Organometallics and Related Molecules for Energy Conversion

With the rapid growth of population and urbanization, the energy demand is increasing annually. Due to the major problems concerning the rapid depleting nature of the extraction of fossil resources, energy conservation and transition to renewable energy supplies have been a hot topic worldwide. Organic light-emitting diodes and solar cells represent two important techniques to allow the efficient utilization of energy resources in energy-saving devices and exploration of using renewable energy in energy-producing devices. In this chapter, the importance of using organometallic and organic molecules in both areas of research is discussed.

Theoretical methods based on density functional theory (DFT) and time-dependent density functional theory (TD-DFT) are increasingly used to rationalise the excited-state properties of metal complexes and to help guide the design of new materials. This chapter provides a brief introduction of the background to such methods, highlighting some of the features that need to be considered, such as the ability of functionals to deal with charge-transfer states and the challenges associated with triplet-state calculations. Examples are drawn from recent studies on (1) the ground-state and light absorption properties of ruthenium(II) complexes as sensitisers for dye-sensitised solar cells (DSSCs) and (2) the triplet excited states of luminescent platinum(II) complexes that are potential phosphors for organic light-emitting diodes (OLEDs).

Ruthenium and iridium complexes have been widely used as sensitizer for dye-sensitized solar cells (conversion of light into electricity) and as highly phosphorescent emitters for organic electroluminescence (conversion of electricity into light). The high costs and limited availability of these platinoid metals have motivated the search for alternatives based on first-row transition metals. First-row transition metal complexes have also been used as alternatives to existing materials as redox mediator. This chapter provides an overview of such materials used as an active component of the aforementioned devices.

Dye-sensitized solar cell (DSSC) is a type of excitonic solar cells with photoanode sensitized by organic molecules, which serve as light harvester. Ruthenium complex-based dyes are a significant class of molecules in the development of DSSCs since 1991. Basically, ruthenium complex is composed of a ruthenium metal center and ancillary ligands. They have been intensively studied because of their excellent photovoltaic properties. In this chapter, we extensively summarize the efforts made to the development of Ru-based dye molecules, such as the approaches to change ancillary ligands of ruthenium complexes and tune the energy levels of the lowest unoccupied molecular orbital and the highest occupied molecular orbital. We also summarize the stability improvement of the dye molecules by introducing hydrophobic ligands. Finally, alternative approaches to expand light response of the Ru dye-sensitized devices including co-sensitization and plasmon-enhanced light harvesting are discussed.

Bulk-heterojunction (BHJ) solar cells using n-type semiconducting polymers, instead of the conventional fullerene derivatives, are known as all-polymer solar cells. Their significant advantages include designable structures of both p-type and n-type semiconducting polymers. In this chapter, recent advances in all-polymer solar cells are introduced. Particular attention is focused on the development of high-performance n-type semiconducting polymers. However, organometallic polymers have generally been employed as p-type semiconductors in BHJ solar cells. Therefore, there are a few examples of all-polymer solar cells based on organometallic polymers. In order to solve this problem, a novel method of inverting the semiconducting feature is applied to produce promising Pt-polyyne polymers with potentially n-type energy levels. The main chain alkynes of the precursor Pt-polyyne polymers are modified by the high-yielding transformation into tetracyanobutadiene units through a [2 + 2] cycloaddition-retroelectrocyclization with tetracyanoethylene (TCNE). All-polymer solar cells composed of the p-type poly(3-hexylthiophene) (P3HT) and n-type Pt-polyyne polymer are successfully fabricated, and the photocurrent generation is demonstrated.

In recent years, significant progress has been achieved in the field of triplet–triplet annihilation (TTA)-based energy upconversion, in which transition-metal complexes as the sensitizers play a key role. These complexes are different from organic fluorophores because the triplet excited states, instead of the singlet excited states, are populated with a high intersystem crossing (ISC) efficiency upon photoexcitation. Meanwhile, the long-lived triplet excited states in the microsecond range are observed for these complexes. All these properties are favorable when transition-metal complexes, including Ir(III), Pd(II), Pt(II), Ru(II), Zn(II), Re(I), Cu(I), and Au(III) complexes summarized herein, are used as sensitizers for TTA upconversion. Moreover, some examples of organic sensitizers and the applications of TTA upconversion systems are also summarized.

Transition metal complexes containing Pt(II), Ir(III), Ru(II), and Re(I) atoms and showing visible light-harvesting ability and long-lived T₁ excited states have been developed and used as photosensitizers for triplet–triplet annihilation (TTA) upconversion, which is a promising upconversion method due to its low excitation power density (solar light is sufficient), high upconversion quantum yield, readily tunable excitation/emission wavelength, and strong absorption of excitation light. TTA upconversion involves triplet energy transfer between a photosensitizer (donor) molecule and an acceptor/annihilator/emitter. This chapter is based on upconversion examples, aiming to cover the challenges that are faced by the developments of TTA upconversion and the molecular structure designing rationales for the triplet photosensitizers and triplet acceptors.

Triarylboron functionalization was found recently to be a highly effective strategy in greatly enhancing the phosphorescence efficiency of Pt(II) compounds and the performance of Pt(II) compounds in electrophosphorescent OLEDs. This chapter examines the role of a dimesitylboron (BMes₂) group in the phosphorescence and electrophosphorescence of N^C-, C^C, and N^C^N-chelate Pt(II) compounds. The influence of the location of the BMes₂ group, substituent groups, ancillary ligands, and intramolecular hydrogen bonds on phosphorescent color/efficiency and the stability of the Pt(II) compounds are discussed in detail. The key focus is on the strategies of achieving efficient blue phosphorescent Pt(II) compounds. Preliminary electrophosphorescent data for BMes₂-functionalized Pt(II) compounds are also presented.

White organic light-emitting diodes (OLEDs) are regarded as one of the most ideal semiconducting solid-state lighting sources to replace the conventional incandescent bulbs and fluorescent lamps. Phosphorescent OLEDs are advantageous over fluorescent ones in terms of higher efficiency by harvesting both singlet and triplet excitons. Transition-metal complexes represent the most successful phosphorescent materials for use in OLEDs. This chapter highlights the basic knowledge and application of the transition-metal complex phosphors that are frequently used in monochromic and white OLEDs, mainly including complexes of iridium(III), platinum(II), and copper(I). The elaboration generally covers the synthesis, excited states, photophysical properties, OLEDs performance, etc. Special emphasis is placed on structure-property-performance relationship of each organometallic phosphor. The advantages and challenges faced by both noble metals Ir and Pt and the normal metal Cu are discussed. How to achieve high-performance and low-cost white OLEDs based on organometallic phosphors is suggested.

Phosphorescent white organic light-emitting diodes (WOLEDs) employing organometallic phosphors as the emitters have attracted considerable attention in the past decade. Due to their capability of harvesting both singlet and triplet excitons to generate highly efficient devices, phosphorescent WOLEDs present potential applications in the next-generation solid-state lighting sources and in the flat-panel display with the assistance of the color filters. In this chapter, we attempt to give a brief overall introduction to the phosphorescent WOLEDs. The basic concepts, like electric-light conversion efficiency, parameters to assess the color quality of the emissive white light, device strategies to fabricate WOLEDs, and the common device fabrication procedures, are introduced firstly. These fundamental understandings are also favorable to comprehend the monochromatic OLEDs and the WOLEDs comprised of other emitters like fluorescent dyes. In particular, we focus on the discussion of phosphorescent WOLEDs with various device architectures and their corresponding device performances. We also note that further enhancement of the device lifetime with simultaneous realization of high efficiency and high quality of the emissive white light could make phosphorescent WOLEDs promising candidates for the alternative next-generation lighting sources.

Kinetics and mechanisms of reduction of protons and CO₂ catalyzed by metal complexes and nanoparticles have been discussed in this chapter. Kinetic studies including deuterium kinetic isotope effects on heterogeneous catalysts for hydrogen evolution by proton reduction have been demonstrated to provide essential mechanistic information on bond cleavage and formation associated with electron transfer. The rate-determining steps in the catalytic cycles are clarified by kinetic studies, providing valuable information on observable intermediates. The most important intermediates in the catalytic reduction of protons and CO₂ are metal-hydride complexes, which can reduce protons and CO₂ to produce hydrogen and formic acid, respectively. The catalytic interconversion between hydrogen and a hydrogen storage compound has been made possible by changing pH, providing a convenient hydrogen-on-demand system in which hydrogen gas can be stored as a liquid (e.g., formic acid) or solid form (NADH) and hydrogen can be produced by the catalytic decomposition of the hydrogen storage compound.

Water can be used as a cheap and renewable source of electrons and protons to make nonfossil fuel-based chemical energy carriers for a sustainable power supply. However, water oxidation is an intricate chemical process and an energy-intensive reaction involving the removal of four electrons with the release of four protons at the same time. Inside the thylakoid membrane in plant leaves is embedded a manganese-calcium molecular cluster in natural photosystem II (PS-II), which represents an excellent model for designing an artificial equivalent of the photosynthesis for light-to-fuel conversion via water splitting. Inspired by the natural PS-II, the scientific community has been striving hard during the last two decades to develop a bio-inspired catalytic system for water oxidation. However, a truly biomimetic catalytic system matching the performance of photosystem for efficient water splitting operating with four consecutive proton-coupled electron transfer (PCET) steps to generate oxygen and hydrogen for hundred thousands of cycles at high rate is yet to be demonstrated. In this chapter, we provide an insight regarding the biomimetic approaches to make molecular and organometallic water oxidation complexes that have been investigated recently in homogeneous solution catalysis using chemical oxidants or as surface-immobilized heterogeneous species for electro-assisted catalytic systems. After comparing their catalytic activities and stabilities, an overview of the mechanistic aspects is also discussed.

Energy directly harvested from sunlight offers an ultimate method of meeting the needs for clean energy with minimal impact on our environment. Intensive research efforts are currently being put on the development of efficient conversion system that can transform solar energy into fuel via light-driven water splitting to generate H₂ and O₂, learning from Nature’s photosynthesis to collect and store solar energy in chemical bonds. Especially, the development of efficient water oxidation catalysts is one of the key issues for achieving artificial photosynthetic devices. From a practical point of view, it is highly desirable to replace noble metal catalysts, which have been quite successful so far, by earth-abundant metal catalysts. In recent years, there has been noticeable progress in the development of water oxidation catalysts (WOCs) based on earth-abundant metals. This review chapter covers the most significant achievements in WOCs based on manganese and cobalt complexes, with emphasis on recent developments in the last three years.

Hydrogen has found important applications as reducing agent for chemical transformations and is nowadays considered as one of the most promising energy vectors able to fuel devices to produce electricity on demand (direct hydrogen fuel cells). Crucial to its application is the understanding at the molecular level of how hydrogen interacts with (transition) metals which are commonly used as catalysts to lower the energy barrier to split the H₂ molecule into its components and allow transfer and reactivity. In this chapter, selected examples of hydrogen activation by water-soluble organometallic complexes are summarized.

Metal-organic frameworks (MOFs), a new class of emerging materials with porosity, crystalline, high interior surface area, controllable structures, high thermal stability, and high yield with low cost, are showing the potential applications for hydrogen storage/release. With respect to physical hydrogen storage (compression, liquefaction, and physisorption), the chemical hydrogen storage is free from extreme processing conditions and safety risk. In this chapter, we select recent and significant advances in the development of MOFs as platforms for hydrogen generation from chemical hydrides and highlight special emphasis on enhanced kinetics and thermodynamics for (1) hydrogen generation from chemical hydrides confined in MOFs, (2) MOF-supported metal nanoparticle-catalyzed hydrogen generation from chemical hydrides, and (3) hydrogen generation from chemical hydrides catalyzed by catalysts formed using MOFs as precursors.

In this chapter, the role of organometallic compounds for hydrogen storage applications is highlighted. In this context, the focus is on transition metal complex-catalysed dehydrogenations of amine-borane adducts as a special class of so-called chemical hydrides as well as dehydrogenation reactions of formic acid and alcohols, which are also discussed as possible fuel alternatives.

The appealing photophysical properties of cyclometalated iridium complexes, such as their intense and highly efficient luminescence and long-lived excited states, render them highly desirable for the conversion and storage of solar energy. In this chapter, we describe general considerations in terms of pursuing these applications and track the successful use of cyclometalated iridium complexes as photosensitizers in dye-sensitized solar cells (DSSCs) and light-driven hydrogen production, which have gained a large amount of attention in recent years. Particular emphasis is placed on the systematic elucidation of the correlation between the complex architectures, the corresponding photophysical behavior, and the aforementioned potential promising applications of cyclometalated iridium complexes, which are expected to contribute possible design implications to the development of superior light-harvesting components for efficiently driving photosynthesis.