Molecules and 2D Materials€¦ · Molecular Charge-Transfer Complexes • TTF is donor • TCNQ is...
Transcript of Molecules and 2D Materials€¦ · Molecular Charge-Transfer Complexes • TTF is donor • TCNQ is...
Molecules and 2D Materials
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Advantages
• Well-defined small molecules• Can be prepared with high purity
• Can be deposited in vapor phase or by solution
• Small molecules are crystalline, easy to organize
• Engineer solid-state molecular structure• Model gas-phase structure
Pentacene
• Highly Crystalline• Well defined arrangement
• Crystal structure facilitates movement of is important for electronic properties
AFM image from Vecco Inc.
Brédas J L et al. PNAS 2002;99:5804-5809
Substituted Pentacenes
• Substitution changes packing arrangement and leads to different solid-state properties
• Affects the pi-overlap in the materials
• May also affect the orientation
“TIPS” Pentacene
• Developed by J. Anthony (U Kentucky)
• Good solubility properties
• Desirable solid state organization
• Maximizes pi-overlap
• Excellent material– Processable
– Simple to synthesize
– Has desirable properties
Oligomers: Structure-Property Relationships
• HOMO-LUMO level narrows as conjugation length is increased
• Absorption spectroscopy: can estimate the HOMO-LUMO gap
• Also interested in their positions
E
The Convergent Synthesis Approach
• Convergent synthesis of oligomers
• Note the complementary chemistry– In (i) iodine couples
with vinyl group (Heck reaction)
– In (ii) Aldehyde couples with C-P nucleophile (Wittig reaction)
– Functional groups are tolerated under reaction conditions
Todd Maddux, Wenjie Li, and Luping Yu. Am. Chem. Soc., 1997, 119 (4), pp 844–845
Optical Absorption Properties
• Red shift is observed as conjugation length is increased (bathochromic effect)
• Consistent with narrowing the HOMO-LUMO gap
• Saturation occurs after ~8 repeat units
• Extinction coefficient increases (hyperchromic effect)
6 = four repeat units8 = eight repeat units10 = twelve repeat units
Optical and Materials Properties
6 7 8 9 10
Tm(°C) 97 67 90 113 105
Tc(°C)a 87 158 176 185
Φfl(%)b 75 87 81 89 82
λmax(nm) 431 441 457 460 463
• Optical properties saturate at ~10 repeat units• Liquid crystal: properties of liquid (flow); properties of solid
crystal (order)
• Tc is the transition temperature from liquid crystal to isotropic phase
• Φfl(%) is the quantum yield of fluorescence
Sexithiophene
• Herring-bone packing structure
• Herringbone angle 63 deg
• Preferentially organize with long-axis toward surface
Oligofurans
• More tightly packed than sexithiophene (in parentheses)• Herringbone angle 58 deg• Pentacene packs in a similar manner (55 deg herringbone angle)
Characterization Data
Compound εmaxa (M
−1 cm−1) λabsa (nm) λflua (nm) Φfa,b
HOMOe (eV)
3F 28700f 331 352, 371f 0.78f (0.07)
−4.91
4F 28600f 364 391, 413 0.80 (0.18)
−4.73
5F 51000 388 421, 449 0.74 (0.36)
−4.62
6F 53000 404 442, 472 0.69 (0.41)
−4.55
7F 56000 417 455, 485 0.67 −4.51
8F 56000 423 467, 499 0.66 −4.48
9F 430 473, 507 0.58 −4.45
Tetrathiofulvalene (TTF)
• Investigated by Wudl in the 1970s• Neutral compound is non-aromatic • Oxidation forms aromatic species• Count pi-electrons and remember Huckle’s rule!• Donor material – High lying HOMO level relative to
vacuum (easily oxidized)
Bendikov, Wudl, and Perepichka Chem. Rev., 2004, 104 (11), pp 4891–4946
Molecular Charge-Transfer Complexes• TTF is donor• TCNQ is acceptor
• Forms highly organized co-crystals
• TTF is partly oxidized; TCNQ partly reduced
Denis Jérome Chem. Rev., 2004, 104 (11), pp 5565–5592
Charge-Transfer Salts
• Electrons and holes are permanently separated
• Electrons reside in TCNQ domain; holes in TTF domain
Electronic Properties
• Individual crystals act like wide band-gap semiconductors
• Combined materials behave like metals
Helena Alves, Anna S. Molinari, Hangxing Xie & Alberto F. Morpurgo Nature Materials 7, 574 - 580 (2008)
Carbon Nanotubes
http://upload.wikimedia.org/wikipedia/commons/archive/5/53/20090124143629!Types_of_Carbon_Nanotubes.png
Carbon Nanotubes
R H Baughman et al. Science 2002;297:787-792
Sorting Carbon Nanotubes
Hersam and coworkers, Nature Nanotech. 2006, 60
Graphene
http://www.nanowerk.com/spotlight/spotid=2340.php
The Chemistry of Graphene
• Ideal graphene is 2D sheet
• Real graphene has may defects and reactive sites – A: Pi-system – B: Zig-zag/Armchair
edges – C: Monovacancy – D: Curve
Weiss and Zhao, Chem. Soc. Rev. 2012, 97-‐117
Synthesis of 2D Materials
• Well-defined 2-D interactions
• Conjugation through core?
• Note the meta linkage
Alexander L. Kanibolotsky,†‡ Rory Berridge,† Peter J. Skabara,*† Igor F. Perepichka,*§ Donal D. C. Bradley, and Mattijs Koeberg J. Am. Chem. Soc., 2004, 126 (42), pp 13695–13702
2D Materials
2D Polymers
Molecular Arrangement
Exfoliated Sheets
Acene Photochemistry
Another 2D Polymer
Discovery of Fullerene
• 1996 Nobel Prize in Chemistry to Curl Jr., Kroto, and Smalley • Huge impact as a research tool • Applications in electronics, energy, medicine are being
developed
Fullerene as a 3D Electronic Material
• Electron deficient• Strong pi-pi interactions
• Leads to clusters• Lots of derivatives
http://www.godunov.com/bucky/fullerene.html