[60]fullerene can reinforce the organogel structure of porphyrin-appended cholesterol derivatives:: Novel odd-even effect of the (CH2)n spacer on the organogel stability

This paper reports the unique influence of added [60]fullerene on the gelation ability of Zn(II)porphyrin appended cholesterols 5a-d. At 5 degreesC 5a with a (CH2)(2) spacer gelated aromatic hydrocarbons such as benzene, toluene, and p-xylene (2.55 x 10(-2) mol dm(-3)), whereas at 20 degreesC these transparent gels were changed into sols. In the presence of [60]fullerene (0.5 equiv) the gel structure of 5a was maintained even at 20 degreesC in these solvents. A similar gel stabilization effect was also observed for 5c, which has a (CH2)(4) spacer. In contrast, 5b and 5d with a (CH2)(3) and a (CH2)(5) spacer, respectively, could not gelate these solvents even in the presence of [60]fullerene. Detailed spectroscopic studies established that the reinforcement of the gel structure is rationalized in terms of the intermolecular Zn(II) porphyrin[60]fullerene interaction, which is possible only in the gel phase. When the concentration of 5a and 5c in toluene was kept constant (0.20 mol dm-3), the sol-gel phase transition temperature increased with an increase in the number of equivalents of added [60] fullerene up to 0.5 equiv and then maintained constant above 0.5 equiv, These results consistently support the view that two porphyrin planes in 5a and 5c interact with one [60]fullerene molecule to form a 2:1 Zn(II) porphyrin/[60]fullerene sandwich complex. The distinct bathochromic shift of the Soret absorption band, which was found in the gel samples of 5a and 5c in the presence of [60]fullerene in toluene, indicates that the intermolecular electronic interaction does exist between the Zn(II) porphyrin moiety and the [60]fullerene in the gel phase. The circular dichroism (CD) spectra of the gel samples of 5a and 5c in the presence of [60]fullerene showed the reproducible and reliable Cotton effects around the Soret absorption band. The observed CD intensities were much stronger than those obtained from the sol samples of 5a and 5c in the absence of [60]fullerene. The results suggest that the Zn(II) porphyrin moieties in 5a and 5c are enforced to orient chirally in the gel phase by the interaction with [60]fullerene. In particular, the highly stabilized gel sample of 5a obtained by the addition of [60]fullerene, showed a negative excitation-coupling-type CD spectrum, indicating that the porphyrin chromophores are oriented in an anticlockwise direction. The aggregate structures formed in the organogel systems of 5a and 5c were also discussed by means of infrared spectroscopic measurements and scanning electron microscopy observations. Thus, this is the first example for organogel stabilization utilizing a Zn(II) porphyrin-[60]fullerene interaction.