hade pyridyl tienopyridiner som ligander. De komplex vars ligander var baserade på 4-tiofen-2-yl-2,2´-bipyridin uppvisade 2+ längre luminiscenslivslängd än [Ru(bpy)3] . Detta indikerar att kom- plex av denna typ bör vara den mest användbara för komplex som ska ingå i system där elektroner eller energi ska överföras mellan olika punkter. En mycket viktig typ av föreningar inom molekylärelektroniken är de som kan fungera som ”strömbrytare”, föreningar som beroende på extern stimulans kan växla mellan två eller flera lägen. Vi har synteti- serat en ny reversibel dubbelt cyklometallerad brytare baserad på rutheniumterpyridinkomplexet av föreningen 3,8-bis-(6-tiofen-2-yl- pyridin-2-yl)-[4,7]fenantrolin. Genom behandling med syra/bas kan detta komplex fås att växla mellan den cyklometallerade formen till den S-bundna formen och tillbaka igen. Denna prototyp har potentiellt tre olika lägen, vilket skulle möjliggöra trinär datalogik. Publications This thesis is based on the following publications: I: Synthesis of the Fused Heterobicycles, 5-pyridin-2-yl- thieno[3,2-b]pyridine, 6-pyridin-2-yl-thieno[2,3-b]pyridine and 6-pyridin-2-yl-thieno[3,2-c]pyridine; L. J. Nurkkala; R. O. Steen; S. J. Dunne; Synthesis, 2006 (8) 1295-1300. II: The Role of Isomeric Effects on the Fluorescence Lifetimes and Electrochemistry of Oligothienyl-bridged Binuclear Ruthe- nium(II) Tris-Bipyridine Complexes; R. O. Steen, L. J. Nurkka- la, S. J. Angus-Dunne, C. X. Schmitt, E. C. Constable, M. J. Ri- ley, P. V. Bernhardt, S. J. Dunne; Eur. J. Inorg. Chem.; submit- ted August 2007. The contributions of the author of this thesis to these papers are: I: Major part of the experiments and major part of writing. II: Half of the experiments and part of writing. "The object of life is not to be on the side of the majority, but to escape finding oneself in the ranks of the insane." -Marcus Aurelius, Roman Emperor 161-180 A.D "Chemists are a strange class of mortals, impelled by an almost maniacal impulse to seek their pleasures amongst smoke and vapour, soot and flames, poisons and poverty, yet amongst all these evils I seem to live so sweetly that I would rather die than change places with the King of Persia.” -Johann Joachim Becher, German chemist/alchemist, 1635-1682 “A tidy laboratory means a lazy chemist.” -Jöns Jacob Berzelius, Swedish chemist, 1779-1848 Contents Introduction: The Path to Molecular Electronics .................................... 10 1.0 The integrated circuit.......................................................................... 10 1.2 On Moore’s Law ................................................................................ 13 1.3 Molecular Electronics......................................................................... 18 1.4 Advantages and disadvantages of organic semiconductors ................ 22 1.5 On the philosophy of things very small .............................................. 25 Background Theory .................................................................................... 28 2.0 Conductivity ....................................................................................... 28 2.1 Inorganic semiconductors .............................................................. 28 2.2 Organic semiconductors ................................................................ 31 3.0 Poly- and Oligothiophenes ................................................................. 33 3.1 Synthesis ........................................................................................ 34 3.2 Conductivity .................................................................................. 35 3.3 Applications ................................................................................... 36 4.0 Ruthenium(II) polypyridyl complexes ............................................... 38 4.1 Photophysical properties ................................................................ 39 4.2 Redox properties ............................................................................ 43 4.3 Charge transfer............................................................................... 44 5.0 Cyclometallation ................................................................................ 46 5.1 Cyclometallated Ru(II) complexes ................................................ 48 Results and discussion ................................................................................ 53 6.0 General methods and mechanisms ..................................................... 53 6.0.1 Stille cross-coupling ................................................................... 53 6.0.2 Negishi cross-coupling ............................................................... 55 6.0.3 Kröhnke pyridine synthesis ........................................................ 56 6.0.4 Beckmann rearrangement ........................................................... 57 6.0.5 Vilsmeier-Haack reaction ........................................................... 59 6.0.6 Skraup synthesis ......................................................................... 60 6.1 Synthesis of three new thienobipyridyls ............................................ 63 6.1.1 Synthesis of 6-pyridin-2-yl-thieno[3,2-b]pyridine (1) ................ 65 6.1.2 Synthesis of 5-pyridin-2-yl-thieno[2,3-b]pyridine (2) ................ 66 6.1.3 Synthesis of 6-pyridin-2-yl-thieno[3,2-c]pyridine (3) ................ 71 6.2 The effect of bridge-ligand conformation upon metal-metal coupling in oligothiophene-bridged
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