Analysis of Electrogenerated Chemiluminescence of PPV type Conducting Polymers

DISSERTATION

zur Erlangung des akademischen Grades
doctor rerum naturalium
(Dr. rer. nat.)
im Fach Chemie

eingereicht an der
Mathematisch-Naturwissenschaftlichen Fakultät I
der Humboldt-Universität zu Berlin

von
Umamaheswari Janakiraman

geb. am 18.11.1974 in Indien

Präsident der Humboldt-Universität zu Berlin

Prof. Dr. Jürgen Mlynek

Dekan der Mathematisch-Naturwissenschaftlichen Fakultät I
Prof. Dr. Michael Linscheid

Gutachter:
1. Prof. Dr. Erhard Kemnitz
2. Prof. Dr. Werner Abraham
3. Prof. Dr. Michael Linscheid

Tag der mündlichen Prüfung:20.05.2003

Dedicated to my parents

Table of contents

• 1.  Introduction
  • 1.1. Electrogenerated chemiluminescence
  • 1.2. Energetics of the ECL
  • 1.3. Analysis of the ECL reaction mechanism
    • 1.3.1. Distinguishing singlet route from triplet route
    • 1.3.2. Identification of other possible mechanisms for the production of an excited state
  • 1.4. Factors affecting the ECL and the precautionary measures
  • 1.5. Conducting polymers
  • 1.6. Electroluminescence
  • 1.7. Objectives of the Present Study
• 2.  Experimental Techniques and Procedures
  • 2.1. Electrochemical techniques
    • 2.1.1. Cyclic voltammetry
    • 2.1.2. Potential step experiments
      • 2.1.2.i. Electrode reaction kinetics
      • 2.1.2.ii.  Measurement of double layer parameters
      • 2.1.2.iii. Double layer charging and charge transfer
  • 2.2. Experimental procedures
  • 2.3. Optical methods
    • 2.3.1. Absorbance and fluorescence spectroscopy
    • 2.3.2. Raman spectroscopy
    • 2.3.3. Photomultiplier Tubes
  • 2.4. Polymer cross-linking by synchrotron radiation
• 3.  The Electrochemiluminescence Experiments
  • 3.1. Solution phase ECL: DPA
    • 3.1.1. Experimental details
    • 3.1.2. Cyclic voltammetry
    • 3.1.3. Current transients
    • 3.1.4.  The ECL experiment
    • 3.1.5. The ECL Emission spectrum
  • 3.2. Polymer phase ECL: MEH-PPV
    • 3.2.1. Experimental details
    • 3.2.2. Cyclic voltammogram
    • 3.2.3. Current transients
    • 3.2.4.  The ECL experiment
  • 3.3.  Polymer phase ECL: DB-PPV
    • 3.3.1. Experimental details
    • 3.3.2.  Cyclic voltammogram
    • 3.3.3. Current transients
    • 3.3.4. The ECL experiment
    • 3.3.5. Studies concerning the stability of DB-PPV
    • 3.3.6. Energetics of the ECL in DB-PPV
• 4.  Theoretical Analysis of the Kinetics of ECL
  • 4.1. Solution phase ECL - 9,10 diphenylanthracene (DPA)
    • 4.1.1. Kinetics of the ECL process in solution phase
    • 4.1.2. Double layer charging
    • 4.1.3. Digital simulation technique
    • 4.1.4. Conditions for simulation
    • 4.1.5. Inclusion of the IR drop in the electrochemical solution and double layer charging at the electrode/solution interface
    • 4.1.6. Simulation of the experimental ECL transient for DPA
  • 4.2. Polymer Phase ECL (MEH-PPV)
    • 4.2.1. Kinetics of the ECL process in the polymer phase
    • 4.2.2. Double layer charging
    • 4.2.3. Digital simulation technique
    • 4.2.4.  Conditions for simulation
    • 4.2.5. Inclusion of the IR drop in the conducting polymer and double layer charging at the electrode/polymer interface
    • 4.2.6. 4.2.6 Simulation of the experimental transients of MEH-PPV
  • 4.3.  Simulation of the experimental transients of DB-PPV
• 5.  Summary, conclusions and outlook
  • 5.1. Symmetrical ECL
  • 5.2. Stability of the polymers
  • 5.3. Influence of counterions
  • 5.4. Kinetics of the ECL process
  • 5.5. Mechanism of ECL in PPV type polymers
  • 5.6. Outlook
• Literature
• Abbreviations
• List of publications
• Acknowledgements
• Lebenslauf
• Selbständigkeitserklärung

Images

• Fig 1.2a: Scheme of potential energy surfaces for an electron transfer reaction.
• Fig. 1.2b: Molecular orbital representation of an electron transfer reaction.
• Fig. 1.5a: Configuration of the exciton in PPV.
• Fig. 1.5b: Scheme of the valence and conduction bands of polarons and bipolarons.
• Fig. 1.5c: Ground and excited states of PPV polymer.
• Fig. 1.6a: Construction of a light emitting diode (LED).
• Fig. 1.6b: Absorption, photoluminescence (PL) and electroluminescence (EL) spectra of PPV.
• Fig. 2.1a: Scheme for controlled potential experiments.
• Fig. 2.1.1a. Triangular waveform of the potential in potential sweep techniques.
• Fig. 2.1.2a: Scheme of a single positive potential step.
• Fig. 2.1.2.b: Equivalent circuit description of an electrochemical system in the absence of electrode reaction.
• Fig. 2.1.2.c: Equivalent circuit description of an electrochemical system in the presence of an electrochemical reaction.
• Fig. 2.2a: Scheme of the setup for the ECL experiment.
• Fig. 2.3.1a: Scheme of the interaction of radiation with energy levels of a substance.
• Fig. 2.3.3a: Cross section of a photomultiplier tube.
• 9,10 - diphenylanthracene (DPA).
• Fig. 3.1.2a: Cyclic voltammogram (CV) of Pt in 1mM DPA + 0.1M TBAClO4 in acetonitrile at a scan rate of 100 mV/s.
• Fig. 3.1.3a: Cottrell plot for 1 mM DPA in 0.1 M TBAClO4-AN when the potential step was made from 0 V to -1.95 V of duration 50 ms.
• Fig. 3.1.4a: Scheme of the sequential anodic and cathodic potential steps in ECL experiments.
• Fig. 3.1.4b: The potential step from Ef to Er was made from -1.9 V (Ef) to a positive (Er) value given in the inset. tf, tr = 0.05 s and tw = 0.2 s.
• Fig. 3.1.4c: The potential step was made from 1.55 V (Ef) to - 2.05 V (Er). tf, tr = 0.01 s and tw = 0.02 s. No ECL was observed when reversing the potential step.
• Fig. 3.1.4d: The potential step was made from 1.55 V (Ef) to -2.05 V (Er). tf, tr = 0.01 s and tw = 0.28 s.
• Fig. 3.1.4e: The potential step was made from - 2.1 V (Ef) to 1.6 V (Er). tf, tr = 0.05 s and tw = 0.20 s.
• Fig. 3.1.4f: The potential step was made from - 2.1 V (Ef) to 1.6 V (Er). tf, tr = 0.05 s and tw = 1.80 s.
• Fig. 3.1.4g: Anodic ECL when the potential step was made from - 2.05 V (Ef) to 1.6 V (Er). tf, tr = 0.05 s and tw = 0.24 s.
• Fig. 3.1.4h: Cathodic ECL under the similar condition as in Fig. 3.1.4g; potential step was from 1.6 V (Ef) to - 2.05 V (Er). tf, tr = 0.05 s and tw = 0.24 s.
• Fig. 3.1.4i: Anodic ECL for potential step from - 2.05 V (Ef) to 1.5 V (Er). tf, tr = 0.05 s and tw = 0.24 s.
• Fig. 3.1.4j: Cathodic ECL under the similar condition as in Fig. 3.1.4g; potential step was from 1.5 V (Ef) to - 2.05 V (Er). tf, tr = 0.05 s and tw = 0.24 s.
• Fig. 3.1.5a: ECL emission spectrum of DPA.
• poly(2-ethylhexyloxy-5-methoxy-1,4-phenylenevinylene) (MEH-PPV).
• Fig. 3.2.2.b: Cyclic voltammogram (CV) of Pt/MEH-PPV in 0.1 M TBABF4 in acetonitrile at a scan rate of 100 mV/s.
• Fig. 3.2.3a: Cottrell plot for Pt/MEH-PPV in 0.1 M TEABF4¯AN, when the potential step was made from 0 V to -1.9 V of duration 1 s.
• Fig. 3.2.4a: Anodic ECL when the potential was switched between ¯1.75 V and 1.45 V of duration 4 s each for MEH-PPV film on Pt in 0.1M TEABF4¯AN.
• Fig. 3.2.4b: Cathodic ECL when the potential was switched between 1.45 V and ¯1.75 V of duration 4 s each for MEH-PPV film on Pt in 0.1 M TEABF4¯AN.
• Fig. 3.2.4c: Anodic ECL, when the potential was switched between ¯1.85 V and 1.35 V of duration 1 s each for MEH-PPV film on Pt in 0.1 M TEAPF6¯AN.
• Fig 3.2.4d: Cathodic ECL, when the potential was switched between 1.35 V and ¯1.85 V of duration 1 s each for MEH-PPV film on Pt in 0.1 M TEAPF6¯AN.
• Fig. 3.2.4e: Comparison of anodic ECL and current decay for Pt/MEH-PPV in 0.1M TEABF4¯AN.
• poly(2,3dibutoxy 1,4-phenylenevinylene) DB-PPV.
• Fig. 3.3.2a: Cyclic voltammogram (CV) of Pt/DB-PPV in 0.1M TEAClO4 in acetonitrile; scan rate: 100 mV/s.
• Fig. 3.3.2b: Cyclic voltammogram (CV) of Pt/DB-PPV in 0.1 M TBABF4 in acetonitrile; scan rate = 100 mV/s.
• Fig. 3.3.2c.: Cyclic voltammogram (CV) of Pt/DB-PPV in 0.1 M TEABF4 in acetonitrile; scan rate = 100 mV/s.
• Fig. 3.3.2e: Anodic potential sweep of Pt/DB-PPV in 0.1 M TBABF4 in acetonitrile; scan rate = 100 mV/s.
• Fig. 3.3.2f: Potential sweep of Pt/DB-PPV (thickness = 1μm, energy of cross-linking = 400 mJ/cm2) in 0.1 M TBABF4 in acetonitrile; scan rate = 100 mV/s.
• Fig. 3.3.3b: Cottrell plot for the cathodic current on the negative potential step of the square wave between 0 and ¯1.7 V, with step width of 150 ms each, for Pt/DB-PPV in 0.1 M TBABF4 in acetonitrile.
• Fig. 3.3.4a: Cathodic ECL when the potential was switched between 1.3 V and -1.85 V of duration 150 ms each for DB-PPV film of thickness ≈ 0.6 μm on Pt in 0.1 M TBABF4 - AN.
• Fig 3.3.4b: Anodic ECL, when the potential was switched between -1.9 V and 1.3 V of duration 150 ms each for DB-PPV film of thickness ≈ 0.6 μm on Pt in 0.1M TBABF4-AN.
• Fig. 3.3.4c: Anodic ECL when the potential was switched between - 1.9 V and 1.2V of duration 150 ms each for DB-PPV film of thickness ≈ 0.6 μm on Pt in 0.1M TEABF4-AN.
• Fig. 3.3.4d: Scheme of the sequential anodic and cathodic potential steps in ECL experiments.
• Fig. 3.3.5a: Decrease in the anodic ECL intensity with successive potential steps.
• Fig. 3.3.5b: Film morphology before the ECL experiment as seen with a microscope.
• Fig. 3.3.5c: Film morphology after the ECL experiment as seen with a microscope.
• Fig. 3.3.5d: SEM image of the DB-PPV coated electrode surface.
• Fig. 3.3.5e: SEM image of the DB-PPV coated electrode surface at a higher magnification.
• Fig. 3.3.5f: EDS analysis on the darker spots as seen in the SEM image.
• Fig. 3.3.5.g: Energy of cross-linking (mJ/cm2) used to cross-link the DB-PPV films against the magnitude of cathodic charging current.
• Fig.3.3.5i: Comparison of Raman spectra between the powder form and the cross-linked film of the polymer DB-PPV before the ECL experiment.
• Fig. 3.3.5j: Comparison of the Raman spectra of the polymer DB-PPV before and after the ECL experiment.
• Fig. 3.3.5k: DB-PPV in phenylic form.
• Fig. 3.3.5l: DB-PPV in quinonic form.
• Fig. 3.3.6a: Cathodic ECL spectrum.
• Fig. 3.3.6b: Anodic ECL spectrum.
• Fig. 3.3.6c: Absorption spectrum of DB-PPV in CHCl3.
• Fig. 3.3.6d: Flourescence spectrum of DB-PPV in CHCl3.
• Fig. 4.1.2a: Equivalent circuit representation of the electrochemical processes.
• Fig. 4.1.3a Representation of concentration array in point model.
• Fig. 4.1.6 a. Simulation of the anodic transient (potential step between -2.05 V and 1.6V, duration 50ms each) with C° = 1×10-6 mol cm-3 kex =3x1010 lmol-1s-1, D = 5x10-6 cm2 s-1, Ru= 109 Ω and Cdl= 1.5μ F.
• Fig. 4.1.6 b. Simulation of the cathodic transient (potential step between 1.6 V and -2.05V, duration 50ms each) with C°=1×10-6 mol cm-3 kex =1x1010 lmol-1s-1, D = 5x10-6 cm2 s-1, Ru= 109 Ω and Cdl= 1.5μ F.
• Fig. 4.1.6c: Effect of double layer charging on the potential step from -2.05 V to 1.6 V.
• Fig. 4.1.6 d: Effect of double layer charging on the potential step from 1.6 V to -2.05 V.
• Fig. 4.2.1a. Representation of double layer charging in the electrode/polymer/solution system: situation prior to charge transfer.
• Fig. 4.2.1b. State of the polymer during charge transfer and transport maintaining bulk electroneutrality.
• Fig. 4.2.1b. State of the polymer during charge transfer and transport maintaining bulk electroneutrality.
• Fig. 4.2.1c: Plot of current density against t-1/2 under diffusion-migration conditions for the electrode reaction entity="Objekt118" label="88#28"/>. The pure diffusion case (Cottrell) was included for comparison.
• Fig 4.2.6a. Simulation of the anodic transient (for a potential step between -1.75 V and 1.45 V, SE - 0.1M TEABF4) with C° = 5.8×10-4 mol cm-3, kex =1x103 lmol-1s-1, Dap = 3.2x10-12cm2 s-1, Ru= 1x104Ω and Cdl=1 μF.
• Fig 4.2.6b. Simulation of the cathodic transient (for a potential step between 1.45 V and -1.75 V, SE - 0.1M TEABF4) with C° = 5.8×10-4 mol cm-3, kex=3x103 lmol-1s-1, Dap = 8x10-13 cm2 s-1, Ru = 1x104 Ω and Cdl =2 μF.
• Fig. 4.2.6c. Simulation of the anodic transient (for a potential step between -1.85 V and 1.35 V, SE - 0.1M TEAPF6) with C° = 5.8×10-4 molcm-3, kex = 6x103 lmol-1s-1, Dap = 0.5x10-12 cm2 s-1, Ru= 1x104Ω and Cdl= 5μF.
• Fig 4.2.6d. Simulation of the cathodic transient (for a potential step between 1.35 V and -1.85 V, SE - 0.1M TEAPF6) with C° = 5.8×10-4 molcm-3 kex = 5x103 lmol-1s-1, Dap = 1.5x10-13 cm2 s-1, Ru = 1x104 Ω and Cdl = 6 μF.
• Fig. 4.3a. Simulation of the anodic ECL transient (for a potential step between -1.9 V and 1.3 V, SE - 0.1M TBABF4) with C° = 6.1×10-4 molcm-3, kex = 3x104 lmol-1s-1, Dap = 6x10-11 cm2 s-1, Ru= 1200 Ω and Cdl= 1 μF.
• Fig. 4.3b. Simulation of the cathodic ECL transient (for a potential step between 1.3 V and -1.8 V, SE - 0.1M TBABF4) with C° = 6.1×10-4 molcm-3, kex =1x104 lmol-1s-1, Dap = 1x10-11 cm2 s-1, Ru= 1200 Ω and Cdl= 3μ F.
• Fig. 5.1a. Typical shape of the triple potential steps including the waiting period tw in the ECL experiments.