1.1 Porphyrins - Friedrich-Alexander-Universität Erlangen-Nürnberg

Discussion and Results 3
effective quenching. The corresponding energy level diagrams depicting the different
situations are shown in Scheme 35.
S 1
S 0
S 1
T 1 T 1 T 1
S 0
regular porphyrin copper(II) porphyrin nickel(II) porphyrin
Scheme 35. Energy levels in different metalloporphyrins. Red lines represent COULOMB
exchange interactions (CEI), green lines stand for spin-orbit couplings whereat dotted lines
resemble normal measures and straight lines depict enhanced effects. 47,104,107c
In the copper(II) case, a low lying singlet state on the energy level of the corresponding T1
state evolves due to CEI and is strongly coupled with the S1 state above while it does not
couple with T1. So the fluorescence is quenched as consequence of relaxation involving that
singlet state. For nickel(II) porphyrins, low lying triplet states exist which enhance spin-orbit
couplings and thus make relaxation via fluorescence or IC in the singlet system improbable.
So the relaxation will majorly occur in the triplet system either by IC processes or by
phosphorescence which is especially pronounced for other metalloporphyrins of that group
(e.g. palladium species). 47,104,107c
Hence, the following discussion on photophysical parameters will concentrate on the
fluorescent derivatives Zn(II)-53 and In(III)-53 whose data are summarized in Table 10 104 .
Measurements elucidated that both metalloporphyrins fluoresce at lower wavelengths (blue
shifted compared to 53) while they show lowered fluorescence quantum yields and shorter
decay times (see Table 10). These effects are comprehensible if one takes into account that
metallation also affects the energies of the frontier orbitals and those of the excited singlet
and triplet states in regular systems. Additionally, in both cases only one major fluorophore
(proportion > 97 %) was detected serving as further proof for the presence of tautomeric
structures in the free base.
S 1
S 0
65