Charge Separation in Polyoxometalate-Based Systems for Solar Energy Production Open Access

Glass, Elliot Nelson (2016)

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The desire for green, renewable energy has driven many areas of science in recent years. In chemistry, one popular vision for sustainable energy utilizes sunlight to split water molecules into hydrogen and oxygen gas. When used as a fuel, these gases release energy that can be used to power our civilization, with water as the only waste product. To realize this vision, cost-effective and robust materials are being sought to improve upon current approaches. This includes molecular catalysts to promote water oxidation (yielding oxygen) and proton reduction (yielding hydrogen), and robust materials capable of absorbing sunlight to generate a charge separated state, providing the driving force for both catalytic reactions. The research described herein focuses on the latter problem, exploring the underlying photophysical properties of chromophoric polyoxometalates (POMs) as photosensitizers for solar energy production. The first part of this work investigates the photophysical properties of Keggin POM chromophores by principally utilizing ultrafast spectroscopic techniques. While the excited state lifetimes of most POMs are too short to facilitate catalytic reactions, [CoIIW12O40]6- was found to have a far-longer lived excited state generated through a metal-to-polyoxometalate charge transfer (MPCT) transition (Chapter 2). This extended lifetime was ascribed to a transient structural change and charge localization. In Chapter 3, the influence of structural modifications of this POM on the excited state lifetime were investigated by substituting transition metals into the structure, yielding the series [CoII(MxOHy)W11O39](12-x-y)- (MxOHy = VIVO, CrIII(OH2), MnII(OH2), FeIII(OH2), CoII(OH2), NiII(OH2), CuII(OH2), ZnII(OH2)). Substitution maintains and modulates the original MPCT transition, but decreases the excited state lifetime. These complexes serve as useful models for advancing the development of robust photosensitizers, though their photophysical properties limit their direct application in a light-driven chemical system. Chapter 4 describes the synthesis, structure, and photodynamics of three Sn-containing POMs. A charge transfer transition in one complex is attributed to a unique interaction between Sb and Sn ions in the structure. In Chapter 5, additional POM chromophores are investigated by ultrafast spectroscopy, exhibiting short-lived excited states that limit their applications as effect chromophores for solar energy applications.

Table of Contents

Chapter 1: Polyoxometalates in Solar Energy Research. 1

1.1 Fundamental Challenges in Solar Energy Production. 2

1.2 Photosynthesis as a Guide. 7

1.3 Photosensitizers in Solar Fuel Production. 11

1.4 Overview of Polyoxometalate Chemistry. 16

1.5 Applications of Polyoxometalates in Solar Fuel Production. 27

1.6 Goal of This Work and Outline. 33

1.7 References. 34

Chapter 2: Influence of Heterometal Location on Charge Transfer Lifetimes in Keggin Polyoxometalate Chromophores. 47

2.1 Abstract. 48

2.2 Introduction. 49

2.3 Experimental. 50

2.4 Results and Discussion. 60

2.5 Conclusion. 92

2.6 References. 93

Chapter 3: Transition Metal Substitution Effects on Charge Transfer Lifetimes in Keggin Polyoxometalates. 100

3.1 Abstract. 101

3.2 Introduction. 101

3.3 Experimental. 104

3.4 Results and Discussion. 124

3.5 Conclusion. 164

3.6 References. 165

Chapter 4: Charge Transfer in All-Inorganic Networks and a Tetramer Based on Tin(II)-Containing Polyoxometalates. 172

4.1 Abstract. 173

4.2 Introduction. 173

4.3 Experimental. 175

4.4 Results and Discussion. 181

4.5 Conclusion. 201

4.6 References. 201

Chapter 5: Charge Transfer Lifetimes in Other Polyoxometalate Chromophores. 209

4.1 Abstract. 210

4.2 Introduction. 210

4.3 Experimental. 212

4.4 Results and Discussion. 216

4.5 Conclusion. 225

4.6 References. 225

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