Abstract
The prevalence of favourable photophysics, i.e. long excited state lifetimes and high luminescence quantum yields, among complexes of heavier transition metals (TMs) is well-established. Broad applicability of these materials in solar cell and display technology, cancer treatment and more is known, though the declining commercial availability of heavier TMs threatens their sustainable employment. Some literature approaches involve using strong-donor N-heterocyclic carbene (NHC) donors in combination with first-row TMs, alongside tripodal binding frameworks, as a means of improving excited state lifetimes. The work herein hence investigates the influence of tripodal ligand binding on TM photophysics, where a ligand of N-donor character is investigated alongside ligands of NHC-donor character. Novel complexes incorporating both first-row and heavier TMs are reported.Chapter 2 describes preparation of the tripodal N-donor ligand 4,4',4''-((4-methoxyphenyl)methanetriyl)tris(1-benzyl-1H-1,2,3-triazole) (ttz), which was then used to prepare homoleptic, first-row TM complexes (Mn(II), Fe(II), Co(II), Co(III), Ni(II)). Complexes of Fe(II) and Co(III) exhibited MC excited state lifetimes on the nanosecond timescale (5.3 ns and 3 ns, respectively), perhaps inferring applicability to photocatalysis.
Chapter 3 describes the preparation of homoleptic complexes of ttz using group 8 TMs (Fe(II), Ru(II), Os(II)). Transient absorption (TA) spectroscopy in acetonitrile allowed fitting of long-lived time constants to the excited state dynamics of Ru(II) and Os(II) complexes prior to ground state recovery (44 ns and 6.6 μs, respectively). Supporting density functional theory (DFT) and photochemical stability studies inferred that pseudo-penta-coordinate species were responsible for the long-lived traces, where regeneration of the broken M-N bond facilitated ground state recovery under brief periods of irradiation.
Heteroleptic ruthenium(II) complexes [Ru(ttz)(N,N)(py)](PF6)2 (where N,N = 2,2’-bipyridine (bpy), 1,10-phenanthroline (phen), 3,3’-biisoquinoline (biiq) and py = pyridine) are reported in Chapter 4. [Ru(ttz)(bpy)(py)](PF6)2 exhibited photochemical ejection of pyridine, resulting in full conversion into the photoproduct [Ru(ttz)(bpy)(CD3CN)](PF6)2. [Ru(ttz)(phen)(py)](PF6)2 and [Ru(ttz)(biiq)(py)](PF6)2 also exhibited photochemical ejection of pyridine, however a mixture of photoproducts was observed in each case ([Ru(κ3-ttz)(N,N)(CD3CN)](PF6)2 and [Ru(κ2-ttz)(N,N)(CD3CN)2](PF6)2).
Several carbene-donor precursors are reported in Chapter 5, where the borate-containing precursor [PhB(imidazo(1,5-a)pyridine)3](PF6)2 (L1) was used to prepare complexes of Fe(III) (Fe1) and Co(III) (Co). Fe1 exhibited 2LMCT absorption maxima at λ = 712 and 783 nm and an excited state lifetime of 14 ps, which is approximately 140 times shorter than that of the known literature analogue. Co exhibited a 1MLCT absorption band at λ = 350 nm, alongside 77 K emission maxima at λ = 556, 607 and 667 nm, presumably derived from a 3MC state in agreement with literature observations. Initial attempts at preparing an Fe(III) complex of the precursor [PhB(1-phenyl-5-methyl-1,2,3-triazole)3](OTf)2 (L2) led to formation of a mixture of two species. Later studies by Dr Daniel Ross have allowed for identification of these two species as the desired Fe(III) complex (Fe3) and an oxidised Fe(IV) variant (Fe4). Complexes bearing tripodal ligands have been thoroughly investigated throughout, where successful tuning of existing first-row sensitisers represents a key contribution to the field.
| Date of Award | 15 Jan 2025 |
|---|---|
| Original language | English |
| Sponsors | Engineering and Physical Sciences Research Council |
| Supervisor | Paul Elliott (Main Supervisor) & Paul Scattergood (Co-Supervisor) |