1.1 Porphyrins - Friedrich-Alexander-Universität Erlangen-Nürnberg

3 Discussion and Results
phenyl substituents. For 53 and 57 those are clearly lowered since a significant line
broadening is observed representing the presence of a coalescence-like state. Connatural
situations are also found in non-annulated tetraarylporphyrins with distinguishable half-
spaces but at higher temperatures. As for 58, the exocyclic structure seems to be flexible
itself, no definite statement is possible according to the underlying data.
Since the rising tilt of the tethered phenyl ring is supposed to affect the electronic structure
of the porphyrin core by mingling with its orbitals, variations in the porphyrin’s ring current
are to be observed resulting in significant shifts of resonances for protons inside or situated
directly on the periphery of the macrocycle. And indeed, this is detected for all studied
compounds whereas the shifts seem to be the more pronounced the more the tethered
phenyl ring is tilted. Exemplarily, some observed signals are gathered in Table 13.
Table 13. Chemical shifts δH of selected signals in compounds 53, 57 and 58 with given
position number based on measurements at 400 MHz in CDCl3 at rt.
a
86
Compound δH(2) δH(NH) dihedral angle a
53 8.96 -1.62 55°
57 9.16 -1.70 46°
58 9.24 -0.55 15°
angle between the C meso -C α -bond and the plane of the tethered phenyl substituent (see also Figure 32, p. 84) a
3.2.6.5.3 UV/Vis Spectroscopy
Just like the NMR spectra, also the UV/Vis spectra of 53 and 57 appear similar while 58
differs quite a lot (Figure 34). While for all compounds a huge bathochromic shift of all bands
is observed compared to the precursors, it is most pronounced for 58 as the summarized
data in Table 14 depicts.
The rising bathochromic shift nicely reflects the more pronounced interaction of the
tethered phenyl ring’s orbitals with those of the porphyrin macrocycle when the substituent
becomes more tilted. The effect, being in full accordance to literature data of extended π-
systems 46,48a , appears most pronounce for 58 where the tethered substituent is nearly
coplanar and thus in significant conjugation. Since 53 and 57 do not differ that much in their
torsion of the tethered phenyl ring it is also not surprising that their spectra are very similar.
Interestingly, 58 shows a considerable shoulder on the SORET band which is also detectable