shape of the spectra since the extinction coefficient of the fullerene cores in this area are negligible compared with the porphyrin one. Characteristic in this area are the Soret band at about 400 nm and the Q bands at about 515, 550 and 590 nm. Bathochromic shifts of 7-4 nm for all these bands are observed in 72 relative to the parent dyad 53, while shifts of 4-2 nm are observed for hybrid 71. These shifts are unequivocally reflecting that interaction between the porphyrin and the dendrons occurs and that the magnitude of this interaction is increasing with the generation number as was expected. These shifts might derive from microenvironmental effects and/or from the deviation of planarity which the porphyrin could suffer in the resulting crowded surroundings.
Figure 3.28. UV/Vis (CH2Cl2) spectra of porphyrin-dendrimer-[60]fullerenes (a) - - - - - - 71;
(b) –– - –– - –– 72, and (c) –– –– –– –– the parent porphyrin-[60]fullerene 53.
In a preliminary investigation the influence of the dendritic coverage in porphyrin-dendrimer-[60]fullerenes 71 and 72 on the redox potentials of the porphyrin moiety was studied. This project is part of a collaboration with Prof. Echegoyen at Miami University. Cyclovoltammetric studies of compounds 53, 71, 72 and 73[83] (figure 3.29) were performed[88].
These preliminary results are shown in table 4.1 and figure 3.30. The complete assignment of the redox potentials still needs to be corroborated in further studies.
oxidation |
reduction | ||||||
E1 |
E2 |
E3 |
E1 |
E2 |
E3 |
E4 | |
53 |
0.760 |
0.957 |
1.095 |
-0.623 |
-0.997 |
-1.344 |
-1.432 |
73 |
0.778 |
0.982 |
1.115 |
-1.423 |
|||
71 |
0.783 |
1.133 |
-1.381 |
||||
72 |
0.668 |
0.909 |
1.209 |
-1.444 |
|||
Table 3.1. E1/2 (V) values of compounds 53, 71-73. All values are based on the Ag/AgCl reference electrode.
Figure 3.29. Fullerene hexakisadduct 73.
The cyclovoltammogram of 53 presents two first reductions steps with potentials equal to
-0.623 and -0.997 mV corresponding to two reversible reduction processes in the fullerene core. These two peaks are not present in 71-73. In these cases the high substitution of the fullerene cage has decreased their number of 
-electrons and disrupted its -system, being not possible to achieve these low energy reduction steps. The first reduction potentials for 71-73 are assigned to the same reduction process at the porphyrin moiety. The tendency observed is that the values of the first reduction potential become more negative when increasing the size of the addends attached to the porphyrin-fullerene.
The first oxidation potential of these compounds are most likely due to the porphyrin group. In the most crowded porphyrin 72 it appears at 0.668 mV whereas in the naked dyad 53 its value is 0.760 mV.
Figure 3.30. Cyclovoltammograms of 53, 71-73. (a) oxidation, and (b) reduction processes.
Further photochemical and photophysical investigation will help to understand the nature of the processes involved. A plausible interpretation of these results is that the polar microenvironment helps to attain the different oxidation states within the porphyrin. The numerous oxygen atoms of the dendritic branches apparently tend to stabilize a positive charge on the porphyrin[89]. For the same reason, the first reduction step within the porphyrin moiety is energetically more disfavorable with increasing the size of the addends attached to the fullerene core.