1.1 Porphyrins - Friedrich-Alexander-Universität Erlangen-Nürnberg

Discussion and Results 3
Table 24. Determined half-wave potentials E½ for compounds 91 and 92 in comparison to
those obtained for unfunctionalized 53. Values given in V vs. ferrocene E½(Fc/Fc + ) = +0.53 V.
For detailed experimental parameters see paragraph 6.1, p. 138.
Compound E½ Red2 [V] E½ Red1 [V] E½ Ox1 [V] E½ Ox2 [V]
91 -1.02 -0.86 +1.13 +1.35
92 -1.08 -0.79 - a
53 96 -1.23 -0.95 +1.02 +1.25
a only irreversible processes found a
In the cathodic region, both systems show quasi-reversible redox processes whereas in the
anodic region, only for nitro substituted 91 a reversible redox behavior is found. The
irreversible behavior of amino substituted 92 can thereby be due to slowed electron transfer
processes or consecutive chemical reactions leading to detectable cathodic peak potentials
at +0.98 V and +1.08 V of higher peak current ipc and one at +1.37 V of lower ipc while the
corresponding anodic peak potentials at +0.92 V and +0.21 V appear quite separate and only
have very low peak currents. A reliable explanation is not possible based on this data since
several processes might participate in causing that behavior. E.g. a generated porphyrin
radical cation could react with the amino group of another molecule (potentials lie in region
of the first oxidation potential of unfunctionalized 53 96 ) or the amino group could become
oxidized itself and show further reactions. For 91 three reduction processes are detected
whereat the ones of higher potentials show peak currents comparable to the ones observed
for the oxidation processes. For the third process, the half-wave potential can only be
estimated to be around -1.25 V since no clear peaks are observable. The additional reduction
process is thereby due the presence of the reducible nitro group.
Interestingly, for both 91 and 92, the obtained negative half-wave potentials E½ Red are
significantly shifted to higher values compared to 53 (see Table 24) meaning that both
systems are easier to reduce. This effect seems to arise from the higher group
electronegativity of -NH2 and -NO2 (calc. ENG(NH2) = 2.70 and ENG(NO2) = 3.21) compared to
-t-Bu (calc. ENG(NH2) = 2.48). 140 Thus, -NH2 and -NO2 have negative inductive effects easing
electron uptake processes. The same effect is considered to shift the positive half-wave
potentials E½ Ox to higher values for 91 compared to 53 meaning that oxidation is more
difficult to achieve. Thus, the peripheral substituents do indeed influence the electronic
structure of the core system.
- a
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