Electrochemistry of Phthalocyanines

Transcription

1 Electrochemistry of Phthalocyanines

2 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)

3 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?

4 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

5 Redox properties of phthalocyanines Reduction/oxidation may occur in two locations Pc ring itself Central metal Ring oxidations (2) Pc 2- Pc - or Pc 0 Ring reductions (4) Pc 2- Pc 3-,Pc 4-, Pc 5-, Pc 6-

6 Redox properties of phthalocyanines 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.

7 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.

8 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?

9 Voltammetry and Spectroelectrochemistry Voltammetry - measures current as potential is varied. Spectroelectrochemistry spectra of sample during in situ reduction/ oxidation at set potentials

10 First reduction (-0.7 V) Second reduction (-1.00 V) First oxidation (0.8 V) CV and SEC results for an H 2 Pc Koca A, Özkaya AR, Selçukoğlu M, Hamuryudan E (2007) Electrochim Acta 52(7):

11 Red 1 Red 2 CV and SEC results for an CoPc Red 1: [Co II Pc 2- ] to [Co I Pc -2 ] -1 Red 2: [Co I Pc -2 ] -1 to [Co I Pc -3 ] -2 Ox 1 Ox 1: [Co II Pc 2- ] to [Co III Pc -2 ] +1 Arican D, Arici M, Ugur AL, Erdogmus A, Koca A (2013). Electrochim Acta 106:

12 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

13 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

14 Modifying an electrode Chemically Covalent bonding, click chemistry Chemisorption self assembled monolayers Composite Polymer film coating/ Electropolymerisation Physically Drop dry / dip dry

15 Cyclic Voltammetry CoTPP-rGONs CoTPP-CoNiNP-rGONs Hydrazine oxidation: N 2 H 4 N 2 + H + + 4e - Modified Glassy Carbon Electrodes (GCEs) CoTPP 1 mm Hydrazine in 0.1 M NaOH

16 Acknowledgements and Resources Prof Dr Khene Munya, Charlie, Xoli Book by Zagla and Bedoui - Electrochemistry of N4 macrocycles Chapters by Prof and A. Koca