1.1 Porphyrins - Friedrich-Alexander-Universität Erlangen-Nürnberg

Discussion and Results 3
To explain this behavior, electronic and structural effects of the metal centers have to be
taken into account.
Nickel(II) being the most electronegative metal center (EN(Ni) = 1.91) 115 and also the
smallest one (rNi(II) = 0.63 Å) 116 is thus perfectly fitting within the macrocycle allowing an
efficient orbital overlap and therewith a pronounced electronic communication leading to
the huge shifts observed for the signals of the β-protons. Due to the filled dz2 orbital, axial
ligands are repelled and such complexes exist as bare metal species in mostly square planar
coordination geometry while distortions of the usually planar macrocycle are common to
provide saddled or ruffled conformations. 40 As this eases rotational processes, the intense
line broadening for the arylic resonances becomes well comprehensible.
Indium(III) and zinc(II) being less electronegative (EN(In) = 1.65, EN(Zn) = 1.78) 115 represent
larger ions (rIn(III) = 0.74 Å, rZn(II) = 0.82 Å) 116 which adopt out-of-plane conformations when
they form porphyrin complexes being five-coordinate in a square pyramidal fashion. This is
leading to differentiable half-spaces within the porphyrin molecule affecting the rotational
barriers of the meso-phenyl substituents and also the chemical shifts of the protons lying in
those half-spaces. With rising rotational barriers, the signals become sharper while the then
differentiable chemical surroundings above and below the macrocycle’s plane provide larger
signal differences (Δδ). These facts explain the better resolution observed in those spectra
whereas the effects appear strongest for zinc(II) complex Zn(II)-53. That fact becomes
plausible by taking into consideration that indium(III) is smaller and that the additional
ligand in indium(III) porphyrin complexes is very labile (binding constants too small to be
measured in most cases) leading to fast exchange processes on the NMR timescale. 40
In connection with the photophysical properties, UV/Vis and steady-state fluorescence
spectra were recorded and investigations were conducted concerning the different
relaxation pathways from the first exited singlet state and singlet oxygen generation.
The UV/Vis spectra have the typical shape for metallated porphyrins consisting of the SORET
absorption in the blue region and two Q-band absorptions in the red region of the spectrum
(Figure 27). For Zn(II)-53 and In(III)-53, the whole spectra are significantly bathochromically
shifted by 8 and 10 nm, respectively, for the SORET band and between 32 and 49 nm for the
Q-bands. In the spectra obtained for Ni(II)-53 and Cu(II)-53 the corresponding shifts are non-
uniform as only the QI-band depicts a red shift by 17 and 28 nm, respectively, while the
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