Abstract
A number of novel ruthenium(II) polypyridine complexes have been designed and synthesized for use as photosensitizers in dye-sensitized solar cells (DSSCs) due to their rich photophysical properties such as intense absorption, long-lived lifetimes, high emission quantum yields and unique redox characteristics. Many of these complexes exhibit photophysical behavior that can be readily controlled through a careful choice of ligands and/or substituents. With this perspective, we review the design and general synthetic methods of some polypyridine ligands based on bipyridine, phenanthroline, terpyridine and quaterpyridine with/without anchoring groups with a view to correlate functionality of ligand structures with the observed photophysical, electroredox and power conversion efficiency of some examples of Ru(II) polypyridyl complexes that have been reported and particularly used in the DSSCs applications. The main interest, however, is focused on showing the development of new polypyridine ligand materials containing long-range electron transfer motifs such as the alkenyl, alkynyl and polyaromatic donor functionalities.
Highlights
The recent interest generated by the study of Ru(II) polypyridine complexes has stimulated the growth of several branches of pure and applied chemistry
It has been reported that the choice of ligand, structure and/or substituents on nitrogen-based coordinated ligands are paramount in order to influence the photophysical and electro-redox properties of the coordinated ruthenium complexes, which may lead to enhanced incident photon to current efficiency (IPCE) in the resulting dye sensitized solar cells
The observed trend in the UV-Vis absorption will be for the chromophore to absorb all the wavelength coverage concomitant with good molar extinction coefficient at the near visible region (300–405 nm) that covers the vibronic peaks for the intra-ligand (π→π*) charge transfer transitions characteristics of anthracene derivatives, which may compliment the 1MLCT absorption of the metal complex for a better photon absorption and possibly enhance the IPCE values of the dye-sensitized solar cells [87,90,92]
Summary
The recent interest generated by the study of Ru(II) polypyridine complexes has stimulated the growth of several branches of pure and applied chemistry. A complete charge-separation, absorption of solar radiation wavelengths in the visible or near-IR region, appropriate ground and excited state redox potentials, presence of an anchoring group such as the carboxylic or phosphonic acid needed to bind strongly to the TiO2 semiconductor, and efficient electron injection into the TiO2 conducting band. It is imperative to further examine and understand the chemical properties and synthetic approaches used to obtain materials for the construction of dyes for DSSCs. It has been reported that the choice of ligand, structure and/or substituents on nitrogen-based coordinated ligands are paramount in order to influence the photophysical and electro-redox properties of the coordinated ruthenium complexes, which may lead to enhanced incident photon to current efficiency (IPCE) in the resulting dye sensitized solar cells. According to Keene and co-workers, bpy-based complexes induce chirality at the metal centre, and separation of the enantiomers in polymetallic complexescan be complicated, leading to the development of achiral tridentate 2,2':6',2"-terpyridine (tpy)-type ligands [48,49]
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