[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 gelat ion ability of Zn(II)porphyrin appended cholesterols 5a-d. At 5 degreesC 5a with a (CH2)(2) spacer gelated aromatic hydrocarbons such as benzene, tolu ene, 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 th ese 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 p resence of [60]fullerene. Detailed spectroscopic studies established that t he reinforcement of the gel structure is rationalized in terms of the inter molecular 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 increase d 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 co nsistently 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]fullere ne sandwich complex. The distinct bathochromic shift of the Soret absorptio n 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 inte raction does exist between the Zn(II) porphyrin moiety and the [60]fulleren e 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 r eliable Cotton effects around the Soret absorption band. The observed CD in tensities 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 ge l phase by the interaction with [60]fullerene. In particular, the highly st abilized gel sample of 5a obtained by the addition of [60]fullerene, showed a negative excitation-coupling-type CD spectrum, indicating that the porph yrin chromophores are oriented in an anticlockwise direction. The aggregate structures formed in the organogel systems of 5a and 5c were also discusse d by means of infrared spectroscopic measurements and scanning electron mic roscopy observations. Thus, this is the first example for organogel stabili zation utilizing a Zn(II) porphyrin-[60]fullerene interaction.