Molecules and 2D Materials

1

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