Modeling the stacking in Thiophene Oligomers

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Modeling the ?-stacking in Thiophene Oligomers

• Damian Scherlis, Nicola Marzari

• OUTLINE
• Overview of molecular actuators
• The ?-stacking in oligothiophenes confrontation
  of methods
• Solvent effects
• The calixarene-thiophene complex

• METHODOLOGY
• First principles approaches (DFT, HF, MP2,
  CASSCF)
• Modeling the solvent QM-MM and continuum methods

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Conducting polymers as actuators
Polypyrrole
Polyaniline
Polythiophene
Mammalian Skeletal Muscle
Polypyrrole
Stress 0.35 MPa
40 MPa Strain
20 2 - 7 Max Power 50-100
Wkg-1 40 Wkg-1
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Actuators in Ionic Liquids
Strain-generating mechanism cation
incorporation/expulsion
0.8 V contracted
-0.4 V expanded
L. Wu et al. Science 297, 983. August 2002.
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p-STACKING
T T ? T22
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Thiophene Oxidation - Dimerization
Quaterthiophene p-stacking

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Calix4arene
Highly versatile building block in molecular
design
quaterthiophene
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Calix-4-arene quaterthiophene complex
quaterthiophene
calix-4-arene
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ACTUATION MECHANISM
oxidation
reduction
Calix-4-arene quaterthiophene polymer
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Density Functional Theory (DFT)
kinetic energy of electrons
Exchange-correlation energy
electron-electron
nuclei-electrons
?
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Local Density Approximation (LDA) homogeneous
electron gas
Exchange-correlation Functionals
Generalized Gradients Approximation (GGA) BP86,
PW91, BLYP, PBE
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Charged quaterthiophene dimer Energy as a
function of the distance
electrostatic
?
covalent
Van der Waals
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Higher accuracy in the description of the
electronic correlation
Perturbation Theory MP2
Taylor expansion of En and ?n
EMP2 E0(2)
EHF
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Multiconfiguration methods Complete Active Space
SCF (CASSCF)
?i orbitals ACTIVE and INACTIVE
bi and ci are optimized self consistenly
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Dithiophene Dimer
CAS(10,8) 10 electrons and 8 orbitals
HOMO-4 HOMO-3
HOMO-2 HOMO-1
HOMO LUMO
LUMO1 LUMO2
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DITHIOPHENE DIMER
Comparison of methods
distance
Gaussian basis sets Basis Set Superposition Error
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SOLVENT EFFECTS
Hybrid QM-MM Scheme
Solute Quantum Mechanics description (electronic
structure)
Solvent Molecular Mechanics treatment
ETOT EQM EMM EQM-MM
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ETOT EQM EMM EQM-MM
Coupling Energy
Interaction between point charges and electronic
density
Interaction between point charges and nuclei
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Conducting polymers as actuators
Polypyrrole
Polyaniline
Polythiophene
Mammalian Skeletal Muscle
Polypyrrole
Stress 0.35 MPa
40 MPa Strain
20 2 - 7 Max Power 50-100
Wkg-1 40 Wkg-1
19
Actuators in Ionic Liquids
Strain-generating mechanism cation
incorporation/expulsion
0.8 V contracted
-0.4 V expanded
L. Wu et al. Science 297, 983. August 2002.
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p-STACKING
T T ? T22
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Thiophene Oxidation - Dimerization
Quaterthiophene p-stacking

2

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ETOT EQM EMM EQM-MM
Coupling Energy
Interaction between point charges and electronic
density
Interaction between point charges and nuclei
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Solvated Dimer
Solvation cage of 220 water molecules
DFT / QM-MM curve relaxing the solvent
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MOLECULAR DYNAMICS at 350 K
Solvation cage of 220 water molecules
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Polarizable Continuum Model IPCM
Solvent infinite, continuous dielectric
Charge density and reaction field adjust to each
other self consistently
HM H0M Vint(?)
The molecule is surrounded by an isodensity
surface, which is split in surface elements
containing point charges
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IPCM binding energies in cloromethane
dithiophene
terthiophene
p-stacks stabilized by the solvent
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MIXED VALENCE SINGLY CHARGED DIMER
T T0 ? T2


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T T0 ? T2
Optimization of quaterthiophene dimer
Optimization of monothiophene dimer
4.0 Å
DEPW91 -23 kcal/mol
DEPW91 -14.6 kcal/mol
DEMP2 -23 kcal/mol
DEPW91(dcm) -3.6 kcal/mol
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Attention! DFT shows good agreement with MP2, but
delocalizes the charge, regardless of the
separation unreal description of the singly
charged dimer
MP2
DFT
1/2

1/2
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Car-Parrinello Molecular Dynamics on the
calixarene-quaterthiophene complex
Conventional DFT
the electronic problem is solved SC at every
ionic configuration, compute forces acting on
ions and solve Newton Eq of motion
Car-Parrinello DFT
Ionic trajectories and electronic ground state
are generated at the same time, without solving
SC the KS at each MD step
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Oxidation of the Calixarene polymer the change
in the electronic density
? difference between the neutral and the charged
species shows that oxidation mostly affects the
quaterthiophene
?neutral - ?chgd
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Summary

• LDA shows important overbinding, while GGA is in
  good agreement with higher level QC methods
• No binding for the doubly charged
• Solvent stabilizes the ?-stacks. Longer
  oligomers are more bound
• Singly charged dimer appears to be a bound
  species in vacuum, and partially destabilized by
  the solvent