Photogenerated avenues in macromolecules containing Re(i), Ru(ii), Os(ii), and Ir(iii) metal complexes of pyridine-based ligands

Abstract

Pyridine-based ligands, such as 2,2'-bipyridine and 1,10-phenanthroline, have gained much interest in the fields of supramolecular chemistry as well as materials science. The appealing optoelectronic properties of their complexes with heavy d(6) transition metal ions, such as Ru(ii), Os(II), Re(I) and Ir(III), primarily based on the metal-to-ligand charge-transfer (MLCT) nature featuring access to charge-separated states, have provided the starting point for many studies in the field of dye-sensitized solar cells (DSSCs), organic light emitting diodes (OLEDs), artificial photosynthesis and photogenerated electron as well as energy transfer processes. This critical review provides a comprehensive survey over central advances in the field of soluble metal-containing macromolecules in the last few decades. The synthesis and properties of functionalized 2,2'-bipyridyine- and 1,10-phenanthroline-based d(6) metal complexes, in particular, their introduction into different prevailing polymeric structures are highlighted. In the most part of the review metal complexes which have been attached as pendant groups on the polymer side chain are covered. Selected applications of the herein discussed metal-containing macromolecules are addressed, particularly, with respect to photogenerated electron/energy transfer processes. In order to enable a deeper understanding of the properties of the ligands and metal complexes, the fundamentals of selected photophysical processes will be discussed (223 references).

Towards Water Soluble Mitochondria-Targeting Theranostic Osmium(II) Triazole-Based Complexes

“…This is significantly blue-shifted relative to that observed for 2 (491 nm) consistent with the btzpy ligand, and hence its complex, having a much higher energy LUMO as indicated from the CV data described above. Similarly to the data for 2, complex 1 also exhibits absorptions of lesser intensity at longer wavelengths corresponding to spinforbidden direct 3 MLCT excitations (λ max = 526 nm) from the singlet ground state enabled by the large spin-orbit coupling constant associated with the osmium centre [32,33]. In stark contrast to its ruthenium(II) analogue, 1 is emissive at room temperature in de-aerated acetonitrile solutions, with the emission being characterized by a broad featureless band at 595 nm (λ ex = 500 nm) and a lifetime of 937 ns ( Figure 1b and Table 1) attributed to an emissive 3 MLCT state.…”

“…This is significantly blue-shifted relative to that observed for 2 (491 nm) consistent with the btzpy ligand, and hence its complex, having a much higher energy LUMO as indicated from the CV data described above. Similarly to the data for 2, complex 1 also exhibits absorptions of lesser intensity at longer wavelengths corresponding to spin-forbidden direct 3 MLCT excitations (λ max = 526 nm) from the singlet ground state enabled by the large spin-orbit coupling constant associated with the osmium centre [32,33]. [42][43][44].…”

“…Bis(tridentate) complexes of ruthenium(II) typically show little or no emission at room temperature as the deviation from an ideal octahedral-like coordination geometry results in stabilization of triplet metal-centred ( 3 MC) states relative to the 3 MLCT state [46,47]. As such, 3 MC states are efficiently populated from photoexcited 3 MLCT states thereby quenching emission. The observed emission for 1 must therefore arise from the destabilization of the 3 MC states due to the typically larger ligandfield splitting associated with the 5d metal centre over its 4d counterpart, such that non-radiative depopulation of the emissive 3 MLCT state is comparatively disfavoured.…”

“…Since phosphorescence is longer-lived than the autofluorescence from biological organic compounds, emissive complexes are amenable to use in time-gated imaging microscopy [29][30][31]. Osmium(II) complexes, however, often exhibit lower energy spin-forbidden direct 3 MLCT optical absorption bands of moderate extinction coefficient due to the high spin-orbit coupling constant for the osmium centre [32,33]. This offers the advantage of enabling efficient excitation at lower energies that therefore avoids potential cellular damage and negates autofluorescence and the necessity of the added expense of time-gated apparatus.…”

“…Oligopyridyl complexes of kinetically inert d 6 metals, e.g., Ru(II), Os(II), have attracted enormous interest in recent decades due to their attractive photophysical properties [1][2][3][4]. These complexes typically exhibit relatively long-lived triplet metal-to-ligand charge transfer ( 3 MLCT) states.…”