electrochemistry of phthalocyanines - rhodes university · electrochemistry a physical measure of...

Electrochemistry of Phthalocyanines

Electrochemistry A physical measure of electron movement in chemical reactions Why is this important? ESSENTIAL PROCESS! Energy storage, chemical reactions,

manufacturing, neurobiology, telecommunications ….. Try imagining a world without it!

What does it involve? Movement of charged particles (ions and electrons) across an

interface Interface usually involves two phases of matter – an electrode

(solid) and an electrolyte (conductive solution)

Moving electrons between chemical species…Redox

REDOX = Reduction + Oxidation Oxidation is LOSS, Reduction is GAIN (OIL RIG)

Some important questions electron sink or electron source? where/ how are the electrons moving how much energy is needed to make electrons move? how much energy do they produce when electrons move?

Phthalocyanines • Heteroatomic macrocycle with 18 π electrons • Pc is a dianionic ligand with an oxidation state charge of 2-

Electrochemistry of phthalocyanines – two areas of interest Reduction/ Oxidation activity of the

Pc itself Using the Pc’s ability to transfer

electrons for electrocatalysis

Redox properties of phthalocyanines Reduction/oxidation may occur in two locations Pc ring itself Central metal

Ring oxidations (2)

Pc2- → Pc- or Pc0

Ring reductions (4) Pc2- → Pc 3- ,Pc4- , Pc5- , Pc6-

Can class Pcs into groups Metal free Pcs MPcs with redox inactive metals (includes Zn(II), Ni(II), Cu(II) and Pb(II)) MPcs with redox active metals (Ag, Co, Cr, Ir, Fe, Mn, Mo, Os, Re, Rh, Ru,Ti) MPcs with redox active active substituents Sandwich complexes…

Redox active metal centres If there are empty/full metal d orbitals between the HOMO – LUMO orbitals of the PC ring, the metal ions can reduce/ oxidize before the Pc ring.

Redox properties of phthalocyanines

Relating redox properties to spectroscopy In main group metal and metal free Pcs, first ring oxidation and first

ring reduction energy approximately = HOMO – LUMO gap energy. So… we can relate redox activity to frontier molecular orbitals and

therefore ….to UV-Vis spectra! Characteristic bands on UV-Vis spectra for ring oxidized and ring

reduced Pc complexes. This includes the appearance of new bands.

Relating redox properties to spectroscopy

Redox processes and potentials affected by: Type and position of

substituents Electrolyte media Axial ligands Extent of π conjugation

A small HOMO – LUMO gap means redox reactions at smaller potentials

Techniques for looking at this…?

Voltammetry and Spectroelectrochemistry

Voltammetry - measures current as potential is varied.

Spectroelectrochemistry – spectra of sample during in situ reduction/ oxidation at set potentials

First reduction (-0.7 V)

Second reduction (-1.00 V)

First oxidation (0.8 V)

CV and SEC results for an H2Pc Koca A, Özkaya AR, Selçukoğlu M, Hamuryudan E (2007) Electrochim Acta 52(7):2683–2690

Red 1

CV and SEC results for an CoPc Red 1: [CoIIPc2-] to [CoIPc-2]-1 Red 2: [CoIPc-2]-1 to [CoIPc-3]-2 Ox 1: [CoIIPc2-] to [CoIIIPc-2]+1 Arican D, Arici M, Ugur AL, Erdogmus A, Koca A (2013). Electrochim Acta 106: 541-555

Red 2

Ox 1

Trends in redox properties

• Redox couples seen on CV can be attributed to the Pc ring in free base or redox inactive species or to the ring and metal centre in redox active species.

• Electron withdrawing vs electron donating substituents – can shift redox potentials

• Position of substituents – affects electron density in frontal molecular orbitals

• Solvents …. Polar, non-polar, coordinating

Electrocatalysis

Catalysing oxidation/ reduction of an analyte Using redox properties of Pcs on a modified working

electrode Types of working electrodes Gold Glassy Carbon

Monitor changes in analyte redox properties

Modifying an electrode

Chemically Covalent bonding, click chemistry Chemisorption – self assembled monolayers Composite Polymer film coating/ Electropolymerisation

Physically Drop dry / dip dry

Cyclic Voltammetry

Modified Glassy Carbon Electrodes (GCEs) 1 mM Hydrazine in 0.1 M NaOH

CoTPP

CoTPP-CoNiNP-rGONs CoTPP-rGONs

N2H4 N2 + H+ + 4e- Hydrazine oxidation:

Acknowledgements and Resources

• Prof • Dr Khene • Munya, Charlie, Xoli

• Book by Zagla and Bedoui - Electrochemistry of N4 macrocycles

• Chapters by Prof and A. Koca