Dissertation

|1.9 Multicomponent Systems from Fundamental Building Blocks|
(5) small energy gap between relevant excited states
(6) high efficiency of population of the reactive excited state (η)
(7) suitable lifetime of the reactive excited state (τ)
(8) high energy content of the reactive excited state (E 0-0 )
(9) good kinetic factors for outer sphere electron transfer reactions (k ET ).
In the research of multicomponent systems for photocatalytic water reduction, many
chromophores, including acridine dyes, metalloporphyrins, metallophthalocyanines, and
transition metal complexes of Os, Cr, Ir or Ru were screened. [29, 37, 35] Because of their outstanding
properties, especially ruthenium complexes with polypyridine ligands, and [Ru(bpy) 3 ] 2+ in
particular, may be considered as prototype workhorse-chromophores (see figure 17 for examples).
N
N
N
N
N
2+ N
2+ N
2+ N
Ru
N Ru N
N Ru N
N
N
N
N
N
N
a b c
Figure 17: Representative ruthenium polypyridine chromophores: a) [Ru(bpy) 3 ] 2+ , [36]
b) [Ru(tpy) 2 ] 2+ , [38] c) [Ru(bqp) 2 ] 2+[39] .
For a better understanding of the capacity of this type of complex, it is important to understand the
ground state, excited state and redox characteristics. Therefore, it is useful to consider its molecular
orbitals, instead of being uniformly distributed across the molecule, to be localized on a specific
part of it. According to the localized molecular orbital approximation, molecular orbitals of the
complex derive from orbitals of its precursor components (metal ion and free ligand). In general,
upon complex formation, energies of the existing orbitals will be shifted, but their positions remain
predominantly localized on whether metal fragment (M) or ligand (L). As a result of that, although
slightly changed, several features of the components will be retained in the resulting complex (e.g.
possible electronic transitions). [40] |24|