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

DHIDHICA

Eumelanin

R = -H or -COOH

Noosa 2006

The S-P Jigsaw Puzzle for Eumelanin (structure)

OR

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

Many Body Calculations

LUMO

HOMO

Noosa 2006

Many Body Calculations - Oligomers

3.84eV 3.81eV

3.64eV 3.10eV

2.22eV 2.13eV

Noosa 2006

Many Body Calculations - Oligomers

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