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Fe Complexes as Photosensitizers for Dye-Sensitized Solar Cells
Published in Carlito S. Ponseca, Emerging Photovoltaic Technologies, 2019
The RuII bis(terpyridine) complex 16 (Scheme 5.5) is well-known for its much shorter-lived 3MLCT ES compared to the tris(bipyridine) complex 3. This is mainly ascribed to a larger deviation of the coordination sphere from a perfect octahedral geometry as imposed by the rigid tridentate ligand featuring five-membered chelate rings. The thus attenuated overlap between the σ-lone electron pairs of the N-donor and the eg* orbitals of the metal center leads to decreased ligand field strength, resulting in low-lying 3MC state that deactivates the 3MLCT state at a higher rate [18, 89]. This issue has been addressed in a prominent example by Hammarström et al., the RuII bis(dqp) (dqp = 2,6-di(quinolin-8-yl) pyridine) complex 17 (Scheme 5.5), with expanded chelate ring size and thus a close-to-perfect octahedral geometry [90]. Thanks to the significantly enhanced ligand field strength due to the now optimal M–L orbital overlap, and the extended π-conjugation of the ligand, the MLCT energy is beneficially stabilized relative to the 3MC state. This gives rise to a tremendously retarded 3MLCT deactivation, yielding a RT 3MLCT ES lifetime of 3 µs [90–92]. In a similar strategy, combining the expansion of the chelate ring and the extension of the π-conjugation of the ligand, the resulting RuII bis(dcpp) (dcpp = 2,6-bis(2-carboxypyridyl)pyridine) complex 18 (Scheme 5.5) displayed a RT 3MLCT ES lifetime of 1.36 µs [93].
Orbital Tuning of Ruthenium Polyimine Complexes by Ligand Design: From Basic Principles to Applications
Published in Ajay Kumar Mishra, Lallan Mishra, Ruthenium Chemistry, 2018
Joe Otsuki, Guohua Wu, Ryuji Kaneko, Yayoi Ebata
Therefore, the strategy to increase the excited-excited lifetime is to lower the 3MLCT state energy relative to the 3dd state energy. To decrease the 3MLCT state energy, electron-withdrawing groups are introduced into the ligands to lower the π* orbital energies. To extend the π-system of the ligand is another effective strategy to lower the π* orbital energies. In this case, the nuclear displacement associated with the MLCT excitation is small that further helps reduce the nonradiative decay rate. On the other hand, raising the energy of dd state is a complementary method to enlarge ∆E. Introducing a stronger σ-donating ligand, which is not necessarily the ligand that provides the π* orbital for the MLCT state, is a straightforward strategy. Even a π-basic ligand would help, which increases the electron density on the metal that in turn raises the dσ* orbital energy not only the dπ orbital energy, giving rise to a larger ΔE. A few examples of emissive ruthenium complexes with a bis(tridentate) ligand will be given in the next section.
Introduction to Organometallics
Published in Samir H. Chikkali, Metal-Catalyzed Polymerization, 2017
Samir H. Chikkali, Sandeep Netalkar
As explained earlier, even though H2O has two lone pairs of electrons, it cannot function as a bidentate ligand. Hypothetically, if we consider that H2O donates both of its lone pair to single central metal ions, it would still be bound to the metal through a single atom and hence would be considered as a monodentate ligand. For a ligand to function as a bidentate system, two valence pairs of electrons on two separate atom sites are required. For example, ethylene diamine and oxalate dianion bind to the metal through two separate atoms and hence is considered as bidentate ligand (Figure 1.5). Bidentate ligands form one of the most important classes of ligands in metal-catalyzed polymerization. The significance of bidentate ligands is presented in Chapters 2, 3, 4, 6, and 7. Similarly, a ligand with three lone pairs of electrons would serve as tridentate ligand (Figure 1.5) and so on.
Structural conversion of an oxazolidine ligand upon treatment with copper(I) and (II) halides; structural, spectral, theoretical and docking studies
Published in Journal of Coordination Chemistry, 2018
Zahra Mardani, Vali Golsanamlou, Zahra Jabbarzadeh, Keyvan Moeini, Saba Khodavandegar, Cameron Carpenter-Warren, Alexandra M. Z. Slawin, J. Derek Woollins
The DEA ligand acts as tridentate NO2-donor through a secondary amine nitrogen and two alcohol oxygen atoms and forms two five-membered nonplanar chelate rings. Each tridentate ligand can coordinate to the metal in facial or meridional forms. In the mer form, there are two angles of 90° and one at 180°; in the fac form, there are three angles of 90°. In 2, two angles of coordinated DEA are deviating from 90° due the chelating bite angle, while the third one is about 149°, confirming mer form (135°, exactly half way between fac and mer) [49, 50].