Density Functional Theory in the Design of Organometallics for Energy Conversion

Abstract

Theoretical methods based on density functional theory (DFT) and timedependent 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) IntroductionMetal complexes and organometallics play a key role in many devices for energy conversion, particularly those where light is involved. For example, in the new generation of display screen equipment and energy-efficient lighting -organic light-emitting diodes (OLEDs) -the use of complexes of heavy transition metals as phosphorescent emitters allows large gains in efficiency through the harvesting of otherwise wasted triplet states [1]. Meanwhile, in the reverse process of light-toelectrical energy conversion, metal complexes that absorb light to generate chargetransfer states are at the forefront of research into dye-sensitised solar cells (DSSCs) [2]. Metal complexes with long excited-state lifetimes and that are strong excitedstate oxidants or reductants also attract attention in the field of 'artificial photosynthesis' (AP) -the conversion of light-to-chemical energy, for example, in

Computational studies of Ni(II) photosensitizers complexes containing 1,1′-bis(diphenylphosphino)ferrocene and dithio ligands

“…The analysis of the frontier molecular orbitals (FMOs) is very helpful to study electronic and electrochemical properties of the complexes, on which depend their DSSC functions [6,10,24,32]. This analysis allows the identification of the electronic transitions related to the appearance of bands in the electronic absorption spectra.…”

“…To obtain more efficient DSSCs, the improvement of the photoanode, via the development of the sensitizing dye, is one promising approach [4]. Several theoretical and experimental investigations have been performed to find sensitizers with good performance and low manufacturing cost [5,6]. The ideal dye must (i) absorb UV and visible light, extending into the near-IR region with high extinction coefficients, (ii) should exhibit strong adsorption on the surface of semiconductor TiO 2 , and (iii) for the charge-separation lifetime, its unoccupied orbitals (particularly the LUMO) must be higher in energy than the conduction band of the semiconductor while its HOMO energy must be lower than the redox potential of the electrolyte [1,2,7].…”

“…HOMO of the four complexes are based on Ni atomic orbital which indicate that their regeneration occur by electronic transfer from electrolyte to the nickel atom. The electronic transition from HOMO orbital should be of MLCT character as required for dye sensitizer[6]. HOMO-1 in complex 4 results from…”