Noosa 2006
Emergent Macroscopic Properties from Molecular Disorder in Functional Bio-
macromolecules
Paul MeredithSoft Condensed Matter Physics Group
Noosa 2006
Theme: The Structure-Property Jigsaw Puzzle
• Establishment of the Structure-Property Relationships for Bio-macromolecules (proteins, etc.)
• Focusing on the human pigmentary system - melanins
Structure:
Properties:
Primary (Å) Secondary (nms) Tertiary (10snms) Aggregates (s)
QM QM/Classical Classical Classical
Molecular:-relaxed geometries-electronic structure-energies-phonon structure
Mesoscopics:-electron-phonon interactions-free spin dynamics-electron delocalisation-weak dispersive interactions
Macroscopic Observables:-optical-electrical-photochemical-chemical
Noosa 2006
Melanin Background
• Functional bio-macromolecule found throughout nature:– Photoprotectant– Pigment
• Several types– Eumelanin (predominant in humans)– Pheomelanin– Neuromelanins– Allomelanins
• Exotic physico-chemical properties– Broad band monotonic absorption in the UV & visible (black)– Condensed phase electrical and photoconductivity– Efficient non-radiative relaxation of photoexcitations
Noosa 2006
Eumelanin
• Macromolecules of indolequinones
• The important questions:– What is the correct secondary structure?– What are the appropriate mesoscopic models linking this
structure to the macroscopic properties?– Are melanins quantum materials – how do their macroscopic
properties emerge and at what length scales are quantum effects important?
– What are the broader implications fro understanding structure-property relationships in other important bio-macromoleules?
• Molecular engineering of synthetic analogues with the appropriate properties as functional materials
Noosa 2006
The S-P Jigsaw Puzzle for Eumelanin (molecular properties)
S0
S1
S2
S3
hf = E1-E0=LUMO-HOMO
Noosa 2006
The S-P Jigsaw Puzzle for Eumelanin (mesoscopics)
EF
EV
EC
E
N(E)
1.40eV
0.78eV
0.2eV
{From DC conductivity Measurements}
plane
Overlapping pz
forming and *molecular orbitals
Noosa 2006
The S-P Jigsaw Puzzle for Eumelanin (macroscopic observables)
1.0 1.5 2.0 2.5 3.0 3.5
1x105
2x105
3x105
4x105
5x105
1.70
(m
--1 )
Energy (eV)
Solid state optical gap emerging at 1.7eV
Solution absorption, broad &monotonic
Noosa 2006
The S-P Jigsaw Puzzle for Eumelanin (macroscopic observables)
1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4
0.0
2.0x105
4.0x105
6.0x105
8.0x105
1.0x106
1.2x106
Inte
nsity
(cps
)
Energy (eV)
Radiative emission – weak and broad, and Excitation energy dependent
Decreasing Eex
Emission and Excitation
Noosa 2006
Current Paradigm
Extended indoleheteropolymer
Delocalised system
Amorphous SemiconductorBand structure
= Broad absorptionConductivitySolid state gap
1.0 1.5 2.0 2.5 3.0 3.5
1x105
2x105
3x105
4x105
5x105
1.70
(m
--1 )
Energy (eV)
☺
☺
1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4
0.0
2.0x105
4.0x105
6.0x105
8.0x105
1.0x106
1.2x106
Inte
nsi
ty (
cps)
Energy (eV)
X
X
Noosa 2006
Many Body Calculations
• Relaxed geometry, energy and electronic and phonon structure calculations using NRLMOL (DFT calculations using Gaussian Orbital methods - Generalised Gradient Approximation using the Perdew-Burke-Ernzerhof exchange correlation function)
• Predict the HOMO-LUMO gap for a range of monomers and small oligomers
NH
HO
HO NH
O
O
HOMO-LUMO: 3.61eV 2.02eV
Noosa 2006
Broad Monotonic Absorption
Only 11 inhomogeneously broadened transitions with typical polar solvent line widths
HQ QIDDMD
1. Chemical disorder – macroscopic properties are the ensemble average of a number of chemically distinct species2. Consistent with emission and excitation evidence – selective pumping of a part of the ensemble3. “Low cost” strategy for achieving robust functionality
Noosa 2006
Summary & Conclusions
• Key objective: to establish S-P relationships, correct secondary structural model required
• Current theory (conjugated heteropolymer leading to amorphous semiconductivity) not consistent with electrical and optical data
• Subtle changes at the primary and secondary level create significant differences in electronic structure (HOMO-LUMO gap)
• Broad absorption (and all other properties) could be explained through chemical disorder with a small number of chemically distinct species
• Chemical disorder = low cost resource?
Noosa 2006
Next Steps
• Experimental confirmation of oligomer predictions– HOMO-LUMO gaps (absorption edge)– Phonon spectra (INSS)– Relaxed geometry (Xtalography)
• Calculations– Phonon spectra– Electron densities– Oscillator strengths– Electron-phonon coupling strength
Noosa 2006
Acknowledgements
SynthesisEvan Moore (Berkley)Kirsten LaurieRoss McGearySurya Subianto (QUT)Indu MadehavanPaul Burn (Oxford)
DevicesAdam Micolich (UNSW)David Blake
Biological FunctionStephen Nighswander-RempelPeter Parsons (QIMR)
Optical & StructuralJenny RieszBen PowellRoss McKenzieJose Eduardo de Albuquerque (Viscosa)Stephen Nighswander-RempelTad Sarna (Jagiellonian)John Tomkinson (RAL)
TransportClare GiacomantonioBen PowellAdam Micolich (UNSW)Andrew Watt (Oxford)Francis Pratt (RAL)
Quantum Chemistry, Computation & TheoryBen PowellMark Pederson, Tunna Baurah (NRL Washington)Ross McKenzie $$$ARC & UQ
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