1.1 Porphyrins - Friedrich-Alexander-Universität Erlangen-Nürnberg

Discussion and Results 3
Table 16. Quantum yields of fluorescence (Φfl), inter system crossing (ΦISC), internal
conversion (ΦIC) and 1 O2 generation (ΦΔ) for systems 53, 57 and 58 in DMF.
Compound Φfl ± 0.01 a ΦISC ± 0.03 ΦIC ± 0.04 ΦΔ ± 0.03 b
53 0.03 0.88 0.09 0.85
57 0.03 0.75 0.22 0.65
58 0.05 0.23 0.72 0.22
a
excitation at 532 nm, pyropheophorbide a as reference (Φfl = 0.28) a
b
excitation at 515 nm, 5,10,15,20-tetraphenylporphyrin (TPP, 15) as reference (ΦΔ = 0.65) 88
This data nicely illustrates again the differences between six-membered ring derivative 58
and its seven-membered ring analogs 53 and 57. While the latter show a significant
relaxation from the first excited singlet state via ISC (S1 → T1), 58 tends to relax in a
radiationless fashion by IC processes. This could be explained by the higher flexibility in the
structure of 58 compared to 53 and 57 as NMR data have already shown. Nevertheless, all
system exhibit a highly efficient energy transfer (T1 → 1 O2) being estimated to be around 0.9
for 57 and close to 1 for 53 and 58.
3.2.6.5.5 Cyclic voltammetry
The influence of the ongoing increase in conjugation of the tethered phenyl ring’s orbitals
into those of the porphyrin macrocycle is also visible in CV. The obtained half-wave
potentials are given in Table 17 while the corresponding voltammograms are displayed in
Figure 36.
Table 17. Half-wave potentials E½ for the given compounds obtained by measurements in
CH2Cl2 (c = 10 -3 M) vs. ferrocene E(Fc/Fc + ) = 0.53 V as internal standard (for experimental
setup see paragraph 6.1).
Compound E½ Red2 [V] E½ Red1 [V] E½ Ox1 [V] E½ Ox2 [V]
53 -1.23 -0.95 +1.02 +1.25
57 -1.20 -0.84 a +1.10 +1.35
58 -1.01 -0.63 +1.07 +1.38
15 b,112
-1.45 -1.14 +1.05 +1.35
a
process is not fully reversible a
b
literature data on 5,10,15,20-tetraphenylporphyrin (see Scheme 6, p. 7)
89